text:
This action might not be possible to undo. Are you sure you want to continue?
Fundamentals of modern UV-visible spectroscopy
Tony Owen
Copyright Agilent Technologies 2000 All rights reserved. or translation without prior written permission is prohibited. except as allowed under the copyright laws. Printed in Germany 06/00 Publication number 5980-1397E . The information contained in this publication is subject to change without notice. adaption. Reproduction.
Good Laboratory Practice has grown in importance. Computers are used increasingly to evaluate data. We sought to rectify the situation. a good knowledge of the basics of UV-visible spectroscopy. we also want to show that the conventional "single measurement at a single wavelength" approach to obtaining results is insufficient for assuring optimum results. As instrument manufacturers.Preface Preface In 1988 we published a primer entitled "The Diode-Array Advantage in UV/Visible Spectroscopy". and of the pitfalls of sample handling and sample chemistry remains essential for good results. The primer was very well-received. and we felt this was an appropriate time to produce a new primer. and a new generation of diode-array spectrophotometers is characterized by much improved performance. and many thousands of copies have been distributed. Multiple measurements at multiple wavelengths or (preferably) full spectra yield the best accuracy and precision of results and provide the information necessary to detect erroneous results. at Agilent Technologies from whom I have learned so much about UV-visible spectroscopy over the years. their characteristics and their advantages compared with conventional scanning spectrophotometers were not well-understood. With this primer. At the time. With this primer. I would like to take this opportunity to thank my colleagues. we would like to believe that analytical instruments are now easier to operate. Microprocessor and/or computer control has taken much of the drudgery out of data processing and has improved productivity. too numerous to mention by name. Much has changed in the years since the first primer. iii . Despite these advances. of the instrumental limitations. our objective is to review all aspects of UV-visible spectroscopy that play a role in obtaining the best results. although diode-array spectrophotometers had been on the market since 1979.
iv .
.................................................................................................................................................................................................................................................. 6 Obtaining derivative spectra ....................... 32 v ............................... 21 Simple simultaneous equations method ..... 3 Origin of UV-visible spectra .......................................................................................................................................................................................................................................................................................................................................................... 16 Sample requirements .................................... 26 Instrumental requirements ............................................ 14 Enzyme activity.......................... 2 Wavelength and frequency ............................................. 6 Derivative spectra ............................................. 27 Indirect quantification ...................................................................................................................................................................................................... 16 Quantitative analysis ............ 10 Identification—spectra and structure ........ 20 Multicomponent analysis ....... 15 Instrumental considerations................................ 14 Protein and nucleic acid melting temperature ....................................................... 27 Chemical derivatization....................................... 30 Sources ................................................................ 10 Qualitative analysis ..................................................................................................................................................................................................................................................................................................... 13 Other qualitative information ...................................................................................................................................Contents Chapter 1 Principles and applications of UV-visible spectroscopy Basic principles... 28 Chapter 2 Instrumentation Instrumental design.......................................................................................... 24 Other methods .................................... 8 Applications ..................................................................... 21 Least squares method ...... 10 Confirmation of identity............................................................. 21 Principle of additivity........................................................................... 16 Beer's law ............................................................................. 3 Transmittance and absorbance ......................................................................................... 31 Dispersion devices ................ 2 The electromagnetic spectrum...................................... 27 Spectrophotometric titrations................................................................. 28 Enzyme kinetic assays............................................................................ 9 Instrumental considerations ........................................................................... 30 Components.............................................................................. 26 Sample requirements .............................................................................................................................................................. 9 Signal-to-noise...................... 11 Color .....
...................................................................................................................... 43 Measuring a spectrum ....................................................................................................... 68 Correction techniques ............................................................. 44 Wavelength accuracy and precision ...................................................................................................................... 36 The conventional spectrophotometer .............................................................................................................................................................................................................. concentration...... pH.................................................. 49 Noise .................................. 59 Effect of solvent.............................................................................................................................................................. 70 Multicomponent analysis........................................ 57 Sources of error ......................................................................................... 43 Key instrumental parameters................................................................................................................................................................................................................................................................................................................................................................................................. 47 Photometric accuracy and precision.................. 62 No reference .................................................................... 37 Configuration... 62 Refractive index .................................................................................................................... 56 Cell types ............ 52 Chapter 3 Sample handling and measurement Liquid samples ......................................... 56 Cells ...................................................... 39 Dual-beam design .................... 44 Spectral resolution. 70 vi ................ 65 Wavelength averaging................................................................................................................................. 49 Stray light........ and temperature............................................................................................................. 66 Interference ........................................................................................................ 42 Dual-wavelength design.................................................................. 64 Time averaging ........ 56 Material ................. 68 Scattering........................... 63 Sample geometry........................... 64 Changing slit width ............................................................................................................................................................................. 39 Single-beam design.................................................................. 51 Drift........................................ 36 The diode array spectrophotometer................................................................................................................. 33 Optics ............................................. 58 Care of cells. 50 Linear dynamic range ...Contents Detectors ................................................................................................................................................. 69 Isoabsorbance .......................................................................................................................................................... 67 Other absorbing compounds......... 60 Solid samples .......................................................................................... 59 Choice of solvent......................................... 63 Weak absorbance............................. 40 Split-beam design .............................................................................................................................................................................................. 67 Types of interference ................................................................................................. 65 Strong absorbance...........................................
.................................................................... 91 Instrumental requirements ....................................... 94 Test parameters ..................................................................................................Contents Background modeling ............................... 99 Emission standards ...... 100 Regulatory requirements....................................................................................... 105 Recommendations ............................................................. 98 Standards ................................................................................................... 107 Instrument self-test ...................... 95 Photometric accuracy and precision .......................................................................................................................................................................................................................................... 73 Photochemical problems..... 94 Wavelength accuracy and precision.................... 104 American Standard Testing Methods ............................... 87 Range ......................................... 92 Method validation.... 78 Chapter 4 Method development and validation Method development ..................................................................................................................... 97 Noise ...... 96 Resolution.................................................................... 96 Stray light..... 86 Sensitivity....................................... 72 Three-point correction... 77 Fluorescence .............................................................................................. 85 Precision.......... 72 Derivative spectroscopy ................................................................................................................................................................................................................................ 102 GLP/GMP ..... 89 Selectivity............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................. 99 Liquid absorption standards...................................................................... 92 Chapter 5 Routine operation Instrument performance verification............................................................................................................... 99 Solid absorption standards................................................................................................................................ 102 European Pharmacopoeia ............................... 80 Linearity ............................................................................................. 81 Accuracy... 102 United States Pharmacopoeia....................................................................... 112 vii ....................................... 97 Baseline flatness .......................................................... 77 Sample decomposition................................................... 98 Linearity ........................................................................... 71 Internal referencing....................... 89 Ruggedness ..................................................... 98 Stability ................
............. 126 Resolution ..................................................................................................................... 122 Fast spectral acquisition ...................... 125 Measurement statistics........................ 114 Confirmation wavelengths.................................................................................................................................... 115 Statistics ................................................. 128 Errors in measuring fluorescent samples ........................................................................................129 Index........................................................................................................................................................................................................................ 125 Ruggedness and reliability ................................................................ 113 Electronic storage................................................................................................................................................ 126 Disadvantages of diode array spectroscopy ................................... 123 Wavelength resettability ....................................................................................................................... 125 Open sample area............... 114 Standard operating procedures... 127 Sample decomposition ......................................133 viii ................................... 128 References................................................................................................................................................................................................... 120 Appendix B Characteristics of diode array spectrophotometers Advantages of diode array spectroscopy .................................................................... 124 Sensitivity............................................. 115 Full spectra ..................................... 113 Proper operation................ 122 Simultaneous multiwavelength measurement ........................................................................................... 117 Appendix A Accuracy and precision Definition of terms ............................................................... 127 Complexity.............................. 126 Stray light ................................................................................................................................. 114 Collateral data...............................................................................................................................................Contents System suitability .....
chapter 1 Chapter 1 Principles and applications of UV-visible spectroscopy 1 .
Frequency [Hz] Ultraviolet Visible Infrared Microwave Gamma ray Cosmic ray Ultraviolet visible Television Infrared Radar Wavelength [m] Figure 1 The electromagnetic spectrum 2 Radio X ray NMR . infrared (IR). These provide valuable insight into the uses and limitations of this technique for chemical analysis.Principles and applications of UV-visible spectroscopy This chapter outlines the basic theories and principles of UV-visible spectroscopy. which includes such other forms of radiation as radio. and X rays (see Figure 1). The primary applications of UV-visible spectroscopy are also briefly reviewed. Basic principles The electromagnetic Ultraviolet (UV) and visible radiation comprise only a small spectrum part of the electromagnetic spectrum. cosmic.
Origin of UV-visible When radiation interacts with matter. we want only absorbance to occur. h is Planck's constant (6. including reflection. absorption of light by matter causes the energy content of the molecules (or atoms) to increase. absorbance. The total potential energy of a molecule 3 . In UV-visible spectroscopy. In general. Because light is a form of energy. and photochemical reaction (absorbance and bond breaking). wavelength usually is expressed in nanometers (1 nm = 10-9 m). it is the UV component of light that causes sunburn). which are related by the following equation: ν = c⁄λ where ν is frequency (in seconds). Because radiation acts as a wave. when measuring UV-visible spectra.62 × 10-34 Js). the low-wavelength UV light has the highest energy. a number of spectra processes can occur. In UV-visible spectroscopy. it can be classified in terms of either wavelength or frequency. and λ is wavelength (in meters). this energy is sufficient to cause unwanted photochemical reactions when measuring sample spectra (remember. fluorescence/phosphorescence (absorption and reemission).Principles and applications of UV-visible spectroscopy The energy associated with electromagnetic radiation is defined by the following equation: E = hν where E is energy (in joules). c is the speed of light (3 × 108 ms-1). Wavelength and Electromagnetic radiation can be considered a combination frequency of alternating electric and magnetic fields that travel through space with a wave motion. scattering. and ν is frequency (in seconds). It follows from the above equations that radiation with shorter wavelength has higher energy. In some cases.
and rotational energies: Etotal = E electronic + E vibrational + E rotational The amount of energy a molecule possesses in each form is not a continuum but a series of discrete levels or states. Figure 2 shows an example of electronic transitions in formaldehyde and the wavelengths of light that cause them.Principles and applications of UV-visible spectroscopy generally is represented as the sum of its electronic. vibrational. photons of UV and visible light have enough energy to cause transitions between the different electronic energy levels. The differences in energy among the different states are in the order: Eelectronic > E vibrational > E rotational In some molecules and atoms. The wavelength of light absorbed is that having the energy required to move an electron from a lower energy level to a higher energy level. transition transition Figure 2 Electronic transitions in formaldehyde 4 .
the bands are broadened (see Figure 4). This is true for atoms.Principles and applications of UV-visible spectroscopy These transitions should result in very narrow absorbance bands at wavelengths highly characteristic of the difference in energy levels of the absorbing species. The broadening is even greater in solutions owing to solvent-solute interactions. Figure 3 Electronic transitions and spectra of atoms However. vibrational and rotational energy levels are superimposed on the electronic energy levels. for molecules. 5 . Because many transitions with different energies can occur. as depicted in Figure 3.
Derivative spectra If a spectrum is expressed as absorbance (A) as a function of wavelength (λ). the derivative spectra are: 6 . The amount of light absorbed is expressed as either transmittance or absorbance. absorbance values are used since the relationship between absorbance and both concentration and path length normally is linear.Principles and applications of UV-visible spectroscopy electronic energy levels vibrational energy levels rotational energy levels electronic transition Figure 4 Electronic transitions and UV-visible spectra in molecules Transmittance and When light passes through or is reflected from a sample. the absorbance amount of light absorbed is the difference between the incident radiation (Io) and the transmitted radiation (I). Transmittance usually is given in terms of a fraction of 1 or as a percentage and is defined as follows: T = I ⁄ I o or % T = ( I ⁄ I o ) × 100 Absorbance is defined as follows: A = – log T For most applications.
Principles and applications of UV-visible spectroscopy Zero order: A = f(λ ) dA -----. This derivative has a positive and a negative band with maximum and minimum at the same wavelengths as the inflection points in the absorbance band.= f ″( λ ) 2 dλ 2 First order: Second order: Figure 5 on the next page shows the effects of derivatization on a simple Gaussian absorbance band. The most distinctive feature of the second-order derivative is a negative band with minimum at the same wavelength as the maximum on the zero-order band. This bipolar function is characteristic of all odd-order derivatives. This derivative also shows two positive satellite bands on either side of the main band. The derivative spectra are always more complex than the zero-order spectrum.= f ′( λ ) dλ d A -------. passing through zero at the same wavelength as λmax of the absorbance band. The fourth derivative shows a positive band with a maximum at the same wavelength as the maximum on the zero order band. 7 . It starts and finishes at zero. Even-order derivatives show a negative or positive band with minimum or maximum at the same wavelength as λmax on the absorbance band. The first derivative is the rate of change of absorbance against wavelength.
electronic. and a polynomial Aλ = a 0 + a1 λ + … + a l λ l 8 . these have been largely superseded by mathematical methods.Principles and applications of UV-visible spectroscopy Absorbance Absorbance 1st derivative 2nd derivative 3rd derivative 4th derivative Figure 5 Derivative spectra of a Gaussian absorbance band Obtaining derivative spectra Optical. a window of ± n data points is selected. To calculate the derivative at a particular wavelength (λ). and mathematical methods all can be used to generate derivative spectra. Although optical and electronic techniques formed the basis of early UV-visible spectroscopy.
the intrinsic information content is not increased. An unwanted effect of the derivatization process is the decrease in S/N with higher orders of derivatives. If the analyte band has a bandwidth of 20 nm. The smoothing properties of the Savitzky-Golay polynomial technique can be used to mitigate the decrease in S/N. This decrease follows from the discrimination effect (see "Derivative spectroscopy" on page 73) and from the fact that noise always contains the sharpest features in the spectrum.Principles and applications of UV-visible spectroscopy is fitted by the least squares method. and so on. If the polynomial order (l) is less than the number of data points (2n+1) in the window. the noise has a 2-nm bandwidth. Savitzky and Golay developed a highly efficient method to perform the calculations that is the basis of the derivatization algorithm in most commercial instruments. Applications Derivative spectra can be used to enhance differences among spectra. most importantly. the polynomial generally cannot pass through all data points. Thus the least squares fit gives a smoothed approximation to the original data points. to resolve overlapping bands in qualitative analysis (see "Confirmation of identity" on page 11) and. where a1 is the first derivative. if the spectral data used in the derivative calculation are at 2-nm intervals. The coefficients a0 … al at each wavelength are the derivative values. to reduce the effects of interference from scattering. a2 is the second derivative. or other absorbing compounds in quantitative analysis (see "Derivative spectroscopy" on page 73). Although transforming a UV-visible spectrum to its first or a higher derivative usually yields a more complex profile than the zero-order spectrum (see Figure 5). but care must be taken as too Signal-to-noise 9 . it is decreased by the loss of lower-order data such as constant offset factors. matrix. the S/N of the first derivative will be 10 times worse than with the zero-order spectrum. This method also smooths the data. Thus. In fact.
A chromophore is a molecular group usually containing a π bond. The negative effect of derivatization on S/N also places increased demands on low-noise characteristics of the spectrophotometer. Most absorption by organic compounds results from the presence of π (that is. Table 1 lists some chromophores and the wavelengths of their absorbance maxima. Instrumental considerations The higher resolution of derivative spectra places increased demands on the wavelength reproducibility of the spectrophotometer. Qualitative analysis Identification—spectra UV-visible spectra generally show only a few broad and structure absorbance bands. Compared with techniques such as infrared spectroscopy. Small wavelength errors can result in much larger signal errors in the derivative mode than in the absorbance mode. it produces a compound with absorption between 185 and 1000 nm. UV-visible spectroscopy provides a limited amount of qualitative information. which produces many narrow bands. When inserted into a saturated hydrocarbon (which exhibits no UV-visible absorbance spectrum). If the spectrophotometer can scan and average multiple spectra. Table 1 Selected chromophores and their absorbance maxima Chromophore Carbonyl (ketone) Carbonyl (aldehyde) Formula RR'C=O RHC=O Example Acetone Acetaldehyde λmax (nm) 271 293 10 .Principles and applications of UV-visible spectroscopy high a degree of smoothing will distort the derivative spectrum. S/N can be improved further prior to derivatization. unsaturated) bonds.
Confirmation of Although UV-visible spectra do not enable absolute identity identification of an unknown. Conjugating the double bond with additional double bonds increases both the intensity and the wavelength of the absorption band. For some molecular systems. However. the position of the absorbance maximum is not fixed but depends partially on the molecular environment of the chromophore and on the solvent in which the sample may be dissolved. As shown in Figure 6. such as conjugated hydrocarbons or carotenoids. also may cause changes in both the intensity and the wavelength of the absorbance maxima. derivative spectra may be used. they frequently are used to confirm the identity of a substance through comparison of the measured spectrum with a reference spectrum. such as pH and temperature. Other parameters.Principles and applications of UV-visible spectroscopy Table 1 Selected chromophores and their absorbance maxima Carboxyl Amide Ethylene Acetylene Nitrile Nitro RCOOH RCONH2 RCH=CHR RC=CR RC=N RNO2 Acetic acid Acetamide Ethylene Acetylene Acetonitrile Nitromethane 204 208 193 173 < 160 271 The presence of an absorbance band at a particular wavelength often is a good indicator of the presence of a chromophore. This increase in 11 . Transition metal ions also have electronic energy levels that cause absorption of 400–700 nm in the visible region. the relationship between intensity and wavelength has been systematically investigated. the number of bands increases with higher orders of derivatives. Where spectra are highly similar.
the absorbance spectrum of the steroid testosterone shows a single. featureless band centered at around 330 nm. For example. The resolution enhancement effect may be of use as well in identifying an unknown. In the fourth derivative. a single band with a maximum midway between the two component bands results. Absorbance 4th derivative Figure 6 Resolution enhancement 12 . broad. Figure 6 shows a computer simulation. either for characterizing materials or for identification purposes. with maxima centered close to the λmax of the component bands. whereas the second derivative shows six distinct peaks. these two bands are clearly visible. When two Gaussian bands with a 40-nm natural spectral bandwidth (NBW) separated by 30 nm are added in absorbance mode. The two components are not resolved.Principles and applications of UV-visible spectroscopy complexity of the derivative spectra can be useful in qualitative analysis.
