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Infrared Absorption Spectrometers

Introduction

This document describes dispersive and Fourier-transform spectrometers that are used in infrared absorption spectroscopy. Common light sources are tungsten lamps, Nernst glowers, or glowbars. IR detectors consist of a semiconductor such as PbS or liquid-nitrogen-cooled HgCdTe (also called MCT).


Dispersive infrared spectrometer

Dispersive IR spectrometers use a diffraction grating in a monochromator to disperse the different wavelengths of light. Dispersive IR spectrometers have largely been replaced with FTIR instruments. They find some use in specific applications, such as monitoring a single IR wavelength to measure the kinetics of a fast reaction.

Schematic of a dispersive IR absorption spectrometer


Fourier-transform infrared (FTIR) spectrometer

Most modern IR absorption instruments use Fourier-transform techniques with a Michelson interferometer. To obtain an IR absorption spectrum, one mirror of the interferometer moves to generate interference in the radiation reaching the detector. Since all wavelengths are passing through the interferometer, the interferogram is a complex pattern. The absorption spectrum as a function of wavenumber (cm-1) is obtained from the Fourier transform of the interferogram, which is a function of mirror movement (cm). This design does not have the reference cell of a dispersive instrument, so a reference spectrum is recorded and stored in memory to subtract from the sample spectrum.

Picture of an FTIR absorption spectrometer


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