The FT-IR (Fourier Transform Infrared Spectrophotometer) is used to generate a spectra of a given sample over a range of infrared light wavelengths.  Each of the peaks on the spectra corresponds to a specific type of chemical bond, so the spectra is truly a  chemical “fingerprint” of the material. 

By comparing the spectra of an unknown material to those in a database of known materials it is often possible to identify the exact nature of a given material.  This technique is often used to identify an unknown plastic material and can sometimes be helpful in defining any contaminants that may be present.