What is infrared spectroscopy?

The physics of the measurement

The bonds in a molecule are not rigid; they stretch and bend, vibrating at characteristic frequencies that lie in the infrared region. When infrared light passes through a sample, a bond absorbs radiation whose frequency matches one of its vibrational modes, provided the vibration changes the molecule’s dipole moment. The instrument records how much light is absorbed across the infrared range, producing a spectrum of absorption bands. Because each type of bond — such as O–H, C=O, or C–H — absorbs in a fairly predictable region, the spectrum acts as a fingerprint of the molecule’s functional groups. IR is a vibrational branch of spectroscopy.

FTIR and the fingerprint region

Most modern instruments are Fourier-transform infrared (FTIR) spectrometers. Rather than scanning one wavelength at a time, they collect all frequencies at once using an interferometer, then apply a mathematical Fourier transform to recover the spectrum, giving fast measurement and a strong signal.

Spectra have two useful regions: the higher-frequency region where common functional groups give recognisable bands, and the complex lower-frequency fingerprint region, whose detailed pattern is highly specific and can confirm a compound’s identity by comparison with reference spectra.

Uses in research

Infrared spectroscopy is widely used in organic and analytical chemistry, materials science, and polymer research to identify functional groups, confirm compound identity, and monitor reactions. It complements NMR spectroscopy and mass spectrometry in structure determination, each method supplying different information. Reliable identification depends on referencing spectra to standardised libraries and reporting sample-preparation conditions so results can be reproduced.