A sample is illuminated with infrared light, the absorption of which causes molecular vibrations in the sample. These vibrations are characteristic of functional groups in the sample. A representative spectrum is shown to the left with some of the evident functional groups denoted. FTIR is not able to determine connectivity of functional groups.

FTIR-ATR is capable of rapidly determining the major organic chemical species present in a sample, with a detection limit of 1-5% by weight.