When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation corresponding to the energy difference between the two states. The absorption of infrared (IR) radiation causes transitions between vibrational energy levels in a molecule. Therefore, IR spectroscopy is a useful analytical tool for determining the molecular structure of molecules.

Different compounds display unique properties due to their functional groups, which allows IR spectroscopy to be used to determine the present functional groups. Vibrational IR spectroscopy is performed in the 2.5 to 25 µm wavelength range. A molecule is considered IR active if it exhibits a change in dipole moment during vibration when absorbing IR radiation. For a molecule to absorb IR radiation effectively, its vibrations must fluctuate its dipole moments, allowing it to interact with the electromagnetic field of the IR light.

Since different functional groups absorb IR radiation at varying frequencies, the IR spectrum is similar to the "fingerprint"for each molecule. IR spectroscopy is primarily employed in qualitative analysis to identify the functional groups in organic and inorganic compounds by comparing their vibrational frequencies with known compounds. Additionally, It can determine the concentration of a substance by applying the Beer-Lambert law, where absorbance is proportional to concentration.