Principles and applications of UV-visible spectroscopy
Color Color is an important property of a substance. The color of
matter is related to its absorptivity or reflectivity. The human eye sees the complementary color to that which is absorbed, as shown in Figure 7 and Figure 8.
Figure 7 Transmission and color
Wavelength [nm]
Absorbed color red orange
Complementary color blue-green greenish blue purple red-purple red orange yellow yellow-green
yellow-green green bluish green greenish blue blue violet
Figure 8 Absorbance and complementary colors
Principles and applications of UV-visible spectroscopy
In practice, both the generation and sensation of color are highly complex and depend on many factors, including the spectrum of the illuminant and, in the case of solids, the surface structure. Specialized color measurement systems, such as the CIE L*a*b, and instrumentation to measure color have been developed. When equipped with the appropriate software, most spectrophotometers can be used to measure color. An in-depth discussion of color is beyond the scope of this primer. Several well-written publications2,3 discuss color and the measurement of color in detail.
Other qualitative UV-visible spectroscopy can be used to determine many information physicochemical characteristics of compounds and thus
can provide information as to the identity of a particular compound. Two examples follow. Protein and nucleic acid melting temperature The absorbance spectra of proteins result largely from the presence of the aromatic amino acids tryptophan, tyrosine, and phenylalanine. A protein at room temperature has a specific tertiary structure or conformation that in turn creates a specific electronic environment for the aromatic amino acids. If the protein is heated it will, at a certain temperature, unfold or melt and lose its structure. In this process, the electronic environment of the aromatic amino acids changes, which in turn results in spectral changes or shifts. Multicomponent analysis (see "Multicomponent analysis" on page 21) can be used to determine how many of each aromatic amino acid are present in an intact protein.4 Deoxyribonucleic acid (DNA) in its native state comprises two strands of deoxyribose molecules helically wound around the same axis. The strands are linked by hydrogen bonds between the purine and pyrimidine bases—adenine is joined to thymine (A-T), and guanine to cytosine (G-C). These bases are primarily responsible for the UV absorbance of DNA, with a peak maximum at 260 nm. As in
Principles and applications of UV-visible spectroscopy
any multicomponent system, the observed absorption of any DNA molecule should equal the sum of the individual absorbances:
ADNA = A adenine + Aguanine + A cyto sin e + A thymine
However, the observed absorbance is always significantly less than expected because the hydrogen bonding between the bases changes their electronic environment. When a molecule is heated, the hydrogen bonds break, the double helix unwinds, and the absorbance increases so that it approaches that expected from the sum of all bases. This denaturation process also is known as melting. In a DNA melt experiment, the temperature of a DNA solution is increased in a stepwise fashion, and the absorbance at 260 nm at each temperature is measured and plotted as a melting curve. The midpoint of the temperature range over which the melting occurs is the Tm value. The Tm value of a particular DNA sample depends primarily on the percentage of G-C pairs in the sample, each of which contains three hydrogen bonds (in contrast, each A-T pair contains two hydrogen bonds). The higher the percentage of G-C pairs in the sample, the higher the observed Tm. Enzyme activity The activity of an enzyme is a measure of its effectiveness as a catalyst. The concentration of enzyme in an impure preparation can be expressed in terms of units per milliliter, and the specific activity of the preparation can be expressed as units per milligram of protein. As an enzyme is purified, the specific activity increases to a limit (that of the pure enzyme). Because reaction rate varies with such factors as substrate concentration, pH, ionic strength, and temperature, the conditions under which the activity is determined must be defined precisely. These conditions are usually the optimal assay conditions at a fixed temperature (25, 30, or 37 °C),
and in the manufacture of enzyme assay kits. If these 50 photons then pass through an identical cell. particularly for the identification and confirmation of unknowns. or 50 %. Instrumental Absolute wavelength accuracy and absolute photometric considerations accuracy are very important in qualitative analysis. A known weight of the enzyme is added and the rate of reaction determined. Activity measurements are conducted primarily in the research environment as enzymes are isolated and purified. a system is set up with known concentrations of substrate and. only 25 will emerge. In this regard. Often spectra acquired on different instruments at different times are compared. and so forth.Principles and applications of UV-visible spectroscopy with all substrates present at saturating conditions. 16 . coenzyme. if necessary. the transmittance is 0. Figure 9 shows the plot of transmittance against path length. Quantitative analysis Beer's law If 100 photons of light enter a cell and only 50 emerge from the other side. in which the enzyme activity must be consistent from batch to batch. spectra may have to be measured at a defined instrumental resolution. To determine activity.5.
except that it is stated in terms of concentration. Figure 10 shows a plot of transmittance against concentration. although it now appears that Bouguer first stated it in 1729.Principles and applications of UV-visible spectroscopy Path length Figure 9 Transmittance and path length—the Bouguer-Lambert law Lambert (1760) generally is credited with the first mathematical formulation of this effect. e is the base of natural logarithms. k is a constant. and b is the path length (usually in centimeters). I is the transmitted intensity. The amount of light absorbed is proportional to the number of absorbing molecules through which the light passes. Beer's law is identical to Bouguer's law. The mathematical expression is: Transmission T = I ⁄ Io = e – kb where Io is the incident intensity. 17 .
Figure 11 shows a plot of absorbance against concentration. This expression is commonly known as Beer's law. 18 .Principles and applications of UV-visible spectroscopy Transmission Concentration Figure 10 Transmittance and concentration—Beer's law Combining the two laws gives the Beer-Bouguer-Lambert law: T = I ⁄ Io = e – kbc where c is the concentration of the absorbing species (usually expressed in grams per liter or milligrams per liter). This equation can be transformed into a linear expression by taking the logarithm and is usually expressed in the decadic form: A = – log T = – log ( I ⁄ I o ) = log ( I o ⁄ I ) = εbc where ε is the molar absorption or extinction coefficient.
Therefore. For electronic transitions. such as wavelength. and equilibrium is reached very quickly. solvent. For these reasons.Principles and applications of UV-visible spectroscopy Absorbance [AU] Concentration Figure 11 The Beer–Bouguer-Lambert law The extinction coefficient (ε) is characteristic of a given substance under a precisely defined set of conditions. predetermined values for the extinction coefficient usually are not used for quantitative analysis. absorption of UV-visible light is quantitatively highly accurate. it is highly likely that all molecules are in the electronic ground state. the difference in energy between ground and excited states is relatively large. the measured extinction coefficient also depends partially on the characteristics of the instrument used. In practice. The simple linear relationship between absorbance and concentration and the relative ease of measurement of UV-visible light have made 19 . Thus. Instead. a calibration or working curve for the substance to be analyzed is constructed using one or more standard solutions with known concentrations of the analyte. Absorption and return to ground state are fast processes. and temperature. at room temperature.
Taking the difference between the highest maximum and the lowest minimum gives the best S/N but may result in increased sensitivity to interference from other components.= ----dλ dλ d ε d A -------.bc dλ′ dλ′ n n First derivative: n derivative: th at λ.Principles and applications of UV-visible spectroscopy UV-visible spectroscopy the basis for thousands of quantitative analytical methods. the sample to be analyzed must contain only the absorbing component for which the calibration has been performed. a similar linear relationship exists between concentration and amplitude for all orders of derivative spectra: Zero order: A = εbc dε dA -bc -----. If the sample is a solution. but for the odd-order derivatives. The even-order derivatives have a peak maximum or minimum at the same λmax as the absorbance spectrum. It may be possible to correct for an interfering component with a second wavelength.= ------. Sample requirements For accurate results. ε is the extinction coefficient. b is the sample path length. For single-component quantification. the selection of wavelengths is more difficult with derivative spectra than with absorbance spectra since both positive and negative peaks are present. this wavelength is a zero-crossing point. Assuming Beer's law is obeyed for the zero-order spectrum. a pure sample of the solvent should be used as a blank. where A is absorbance. 20 . and c is the sample concentration.
multicomponent UV-visible analyses are becoming more popular. which in turn depends on the extinction coefficient and the concentration of each component. The wavelengths chosen usually are those of the absorbance maximum of each component. modern instruments yield more precise data. they were not widely applied. This principle is true if more than one absorbing species is present. For these reasons. For calibration. However. and modern curve-fitting techniques give more accurate results and—perhaps more importantly—indicate when results are incorrect. The simple approach to multicomponent analysis is based on measurements at a number of wavelengths equal to the number of components in the mixture. All multicomponent quantitative methods are based on the principle that the absorbance at any wavelength of a mixture is equal to the sum of the absorbance of each component in the mixture at that wavelength. the equations are: Simple simultaneous equations method A′ ( x + y ) = A′ x + A′ y = e′ x bc x + e′ y bc y and 21 . Principle of additivity According to Beer's law (see "Beer's law" on page 16). but because the techniques used in multicomponent analysis often gave incorrect results (as detailed below). the absorbance of standards of known concentrations of pure components is measured to determine the extinction coefficient for each component at each wavelength selected. absorbance is proportional to the number of molecules that absorb radiation at the specified wavelength. Thus for two components x and y. The absorbance of the mixture at each wavelength is the sum of the absorbance of each component at that wavelength.Principles and applications of UV-visible spectroscopy Multicomponent Multicomponent analyses using UV-visible spectra have analysis been performed for almost as long as single-component analyses.
Absorbance [AU] Wavelength [nm] Figure 12 A two-component mixture with little spectral overlap In contrast. 22 . Figure 13 shows a simulated two-component mixture with significant overlap of the spectra at the absorbance maxima. In practice. measurement errors always occur. e′ is molar absorptivity at wavelength ′. These equations are easily solved to determine the concentration of each component. Such errors can affect significantly the accuracy of results when spectra overlap significantly. Figure 12 shows a simulated two-component mixture with no overlap of the spectra at the absorbance maxima. If measurements were always perfect.Principles and applications of UV-visible spectroscopy A″( x + y ) = A″ x + A″ y = e″ x bc x + e″y bc y where A′ is absorbance at wavelength ′. accurate results could be obtained even for complex mixtures of components with very similar spectra. c is concentration. e′′ is molar absorptivity at wavelength ′′. and b is path length. however. A′′ is absorbance at wavelength ′′.
1 + 0.4 + 0. A′(x + y) = 0.10 %) and A′′(x + y) = 0.0 + 0.99 23 .99 (.9 If a 10 % error occurs in the measurement of A′(x + y) and A′′(x + y).1 A''(x + y) = 0.9 = 0.1 A''(x + y) = 0. the measured absorbances should be: With little spectral overlap A'(x + y) = 1.5 = 1.0 = 1.6 + 0.9 With substantial spectral overlap A'(x + y) = 0.Principles and applications of UV-visible spectroscopy Absorbance [AU] Wavelength [nm] Figure 13 A two-component mixture with significant spectral overlap For a mixture of x and y where cx = cy = 1.5 = 0. that is.
98 .9 1. In this so-called overdetermined system.Principles and applications of UV-visible spectroscopy (+ 10 %).1 .10 % + 10 % 0.100 % + 98 % Least squares method The effect of random noise can be reduced through the use of additional spectral information. that is.6 Figure 14 depicts a spectrum for the two-component mixture shown in Figure 13 with a 10 % random error at each measurement point. a least squares fit of the standard spectra to the spectrum of the measured sample yields quantitative results. a series of data points can be used for quantification instead of only two.5. 24 . the quantitative calculation yields the results shown in Table 2: Table 2 Comparison of multicomponent analysis results for examples with little and substantial spectral overlap Substantial spectral overlap Little spectral overlap Component x y Nominal Calculated Calculated concentration concentration % error concentration % error 1 1 0.0 1.
as shown in Table 3.995 + 0.0 1.3 % . Table 3 Comparison of multicomponent analysis results from simple simultaneous equations and least squares methods Using 210 and 230 nm only Using 200–240 nm Component x y Nominal Calculated Calculated concentration concentration % error concentration % error 1 1 0.0.Principles and applications of UV-visible spectroscopy True Absorbance [AU] Measured Wavelength [nm] Figure 14 Mixture spectrum with 10 % random error at each wavelength With 21 data points (2-nm intervals over 200–240 nm). 25 .5 % This method enables the analysis of more complex mixtures and of simple mixtures of components with similar spectra.98 . the quantitative results from the least squares method have an error of < 1 % compared with an error of approximately 100 % from the usual measurements at two wavelengths.003 0.100 % + 98 % 1.
0–7. these methods offer some advantages over those described above.7 Figure 15 shows the absorption spectra of hemoglobin derivatives. Absorbance [AU] Sulfhemoglobin Oxyhemoglobin Carboxyhemoglobin Hemoglobin (pH 7. principle component regression (PCR). The simple simultaneous equations and least squares methods yield accurate results only if calibration is performed using pure standards or mixtures of standards Sample requirements 26 . and multiple least squares (MLS) methods.4) Deoxyhemoglobin Wavelength [nm] Figure 15 Absorption spectra of hemoglobin derivatives Other methods Other statistical approaches to multicomponent analysis include the partial least squares (PLS). In theory. An example of multicomponent analysis is the quantification of five hemoglobins in blood with minimum sample preparation. including spectroscopy and titrations. but in practice the calibration process is much more complex.Principles and applications of UV-visible spectroscopy The residual from the least squares calculation is a good indicator of how well the standard spectra fit the sample spectra and is therefore a good indicator of the probable accuracy of the results. This analysis was previously performed using various analytical techniques.
Indirect quantification Chemical derivatization Because many compounds exhibit either very weak or no absorbance in the UV or visible regions. a number of methods using chemical derivatization have been developed. However. data from the sides of absorbance bands is incorporated into the calculation. Accurate multicomponent analyses require excellent S/N. Moreover. fast scanning is necessary for productivity. wavelength reproducibility becomes very important. making absolute wavelength accuracy and absolute photometric accuracy relatively unimportant. In the least squares method. On the other hand. if a wavelength on the side of the band is used. because more data is required. the instrumental linear range is critical. This calibration process should eliminate instrumental bias. as the calibration process relies on a linear relationship. If measurements are performed only at the absorbance maximum. Instrumental Single-component quantification is normally performed by requirements measuring with the same instrument a standard or series of standards followed by an unknown. wavelength reproducibility is also of little importance because the rate of change of absorbance with wavelength is low. Such methods usually involve adding an organic reagent. The unknown sample must not have any additional absorbing capacity. Finally. which forms a complex with strong absorptivity. The final stage of measurement closely resembles that of 27 . especially if the simple simultaneous equations method is used. photometric reproducibility is essential for precise results.Principles and applications of UV-visible spectroscopy for each component in the sample that contributes to the UV spectrum. making excellent wavelength reproducibility essential as well.
28 . If the enzyme is carefully selected. pH. Separation of the compound of interest may be costly and time-consuming and thus impracticable for routine analysis. Enzyme assays can be used in the indirect analysis of one compound or a group of compounds in a complex matrix. the conditions are selected so that the conversion of substrate to product is completed within a reasonable period of time (5–20 min). With this technique. any change in the sample following addition of the enzyme will result only from the reaction of the specific compound or compounds. The use of a spectrophotometer for endpoint detection introduces objectivity into the analysis and lends itself to automation. Enzyme assays can be divided broadly into two types: rate assays and end point assays. This selectivity is the basis of enzyme assays. the choice of an appropriate reagent can enhance significantly both sensitivity and selectivity. enzyme concentration. Spectrophotometric In volumetric analyses. enzyme activity. such as absorbance. The difference between initial absorbance and final absorbance is directly proportional to the amount of substrate. including temperature. This process is inherently subjective and can be a source of error. the rate of reaction is directly proportional to the substrate concentration. if all other parameters are controlled at a constant level. Enzyme kinetic assays Direct UV-visible analysis of one component in biological matrices. With end point assays. and substrate concentration. Interference from other components often makes impossible direct measurement of a specific property. the color changes that signify the titrations end point of a titration are most often detected through visual inspection. The rate of an enzyme depends on many factors. is difficult. for example blood or foodstuffs.Principles and applications of UV-visible spectroscopy the direct methods. However.
chapter 2 Chapter 2 Instrumentation 29 .
but in practice all instruments are subject to error. a waveband) • a sample area • one or more detectors to measure the intensity of radiation 30 . The key components of a spectrophotometer are:8 • a source that generates a broad band of electromagnetic radiation • a dispersion device that selects from the broadband radiation of the source a particular wavelength (or. analytical instruments always yield correct measurments of a chemical or physicochemical parameter. In this chapter we review the basic components of a spectrophotometer and the various instrument configurations available. Instrumental design Components A spectrophotometer is an instrument for measuring the transmittance or absorbance of a sample as a function of the wavelength of electromagnetic radiation. more correctly.Instrumentation Ideally. Key instrumental parameters and their potential adverse effects on the measured values are also discussed.
relay light through the instrument. or the light from the two sources is mixed to yield a single broadband source.000 h. the tungsten-halogen lamp (see Figure 17). The first source. Although modern deuterium arc lamps have low noise. This type of lamp has very low noise and low drift and typically has a useful life of 10. yields a good intensity continuum in the UV region and provides useful intensity in the visible region (see Figure 16). In such instruments. Sources The ideal light source would yield a constant intensity over all wavelengths with low noise and long-term stability. the deuterium arc lamp. Two sources are commonly used in UV-visible spectrophotometers. such a source does not exist. the intensity of light from a deuterium arc lamp decreases steadily. Such a lamp typically has a half-life (the time required for the intensity to fall to half of its initial value) of approximately 1. Spectral irradiance Wavelength [nm] Figure 16 Intensity spectrum of the deuterium arc lamp Spectral irradiance The second source.000 h. Unfortunately. noise from the lamp is often the limiting factor in overall instrument noise performance. however. either a source selector is used to switch between the lamps as appropriate.Instrumentation Other optical components. Over time. Most spectrophotometers used to measure the UV-visible range contain both types of lamps. Wavelength [nm] Figure 17 Intensity spectrum of the tungsten-halogen lamp 31 . yields good intensity over part of the UV spectrum and over the entire visible range. such as lenses or mirrors.
