IRspectrum

An infrared spectrum (IR spectrum) is a plot of the absorbance or transmittance of a sample as a function of infrared wavenumber. It provides information about molecular vibrations and is widely used to identify functional groups and characterize chemical structures. Infrared radiation causes vibrational transitions in molecules; only vibrations that involve a change in dipole moment are IR active.

Most modern IR work uses Fourier-transform infrared spectroscopy (FTIR). A broadband infrared source, an interferometer, and a detector measure interference patterns that are mathematically transformed into a spectrum. In the mid-infrared region (approximately 4000 to 400 cm−1) fundamental vibrations are observed, while near-infrared and far-infrared regions cover overtones, combination bands, and lattice modes.

Sampling methods vary. Transmission spectra require a suitable sample thickness or cell; attenuated total reflectance (ATR) provides surface-access spectra; diffuse reflectance is used for powders and rough surfaces. The spectrum is typically plotted as absorbance or transmittance versus wavenumber (cm−1).

Interpretation relies on characteristic absorption bands. Functional groups produce diagnostic peaks, for example carbonyls near 1700 cm−1, broad O–H or N–H stretches around 3200–3600 cm−1, and C–H stretches near 2850–2960 cm−1. The fingerprint region (approximately 600–1500 cm−1) contains many absorptions unique to a molecule, aiding identification.

Applications span organic, inorganic, pharmaceutical, and polymer analysis, materials characterization, and quality control. Limitations include peak overlap and limited sensitivity for nonpolar bonds, as well as interference from moisture and ambient CO2. Quantitative use is possible with proper calibration and path-length considerations.