in cross-section in the illustrations. Other geometries use prisms, half-spheres, or thin sheets. Applications Infrared (IR) spectroscopy by ATR is applicable to the same chemical or biological systems as the transmission method. One advantage of ATR-IR over transmission-IR is the limited path length into the sample. This avoids the problem of strong attenuation of the IR signal in highly absorbing media such as aqueous solutions. For ultraviolet or visible light (UV/Vis) the evanescent light path is sufficiently short such that interaction with the sample is decreased with wavelength. For optically dense samples, this may allow for measurements with UV. Also, as no light path has to be established single shaft probes are used for process monitoring and are applicable in both the near and mid infrared spectrum. Recently, ATR-IR has been applied to microfluidic flows of aqueous solutions by engineering microreactors with built-in apertures for the ATR crystal, allowing the flow within microchannels to pass across the crystal surface for characterisation, or in dedicated flow cells. Due to the ATR geometry and the resulting evanescent wave, it is possible with this technique to study transport phenomena and sorption kinetics through thin films. The ability to passively characterise samples, with no sample preparation has also led to the use of ATR-FTIR in studying trace evidence in forensic science. ATR-FTIR is also used as a tool in pharmacological research to investigate protein/pharmaceutical interactions in detail. Water-soluble proteins to be investigated require Polyhistidine-tags, allowing the macromolecule to be anchored to a lipid bilayer, which is attached to a Germanium crystal or other suitable optical media. Internal reflection with and without applied pharmaceutical or ligand will produce difference spectra to study conformational changes of the proteins upon binding. See also Fourier-transform infrared spectroscopy Surface plasmon resonance Sources Bibliography Scientific techniques Infrared spectroscopy