Instrumentation Spectral irradiance An alternate light source is the xenon lamp (see Figure 18). these devices can be used to select a particular wavelength (or. a narrow waveband) of light from a continuous source. prisms and holographic gratings. These devices are made from glass blanks. which yields a good continuum over the entire UV and visible regions. the angle of dispersion is temperature sensitive. Wavelength [nm] Figure 18 Intensity spectrum of the xenon lamp Dispersion devices (a) Prism Dispersion devices cause different wavelengths of light to be dispersed at different angles. When combined with an appropriate exit slit. an aluminum coating is applied to create (b) Grating First order Second order Figure 19 Dispersion devices 32 . However. xenon lamps are used only for applications such as diffuse reflectance measurements. because the noise from currently available xenon lamps is significantly worse than that from deuterium or tungsten lamps. are commonly used in UV-visible spectrophotometers. For these reasons. onto which very narrow grooves are ruled. Moreover. A prism generates a rainbow from sunlight. most modern spectrophotometers contain holographic gratings instead of prisms. but modern production methods use a holographic optical process. Traditionally. Finally. Two types of dispersion devices. but the resulting dispersion is angularly nonlinear (see Figure 19a). The dimensions of the grooves are of the same order as the wavelength of light to be dispersed. This same principle is used in spectrophotometers. more precisely. in which high intensity is the primary concern. this task was done mechanically. Prisms are simple and inexpensive.
high sensitivity is associated with low concentrations. To detect accurately small differences between blank and sample measurements. Ideally. Photodiode detectors have a wider dynamic range and. photodiodes are used as detectors in spectrophotometers (see Figure 21). A concave grating disperses and focuses light simultaneously.Instrumentation a reflecting source. A single photomultiplier yields good sensitivity over the entire UV-visible range. In practice. Holographic gratings yield a linear angular dispersion with wavelength and are temperature insensitive. This type of detector yields high sensitivity at low light levels. The width of the band at half its height is the instrumental bandwidth (IBW). are more robust than photomultiplier tube detectors. however. However. filters must be used to ensure that only the light from the desired reflection order reaches the detector. Ideally. as solid-state devices. Light falling on the grating is reflected at different angles. depending on the wavelength. Increasingly. it should give a linear response over a wide range with low noise and high sensitivity. optimally symmetrical in shape. However. the detector must have low noise at high intensity levels. a dispersion device. which in turn result in high intensity levels. As a result. which overlap (see Figure 19b). Detectors A detector converts a light signal into an electrical signal. A monochromator consists of an entrance slit. they reflect light in different orders. which result in low absorbances. the output is always a band. in analytical spectroscopy applications. the output from a monochromator is monochromatic light. Spectrophotometers normally contain either a photomultiplier tube detector or a photodiode detector. and an exit slit. In a Cathode Anode Figure 20 The photomultiplier tube detector 33 . The photomultiplier tube (see Figure 20) combines signal conversion with several stages of amplification within the body of the tube. The nature of the cathode material determines spectral sensitivity.
The limits of detection are approximately 170–1100 nm for silicon-based detectors. Each diode has a dedicated capacitor and is connected by a solid-state switch to a common output line. but this problem has been corrected in modern detectors. Initially. The amount of charge needed to recharge the capacitor at regular intervals is proportional to the intensity of the light. thereby depleting the charge in a capacitor connected across the material. free electrical charge carriers are generated that discharge the capacitors. the capacitors are charged to a specific level. A diode array consists of a series of photodiode detectors positioned side by side on a silicon crystal. When photons penetrate the silicon. Metal contact Photon p layer n layer Figure 21 The photodiode detector Intrinsic region Gold block Some modern spectrophotometers contain an array of photodiode detectors instead of a single detector. light falling on the semiconductor material allows electrons to flow through it. Earlier photodiodes had low sensitivity in the low UV range. The capacitors are recharged at 34 .Instrumentation photodiode. The switches are controlled by a shift register (see Figure 22).
the diode array of the Agilent 8453 spectrophotometer comprises 1024 detector elements. which corresponds to the illumination time. depending on the instrument and its intended application. have high reliability. The readout cycle. Light Photodiode Capacitator Shift register Transistor switch Video line Readout cycle Figure 22 Schematic diagram of a photodiode array The amount of charge needed to recharge the capacitors is proportional to the number of photons detected by each diode.Instrumentation regular intervals that represent the measurement period for each scanning cycle. which in turn is proportional to the light intensity. Photodiode array technology is similar to microprocessor technology.5 mm. The absorption spectrum is obtained by measuring the variation in light intensity over the entire wavelength range. 35 . Photodiode arrays are complex devices but. and the photosensitive area measures approximately 25 × 0. For example. The array typically comprises between 200 and 1000 elements. because they are solid state. is 100 ms.
However. a deuterium lamp and a tungsten lamp. and use either photomultiplier tubes or. As discussed above. Concave mirrors are less expensive to manufacture than achromatic lenses and are completely free of chromatic aberration. more recently. 5–10 % of the light is lost through absorbance or reflection. Such lenses are used in cameras. At each optical surface. Polychromatic light from the source is focused on the entrance slit of a monochromator. However. 36 . resulting in a loss of efficiency. This light then passes through the sample area to the detector. the aluminum surface is easily corroded. Simple lenses are inexpensive but suffer from chromatic aberration. They offer good performance but at relatively high cost. that is. with careful design. and an effective optical system can be constructed with these simple and inexpensive components. Achromatic lenses combine multiple lenses of different glass with different refractive indices in a compound lens that is largely free of chromatic aberration. photodiodes as detectors. which selectively transmits a narrow band of light. The absorbance of a sample is determined by measuring the intensity of light reaching the detector without the sample (the blank) and comparing it with the intensity of light reaching the detector after passing through the sample. light of different wavelengths is not focused at exactly the same point in space. The conventional Figure 23 shows a schematic of a conventional single-beam spectrophotometer spectrophotometer. the chromatic aberrations of individual lenses in an optical system can be used to cancel each other out. Thus spectrophotometers ideally should be designed with a minimum number of optical surfaces.Instrumentation Optics Either lenses or concave mirrors are used to relay and focus light through the instrument. including the interfaces between components in an achromatic lens. most spectrophotometers contain two source lamps.
To perform such tasks with a conventional spectrophotometer. Polychromatic light from a source is passed through the sample area and focused on the entrance slit of the polychromator. parts of the monochromator must be rotated. on which each diode measures a narrow band of the spectrum. for measuring different compounds at different wavelengths or for obtaining spectra of samples. The diode array Figure 24 shows a schematic diagram of a diode array spectrophotometer spectrophotometer.Instrumentation Sample Monochromator Detector Exit slit Dispersion device Source Entrance slit Figure 23 Schematic of a conventional spectrophotometer This design is well-suited for measuring absorbance at a single point in the spectrum. 37 . which introduces the problem of mechanical irreproducibility into the measurements. The bandwidth of light detected by a diode is related to the size of the polychromator entrance slit and to the size of the diode. serial data acquisition is an inherently slow process. The polychromator disperses the light onto a diode array. Each diode in effect performs the same function as the exit slit of a monochromator. It is less appropriate. Moreover. however.
When the measurement is initiated. Because the relative positions of the sample and the dispersive element are reversed compared with a conventional instrument. a shutter is used to block light from the source until a measurement can be performed. the shutter is automatically opened. The difference in the intensities of the light reaching the detector with and without the sample is measured as described in "The conventional spectrophotometer" on page 36. A diode array spectrophotometer is inherently very fast owing to its parallel data acquisition and electronic scanning capabilities. and is highly reliable.Instrumentation Diode array Polychromator Sample Dispersion device Entrance slit Source Figure 24 Schematic of a diode array spectrophotometer The polychromator (entrance slit plus dispersion device) and the diode array are contained in a unit known as a spectrograph. To minimize possible photochemical reactions. 38 . this configuration often is referred to as reversed optics. has excellent wavelength reproducibility. and light passes through the sample to the array of diodes.
39 . Diode array spectrophotometers in particular are well-suited to single-beam configuration because spectra are acquired very quickly and because the time interval between blank and sample measurements is minimized. In addition. and the simple optical system offers high throughput and hence high sensitivity.Instrumentation Configuration Various configurations of spectrophotometers are commercially available. internal referencing can be used to reduce further the effects of lamp drift (see "Internal referencing" on page 72). the Agilent 8453. Figure 25 shows the optical system of a modern diode array spectrophotometer. The reference spectrophotometers used by national standards institutions such as the National Institute of Standards and Technology (NIST) in the United States and the National Physical Laboratory (NPL) in the United Kingdom are single beam. Single-beam instruments are low in cost. Each has its advantages and disadvantages. Single-beam design Both conventional and diode array spectrophotometers are single beam. This single-beam configuration has a minimum number of optical components for highest throughput efficiency and contains a 1024-element diode array for measuring the wavelength range from 190 to 1100 nm with good resolution.
It rotates at a speed such that that the alternate measurements of blank and sample occur several times per second.Instrumentation Shutter Lens Sample Lens Slit Grating 1024-element diode array Tungsten lamp Deuterium lamp Figure 25 Optical diagram of the Agilent 8453 diode array spectrophotometer Dual-beam design In a conventional single-beam spectrophotometer. dual-beam instruments contain more optical components. with an interval of several seconds for a single wavelength measurement and up to several minutes for a full spectrum measurement with a conventional instrument. The dual-beam spectrophotometer was developed to compensate for these changes in lamp intensity between measurements on blank and sample cuvettes. near the light source. 40 . thus correcting for medium. a chopper is placed in the optical path. The chopper switches the light path between a reference optical path and a sample optical path to the detector. Figure 26 shows a schematic of a dual-beam spectrophotometer.and long-term changes in lamp intensity (drift). Lamp drift can result in significant errors over long time intervals. the blank and the sample are measured consecutively. In this configuration. Compared with single-beam designs.
Single-beam instruments offer higher sensitivity and greater ease of use. For high sensitivity. was a multibeam design (see Figure 27). with drift typically only a factor of two worse than that of dual-beam instruments. In addition. The beam director is used to shift the beam alternately through the reference position and as many as four sample positions (for clarity only one is shown in the figure).Instrumentation which reduces throughput and sensitivity. The first commercially available diode array spectrophotometer. 41 . recent advances in lamp and electronics design have improved the stability of the single-beam spectrophotometer and led to the resurgence of this configuration. the higher stability of dual-beam instruments has been a major factor in the design of high-performance spectrophotometers. long measurement times may be required. the HP 8450A. However. Monochromator Exit slit Dispersion device Entrance Source slit Chopper Detector Reference Sample Figure 26 Optical system of a dual-beam spectrophotometer Traditionally. the more complex mechanical design of the dual-beam spectrophotometer may result in poorer reliability.
Instrumentation Visible lamp UV lamp Source ellipse Reference cell Cube corner mirrors Source mirror UV Spectrograph ellipse Upper beam director mirror Sample cell Visible Holographic grating Spectrograph slit and detector arrays Cube corner mirrors Lower beam director mirror Figure 27 Optical system of the HP 8450A diode array spectrophotometer Split-beam design The split-beam spectrophotometer (see Figure 28) resembles the dual-beam spectrophotometer but uses a beam splitter instead of a chopper to send light along the blank and sample paths simultaneously to two separate but identical detectors. This configuration enables the blank and the sample to be measured at the same time. the use of two independent detectors introduces another potential source of drift. 42 . Although the split-beam design is mechanically simpler than the true dual-beam instrument and requires fewer optical elements.
although not as high as a dual-beam instrument since two detectors can drift independently.Instrumentation Monochromator Exit slit Dispersion device Source Entrance slit Reference Detector Sample Figure 28 Optical system of a split-beam spectrophotometer Detector This design provides high stability. and good noise. The monochromator contains two dispersion devices (in effect transforming it into a duochromator). with the output combined into a single beam. two wavelengths can be measured simultaneously for special applications. Measuring a spectrum The degree of interaction of the sample with radiation (transmittance or absorbance) is determined by measuring both the intensity of the incident radiation (without the sample) and the transmitted intensity (with the sample). 43 . such as the study of two concurrent reactions in a sample. Dual-wavelength design With a dual-wavelength spectrophotometer. These complex instruments typically are significantly more expensive than conventional spectrophotometers and have been largely replaced by diode array spectrophotometers. although not as good as a single-beam instrument since the light is split so that less than 100 % passes through the sample. which are multiwavelength instruments.
With a dual. This process eliminates from the sample measurement any absorbance due to the solvent. Both cuvettes are initially filled with pure solvent. Two wavelengths usually are considered resolved if the minimum between the two peaks of the detector output signal is lower than 80 % of the maximum. This condition is known as the Rayleigh criterion. The resulting spectrum is corrected by subtracting the balance spectrum. respectively. The sample cuvette is then filled with the sample solution for measurement. in the equations in "Transmittance and absorbance" on page 6. and the blank is measured. Sources of error in measurements related to sample handling are described in Chapter 3 "Sample handling and measurement". two cuvettes are required. All modern instruments automatically store the reference Io values. the cuvette containing the solvent is placed in the spectrophotometer. Because most samples measured with UV-visible spectroscopy are in solution. This measurement reflects the difference in absorbance between the two optical paths in use. and Io and I are measured virtually simultaneously. Spectral resolution Spectral resolution is a measure of the ability of an instrument to differentiate between two adjacent wavelengths.Instrumentation These intensities are denoted Io and I. The sample solution is then measured in the same cuvette. With a single-beam instrument. which are used to calculate absorbance values for the sample. the blank should be measured on a cuvette containing the pure solvent used to prepare the sample.or split-beam instrument. and a so-called balance measurement is performed. Figure 29 44 . Key instrumental parameters In this section we discuss some instrumental parameters that may affect the accuracy and precision of measured absorbance values (see Appendix A for a detailed definition of the terms accuracy and precision).
Detector output signal Intensity Wavelength Figure 29 Definition of resolution Wavelength Absorbance I Resolution is closely related to instrumental spectral bandwidth (SBW).5 I SBW Wavelength Figure 30 Instrumental spectral bandwidth 45 . The SBW is defined as the width. 0. at half the maximum intensity. of the band of light leaving the monochromator (see Figure 30).Instrumentation shows schematically a case for two closely adjacent emission lines (the input to the instrument) and the actual signal that is output from the detector.
NBW Wavelength Figure 31 Natural spectral bandwidth An SBW/NBW ratio of 0. 0. and significant errors in absorbance values will occur at most wavelengths.2.Instrumentation Absorbance The accuracy of any measured absorbance depends on the ratio of the SBW to the natural bandwidth (NBW) of the absorbing substance. The NBW is the width of the sample absorption band at half the absorption maximum (see Figure 31). and of the dispersion generated by the grating.1 nm are not unusual.8.5 % or better.18 Absorbance Wavelength Figure 32 Effect of varying SBW on measured band shape 46 . the measured spectrum becomes progressively more distorted. as shown in Figure 32.1. SBW is primarily a function of the entrance and exit slit widths of the monochromator. Bands may not be resolved correctly.5.9 At an SBW/NBW ratio of higher than 0. Resolutions of 0. but higher resolutions cause considerable deterioration in S/N.1 or less will yield an absorbance measurement with accuracy of 99. and 0.
the resolution of the instrument will be degraded. which may be difficult to manage. an instrument with a narrower SBW is necessary. will yield accurate quantitative results even for very narrow bands. most UV-visible measurements are used for quantification. Absorbance Wavelength Figure 33 Effect of digital sampling Wavelength accuracy The difference between wavelength accuracy and and precision wavelength precision usually is not well understood (see Appendix A for an explanation of the difference between 47 . it is important to determine what resolution is required. For absolute absorbance measurements. If the sampling interval is large relative to the SBW. an SBW of 2 nm is more than adequate to yield accurate absorbance measurements of bands with an NBW of 20 nm or greater. particularly for samples in solution. the sampling interval used to digitize the spectrum for computer evaluation and storage also affects resolution (in a diode array spectrophotometer. which normally requires only relative measurements (for example. absorption bands in the UV-visible region are normally rather broad. If an instrument with an SBW of 2 nm is used to measure samples with an NBW narrower than 20 nm (for example. A calibration performed using standards. the sampling interval is best set at equal to or slightly smaller than the SBW. benzene). For approximately 99 % of routine measurements. However.Instrumentation Original spectrum Sampling process Absorbance Digitized spectrum In modern spectrophotometers. In practice. A smaller sampling interval improves resolution but results in much larger spectral files. an error in absolute absorbance measurements will result. digitization occurs on the array itself). As discussed in Chapter 1 "Principles and applications of UV-visible spectroscopy". which bracket the concentration of the unknown sample. Figure 33 shows this effect. the absorbance of an unknown concentration relative to the absorbance of a standard). This error increases as the NBW decreases (see Figure 32). When considering instrumental requirements.
will result in significant errors in measured absorbance. and wavelength precision (that is. the small wavelength errors that occur in resetting the spectrophotometer to that wavelength will have a minimal effect on the measured absorbance.0 AU Absorbance [AU] Error = 0. Wavelength accuracy is important for the comparison of measurements made on different instruments. the analytical wavelength must be at the absorption maximum. On the other hand. This method yields the most reproducible quantitative results. measurements are made on the same instrument relative to a standard. even though other 48 .Instrumentation accuracy and precision). for accurate quantification. Error = 0. the choice of a wavelength on the side of the absorption band. however. In this case. with the same wavelength resetting error. In most UV-visible analyses. If a wavelength at the absorption maximum is selected for quantitative measurements. the quantitative results are unreliable. Figure 34 shows the effect of poor wavelength resettability.1 AU (10 %) Wavelength [nm] Figure 34 Effect of poor wavelength resettability All standard texts on UV-visible spectroscopy emphasize that. resettability) is most important.
