Fourier-transform infrared spectroscopy

Fourier-transform infrared spectroscopy measures how a sample absorbs infrared light, deriving its spectrum from an interferogram using a Fourier transform. The spectrum reveals molecular vibrations associated with chemical bonds and functional groups.

Michelson interferometer: An optical instrument that splits a beam into two paths and recombines them to produce interference. Its moving mirror creates the path differences that encode the sample’s infrared absorption.
Infrared spectroscopy: Spectroscopy that studies how matter interacts with infrared radiation. FTIR belongs to this family, whose spectra arise from infrared-active molecular transitions.
Fourier-transform infrared microscopy: A method that combines infrared spectroscopy with microscopy to map chemical composition at small spatial scales. It applies FTIR to localized regions and spatially resolved chemical maps.
Dispersive infrared spectroscopy: Infrared spectroscopy that separates wavelengths with a dispersive element and measures them sequentially. Unlike FTIR’s interferometric acquisition, it scans spectral components one at a time.
Interferogram: A signal recording intensity as a function of optical path difference in an interferometric measurement. It is the raw signal Fourier-transformed to obtain the spectrum.
Molecular vibration: A periodic motion of atoms in a molecule around their equilibrium positions. Characteristic vibrational frequencies produce many of the bands observed in FTIR spectra.
Gas chromatography–infrared spectroscopy: An analytical approach that separates mixture components by gas chromatography and identifies them by infrared spectra. FTIR detection can provide structural information about compounds separated from complex mixtures.
Raman spectroscopy: A vibrational spectroscopy method based on inelastic scattering of monochromatic light. Its selection rules complement infrared absorption, making the methods useful together.
Fourier transform: A mathematical operation that represents a signal as a combination of frequency components. It converts the interferogram’s path-difference signal into intensity across infrared frequencies.
Infrared-active vibration: A molecular vibration that changes the molecule’s dipole moment and can absorb infrared radiation. Only vibrations that meet this condition generate absorption bands in ordinary infrared spectra.

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