Infrared spectroscopy is one tool of choice for the identification of organic based materials and a few inorganic compounds. A beam of infra-red radiation is directed through the specimen. Specific bonds in the molecules of the sample will absorb radiation from the beam at certain frequencies or wavelengths causing them to oscillate faster. These absorptions ("IR absorbance bands") show up as deflections from the baseline and can be assigned to specific groupings of atoms; e.g., C-H, C=O. In many cases, identification of a material can be made based on

the presence of a number of these absorbance bands. The availability of reference spectra of known compounds increases the probability of making a positive identification.

IR spectroscopy is best accomplished with samples that can be removed and transferred to a sample holder and are transparent to IR. The sample should be at least 10, preferably 25 µm across and just a few µm's thick. Theoretically samples can be done on a reflective substrate but the substrate must be a perfect mirror. In general, this is a qualititative technique. Some relative concentration information can be obtained but exact standards are required for even semi - quantitative assays.

A related technique is called Laser Raman Spectroscopy. It uses a beam of laser light to irradiate the specimen providing spectra that appear similar to FTIR spectra. However, LRS can be used on particles as small as 1 µm across and may be analyzed in situ.