The equation used to calculate transmittance and thereby absorbance is: T = ( I + Is ) ⁄ ( Io + Is ) where T is transmittance. The intensity of stray light normally does not depend on that of transmitted light. At high absorption. stray light causes a negative bias in instrument response and eventually is the limiting factor for the absorbance. I is the intensity of transmitted light. The photometric accuracy of the instrument is thus compromised. that can be measured. these texts are based on conventional mechanical scanning techniques and do not apply to diode array instruments. If Is remains near constant. Stray light Stray light is defined as detected light of any wavelength that lies outside the bandwidth of the selected wavelength. it becomes the dominant term at low levels of I. 49 . and thereby concentration. Photometric accuracy Assuming good optical and electronic design. Io is the intensity of incident light. Figure 35 shows the effect of various levels of stray light on measured absorbance compared with actual absorbance. However. and Is is the intensity of stray light.Instrumentation wavelengths may yield better selectivity. only two and precision factors influence photometric accuracy and precision: stray light and noise. which have virtually absolute wavelength resettability.
50 . It is proportional to the square root of the intensity of light.01 % Stray light 0. may introduce errors in accuracy as well.10 % Stray light 1.00 % Stray light True absorbance [AU] Figure 35 Effect of stray light on measured sample absorbance Noise A spectrophotometer typically has two noise elements. When low concentration samples with low absorbances are measured. and so forth) and is independent of the intensity measured. analog-to-digital converter. noise can be reduced by increasing measurement time (see "Time averaging" on page 65).00 % Stray light 0.Instrumentation Measured absorbance [AU] 0. The first element (photon or Schott noise) results from the statistical distribution of the photons emitted by a light source. It can be minimized through good design. However. this element may prevent an accurate measurement of the small difference between two high light levels. Noise negatively affects the precision of measurements and. The second element is inherent to the instrument electronics (detector amplifier. This element becomes significant at high absorbance levels. for any single measurement. where the sample signal is very small.
Figure 35 shows the error curve due only to stray light. Thus. percentage error due to stray light is equal to ( A t – A m )100 ⁄ A t where At is the true absorbance and Am is the measured absorbance which are given by At = – log ( I ⁄ I o ) Am = – log [ ( I + I s ) ⁄ ( I o + I s ) ] where Io is incident light intensity. Potential errors at different absorbances can be calculated from stray light and noise. In addition. Thus. I is transmitted light intensity and Is is the stray light intensity.Instrumentation Linear dynamic range A frequently quoted and often misunderstood specification is instrument range. and T is transmittance as a percentage. error due to noise (%) is equal to ( A t – A m ) ⁄ ( A t × 100 ) where and or A t = – log ( I ⁄ I o ) A m = A t + T ⁄ 100 × A pn + A en T ⁄ 100 × Apn – A en Am = where Apn is photon noise in AU. the instrument range is simply the numerical range an instrument can display. which specifies for a given acceptable deviation from linearity (as a percentage of absorbance) the minimum and maximum absorbance values. In most cases. the measured absorbance may be in error because of the instrument noise. 51 . Aen is electronic noise in AU. A more useful specification is linear dynamic range.
Drift normally results from variations in lamp intensity between the measurement of Io and the measurement of I. Figure 36 depicts the total error for an example with photon noise of ± 0. % Error Stray light error curve Stray light + noise error curve Stray light .0004 AU and electron noise of ± 0. The instrumental dynamic range can be determined from the acceptable measurement error. Changes in the instrument electronics also can cause drift.Instrumentation The total error at any absorbance is the sum of the errors due to stray light and noise. The plot shows that absorbance measurements made from approximately 0.3 to 1. Good 52 .0001 AU.0 AU have the highest accuracy and precision.noise error curve Absorbance [AU] Figure 36 Theoretical absorbance error versus absorbance Drift Another potential cause of photometric error is drift.
but this effect can reduce the accuracy of results. the problem of drift can be minimized using techniques such as internal referencing (see "Internal referencing" on page 72). Error Absorbance [AU] Wavelength [nm] Figure 37 Effect of drift on measured absorbance values 53 .Instrumentation instrumental design can minimize drift. especially over a long period of time (see Figure 37). With mulitwavelength data.
Instrumentation 54 .
chapter 3 Chapter 3 Sample handling and measurement 55 .
5 AU. The cells lowest in cost are made of plastic. For example.Sample handling and measurement Assuming the instrumental limitations are understood and the spectrophotometer is correctly operated. a cell must contain the liquid or solution in the spectrophotometer sample area. only 1. With this technique.5 AU but the cell absorbs 1 AU. This mode is simpler and allows more accurate quantitative analysis than do reflectance measurements on solids. cells would be completely transparent at all wavelengths since any absorbance from the cell itself reduces the effective linear dynamic range for the sample. the remaining usable range for the sample is between 1 and 2. usually an acrylic. making them unsuitable for measurements in this region. In this chapter we discuss potential sources of error and steps to avoid them. Liquid samples Cells UV-visible spectroscopy is used primarily to measure liquids or solutions. These cells are not resistant to all solvents and absorb strongly below 300 nm. Material Ideally.5 AU. the largest sources of error in UV-visible spectroscopy are related to sample handling and sample chemistry. Consistency (absorbance and 56 . if the upper limit of the instrumental linear range is 2. that is.
and only the most important are described here. Figure 38 shows the absorption characteristics of cells of different materials. but the most popular path length by far is 10 mm. with proper care. Note that all materials exhibit at least an approximately 10 % loss in transmission at all wavelengths. The best cells are made of high-purity fused synthetic silica and are reasonably transparent down to 190 nm. Glass cells are slightly more expensive than plastic cells but are more durable and. can provide years of use. depending on the manufacturer. Fused silica Transmittance [%] Fused quartz Acrylic plastic Optical glass Glass Wavelength [nm] Figure 38 Optical transmission characteristics of some cell materials Cell types A wide range of cells are available. Glass absorbs strongly below 320 nm and thus is not suitable for measurements in this area.Sample handling and measurement path length) can vary from cell to cell. These cells are available in path lengths from 1 to 100 mm. Fused quartz cells are reasonably transparent down to 210 nm. The most frequently used cell is the open-topped rectangular cell (see Figure 39a). Almost all 57 .
5 mm). apertured cells are often used. and sensitivity is to some extent compromised. When sample volume is limited. as shown in Figure 40a. microcells can be used that reduce the aperture of the sample area to a very small cross section (typically 2 × 2. Only approximately 60 µl of sample are required for measurement. Flow cells of various aperture sizes and geometries are available in a wide range of path lengths. throughput is reduced. However. Nonflat or nonparallel optical surfaces can deviate the optical beam and cause apparent absorbance errors (see "Sample geometry" on page 63). When sample volume is extremely limited. the analyst should be aware of a few potential sources of error. Figure 39 Standard (a) and apertured (b) cells With apertured cells and microcells. flow-through cells (see Figure 40b) are used. Modern cells are connected to sample transfer tubing with screw fittings. With special ultramicrocells. their contribution to errors in absorbance measurements should be minimal.5 mm. the precision of the path length is important in absolute measurements. Sources of error If cells are properly designed and used. Figure 40 Micro (a) and flow-through (b) cells When placed in the sample beam. a cell becomes an active optical component. Cell path length tolerances for cells of good quality are ± 0. (see Figure 39b). Because measured absorbance depends on path length.11 For maximum quantitative accuracy. The loss of sensitivity depends on the degree of aperturing and on the optical geometry. the same cell should be used for both standard and sample measurements. sample volumes down to 5 µl can be measured. part of the light beam is blocked. For automated application.Sample handling and measurement rectangular cells have an external width of 12. The best cells have very flat and very parallel optical surfaces 58 .01 mm for path lengths from 0.5 to 100 mm.
With apertured cells. If the optical surfaces of the cell become mildly contaminated. With the exception of water. If a cell becomes seriously contaminated. and measurments will be inaccurate. In severe cases. if the optical beam is broad and collimated (parallel). The degree of error depends on the optical geometry in the sample area. Choice of solvent The ideal solvent for the preparation of sample solutions would dissolve all types of compounds. the parts of the cell that do not contain the sample must be properly masked (as shown in the figures) to avoid unwanted transmissions and reflections through the side walls. Distilled water approaches the ideal but is not suitable for many nonpolar organic compounds. On the other hand. treatment with hydrochloric or nitric acid can be used. would be nonflammable and nontoxic. as oil from fingerprints can cause significant absorbance. Table 4 lists some of the most commonly used solvents. it may be cleaned with a mild sulfonic detergent or with special cleaning fluid available from cell manufacturers. these solvents all exhibit a cut-off wavelength in the UV range below which 59 . much of the light will pass through the walls of the cell instead of through the sample. Care of cells Cells should be handled carefully to prevent scratching. the cell can be wiped carefully with photographic tissue. If the optical beam is highly focused so that a large proportion of the light passes through a very small opening in the cell. If unmasked cells are used. and would be completely transparent at all wavelengths.Sample handling and measurement that minimize their influence as optical components. the measurements will be in error. Care should be taken to avoid touching the optical surfaces with the fingers. The cell always should face the same direction in a cell holder to ensure that any cell optical effects are identical for both blank and sample measurements. results with unmasked cells will be reasonably accurate because the optics are in effect self-masking.
5 1.Sample handling and measurement they absorb too strongly for sample measurements to be performed. and temperature of the sample. the magnitude of the shift can be correlated with solvent 60 . Effect of solvent. including the solvent used as well as concentration. can temperature affect the position and intensity of absorption bands of molecules. such as acetone or methylene chloride. In general. H = health hazard ** With volatile organic solvents. pH.0 4. Table 4 Properties of some common solvents Cut-off Solvent Distilled water Hexane Ethanol (absolute) Methanol Cyclohexane Chloroform Dimethylsulfoxide Acetone * Polarity* 78. The polarity of a solvent can modify the electronic environment of the absorbing chromophore. T = toxic. pH.9 24. and the concentration.8 none 20. it is advisable to use a stoppered cell to eliminate evaporation.6 2. which can result very quickly in changes in concentration. These parameters should be controlled to ensure maximum precision and when comparing spectra measured under different conditions. A number of factors.7 wavelength (nm)** Hazard*** < 195 199 207 210 211 246 270 331 none F F F F F/T H F Dielectric constant at ambient temperature Wavelength at which transmittance of 10-mm path length is < 25 % *** F = flammable.3 32.
In addition. For comparative analysis. At higher concentrations. If temperature is found to have a significant effect on the measurements. Temperature also can affect UV-visible measurements. notably for organic solvents. however. that most buffers themselves exhibit significant absorbance. For example. changes in refractive index with temperature can be significant. however. These changes in turn affect the linearity of the concentration versus absorbance relationship and may lead to inaccurate quantitative results. Thus. If the spectrum of the sample under study is found to be affected by pH. This cell holder may be a simple water-jacketed holder used in conjunction with a circulating water bath. Finally. dimerization) may cause changes in the shape and position of the absorbance band. or a more sophisticated Peltier temperature 61 . a buffer should be used to control this parameter. which changes absorptivity. pH indicators visually change color at different pH values. the sample temperature should be controlled using a thermostatted cell holder. depending on the solvent used. The effects of pH on absorbance spectra can be very large and result primarily from the shifting of equilibrium between two different forms. Simple expansion of the solvent. may be sufficient to change the apparent absorbance and thereby the accuracy of quantitative results. Note. When convection currents cause different temperatures to occur in different parts of the cell. which can be either chemical or physical. which may affect the wavelength range over which measurements can be performed. the resulting Schlieren effect can change the apparent absorbance. Concentration normally affects only the intensity of bands. especially for some organic solvents. A good example of a physical equilibrium is the denaturation of nucleic acids as temperature is increased. the absorption maximum of acetone can vary from 259 to 279 nm. for example. molecular interactions (for example.Sample handling and measurement polarity. temperature may affect equilibria. a single solvent should be used for all measurements.
Sample handling and measurement controller. or to quantitate. This process at best will result in a constant offset at all wavelengths and at worst in a measured spectrum that is a composite of the analyte and the matrix. 62 . internal referencing using a reference wavelength at which no analyte absorbance occurs is often a viable alternative. one component in the solid matrix. the blank measurement must be performed on air since a true blank measurement (matrix without analyte) is not possible. a sample of the matrix that does not contain the analyte is not always available. No reference Often solid samples are measured in order to determine the spectrum of. Accurate quantitative analysis is possible if two or more standards are available and if a calibration curve that is not forced through zero is used. However. In this case. If multiple standards are not available. it is advisable to use a stoppered cell to minimize the Schlieren effect. Solid samples A number of factors can interfere with the accurate and precise measurement of transparent solid samples such as glasses or crystals. The intercept on the y-axis then represents the absorbance due to the matrix. With organic solvents.
Such a problem is very difficult to detect and. this effect is unimportant. Such samples are active optical components in the system and will deviate or change the focal length of the light beam. the detector fails to detect some of the light (see Figure 42). As a result. however. sunglass lenses). which is then measured as an apparent absorbance. this effect can be minimized by placing the sample as close to the detector as possible. This effect can be tested for by rotating or reversing the sample in its holder and can be minimized by placing the sample as close to the detector as possible. 63 . If. resulting in an apparent absorbance error. the beam is focused. This change in path length may in turn cause a significant change in the degree of illumination of the detector between the blank and the sample (see Figure 41). if the instrument is a focused-beam design. If the optical beam is collimated perpendicular to the sample. the solid sample becomes an active optical component and alters the optical path length.Sample handling and measurement Refractive index Collimated beam Sample Focused beam Detector The lack of a reference sample also causes a significant change in the refractive index between the blank (air) and the solid sample. However. Light not detected Figure 41 Effect of refractive index Sample geometry Solid samples often may be glass or molded plastic filters or lenses (for example. has no simple solution.
noise in the measurement results in a loss of precision such that any single measurement may be inaccurate. 64 . Changing slit width If the spectrophotometer has a variable slit width.Sample handling and measurement Photosensitive area Detector Detector Planar sample Figure 42 Effect of nonplanar sample geometry "Wedge" sample Weak absorbance A recurring problem in analytical chemistry is low sensitivity. Reducing the noise level directly improves the precision of results. which may be due to low concentrations of the sample or to very weak absorptivity of the analyte. the noise level can be reduced by increasing the slit width to allow more light through the optics. At low absorbances. Noise specification should be a key parameter in selecting a spectrophotometer. This increase in width yields better S/N but reduces instrument resolution. but note that varying the measurement parameters will reduce noise level.
a certain number of data points will yield optimum S/N (see Figure 44).6 sec Wavelength [nm] Figure 43 Effect of integration time on S/N Wavelength averaging Another averaging technique is wavelength averaging. For a given absorption bandwidth. Figure 43 shows the improvement in S/N with increasing integration time for a dyestuff at very low concentration.1 sec Absorbance [AU] S/N = 18 1. the average absorbance is reduced. 65 . The measurements were performed using a diode array spectrophotometer with integration times of 0. Conventional quantitative measurements would be performed at the absorption maximum. Figure 44 shows a broad absorption band. Absorbance [AU] S/N = 5.6 s. If data at all wavelengths is available. Note that extending the integration time will improve sensitivity only until other effects. such as drift. which has a negative effect on the signal. become dominant. additional values on either side of the maximum can be averaged. The reduction in noise is equivalent to the square root of the number of data points. as more data points are added. However.5 0.Sample handling and measurement Time averaging Taking the average of the data points reduces noise by the square root of the number of points averaged.1 and 1. Actual improvement is close to the expected theoretical improvement of four.
Sample handling and measurement Absorbance Wavelength NBW Optimum range Absorbance S/N Signal Noise # of data points Figure 44 Effect of wavelength averaging on S/N Drift also affects the accuracy of absorbance measurements. a maximum concentration of approximately 0. this is not possible. introduces error and thus may be better avoided. where absorptivity is lower (see Figure 45). Strong absorbance When samples absorb too strongly.26 mg/ml can be measured with reasonable accuracy. However. any sample handling step. The easiest solution to this problem is to dilute the sample to an absorbance level within the linear dynamic range. Moreover. switching to the absorbance on the side of the 66 . An alternative is to select one or more wavelengths on the side of the absorbance band. the linear dynamic range of the instrument is exceeded. Internal referencing (see "Internal referencing" on page 72) can be used to improve precision and accuracy at low absorbance levels. even dilution. and the relationship between absorbance and concentration becomes nonlinear (see "Linear dynamic range" on page 51). however. When the wavelength of the absorbance maximum (241 nm) is used for calibration. With solid samples.
0. however.1428 mg/ml spironoloactone Absorbance [AU] Wavelength [nm] Measured absorbance [AU] Concentration [mg/ml] Figure 45 The use of wavelength switching to increase dynamic range Interference Types of interference Ideally. absorbances that interfere with the measurements often occur for chemical or physical reasons. the absorbance that occurs during UV-visible measurements should be due only to the target analyte.Sample handling and measurement band (266 nm) enables concentrations of up to 5 mg/ml to be measured with equal accuracy. 67 . In practice. A prerequisite for use of this technique is excellent wavelength reproducibility (see "Wavelength accuracy and precision" on page 47).
for example a by-product from the synthesis of a pharmaceutical product. as shown in Figure 46. Rayleigh scattering occurs when the particles are small relative to the wavelength of light and is inversely proportional to the fourth power of the wavelength. biological samples such as blood).2. the exponent of the wavelength may range from . less light reaches the detector. Tyndall scattering occurs when the particles are large relative to the wavelength of light and is inversely proportional to the square of the wavelength. In some cases. In pharmaceutical analyses. even if no absorption occurs.4 to . instead of passing through the solution to the detector. may cause a broad matrix absorption. depending on the distribution of particle sizes: Scattering Detector Transparent sample Detector Scattering sample Figure 46 Scattering Ascatter ∝ 1 ⁄ λ n 68 . is scattered at an angle. In other cases. two types of scattering can be observed. only a single known interfering compound. many of them unknown (for example. is present. these particles are usually the excipients or fillers used in tablet or capsule formulations.Sample handling and measurement Other absorbing compounds The presence of any other compound that absorbs in the same region as the target compound will result in an error in the absorbance measurement. In the UV-visible part of the spectrum. Scattering causes an apparent absorbance because light. Filtering samples prior to measurement eliminates scattering but may not always be practicable. The scattering of radiation results in an apparent background absorbance that interferes with absorbance measurements. Therefore. In chemical systems. multiple absorbing species. and the analyst often must work with spectra that include a scattering component. A recurring problem in pharmaceutical and biological analyses is scattering caused by particles suspended in solution.
