FTIR Spectroscopy Basic theory

Infrared (IR) radiation is electromagnetic radiation that encompasses all the wavelengths between the visible and microwave regions of the electromagnetic spectrum. The IR region can also be subdivided into three smaller regions known as near-IR, mid-IR and far-IR, the ranges of which are neatly summed up in the table.

Region Wavenumber Range Vibrational/Rotational Information
near IR 14000 - 4000 Changes in vibrational and rotational levels, overtone region and some low energy electron transitions
mid-IR 4000 - 400 Changes in fundamental vibrational levels of most molecules
far-IR 400 - 20 Rotational energy level changes

In all types of absorption spectroscopy the fundamental governor of intensity is the number of molecules sampled. A simple relationship between the intensity of the transmitted (I) and incident radiation (I0) and the amount of sample in the beam (concentration or thickness) exists and is known as the

Beer-Lambert law,


Or in the logarithmic form

Where in both cases c is the concentration, l is the cell thickness and is the frequency dependent extinction coefficient. The Beer-Lambert law is the foundation for all quantitative infrared spectroscopy.


Traditional (dispersive) infrared techniques experience difficulties due to the '1 wavenumber at a time' nature of data acquisition. This leads to either a poor signal to noise ratio in a spectrum or a very long time needed to obtain a high quality spectrum. Both these situations cause problems with kinetic work. The first gives inherent large errors, the second prohibits in-situ work. These problems can be overcome using Fourier transform infrared spectroscopy (FT-IR) which is based on the interferometer originally designed by Michelson and a mathematical procedure developed by Fourier that converts response from the 'time' to the 'frequency' domain.

Michelson interferometer

In the Michelson interferometer a parallel, polychromatic beam of radiation from a source (A) is directed to a beam splitter (B), made from an infrared transparent material, such as KBr. The beam splitter reflects approximately half of the light to a mirror, known as the fixed mirror (C), which in turn reflects the light back to the beam splitter. The rest of the light passes through to a mirror, moving continuously, at a known velocity, back and forth along the direction of the incoming light and this is known as the moving mirror (D). Upon reflection from the moving mirror, radiation is then directed back to the beam splitter. At the beam splitter some of the light that has been reflected from the fixed mirror combines with light reflected from the moving mirror and is directed towards the sample. After passing through the sample (E) the radiation is focused onto the detector (F). The detectors are sufficiently fast to cope with time domain signal changes from the modulation in the interferometer.

The Schematic of the Michelson interferometer, which forms the basis of most FTIR instruments.

As the distance of the moving mirror from the beam splitter changes, different wavelengths of radiation are in-phase and out-of-phase at a frequency that is dependent both upon the rate at which the mirror moves and the frequency of radiation.


The complex pattern of overlaid sinusoidal waves of light (in the time domain) is known as an interferogram. The interferogram can be converted back to the original frequency distribution (spectrum) by means of a Fourier transform, which can be done very rapidly on a PC.

Traditional FTIR spectroscopy uses simple transmission sampling techniques. However, 'difficult' samples and experiments need specialised sampling accessories. Within MERI we use the following techniques

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