3.3: Fourier Transform Infrared Spectroscopy

3.3.1 Instrumentation

Fourier transform infrared spectroscopy, a versatile and widely used analytical technique, relies on the creation of interference in a beam of light. A source light beam is split into two parts and a continually varying phase difference is introduced into one of the two resultant beams. The two beams are recombined and the interference signal is measured and recorded as an interferogram. A Fourier transform of the interferogram provides the spectrum of the detected light.

A Fourier transform infrared spectrometer consists of an infrared source, an interference modulator (usually a scanning Michelson interferometer), a sample chamber, and an infrared detector. Interference signals measured at the detector are usually amplified and then digitised. A digital computer initially records and then processes the interferogram and also allows the spectral data that result to be manipulated.

The principal reasons for choosing Fourier transform infrared spectroscopy are that (i) the instruments record all wavelengths simultaneously and thus operate with maximum efficiency and (ii) Fourier transform infrared spectrometers have a more conventional optical geometry than do dispersive infrared instruments. These two factors lead to the following advantages:

1. Much higher signal-to-noise ratios can be achieved in comparable scanning times.

2. Wide spectral ranges can be covered with a single scan in a relatively short scan time, thereby permitting the possibility of kinetic time-resolved measurements.

3. Higher resolution is possible without undue sacrifice in energy throughput or signal-to-noise ratios.

4. None of the stray light problems usually associated with dispersive spectrometers are encountered.