In general. Correction techniques A number of correction techniques can be used to eliminate or reduce interference errors. This relationship causes the error due to scattering to increase at lower wavelengths. 69 . However. if the source of the error is known and is consistent from sample to sample. light is lost and the sensitivity and accuracy of quantitative analysis are seriously impaired.Sample handling and measurement where A is absorbance due to scatter. Correction techniques always require data from at least two wavelengths. the error can be eliminated. On the other hand. the error can be reduced but not eliminated. Absorbance [AU] Tyndall scattering Rayleigh scattering Wavelength [nm] Figure 47 Scatter spectra The magnitude of the scattering effect can be reduced by placing the sample as close as possible to the detector. with significant scattering. and n is the order of scattering. The more sophisticated correction techniques require multiwavelength or spectral data. if the source is unknown and varies from sample to sample. λ is wavelength. as shown in Figure 47.
as described in "Multicomponent analysis" on page 21. Multicomponent analysis An extension of isoabsorbance is multicomponent analysis. spectra of the interferent or interferents must be measured as standards. The residual absorbance is the true absorbance of the analyte. as shown in Figure 48.Sample handling and measurement Isoabsorbance When a known interfering component with a known spectrum is present. The absorbance at this reference wavelength is subtracted from the absorbance at the analytical wavelength. 70 . it can correct for only one interferent. the error introduced by this component at the analytical wavelength for the target analyte can be eliminated by selecting a reference wavelength at which the interfering compound exhibits the same absorbance as it does at the analytical wavelength. Moreover. Here. This technique can be applied successfully when the spectra overlap considerably and when more than one interferent is present. Absorbance [AU] Analyte spectrum Interferent spectrum Measured spectrum Wavelength [nm] Figure 48 Isoabsorbance correction This technique is less reliable when the spectra of the analyte and of the interferent are highly similar.
The background absorbance at all other wavelengths can be estimated using the coefficients determined from the fit. A portion of the spectrum where the apparent absorbance is due only to interference is selected. and a is a constant. λ is wavelength. These values are then subtracted from the measured values to give the absorbances due to the analyte. n is the order of the relationship between absorbance and wavelength. as shown in Figure 49.Sample handling and measurement Background modeling Background modeling is appropriate when the interference is due to a physical process (most often scattering) in which the interference at the analytical wavelength can be estimated by extrapolation from modeling at another wavelength range. Measured spectrum Absorbance [AU] Extrapolated scatter spectrum Wavelength [nm] Figure 49 Background modeling 71 . A polynomial is then fitted to this part of the spectrum using a least squares fit to the logarithm of the absorbance: A = aλ n log ( A ) = log ( a ) + n log ( λ ) where A is absorbance.
as shown in Figure 50. it is best to choose a reference wavelength as close as possible to the analytical wavelength but with no significant absorbance from the analyte. The absorbance at the reference wavelength is subtracted from the absorbance at the analytical wavelength. Any interference that is constant at all wavelengths is corrected for. In general. Spectrum a Spectrum b Absorbance [AU] Wavelength [nm] Figure 50 Internal referencing Three-point correction The three-point. correction uses two reference wavelengths. or Morton-Stubbs. Although in practice interference usually is not constant at all wavelengths. usually those on either side of the analytical wavelength. this simple technique often yields surprisingly good improvements in accuracy. which uses a single reference wavelength. This method is most often used when a baseline shift occurs between measurements. The background interfering absorbance at the analytical wavelength is then estimated using linear interpolation (see Figure 51). This method represents an improvement over the single-wavelength reference technique because it corrects for any background 72 .Sample handling and measurement Internal referencing One of the simplest correction techniques is internal referencing.
In many cases.Sample handling and measurement absorbance that exhibits a linear relationship to the wavelength. Unfortunately. Thus a constant baseline offset (a0) is eliminated by the first derivative. 73 . a background absorbance that increases linearly with wavelength is eliminated by the second derivative. it will be a reasonable correction for nonlinear background absorbances such as that resulting from scattering or from a complex matrix. and so on. derivative spectroscopy can be used to reduce or eliminate background interference from a wide range of sources. if the wavelength range is narrow. any interference components that exhibit a directly proportional relationship to different orders of wavelength with the general form A = a 0 + a1 λ + … + a n λ 1 n are eliminated through the use of increasingly higher orders of derivatives. only the constant baseline offset is common in UV-visible spectroscopy. Measured spectrum Absorbance [AU] Wavelength [nm] Figure 51 Three-point (Morton-Stubbs) correction Derivative spectroscopy Owing to two of its properties. First.
these bands have equal amplitude. The latter may be an interfering component. This property improves the accuracy of quantification of any narrow band component in the presence of any broadband component.Sample handling and measurement Second. as is shown in Figure 52. the narrower band has 4 times greater amplitude. derivatives discriminate against broad absorbance bands relative to narrow absorbance bands. This discrimination results from the fact that the amplitude (Dn) of a Gaussian band in the nth derivative is inversely proportional to the original bandwidth (W) raised to the nth degree: 1 n D ∝ -----.= --------n n DY DX Figure 52 shows the effect of taking derivatives of two bands with NBWs of 160 and 40 nm. n W Thus for two coincident bands of equal intensity but different bandwidth in the zero order. In absorbance mode. and perhaps more important than the first property. and in the second derivative it has 16 times greater amplitude. respectively. n n 74 .. In the first derivative. the nth derivative amplitude of the sharper band (X) is greater than that of the broader band (Y) by a factor that depends on the relative bandwidth and on the derivative order: DX WY -------.
the signal in the presence of scattering is 0.1. Quantification at this wavelength results in an error of + 9.02992 A instead of 0.Sample handling and measurement Absorbance First derivative Second derivative Wavelength [nm] Figure 52 Discrimination against broad bands by derivative spectroscopy The spectrum resulting from scattering is also broadband. and the use of derivatives can reduce its contribution as well.0920 A instead of 1. the amplitude at 500 nm is 1.2 %. Without any correction. For example. Taking the first derivative reduces the contribution from the scattering component such that. using peak maximum to minimum. Figure 53 shows an absorbance band with an NBW of 40 nm and the same band in the presence of a scattering background.0 A because of the scattering contribution.1 %. 75 .03024—a quantification error of only .
the analytical problem cannot be defined simply as scattering. a combination of two or more of these effects results in a broad absorbing background matrix. Rather. however. Derivative spectroscopy is an excellent tool for reducing or eliminating these difficult-tocharacterize sources of error. baseline shift. 76 . or unwanted broad absorbing components.Sample handling and measurement Absorbance Measured spectrum Actual spectrum First derivative Wavelength [nm] Figure 53 Scatter correction by derivative spectroscopy Usually.
Sample handling and measurement
Photochemical problems
Fluorescence Some samples fluoresce, that is, they emit light over a
wavelength range when irradiated with light of a shorter (more energetic) wavelength. This emitted light results in an error in the absorbance measurement. The position and magnitude of the error depend on whether the instrument has conventional forward optics or reversed optics. In a conventional forward optics instrument, the sample is illuminated with light of varying wavelengths over time. As the excitation wavelength range is scanned, absorption occurs, initiating the fluorescence process that emits light at longer wavelengths. Because the detector cannot differentiate among the individual wavelengths, the absorbance measured at the excitation wavelength is too low (see Figure 54). As the emission wavelength range is scanned, no fluorescence occurs, and the absorption measurements are thus accurate.
Excitation wavelength Absorbance [AU]
Spectrum measured with forward optics Spectrum measured with reversed optics
Emission wavelength
Wavelength [nm] Figure 54 Effect of fluorescence on the measured absorbance spectrum
Sample handling and measurement
In a reversed optics instrument, the sample is illuminated with all wavelengths of light simultaneously, including light at the excitation wavelength. The sample therefore fluoresces and emits light but, because the wavelength selection occurs after the light has passed through the sample, the emitted light is directed to the correct wavelength. The absorbance measured at the emission wavelength is therefore too low (see Figure 54), but the absorbance measured at the excitation wavelength is correct. An additional factor that affects the magnitude of the error is the so-called acceptance angle of the detector, as shown in Figure 55. The fluorescent light is emitted in all directions. If the acceptance angle is wide, a significant portion of the fluorescent light will reach the detector. Conversely, if the detector acceptance angle is narrow, only a small amount of the fluorescent light will reach the detector, and the absorbance error will be correspondingly small. Reversed optics instruments have a narrow detector acceptance angle and thus are less susceptible to fluorescence error. Placing a filter in the light beam can eliminate the error due to fluorescence. In a conventional instrument, the filter is placed between the sample and the detector to filter out the emission wavelength or wavelengths, whereas in a reversed optics instrument, the filter is placed between the source and the sample to eliminate excitation wavelengths.
Forward optics Detector
Reversed optics Slit Detector
Figure 55 Acceptance angles and magnitude of fluorescence error
Sample decomposition
Some samples are sensitive to photochemical reaction, especially when exposed to low-wavelength UV light. In extreme cases, a filter may be necessary to eliminate this light.
chapter 4
Chapter 4
Method development and validation
instrumentation limited the choice of wavelength (owing mainly to problems of wavelength reproducibility) and/or suitable tools for the comparison of choices were not available. best is defined by such parameters as accuracy. NOTE: Method development involves the use of various statistical tools. Because all of these parameters cannot be optimized at the same time. Method development involves selecting the wavelength or wavelengths that yield the best results for a particular analysis on a particular instrument. and ruggedness. Until recently.12–15 80 . Method development Because most UV-visible applications are single-component quantitative analyses. These limitations have been eliminated in modern instruments. the parameter or parameters for optimization must be determined and the requirements defined prior to analysis. In this chapter we review criteria for defining a good method and strategies for finding optimum method parameters. linearity. range. An in-depth discussion of how these tools are calculated and of their relative advantages and disadvantages is beyond the scope of this primer. selectivity. sensitivity. but detailed information may be found in the references. Here. precision. in this chapter we focus on the optimization of such methods. by sample handling and by the sample itself enables you to develop analytical methods that minimize their effect on results.Method development and validation An awareness of the errors that can be introduced by the instrument.
Method development and validation Linearity Linearity is the ability of the method to produce test results that are proportional. and significant quantitative errors can result if the calibration curve is not accurately characterized (see Figure 56). 81 . because these deviations are wavelength dependent. which states that the absorbance of a solute is directly proportional to its concentration (see "Beer's law" on page 16). to the concentration of analyte in samples within a given range. an absorbance measurement of only one standard of known concentration is required in order to calibrate for quantification.16 For UV-visible measurements. and k is the calibration factor (the slope of the calibration curve). A number of instrumental and sample parameters (see Chapter 2 "Instrumentation" and Chapter 3 "Sample handling and measurement") can cause deviations from Beer's law. However. either directly or by a well-defined mathematical transformation. the usual linear relationship is Beer's law. A linear calibration curve relating absorbance to concentration should have the form: A = kc where A is absorbance. c is concentration. The measured absorbance value divided by the concentration gives the slope. selection of the appropriate wavelength can minimize their influence on results. Thus testing for linearity in effect tests how well our theoretical model (Beer's law) fits the actual measurements. Theoretically.
Ideally. with A increasing. This value always lies between + 1 and .Method development and validation Absorbance [AU] Measured standard absorbance Th e tic ore br al i lc a on ati cu rve Possible true calibration curves Measured sample absorbance Th a tic ore lc tio br a ali nc ur v e e Known concentration of standard Concentration Possible quantification results Concentration Figure 56 Potential errors resulting from inadequate calibration Concentration To construct a calibration curve. standard error of regression. A value of + 1 indicates a perfect linear relationship between absorbance and concentration. the spectra of a set of at least three standard analyte solutions should be measured. including correlation coefficient.1. The concentrations of the standard solutions should bracket the expected concentration range of the samples for analysis. all measured standard values would lie on a straight line. Of these. and uncertainty can be used to obtain this measurement. To compare two calibration curves. which is also known as the least squares method. Several statistical values. to determine which type of calibration curve gives the best fit. a measure of the goodness of the fit of the standards to the line is required.1 also indicates a perfect linear Absorbance [AU] Absorbance [AU] Concentration Figure 57 Calibration data sets 82 . A statistical method must be applied to find the best fit of the calibration curve to the data and. A value of . but in practice the values always exhibit some scatter (see Figure 57). in a second step. the correlation coefficient is the most popular. The statistical method most often used is linear regression.
Typically. the spectra of a set of pure standards with a wide range of concentrations are measured. correlation coefficient values of better than 0. A value of 0 indicates that there is no correlation between absorbance and concentration. but with A decreasing (which can occur if derivative data is used).Method development and validation relationship. The best calibration in terms of linearity is achieved at the point at which the correlation coefficient approaches unity.999 can be expected. the wavelength of maximum absorbance (414 nm) is not identical to the wavelength (402 nm) that gives the best linearity. Figure 58 depicts the spectra of four yellow dye standards and the corresponding correlation coefficient spectrum for a simple linear calibration curve. To determine the best wavelength or combination of wavelengths. and the chosen statistic for the assessment of linearity is calculated. A linear calibration curve is applied to each wavelength. For quickest evaluation. Correlation coefficient Absorbance [AU] Optimum range Wavelength [nm] Figure 58 Selecting the wavelength or wavelengths for best linearity 83 . a graphical plot of the linearity statistic versus wavelength is useful. In this example.
Again. If the 84 .Method development and validation If an individual wavelength does not give the required degree of linearity. the use of an internal reference wavelength to eliminate baseline shift errors in measurements will improve results. If the desired degree of linearity cannot be achieved with a simple linear calibration curve. For example. Note that when calibration curve types with higher degrees of freedom are used. The simple linearity test described above cannot be used. if the standards can be fitted perfectly to the measured spectrum—Beer's law and the law of additivity hold for the system under study. This value is the sum of the squares of the differences at each wavelength between the measured spectrum and the calculated spectrum. Beer's law is also assumed. Instead. equations of the form: A = a + kc A = kc + k′c and 2 A = a + kc + k′c 2 are used. a combination of wavelengths may be used. the correlation coefficient or another statistical tool should be used to assess the relative quality of the calibration curves. It may be assumed that if the residual is zero—that is. a different type of calibration curve can be applied to minimize the influence of the nonideality on the results. Here. Typically. more standards are required in order to characterize properly the curve. the standard deviation of the residual can be used to test how well our standard set fits the measured spectrum. For multicomponent analyses. the calculation model includes the law of additivity to explain total absorbances at each wavelength. however.
A sample of known concentration is then required. The quantitative results at each wavelength are then compared with the known value. The spectrum of the sample is measured. Accuracy Accuracy of a method is the degree of agreement between an individual test result generated by the method and the true value. a synthetic sample containing a known weight of the sample is prepared. Figure 59 shows the results for a yellow dye sample. however. as described above. and the acceptable value for an analysis must be determined empirically using analytical results that meet the requirements for accuracy and precision.Method development and validation standard deviation of the residual is not zero. the theoretical model does not reflect the system under study because one of the laws is not obeyed or because an additional component is present for which there is no calibration standard. if such a sample is not available.) To determine which wavelength or wavelengths give the best accuracy. 85 .16 (See Appendix A for an explanation of the difference between accuracy and precision. the wavelength or wavelengths that give the best accuracy lie in the region of 400 nm. the residual is never zero. However. In practice. Although the analytical wavelength would have been set at 414 nm using traditional methods. and quantification is performed at all wavelengths over the measured wavelength range. A graphical plot of the quantitative results versus wavelength enables quick evaluation. This sample is ideally one for which the concentration of the analyte has been determined using a different technique. the spectra of a set of standards are measured and calibration curves constructed at all wavelengths.
however. In multicomponent analyses with spectral ranges. Standard deviation.16 (See Appendix A for an explanation of the difference between accuracy and precision. This method reduces the contribution of noise to errors in accuracy. Instead. it is preferable to perform a series of measurements on the sample and then calculate the average.Method development and validation Concentration Absorbance [AU] Measured concentration True concentration Optimum range Wavelength [nm] Figure 59 Selecting the wavelength(s) for best accuracy Because noise may bias the accuracy of any individual measurement. Precision Precision of a method is the degree of agreement among individual test results when the procedure is applied repeatedly to multiple samplings. percent relative standard 86 .) A statistical value is required in order to determine precision. the optimum wavelength range cannot be derived using such a simple technique. the best range must be determined through trial-and-error variation of the wavelength range and through comparison of the calculated results with actual values.
and confidence interval are the most popular tools for assessing the precision or repeatability of a set of values. The same techniques can be applied in multicomponent analyses. To determine the precision of a method. a set of typically 10–20 samples with the same concentration are prepared. Sensitivity Sensitivity refers to the response obtained for a given amount of analyte and is often denoted by two analytical factors: the limit of detection (LOD) and the limit of quantification (LOQ).022 mg/l Measurement number Figure 60 Determining the precision of an analysis If the desired level of precision is not achieved.16 87 . Figure 60 depicts an example of the scatter of results and the standard deviation that may be expected for a good analysis. These samples are then measured.Method development and validation deviation (obtained by dividing the standard deviation by the average value and multiplying by 100).41 mg/l Standard deviation = 0. and the amount of analyte is calculated. time averaging. Value [mg/l] Average = 31. noise reduction techniques such as wavelength averaging. and internal referencing should be used to improve the values. The standard deviation of the results is a measure of the precision.
The LOD is approximately three times the standard deviation. the best sensitivity is obtained at 220 nm. the LOD is the point at which the signal from the analyte is equal to three times the noise in the measurement. A better way to determine the wavelength or wavelengths of optimum sensitivity is to measure the spectrum of a sample of low concentration several times. The tools described above then can be used to determine the acceptable limits. Although wavelengths between 400 and 450 nm give excellent sensitivity. Figure 61 illustrates this technique for a yellow dye sample. It is often assumed that the wavelength with maximum absorbance will give the best sensitivity.Method development and validation The LOD is the lowest concentration of analyte that can be detected but not necessarily quantified in sample matrices. To calculate the LOQ. The average and percent relative standard deviation of the measured values at each wavelength are then calculated. this is not necessarily the case. In general. 88 . However. The wavelength with the lowest percent relative standard deviation likely will yield the best sensitivity. Measurement results from some spectrophotometers list standard deviations based on the noise in the measurement. because instrumental noise can vary significantly with wavelength. The LOQ is the lowest concentration of analyte that can be determined with acceptable precision and accuracy in sample matrices. the acceptable limits of precision and accuracy (which depend on the objectives for the analysis) must be defined.
