used: 1) The chromatogram of the unknown is recorded 2) a known amount of the analyte(s) of interest is added 3) the sample is analyzed again under the same conditions and the chromatogram is recorded. From the increase in the peak area (or peak height), the original concentration can be computed by extrapolation. The detector response must be a linear function of analyte concentration and yield no signal (other than background) at zero concentration of the analyte. Procedure A typical procedure involves preparing several solutions containing the same amount of unknown, but different amounts of standard. For example, five 25 mL volumetric flasks are each filled with 10 mL of the unknown. Then the standard is added in differing amounts, such as 0, 1, 2, 3, and 4 mL. The flasks are then diluted to the mark and mixed well. The idea of this procedure is that the total concentration of the analyte is the sum of the unknown and the standard, and that the total concentration varies linearly. If the signal response is linear in this concentration range, then a plot similar to what is shown above is generated. Error The x-intercept gives the concentration of the unknown. Note this value is the diluted concentration. In the procedure section above, 10 mL of unknown was diluted to 25 mL. It is this diluted concentration that is found by the x-intercept. To find the original concentration of the unknown, one must back calculate that value. The error in the x-intercept is calculated as shown below. is the standard deviation in the residuals is the slope of the line is the y-intercept of the line is the number of standards is the average measurement of the standards are the concentrations of the standards is the average concentration of the standards See also Standard Curve Isotope dilution Internal standard References Analytical chemistry Laboratory techniques