Molecules absorb light in the infra-red region of the electromagnetic spectrum. This absorption corresponds to the chemical (covalent, other hydrogen bonds) bonds present in the molecule. The frequency range are measured as wave numbers typically over the range 4000 – 600 cm-1.The ratio of the sample spectrum to the background spectrum is directly related to the samples absorption spectrum. The resultant absorption spectrum from the bond natural vibration frequencies indicates the presence of various chemical bonds and functional groups present in the molecule. FT-IR is particularly useful for identification of organic molecular groups due to the range of functional groups, side chains and cross-links involved, all of which will have characteristic vibrational frequencies in the infra-red range.