Range Range is the interval between (and including) the upper and lower levels of analyte that have been calculated with the required precision. and linearity.16 The presence of any other compound that absorbs at the wavelength used to quantify the analyte will result in 89 . Selectivity Selectivity is the ability of a method to quantify accurately and specifically the analyte or analytes in the presence of other compounds. precision.Method development and validation % RSD Absorbance [AU] Optimum range Wavelength [nm] Figure 61 Selecting the wavelength or wavelengths for best sensitivity For multicomponent analysis. These wavelengths can be applied directly in the simple simultaneous equations method or as a guide for the optimum wavelength range in the least squares method. this technique can be used to identify the optimum wavelength or wavelengths for each component. and accuracy of the results. accuracy.16 The range is determined by first analyzing samples that contain varying concentrations of the analyte and then using the tools described above to calculate the linearity.
excipients. the wavelength that gives the result closest to the known value yields the best selectivity. the wavelength that gives the lowest concentration usually yields the best selectivity (impurities always add absorbance. If the concentration of the analyte in the sample is known. If the concentration of the analyte is unknown. In the example in Figure 62. If the identity and/or spectra of possible interferents are unknown. owing to the contaminant. an appropriate correction technique such as use of an isoabsorbance wavelength (see "Isoabsorbance" on page 70) may be applied. an empirical approach almost identical to that described above may be used to determine which wavelength or wavelengths give the best accuracy. If the choice of wavelength does not suppress sufficiently the effect of the interferent.Method development and validation quantitative error. or degradation products in the sample matrix. If the type of interferent is known. The plot of concentration versus wavelength shows that wavelengths between 390 and 420 nm yield the best selectivity. 90 . causing erroneously high results). the yellow dye sample has been contaminated by a red dye. the test samples must contain the interferent. the method developer can examine the spectra of the analyte and of the interferent to select a wavelength at which the analyte has significant absorbance but the interferent has little absorbance. Note that. In this case. These other compounds may be synthesis precursors. the accuracy error range is much wider than it is for pure yellow dye (see Figure 59). known impurities.
91 . The degree of reproducibility of the results is then calculated as a function of the assay variables. at different assay temperatures. This value can be compared with the precision of the method under normal conditions to obtain a measure of its ruggedness. and at different elapsed assay times.Method development and validation Concentration [mg/l] Absorbance [AU] Measured concentration True concentration Optimum range Wavelength [nm] Figure 62 Selecting the wavelength or wavelengths for best selectivity Ruggedness Ruggedness is the degree of reproducibility of test results obtained by analyzing the same samples under a variety of normal test conditions. These tests should be performed by different analysts under operational and environmental conditions that may vary but that fall within the specified parameters for the method. The ruggedness of an analytical method is determined by analyzing subsamples of a homogeneous sample in different laboratories and on different instruments. for example with different reagents batches. The reproducibility of the method should be established under various conditions.16 The method should not be affected by changes in time or place.
for the evaluation of all possible choices.Method development and validation Instrumental In developing an analytical method. such as a pharmaceutical quality control laboratory. accuracy or recovery. linearity. if not essential. 92 . method validation is used to confirm that the performance characteristics of the method are in keeping with the intended application in laboratory studies. must be validated.17 Method validation is similar to method development in that it includes studies on specificity.16 Whereas method development involves the selection of specific parameters and conditions. Calculation by hand or the transferring of data from the spectrophotometer to other applications for evaluation seriously limits productivity. Method validation is the process for determining whether performance characteristics of the method are suitable for the intended purpose. A spectrophotometer that automatically measures multiple spectra and calculates average and standard deviation values for each absorbance value can greatly improve productivity as well. and precision or reproducibility. Finally. The United States Pharmacopoeia (USP) contains specific guidelines for method validation. However. full spectral data is requirements valuable. the instrumental software must be able to process a large amount of data and use appropriate statistical tools. Method validation Any analytical method designed for use in a regulated environment. method validation must be kept separate from method development and selection. sensitivity. The wavelength reproducibility of the instrument also must be excellent so as not to restrict the choice of wavelength or wavelengths.
chapter 5 Chapter 5 Routine operation 93 .
Good Manufacturing Practice (GMP). Verification of the continued proper performance of UV-visible spectrophotometers is a key element of these quality requirements. This chapter reviews steps to ensure accurate and precise results on a regular basis and. in the pharmaceutical industry. quality requirements as outlined by ISO 9000 (BS 5750). and the United Kingdom National Measurement Accreditation Service (NAMAS) have assumed increasing importance. 94 . Instrument performance verification In recent years. Test parameters Although there is no clear definition as to which parameters should be tested to verify instrument performance. Yet neither the considerations discussed in previous chapters nor the best method development guarantees good results. Good Laboratory Practice (GLP). perhaps more importantly. Consequently.Routine operation UV-visible spectroscopy is considered a routine technique and is used extensively in QA/QC and similar laboratories. manufacturers and users generally agree that the criteria discussed below merit inclusion. the recommendations contained in pharmacopoeias also have become more influential. to detect erroneous results.
This ideal standard does not exist. To measure wavelength accuracy and precision. Because these factors are wavelength dependent. although some standards approach the ideal.Routine operation Wavelength accuracy and precision Wavelength accuracy is the most important performance criterion for differentiating spectra when the spectra are measured on different instruments. When comparing spectra or absorbance values measured on the same instrument (as in most quantitative analyses in which a standard is measured). wavelength precision is the critical parameter (see "Wavelength accuracy and precision" on page 47). a reference standard is required. this standard would have very narrow. however. well-defined peaks at a series of wavelengths throughout the UV and visible ranges. changes in the slit width of the instrument will affect slightly the measured position of the peaks. Absorbance standard Absorbance [AU] Wavelength standard Wavelength [nm] Figure 63 Ideal spectra of absorbance and wavelength standards 95 . as shown in Figure 63. because the peaks are not ideally symmetrical. measurements should be performed at exactly the same wavelength at which the factors were originally determined. It also plays a role in quantitative analysis when extinction coefficients or factors are used. For actual wavelength accuracy standards. Ideally.
this filter would absorb all light of the wavelength at which the measurement is to be performed and transmit higher and lower wavelengths (the sources of the stray light. photometric precision is the critical parameter (see "Photometric accuracy and precision" on page 49). In practice. cut-off filters that transmit all light above or below a certain wavelength and that block all light in the wavelength range are used. Standards used typically have broad peaks and valleys. however. To measure stray light. A reference standard is necessary in order to measure photometric accuracy and precision. no such standard exists. however. a filter is needed. although neutral density glass approaches the ideal in the visible wavelength range. such a filter does not exist. Ideally. 96 . as shown in Figure 64). For comparative measurements (as above). Ideally. It introduces a systematic bias to lower absorbances at increasing concentrations. this standard would have constant absorbance at all wavelengths throughout the UV and visible ranges (see Figure 63) so that any errors in wavelength accuracy do not affect results. Stray light Stray light is the factor that affects most strongly the linear relationship between absorbance and concentration at high absorbances.Routine operation Photometric accuracy and precision Photometric accuracy is the most important criterion for quantitative analysis when extinction coefficients or factors are used. In practice. Instead. Stray light is also the primary influence on the upper limit of the linear dynamic range for an analysis (see "Stray light" on page 49).
as discussed in "Spectral resolution" on page 44. that is. 97 . and empirical approaches. Noise Noise is the major factor affecting the precision of absorbance measurements. with no sample in the light path. such as measuring the relative heights of peaks and valleys for a sample with a narrow band. This bias plays a key role if absolute accuracy is essential but. Noise typically is measured at zero absorbance. are often used (see "Resolution" on page 118). If the instrumental resolution is not sufficient. the absorbance measured at the wavelength maximum will be too low. Thus photometric accuracy has a systematic bias. In general. Measuring resolution is difficult. It is especially significant at low absorbances. it is less important for relative measurements such as quantification.Routine operation Transmittance [%] Wavelength [nm] Figure 64 Ideal spectrum of a stray light filter Resolution Resolution is a critical factor in determining the shape of measured peaks. at which it is used determine the detection limit (see "Noise" on page 50). the instrumental resolution should be approximately 10 times better than the bandwidth of the peak measured.
sample dependent. The results of these tests should be compared with absolute performance specifications for the methods in use. the desired accuracy and linearity can be achieved only with an appropriate system suitability test performed on the sample itself. 98 . Because it is not practicable to perform all tests at all wavelengths. baseline flatness indicates relative noise at all wavelengths and reveals wavelengths with instrumental problems resulting from the switching of filters or source exchange. Drift in absorbance measurements introduces systematic errors in photometric accuracy (see "Drift" on page 52). a few wavelengths representative of the intended purpose should be selected. Linearity Linearity often is considered an important parameter for performance verification. Performance verification then will demonstrate effectively whether the performance characteristics have changed in a way that could affect the goodness of the analytical results. Because many parameters are sample dependent. However. in the case of stray light. as described below. Stability typically is measured at zero absorbance. however. because linearity is strongly sample dependent. Stability Stability affects the accuracy of absorbance measurements as a function of time. we feel it is best measured during a system suitability test. However. Baseline flatness typically is measured at zero absorbance. The main problem in performance verification is the fact that all of the above-listed parameters are wavelength dependent and. Compliance with the above criteria (as determined using an appropriate set of reference standards) does not guarantee that a particular analysis can be performed with the required accuracy and linearity.Routine operation Baseline flatness To measure noise at all wavelengths usually is not practicable. for example.
in many modern spectrophotometers. with solid standards. However. deuterium lines at 486. since solid standards are not absolutely stable. are easy to use and maintain. Solid standards do not require any preparation. and if the instrument has been self-calibrated using its built-in deuterium lamp. because emission sources are not typical samples. such as mercury and deuterium arc lamps. homogeneity cannot be ensured from one standard to another or from one batch of material to another. they must be returned for recalibration at regular intervals. Emission standards Certain emission sources. Because this process is time-consuming. Each standard therefore must be calibrated individually on a reference spectrophotometer. and have good stability over time. However.18. emission sources require electrical power supplies.19 Some general comments on the relative merits of the three main types of standards follow. exhibit sharp lines at specific wavelengths that are ideal for wavelength accuracy and precision testing. these samples must be kept scrupulously clean and cannot be used for testing flow systems.Routine operation Standards A detailed discussion of all standards and their relative advantages and disadvantages is beyond the scope of this primer. In fact. are relatively insensitive to temperature. Finally. they do not test the full system.0 and 656. Several excellent publications cover these topics in depth. Moreover.1 nm of the built-in source are used to check and recalibrate for wavelength accuracy. another standard should be used for verification. solid standards tend to be expensive. Solid absorption standards 99 . Moreover.
they must be freshly prepared. and so on required for accuracy checks. However. These standards also can be used to check flow systems. if liquid standards are prepared using appropriately pure materials. Because calibration of the solutions is unnecessary. and their advantages and disadvantages.Routine operation Table 5 summarizes some of the best-known solid standards. must be recalibrated at intervals Frequent interreflection error problems Temperature sensitive Not very stable. peak maxima. the process of using a solution as a standard closely resembles that for a normal sample. because the cuvette itself contributes 100 . their uses. Table 5 Standard Holmium oxide glass Didymium oxide glass Neodymium yttrium aluminum garnet Neutral density glass Use Wavelength standard Wavelength standard Wavelength standard Photometric standard Some solid standards Advantages Disadvantages Peaks at many wavelengths from 280 to Position of peaks varies from batch to batch 2000 nm Peaks at many wavelengths from 400 to Position of peaks varies from batch to batch 1920 nm No peaks below 400 nm Peaks at many wavelengths from 350 to No peaks below 350 nm 1000 nm Very flat absorbance profile in visible wavelength range Very flat absorbance profile in UV and visible wavelength ranges Not suitable for UV range Not absolutely stable. The main disadvantage of liquid standards is that. must be recalibrated at intervals Metal on quartz Photometric standard Liquid absorption standards Liquid standards normally are absolute physical standards. This preparation is time-consuming and requires a reasonable degree of skill on the part of the operator. in general. Moreover. liquid standards can be relatively inexpensive. they possess inherently the properties of absorptivity. Some suppliers have attempted to overcome this problem by supplying standards sealed in cuvettes. In other words.
and their advantages and disadvantages. which increases costs. 512. their uses.Routine operation some absorbance. 395. Table 6 Standard Holmium oxide in perchloric acid Samarium oxide in perchloric acid Benzene (vapor) Use Some liquid standards Advantages Disadvantages Wavelength standard Wavelength standard Wavelength standard Peaks at many wavelengths from 240 No usable peaks above 650 nm to 650 nm Peaks at many wavelengths from 300 No usable peaks above 500 nm to 500 nm Very narrow peaks from 230 to 260 nm Broad peaks at 257 and 350 nm and broad valleys at 235 and 313 nm Peaks at 302. 390 nm Used to measure stray light at 340 nm or below Cuts off at ca. and 678 nm cover both UV and visible regions Cuts off at ca. because liquid standards are less stable than solid standards. Furthermore. 260 nm Used to measure stray light at 200 nm or below Very limited wavelength range No absorbance in visible range Very pH sensitive and. these standards must be calibrated and recalibrated individually. they must be recalibrated more often.20 Table 6 summarizes some of the best-known liquid standards. as a powerful oxidizing agent. Prepared solutions sealed in ampoules for use with normal cuvettes are now available that are both easy to use and cost-effective. wavelength accuracy can affect results None Potassium dichromate in perchloric Photometric standard or sulfuric acid Mixture of cobalt and nickel salts Sodium nitrite solution (50 g/l) Photometric standard Stray light Potassium or sodium iodide solution (10 g/l) Stray light Tendency to decompose 101 . can be unstable Because bands are rather narrow.
.). wavelength accuracy errors can affect the measured stray light Can only be used to determine resolution at one point in the spectrum May vary with wavelength Toluene in hexane Resolution Regulatory In the following section we review some of the more requirements important regulatory requirements governing the use of UV-visible spectrophotometers. alignment and wavelength accuracy of spectrophotometers etc.22 European Pharmacopoeia The requirements governing UV-visible spectrophotometers used for pharmaceutical analyses in Europe are contained in the European Pharmacopoeia (EP).Routine operation Table 6 Standard Potassium chloride solution (12 g/l) Use Stray light Some liquid standards Advantages Cuts off at ca.21 For spectroscopy. the following guideline is given: Where appropriate. the resolution.23 These requirements are based on—and virtually identical to— those listed in national pharmacopoeia such as the British Pharmacopoeia (BP) in Great Britain and the Deutsche Arzneimittelbuch (DAB) in Germany: Control of wavelengths—Verify the wavelength scale using the absorption maxima of holmium perchlorate solution. periodic performance checks should be carried out (for example. 200 nm Used to measure stray light at 220 nm or below Easy empirical approach using peak (269 nm) and valley (266 nm) in toluene spectrum Disadvantages Because the measurement is performed on the side of the cut-off slope. .. GLP/GMP GLP and GMP requirements for the validation of instruments can be summarized as follows: Documented verification that the system or subsystem performs as intended throughout representative or anticipated operating ranges. the line of a hydrogen or deuterium discharge 102 .
1 cm) and the permitted limits.16 nm (Hg) 334.70 nm (Hg) 287.4 to 145. Wavelength (nm) 235 257 313 350 A (1 per cent.9 to 108.07 nm (Hg) Control of absorbance—Check the absorbance using potassium dichromate solution R at the wavelengths indicated in the following table. 1 cm) 124.0 to 50. The permitted tolerance is ± 1 nm for the ultraviolet range and ± 3 nm for the visible range.6 Maximum tolerance 122.2 142.00 nm (Dβ) 486.0 48. measure the resolution of the apparatus as follows: record the spectrum of a 0.96 nm (Hg) 579.30 nm (Ho) 546. Resolution power—When prescribed in a monograph.5 144. 241.3 104.7 47.15 nm (Ho) 253.Routine operation lamp or the lines of a mercury vapor arc shown below.66 nm (Hg) 435.2 Limit of stray light—Stray light may be detected at a given wavelength with suitable filters or solutions: for example the absorbance of a 1.10 nm (Hβ) 536.2 per cent m/V solution of potassium chloride R in a 1 cm cell should be greater than 2 at 200 nm when compared with water as compensation liquid. which gives for each wavelength the exact value of A (1 per cent.48 nm (Hg) 404.9 to 126.83 nm (Hg) 486.07 nm (Hg) 576.25 nm (Hg) 313.6 106.15 nm (Ho) 302.15 nm (Hg) 361.02 % V/V solution of toluene R in 103 .50 nm (Ho) 365.
Section 831 ("Spectrophotometry and light scattering") states: Check the instrument for accuracy of calibration.Routine operation hexane R.. For checking the photometric scale. The wavelength scale may be calibrated also by means of suitable glass filters.. The latter contains a cross reference: For further details regarding checks on both wavelength and photometric scales of a spectrophotometer. Glass containing holmium is considered superior.. The minimum ratio of the absorbance at the maximum at 269 nm to that at the minimum at 266 nm is stated in the monograph. The United States Pharmacopoeia24 (USP) XII. the regulatory requirements for UV-visible spectrophotometers are not as clearly defined as in Europe. reference may be made to the following publications of the National Institute of Standards and Technology .." United States Pharmacopoeia In the United States. Standard glasses containing didymium (a mixture of praseodymium and neodymium) have been widely used. be not less than 1.. The National Institute of Standards and Technology (NIST) gives a range of solid and liquid standards for determining 104 ..5 unless otherwise specified in the monograph. NOTE: The BP states that the ratio should ". a number of standard inorganic glass filters as well as standard solutions of known transmittance such as potassium chromate or potassium dichromate are available. . which have useful absorption bands through the visible and ultraviolet regions.
