In pharmaceutical analysis the region of the electromagnetic spectrum used is 4000-600 cm-1 (wavelength 2.5-16.7 µm), i.e. the mid-infrared. Spectrophotometric measurements in this region are mainly used for identification purposes. Except for enantiomers, which have identical spectra in solution, the infrared spectrum of any given substance is unique. Polymorphism and other factors, such as variations in crystal size and orientation, the grinding procedure, and the possible formation of hydrates may, however, be responsible for minor, and occasionally substantial, variations in the infrared spectrum of a substance in the solid state. The infrared spectrum is not usually greatly affected by the presence of small quantities of impurities in the substance tested. For identification purposes, the spectrum may be compared with that of a reference substance, concomitantly prepared, or with a reference spectrum.
The terms absorbance, transmittance, absorptivity and absorption spectrum are defined in The international pharmacopoeia, 3rd ed., Vol. 1, pp. 33-34, in the chapter "Spectrophotometry in the visible and ultraviolet regions".