The different possible vibrational states of a molecule translate into a set of energy levels. The separation between these energy levels falls within the infrared region of the electromagnetic spectrum. The separation between these energy levels falls within the infrared region of the electromagnetic spectrum.

With the exception of the enantiomers, there are not two molecules that have the same spectrum of IR. We can distinguish two zones in the spectrum from IR according to the vibrations that are observed there:
 

• Between 4000 and 1300 cm^-1: a series of bands of absorption are usually observed provoked by simple tension vibrations, that is between two atoms of the molecule. They identify functional groups.
• Between 1400 and 600 cm^-1: absorption bands of molecular vibrations are usually observed, the assignment of which is more difficult, since each of them is generated by individual absorptions added together (complex vibrations). On the other hand, in this area, small differences in the structure of the molecule give rise to important variations in the absorption maxima. Therefore, they are characteristics of a given compound and that is why this area of the spectrum is called a fingerprint.

FT-IR is one of the most versatile spectroscopic techniques and of greatest application in the characterisation and identification of materials (polymers, plastics, inorganic solids), analysis of pharmaceutical and synthesis products, analysis of pollutants, forensic science, biomedicine, agriculture and food among others.

• Pamphlet of the Bruker Alpha FT-IR Spectrometer
• Specifications for the Bruker Alpha FT-IR Spectrometer