Routine operation wavelength accuracy. benzene vapor (UV region). or holmium oxide glass or holmium perchlorate solution (UV and visible regions). The SRM is for the verification and calibration of the absorbance scales of ultraviolet and visible absoprtion spectrometers having narrow bandpasses. 2031. and stray light. Photometric accuracy: determined using NIST 930. 105 . Linearity: determined by preparing an analytical working curve with the target analyte (see "System suitability" on page 113). This SRM is for the verification and calibration of the absorbance scales of ultraviolet absorption spectrometers. This SRM is for the verification and calibration of the transmittance and absorbance scales of ultraviolet and visible absorption spectrometers. American Standard Testing Methods The American Standard Testing Methods (ASTM)26 publishes test methods for measuring the key performance characteristics of UV-visible spectrophotometers: Wavelength accuracy and precision: determined using a mercury vapor discharge lamp (UV region). photometric accuracy. a deuterium or hydrogen arc lamp (visible region). The ASTM recommends that the calibration wavelengths used bracket the analytical wavelength. This SRM is for use in the verification and calibration of the wavelength scale of ultraviolet and visible absorption spectrometers having nominal spectral bandwidths not exceeding 3 nm.25 Table 7 summarizes the most important of these. Table 7 SRM # 930 931 935 2031 2034 2032 Type Neutral density glass filters Cobalt and nickel solution in nitric/perchloric acid mixture Potassium dichromate solid for preparation of test solution Metal on quartz Holmium oxide solution in perchloric acid Potassium iodide solid for preparation of test solution NIST standards NIST description This SRM is for the verification and calibration of the transmittance and absorbance scales of visible absorption spectrometers. This SRM is for use in the assessment of heterochromic stray radiant energy (stray light) in ultraviolet absorption spectrometers. or 935 standards.
Table 8 summarizes the requirements and recommendations of the various regulatory bodies. but no practical method for measuring these parameters is given.Routine operation Photometric precision: determined using a metallic screen or a suitable glass filter. Table 8 Test parameters and standards used by the main regulatory agencies NIST USP SRM — X X X — X — — — — — — — — 2034 — — — — 935 930 931 2031 — 2032 — — — Test type Wavelength accuracy Standard Holmium perchlorate solution holmium oxide glass Deuterium arc lamp Mercury arc lamp Benzene vapor EP ASTM S — S S — S — — — S — — — — S X X — X X X X X X X X X X Photometric accuracy and linearity Potassium dichromate solution Neutral density glass Cobalt and nickel salts solution Metal on quartz Stray light Potassium chloride solution Potassium iodide solution Sodium iodide solution Sodium nitrite solution Resolution Toluene in hexane solution USP: Test method only NIST SRM: Standard material and test method EP: Test method and minimum performance specification ASTM: Test method only 106 . An additional ASTM publication27 describes standards and procedures for measuring stray light. The importance of slit width and resolution are mentioned in ASTM documentation.
preferably at several wavelengths. Recommendations Although there is no definite set of standards for verifying the performance of a spectrophotometer. photometric accuracy should be verified not in the visible range but in the UV range. for example. it is not appropriate to verify wavelength accuracy only at the 656. Thus. by ASTM recommendations that wavelengths used for calibration bracket the analytical wavelength. 107 . and by NIST regulations. Similarly. This condition is highlighted by GLP requirements that verification should be performed for the intended and anticipated operating ranges. Figure 65 shows the spectrum of this solution as measured on an Agilent 8453 spectrophotometer.1-nm deuterium line since this is not a reliable indicator of wavelength accuracy in the UV region. Wavelength accuracy: holmium perchlorate solution (40 g/l holmium oxide in 10 % v/v perchloric acid).Routine operation The choice of standards depends on the analyses to be performed on the instrument. the following tests are recommended for those users who analyze primarily liquid samples and who need to comply with general regulatory requirements. if the intended purpose is to measure absorbance in the UV region (as in most pharmaceutical analyses).
Routine operation Absorbance [AU] Wavelength [nm] Figure 65 Spectrum of holmium perchlorate solution Table 9 shows the values specified by NIST for the usable reference peaks for three different instrumental spectral bandwidths (SBWs). These values are preferable to EP values because the EP does not specify the SBW or 108 .
Routine operation
temperature used and because the EP and NIST values exhibit significant discrepancy.
Table 9
NIST values for holmium perchlorate solution peaks28 SBW 0.5 nm 241.01 249.79 278.13 287.01 333.43 345.52 361.33 385.50 416.09 — 467.80 485.27 536.54 640.49 1.0 nm 241.08 249.87 278.10 287.18 333.44 345.47 361.31 385.66 416.28 451.30 467.83 485.29 536.64 640.52 2.0 nm 240.90 249.98 278.03 287.47 333.40 345.49 361.16 385.86 416.62 451.30 467.94 485.33 536.97 640.48
Photometric accuracy: potassium dichromate solution (approximately 60 mg/l in 0.01N sulfuric acid). Figure 66 shows the spectrum of potassium dichromate. The EP values for this standard are given on page 112.
Routine operation
Absorbance [AU]
Wavelength [nm] Figure 66 Spectrum of potassium dichromate
If measurements are to be performed in the visible region, additional testing with neutral density glass filters such as the NIST SRM 930 is advisable. Stray light: solutions of sodium nitrite (50 g/l), sodium iodide (10 g/l), and potassium chloride (12 g/l) in water. These three solutions enable the measurement of stray light at three different wavelengths. Figure 67 shows the spectra of these solutions. In general, sodium nitrite and sodium or potassium iodide is recommended but, for those users who need to comply with EP requirements, potassium chloride also may be necessary.
Routine operation
Transmittance [%]
Wavelength [nm] Figure 67 Spectra of stray light standard solutions
Resolution: solution of toluene in hexane (0.02 % v/v), as specified in the EP. Figure 68 depicts the spectrum of this solution.
Absorbance [AU]
Wavelength [nm] Figure 68 Spectrum of toluene in hexane
The resolution is estimated by taking the ratio of the absorbance of the maximum near 269 nm to that of the
25 0.10 1. This ratio is empirically related to the SBW.00 1.00 Noise.29 Table 10 SBW and 269/266 nm ratio (toluene) Ratio of absorbance at 269 nm to absorbance at 266 nm 2. the instrument should be equipped with self-test routines that can be run on a daily basis.50 1. No standard is required. and drift also should be measured with a clear sample area.40 1.00 2. Instrument self-test Full performance verification of a spectrophotometer is time-consuming and thus normally done only at periodic intervals.20 2. These intervals are determined according to the stability of the instrument.30 2. 112 . To ensure that any deviations from performance occurring between verifications are detected in a timely fashion. These routines should include a check of the electronic and optical operation of the spectrophotometer as well as wavelength accuracy checks with one or both lines from the deuterium lamp. baseline flatness. as shown in Table 10.00 3.00 SBW (nm) 0.00 4.Routine operation minimum near 266 nm.
system suitability tests are performed on a given system periodically to determine its adequacy or effectiveness. Measure and calibrate using one standard with a concentration equal to 100 % of the expected component concentration. Whereas method validation is performed once at the end of method development. B. System suitability should not be confused with method validation. System suitability requirements are well-defined for chromatography systems but not for UV-visible spectroscopy systems. Although this factor can never be eliminated. accurate and precise results remain highly dependent on the human factor. analysts have developed their own strategies for performing system suitability tests on UV-visible instruments. Then measure and quantify the standard and the standard diluted by a factor of two. Measure first the standard and then a series of dilutions of the standard and calculate the extinction coefficient (absorbance divided by concentration) for each concentration. Two examples follow: A. The values of the extinction coefficients should not vary by more than a specified percentage. steps nonetheless can be taken to reduce human error.Routine operation System suitability System suitability is designed to evaluate the components of the analytical system in order to verify that system performance meets the standards required by the method. The results of both samples should fall within a specified percentage of the known concentration. 113 . Proper operation However good quality control measures such as instrument performance verification tests and method validation. In practice. Remeasuring the standard confirms the accuracy of the initial measurement.
emptying. all relevant parameters should be stored in a single method file under a unique name. Standard operating Although spectrophotometers can store and automatically procedures set most method parameters. and operator training. thus eliminating potential error. rinsing.Routine operation Electronic storage Modern spectrophotometers are almost invariably microprocessor or computer controlled and should provide for electronic storage of method parameters. Ideally. In a regulated environment. some procedures—for example. Collateral data Errors or entirely incorrect results can occur despite the best instrumentation. When the operator enters the name of the method. The data also can be used to investigate the reason for an incorrect result. 114 . however. Collateral data and multiple measurements at a single wavelength or (preferably) at multiple wavelengths can help ensure that a result is correct. Most analytical results obtained with UV-visible spectroscopy are based on a single measurement at a single wavelength. the sample may be contaminated. Whereas an incorrect result in itself is not necessarily problematic. these steps are known as standard operating procedures (SOPs). all parameters should set automatically. method development and validation. failing to recognize that a result is incorrect can have serious consequences. With a single data point. Operators should be properly trained in these procedures. and filling cuvettes with samples—must be performed by the operator and can introduce substantial errors. For example. Regulated environments in particular would benefit from a system that prevents operators from changing method parameters or that keeps track of and reports any method changes. there is virtually no way to detect whether a result is suspect unless a typical result is known and the actual result deviates significantly from the known value. and all steps should be clearly documented in working instructions.
the results at the analytical wavelength and at the confirmation wavelength or wavelengths will be identical. Caffeine Absorbance [AU] Salicylic acid Wavelength [nm] Figure 69 Confirmation analysis Full spectra For optimum results. The sample spectra can be overlayed onto the standard spectra to check for any differences. Quantification is performed at all measured wavelengths. In confirmation analysis. If the sample is pure. or mathematical methods can be used to obtain 115 . and results will differ. Confirmation analysis also can be used to detect whether the correct sample has been measured and whether measurements lie outside the linear dynamic range of the instrument. the absorbance at one or more wavelengths in addition to the analytical wavelength is measured both on standards and on unknown samples. If the sample is contaminated. it is highly probable that the contaminant will contribute a different absorbance value at the analytical and conformation wavelength or wavelengths.Routine operation Confirmation A simple way to verify quantitative analytical results is wavelengths through confirmation analysis. An example is shown in Figure 69. the full spectra of the sample for analysis should be acquired.
the plotted points will fall on a straight line. linear regression can be used to fit the best straight line. The match factor is obtained by plotting the absorbance values at each wavelength of the sample against the standard. whereas for completely dissimilar spectra. Figure 70 shows plots of both similar and dissimilar spectra. In both cases. the points will not fall on a straight line but exhibit scatter. it will be close to 0. The slope of this line is the ratio of the concentration of the sample to the standard. Correlation = 0. If the spectra are not identical.Routine operation a match factor. which indicates the similarity of samples to standards.999 Normalized Standard Unknown Wavelength [nm] Unknown Correlation = 0.056 Normalized Standard Unknown [mAU] Wavelength [nm] Unknown [AU] Figure 70 Comparative plots of similar and dissimilar spectra 116 . The correlation coefficient is a measure of the similarity of the sample spectra to the standard spectra. the correlation coefficient will be 1. and the correlation coefficient can be calculated. If the spectra are identical. If spectra are perfectly similar.
All measurements include some degree of bias and random variation due to noise. 117 . The standard deviation is an indicator of the validity of a measurement. all analytical information is available for reevaluation of results and for determining why a result was incorrect. Although bias can be detected using techniques such as those described above. The precision of results always should be estimated by performing multiple measurements and then calculating the average and the standard deviation of the average. Monitoring of changes in the standard deviation of results over time can be a useful diagnostic for many measurement problems. noise also can result in significant errors. Statistics A single measurement does not have a high degree of validity.Routine operation If full spectra are used.
Routine operation 118 .
Appendix A Appendix A Accuracy and precision 119 .
the shots are neither accurate nor precise. Figure 71 shows the performance of a marksman on a rifle range. As an analogy. but not interchangeably. In (c). 120 . but a consistent bias is evident. the shots are both accurate and precise. the shots are accurate but imprecise: the average of the shots would lie in the center of the target. It is therefore important to clearly understand the difference between them. Figure 71 Example of precision and accuracy In (a). the marksman is peforming well. the shots are precise but inaccurate.Accuracy and precision Definition of terms The terms accuracy and precision are used throughout this primer. In (d). but the individual shots deviate significantly. In (b).
Appendix B Appendix B Characteristics of diode array spectrophotometers 121 .
we feel this technology offers considerable advantages over conventional scanning spectrophotometers. this method enables the acquisition of full spectra even when the analysis requires only a single 122 . Fast spectral Fast spectral acquisition makes diode array spectroacquisition photometers the first choice for measurement of dynamic systems such as flow injection analysis detection. Absorbance [AU] Wavelength [nm] Figure 72 Spectra of hydrolysis of sultone For most UV-visible spectrophotometer users who are not performing measurements on dynamic systems. For example. the disappearance of the reactant and the appearance of the product can be monitored simultaneously. Figure 72 shows spectra measured at intervals of 1 s during a hydrolysis reaction of sultone. Because it is so fast. process control. and kinetic measurements.Characteristics of diode array spectrophotometers Advantages of diode array spectroscopy As the leading manufacturer of diode array spectrophotometers. In the following sections we review the advantages of diode array technology and comment on some of the real or perceived disadvantages. With this data. fast full spectral acquisition still offers significant advantages.
The absorption maxima can be used for high-sensitivity measurements. it provides collateral data for confirming data quality (see "Confirmation wavelengths" on page 115) and enhances productivity when the simple simultaneous equations method is used (see "Multicomponent analysis" on page 21).Characteristics of diode array spectrophotometers wavelength. With multiwavelength measurement. However. Simultaneous Most conventional spectrophotometers can perform multiwavelength multiwavelength measurements but must physically move measurement from one point in the spectrum to another to do so. smaller quantities of data need to be stored than with full spectral measurement. and "Three-point correction" on page 72). full spectra can be obtained with high productivity for use in method development (see "Method development" on page 80) and for multicomponent analysis (see "Multicomponent analysis" on page 21). and a wavelength with lower absorption on the side of the absorption band prevents errors due to stray light. Multiple wavelength measurements thus are completed in the time a conventional instrument requires to perform a single wavelength measurement. Finally. Through the use of alternative wavelengths for high and low concentration ranges. which takes time. all points in the spectrum can be measured simultaneously. a diode array spectrophotometer can be used in many applications where an expensive dual-wavelength 123 . This technique facilitates the use of various techniques for eliminating errors (see "Isoabsorbance" on page 70. dynamic range can be maximized with diode array spectrophotometers. The full spectral data can be used for error correction (see "Background modeling" on page 71 and "Derivative spectroscopy" on page 73) and as collateral data for confirming data quality ("Full spectra" on page 115). error-eliminating techniques are somewhat less effective than those that can be used with full spectra. "Internal referencing" on page 72. Finally. Moreover. With a diode array spectrophotometer.
They are also susceptible to wavelength drift over long periods of time. confirmation of data quality (see "Confirmation wavelengths" on page 115). unaffected by wavelength resettability errors. Positive identification of compounds is thus possible even when the spectra are virtually identical but exhibit a small wavelength shift. to decrease sensitivity for strongly absorbing compounds (see "Strong absorbance" on page 66). Data from all parts of the spectrum. no significant mechanical error or drift with time occurs. Moreover. and derivative spectroscopy (see "Derivative spectra" on page 6 and "Derivative spectroscopy" on page 73). method development (see "Method development" on page 80). Because in diode array instruments no parts are moved to change wavelength or to scan. It allows selection of the optimum wavelength for improved dynamic range. or even months later for use in quantitative analysis without remeasuring the 124 . the spectra of standards can be measured. Moreover. Wavelength Conventional mechanical scanning spectrophotometers resettability have an inherent wavelength resettability error (see "Wavelength accuracy and precision" on page 47) that increases with time as mechanical linkages wear down. Other techniques that use full spectral data include multicomponent analysis (see "Multicomponent analysis" on page 70).Characteristics of diode array spectrophotometers spectrophotometer (see "Dual-wavelength design" on page 43) was previously required. sensitivity. Excellent wavelength resettability also ensures that any differences in measurements result from the sample and not from instrument resettability errors. it is essential in techniques to improve sensitivity using wavelength averaging (see "Wavelength averaging" on page 65). and recalled days. and to confirm data quality with confirmation wavelengths (see "Confirmation wavelengths" on page 115). stored on disk. and selectivity in method development (see "Method development" on page 80). weeks. is of benefit to the analyst.
light throughput is higher and noise levels are lower. This method improves productivity. these instruments are highly reliable. cost of ownership is lower than with conventional instruments because diode array spectrophotometers do not require regular maintenance or recalibration. Modern diode array 125 . The standard deviation is a measure of the reliability of the data point and is obtained for every absorbance value in the spectrum. Ruggedness and Diode array spectrophotometers are mechanically simple in reliability that they have almost no moving parts. especially in the case of expensive. The user benefits from minimal down-time. Diode array instruments typically are less complex and have fewer optical surfaces than conventional ones. The statistical data is a valuable diagnostic tool for evaluating the quality of the analysis results (see "Statistics" on page 117) and also should be used during method development (the software that comes with Agilent spectrophotometers automatically makes use of such data). Measurement statistics Diode array spectrophotometers scan so quickly that multiple measurements normally are performed and averaged together. or unstable compounds that cannot be used routinely in the laboratory. As a result. The ability to acquire full spectra quickly also can help improve spectral sensitivity using time averaging (see "Time averaging" on page 65) and wavelength averaging (see "Wavelength averaging" on page 65) techniques. Because few pieces will wear out or break. In addition. difficult-to-obtain. obtaining multiple measurements is too time-consuming. Both the average and standard deviation for each data point are calculated. Sensitivity The dynamic range of a diode array spectrophotometer can be extended further at low absorption levels.12 With conventional spectrophotometers. It has been said that no statistical significance can be attached to a single measurment.Characteristics of diode array spectrophotometers standards. however.
