Infrared spectrophotometry (IR) uses similar instrumentation to UV-vis spectrophotometry but it uses a different light source (infrared frequencies of ~1011 to 1014 Hz). The instrumentation consists of an infrared beam that is split into two identical beams (reference beam and analytical beam).
The analytical beam passes through the sample so that molecular vibrational absorption can occur. The intensity of the two beams are compared over the range 2.5 µm to 16 µm and plotted as a function of wavenumber (the reciprocal of the wavelength in cm-1).
The result is an IR Spectrum plot where intensity is plotted as a function of wavenumber over the range of 4000 - 625 cm-1. The important thing to remember is that the IR plot is a series of downward facing peaks. That is, absorption of the IR beam is being measured.
The position, width and heights of the peaks can be used to assist the identification of structural features of molecules. IR analysis provides a fast and relatively cheap method of organic functional group identification.
One benefit of this analysis is that solids, liquids and gases can all be examined.