Gas Chromatography - Tandem Techniques - The Characteristics of Infrared Absorption > IR Spectroscopy Instrumentation > The Fourier Transform IR Spectrometer. > Page 29



Light from the IR source is reflected from a mirror, through the reference cell, through an attenuator system and onto a second mirror. The beam is attenuated by interposing a device that removes a continuous, but controllable fraction of the light from the reference beam. The attenuator usually takes the form of a comb, the teeth of which are cut so that the amount of light attenuated is linearly related to the lateral movement of the comb through the beam. After passing through the attenuator to the second mirror, the attenuated light passes through a chopper to a grating, where light of a specific wavelength is selected and passes to an IR sensor. A second beam of light from the source is reflected through the sample cell by a plane mirror and then, by means of another mirror, also passes through the chopper to the grating and then to the IR sensor.


The chopper arranges for the sensor to alternately receive light that has been transmitted through the cell, and light that has passed through the attenuator. The servo system of the attenuator adjusts the light transmission until both beams have the same intensity. The amount of light that is absorbed is indicated by the position of the attenuator. More simply, the amount of light of the chosen wavelength which is absorbed by the sample, is measured by attenuating the reference beam until its intensity is equivalent to that of the beam transmitted through the sample. The resolution is controlled by the width of the slit which is adjustable. In the older versions of this type of IR spectrometer, an analog plotter, mechanically associated with the attenuator, recorded the spectrum. Even if modified to provide an output that is proportional to absorption, the big disadvantage of this type of spectrometer for use in combined systems is its very slow rate of scanning.



The Fourier Transform IR Spectrometer.


The Fourier transform IR spectrometer (FTIR) involves much simpler instrumentation but more complicated data processing. As a consequence, the FTIR spectrometer can scan a sample far more rapidly than the dispersive instrument. The faster scan speed, obviously, makes it more suitable for operation in conjunction with a gas chromatograph. The basic difference is that the dispersion instrument scans the sample at one increment of wavelengths at a time, whereas the FTIR spectrometer examines the sample using all the wavelengths coincidentally. A diagram of the basic system is shown in figure 19.