the sample area of a diode array spectrophotometer does not need to be covered because the instrument is not susceptible to interference from ambient light. diode array instruments had some resolution limitations compared with 2-nm resolution instruments. Changing or adding specialized sampling accessories is also easier. Disadvantages of diode array spectroscopy Because diode array spectrophotometers differ from conventional scanning spectrophotometers. this design increases productivity by facilitating the exchange of samples. In this section we address briefly these objections. The sampling interval in turn depends on the number of diodes in the array and on the wavelength range measured. with higher resolution. if a spectrum 126 . Moreover. less light reaches the detector and sensitivity is lower. For example. This problem has been corrected in modern diode arrays. Until recently. resolution can be changed easily by varying the slit width. Resolution With a conventional scanning spectrophotometer. such as the Agilent 8453 (see "Dual-beam design" on page 40).5 nm or less as can some conventional instruments. Although diode array instruments cannot achieve resolutions of 0. and a wider range of sample types can be measured because size restrictions are fewer. resolution depends on an additional factor: the sampling interval of the diode array. With a diode array spectrophotometer. many objections have been raised concerning their performance. such high resolution may not be necessary in many analyses (see "Spectral resolution" on page 44). Moreover. Open sample area Owing to the reversed optics design of the instrument (see "The diode array spectrophotometer" on page 37).Characteristics of diode array spectrophotometers spectrophotometers are estimated to have only one failure in 10 years (excluding lamp changes). which contain more elements. however.
it can take as much as an hour or more to obtain a spectrum with good S/N. Sample decomposition Because a diode array spectrophotometer irradiates the sample with all wavelengths of light. stray light is reduced by moving filters into the light path in front of the monochromator (see "Stray light" on page 49). diode array spectrophotometers exhibited higher levels of stray light than conventional instruments. Instead. First.1 nm. These filters must be changed in accordance with the wavelength of the incident light. A filter that blocks all light below 430 nm is then dropped into the light path. The only light detected is stray light. which is then subtracted from the first measurement to yield a stray light corrected spectrum. Until recently. However. the intensities of light used with a diode array spectrophotometer are no higher than with a conventional instrument. when a full spectrum is scanned sequentially with a conventional spectrophotometer.5-s measurements) and reduces stray light to levels equivalent to or better than those obtained with comparable single monochromator conventional scanning instruments. Thus. the full spectrum is scanned. However. The entire process takes only 1. because all wavelengths of light are always in the detector area. and the spectrum in the region up to 430 nm is measured.5 s (with two 0. it has been argued that the sample will be degraded. the total amount of light (the sum of all light at each wavelength) to which the sample is subjected 127 . filters are placed over the array itself to reduce stray light.Characteristics of diode array spectrophotometers is scanned with a resolution of 0. the Agilent 8453 is based on a patented process that uses the fast full spectrum scanning capability of diode array technology to reduce significantly stray light. Stray light In a sequentially scanning instrument. stray light is more problematic than it is with conventional spectrophotometers. using filters in this way usually limits the range of wavelengths that can be measured simultaneously for a spectrum. With a diode array spectrophotometer. However.
Complexity Some users are intimidated by diode array instruments because they believe them to be too complicated. conventional scanning instruments are subject to error as well (see "Fluorescence" on page 77 for an explanation). however. both instruments will decompose it to the same extent. problems of photodegradation are extremely rare. whereas diode array spectrophotometers generate much more data than conventional instruments. we are not aware of any documented cases.Characteristics of diode array spectrophotometers is the same as with a diode array spectrophotometer. When measuring a single wavelength. 128 . If the sample is photosensitive. And. Yet although the internal data processing associated with diode array spectrophotometers is indeed far more complex than that of conventional instruments. the additional data creates numerous advantages. a conventional instrument should have a clear advantage over a diode array spectrophotometer since it subjects the sample to much less irradiation. the user is never made aware of the difference. Some circumstantial evidence indicates that a diode array instrument in some cases can acquire spectra of highly photosensitive compounds that cannot be obtained with a conventional instrument. Moreover. At the time of writing. this data is easily managed with modern software. In practice. as discussed throughout this primer. Errors in measuring Although diode array spectrophotometers can produce fluorescent samples incorrect results when measuring fluorescent samples.
References 1 References 129 .
. A. Chem. 82. Lavery.. 1980. 1983.. "Multiple analytical frequencies and standards for the least-squares analysis of serum lipids".B. Vol. H. van Kampen.. Strong III. Brown. 4. 1694. A. C. 1983. 9. 2. Anal. F. 1627–1639. Sharaf. B.. 82..G.. 42. Chemical Analysis.A. 30. Chem. Clin. Burgess. W. Anal. 2600– 2606. Zwart. Hellma: Müllheim/Baden. Chamberlain. M. Eds...A. Colour. 21.C.. A. 48. Chem. Techniques in Visible and Ultraviolet Spectrometry: Volume 3. Ilman.J. 1984. 1964. Heyden: London. Springer: Berlin. W.M. 8.. 1986.1. Chamberlain..B. G. 1703.J. Int. Practical Absorption Spectrometry.L.L. Lab. Anal. Macadam. Brown. 44–45. 7. Anal..R. Zijlstra. D. "Multicomponent analysis of hemoglobin derivatives with a reversed-optics instrument". Kisner.L.J... Kowalski. Johnson. 55. 1981. model compounds for second derivative spectroscopy". Hellma Catalogue. 6. Wiley: New York.W. 11. Knowles. M. G. computation and application.. T. "Correlation of measurements of absorbance in the ultraviolet and visible regions at different slit widths". 3. 55. Federici. Chapman and Hall: London. 12(7). D. 1994.. 1982. Maris M. 10. "Nonlinear multicomponent analysis by infrared spectrophotometry". Biochemistry. 130 . R.. 36. C.S. 1976. 12. Colour Measurement. 373. Kavarnos.J. "Quantification of aromatic residues in proteins. Chem. D. Golay. E. D. Chemometrics. 1984. its measurement.J. Savitzky.. Chem. 2155. M.G. Levine. 1982. 5.
C. Wieteska. Understanding Your Advanced UV-visible Software.. "PMA's computer system validation committee.. 131 . Elsevier: Amsterdam. Miller. Nowicka-Jankowska. 1990. Technol. J. Chemical Analysis. Gorczynska. National Formulary. 14. Analytical Visible and Ultraviolet Spectrometry. "USP perspectives on analytical methods validation". Rockville. Alford. W. 17. 1991. 15(3). 130–141.. Standards in Absorption Spectrometry. 1991. Harwood: Chichester. Inc.. Technol. 2nd ed. publication number 5965-7438E. XVII. 15.. J. Pharm. Mark. 18. computer system validation— staying current: installation qualification". 1993. 1990. Statistics for Analytical Chemistry. 2000. part number G1116-90003. J. Validation of Compendial Methods. 1710–1712. 20. Eds. MD. Pharm. 1988. A. WGD 2: Accreditation for chemical laboratories: Guidance on the interpretation of the EN 45000 series of standards and ISO/IEC Guide 25.. 1/WELAC Guidance Document No. A. 118.. F.. Comprehensive Analytical Chemistry. 1981. Volume XIX. C. 21. Vol. Techniques in Visible and Ultraviolet Spectrometry: Volume 1. 1986.. 16.N.. Paul.. 19. 22. A. 14. Agilent Technologies. 88–104. Knowles. The United States Pharmacopoeial Convention. Burgess.13. T. EURACHEM Guidance Document No. Agilent Technologies. General Chapter <1225>.S. Chapman & Hall: London. Michalik. United States Pharmacopoeia XXII.L..L. E. H. Principles and Practice of Spectroscopic Calibration. Cline. Operational Qualification and Performance Verification of UV-Visible Spectrophotometers. Wiley: New York. Miller. 2000.
28. Special Edition. United States Pharmacopoeia XXII. ASTM Standard E-387. and Near-Infrared Spectrophotometers. 29. 26. National Formulary. 24. 1990. MD.. 3rd ed. Strasbourg. National Institute of Standards and Technology: Gaithersburg. 1993. 1990. ASTM Standard E 275-83. Spectrophotometry and Light Scattering.. 25. Rockville. SRM 2034 Certificate. Standard Practice for Describing and Measuring Performance of Ultraviolet. General Chapter <851>. 1992. 132 . Technical Guide. 1996. MD. MD. The United States Pharmacopoeial Convention. Standard Test Method for Estimating Stray Radiant Power Ratio of Spectrophotometers by the Opaque Filter Method. National Institute of Standards and Technology: Gaithersburg. 104–106. Pharmeuropa. Inc.23. 27. Visible. 1995. Standard Reference Materials Catalog. 1609–1614. France. Council of Europe: Strasbourg. European Pharmacopoeia. XVII. 1994.
Index IndexIndex 133 .
90 multicomponent analysis. 94. 71 derivative spectroscopy. 58 unmasked. 105 holmium perchlorate solution. 123. 71 balance measurement. 31. 99. 58 open-topped rectangular. 33–34 photomultiplier tube. 104 D derivative spectra. 10–11. 58 fluorescence. 16–20. 36. 102 didymium oxide glass. 100 diode array detector. 81–83 cell types. 39. 105. 70. 68. 28 European Pharmacopoeia (EP). 73–76 detector diode array. 52. 72. 19. 105. 77–78 forward optics. 78. 44 baseline flatness. 21. 102–104. 106 in perchloric acid. 7. 62. 17 British Pharmacopoeia (BP). 43 G GLP (Good Laboratory Practice). 14. 21. 58 microcells. 4 enzyme activity. 105. 98. 106 blank. 36–37 correction techniques. 77 frequency. 101. 84. 37– 38 dispersion devices. 58. 106 Deutsche Arzneimittelbuch (DAB). 57–58 apertured. 49–52. 102 Golay. 42. 58. 20 derivative spectroscopy. 4 rotational. 34–35 photodiode. 56–59 H holmium oxide. 107 GMP (Good Manufacturing Practice). 112 Beer's law. 87 isoabsorbance. 105 collateral data.Index A absorbance. 3. 3. 96. 105. 98 photometric. 70. 110 flow-through cells. 59 flow through. 6–10. 73–76 internal referencing. 41. 39. 63 Bouguer-Lambert law. 74. 51–52. See Savitzky-Golay polynomial technique C calibration curve. 30 spectrum. 59 ASTM (American Standard Testing Methods). 87 confirmation wavelength. 106. 44. 109 holographic gratings. 13–14 concave mirrors. 56–57 chemical derivatization. 42. 32–33 HP 8450A spectrophotometer. 40– 41 dual-wavelength spectrophotometer. 105. 40. 64–66 acceptance angle. 102. 106. 32–33 DNA (deoxyribonucleic acid). 33. 21. 66–67. 15–16. 113 F filters. 60 cobalt salts. 101. 94. 99. 57 ultramicrocells. 103 and color. 11–12. 106. 78 accuracy. 109 wavelength. 40. 58– 59. 21– 26. 106 in nitric/perchloric acid. 69–76. 34–35 diode array spectrophotometer. 112. 105. 39. 63. 66. 36 chromophore. 96 neutral density glass. 4 vibrational. 98. 115. 123 background modeling. 13 bands. 100. 84 Agilent 8453 spectrophotometer. 111 extinction coefficient. 105–107 care of. 59 cells. 107. 14 apertured cells. 53. 124 weak. 116 E electromagnetic radiation. 101. 18–19 benzene (vapor). 96. 83. 20. 62. 3 B background modeling. 124 three-point correction. 42 134 . 105. 72 correlation coefficient. 108. 90. 95. 14–15 drift. 28 enzyme kinetic assays. 81 Beer–Bouguer-Lambert law. 95. 27 chromatic aberration. 18–19. 33 deuterium arc lamp. 123 color. 40 amino acids. 81. 21. 98. 124 conventional spectrophotometer. 47–49. 77 strong. 102. 75 interfering. 99 energy electronic. 82. 107 glass. 124 dual-beam spectrophotometer. 106 additivity. 72 measurement errors in. 6. 3. 67. 85–86. 4 emission standards. 97. 101. 2 electronic energy. 114–117. 59 material. 36 confidence interval. 11.
101. 108. 92. 106 in nitric/perchloric acid. 107 pH. 3 refractive index. 106. See three-point correction multicomponent analysis. 15. 97. 3 photochemical reaction. 117 NPL (National Physical Laboratory). 104–105. 106. 28 types of. 77 reversed. 9. 99. 124 and isoabsorbance. 100. 109 nitric acid. 109 photometric precision. 106 mercury vapor discharge lamp. See Bouger-Lambert law lamp deuterium arc. 108. 81–85. 70 monochromator. 124 linearity. 94 isoabsorbance. 106. 122 I identification. 9. 86–87. 10–11. 91. 108. 96 values. 63 resolution. 26 N NAMAS (National Measurement Accreditation Service). 68 reaction photochemical. 45–47. 32 proteins. 105. 106 Q qualitative analysis. 99. 37 potassium chloride. 56. 77. 105. 94 natural spectral bandwidth (NBW). 20. 68. 126 R range. 56. 104 European. 72. 123. 87 ISO 9000. 27. 62. 31–32 limit of detection (LOD). 101 potassium iodide. 105 principle component regression (PCR). 49–52. 51–52. 53. 104–105. 106 photomultiplier tube detector. 47–49. 126–127 spectral. 14 L Lambert. 10–16 quantitative analysis. 102. 107. 105. 106 phosphorescence. 82. 89 Rayleigh scattering. 32 least squares method. 98. 117 photometric. 102. 110 potassium dichromate. 111 135 . 76 mercury arc lamp. 16–27. 9. 99. 70 multiple least squares (MLS) method. 126 rotational energy. 101. 123 method validation. 106 in perchloric acid. 26 Peltier temperature controller. 84 reflection. 21– 26. 87–88 limit of quantification (LOQ). 62. 44–47 reversed optics. 31 xenon. 64. 100. 36 light sources. 70. 106. 11. 57 optics. 36. 96. 97. 112 interference. 26 prisms. 127 hydrolysis reaction of sultone. 103. 49–52. 123. 105 noise. 98. 77. 125–126 M matrix. 101 in sulfuric acid. 39. 92. 26. 64. 89 lenses. 44 wavelength. 105 tungsten-halogen. 14. 3. 105 NIST (National Institute of Standards and Technology). 78 reference standard. 38. 105. 72. 98. 106 mercury vapor discharge. 4 ruggedness. 106 wavelength. 31. 61 perchloric acid. 95. 92 microcells. 33 polychromator. 46–47 neodymium yttrium aluminum garnet. 12. 66. 39 United States. 78 photodiode detector. 59 forward. 16 instrumental spectral bandwidth (SBW). 106. 70. 49–52. 95. 67–76 internal referencing. 106. 106 filters. 105. 110 nickel salts. 102–104. 109. 3. 96. 106 mercury arc. 110 precision. 60–61 pharmacopoeia British. 95. 96. 100 neutral density glass. 58. 33–34 photometric accuracy. 39. 112. 96. 38. 99. 105. 106 method development. 78. 78. 50. 105. 101. 90 and multicomponent analysis. 62. 80–92. 87–88 linear dynamic range. 62. 86.Index HP 8453 spectrophotometer. 105. 124 O open-topped rectangular cells. 28. 24–25. 105 metal on quartz. 33 Morton-Stubbs correction. 58 P partial least squares (PLS) method. 96.
43. 124. 68–69. 82 uncertainty. 82. 39 single-component quantification. 41. 26. 95. 38. 39 split beam. 26 principle component regression (PCR). 101 sultone. 92 resettability. 47–49. 20 slit width. 99. 26. 125 stray light. 42 HP 8453. 125 signal-to-noise (S/N). 49 tungsten-halogen lamp. 9. 58 uncertainty. 103. 127 single beam. 110 solid absorption standards. 28 split-beam spectrophotometer. 107. 82 statistics. 58. 31 Tyndall scattering. 32 T temperature. 25. 48. 65 simple simultaneous equations method. 101. 64–66 working curve. 49. 26. 82 correlation coefficient. 114 spectral resolution. 110–111. 19. 42 stability. 59 V vibrational energy. 116 least squares. 60– 62 three-point correction. 42 spectrophotometric titrations. 106. 82. 60–62 SOP (standard operating procedure). 28. 9. 116 Rayleigh scattering. 87. 31. 101. 98 standard deviation. 78. 124 sulfuric acid. 125 confirmation. 62 Schott noise. 64. 110 sodium nitrite. 47–49. 89 single-beam spectrophotometer. 72. 82 statistical methods. 21–24. 39. 68 Schlieren effect. 124. 106 averaging. 9–10. 66–67. 101. 21–24. 28. 24–25. 104–105. 113 W wavelength accuracy. 68 Tyndall scattering. 106. 6. 43 HP 8450A. 67.Index S samarium oxide in perchloric acid. 125 standard error of regression. 92. 84. 82. See calibration curve X xenon lamp. 86. 96–97. 65. 82 United States Pharmacopoeia (USP). 25. 27. 112 sensitivity. 84 reproducibility. 106 unmasked cells. 49–50. 127 strong absorbance. 68 U ultramicrocells. 95. 124–125 weak absorbance. 87. 37–38 dual beam. 26. 63 Savitzky-Golay polynomial technique. 18. 126 sodium iodide. 87–89. 26 partial least squares (PLS). 50 selectivity. 36–37 diode array. 27. 65. 61. 99. 14–15. 87. 111 transmittance. 40 conventional. 3. 41. 9 scattering. 117. 4 136 . 99 solvent choice of. 40–41 dual wavelength. 124 precision. 27. 28. 44–47 spectrophotometer Agilent 8453. 62. 72 time averaging. 88. 83. 105. 124 self-test. 27. 49. 87. 105 reference. 125 toluene in hexane. 59–60 effect of. 10. 89 multiple least squares (MLS). 11. 117. 84–85. 69. 106. 122 system suitability. 101. 89 standard error of regression. 89–91. 70. 102. 115. 127–128 sample geometry. 101 sample decomposition. 26 simple simultaneous equations.
adaption. Reproduction. Printed in Germany 06/00 Publication number 5980-1397E . except as allowed under the copyright laws. or translation without prior written permission is prohibited. The information contained in this publication is subject to change without notice.s1 Copyright Agilent Technologies 2000 All rights reserved.