There are two techniques to measure how much light a sample absorbs or emits at each wavelength in the IR region- Dispersive spectroscopy and second is Fourier transform spectroscopy.
Dispersive spectroscopy is a straightforward
technique, where a monochromatic beam (single wavelength) of light shines on
the sample and absorption is measured. This is repeated for each wavelength.
In FT-IR spectroscopy, a light
beam consisting of many frequencies shines on the sample and how much of that
beam is absorbed is measured. Next the light beam is modified using ‘Michaelson
Interferometer’ – it is a set of mirrors one of which is moving, and the other
mirror is fixed. The moving mirror blocks each wavelength of light periodically
(blocked, transmitted, blocked……). This is how the beam is modified to contain
different combination of wavelengths or frequencies giving a second data set.
This is repeated many times over a short span of time giving many data sets
corresponding to different frequencies. A computer takes this raw data and
converts it into spectrum using a mathematical process known ‘Fourier
Transform’. Here, raw data is light absorption at each mirror position into
desired result which is light absorption at each wavelength.
FT-IR has many advantages over
dispersive IR such as high signal-to-noise spectrum ratio, short scan time,
high resolution, reduced interference from stray light.
