Page 83 - Tandem Techniques

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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 attenuator is controlled by a servo mechanism
that 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. In other words, the amount of light, of a
particular 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 tandem systems is its very slow rate of scanning.

The Fourier Transform IR Spectrometer.

The Fourier transform IR (FTIR) spectrometer works on an entirely different principle, and involves
much simpler instrumentation but more complicated data processing. The FTIR spectrometer can scan a
sample far more rapidly than the dispersive instrument. The faster scan speed, obviously, makes it more
suitable for tandem operation in conjunction with a chromatograph. The basic difference is that the
dispersion instrument scans the sample one wavelength 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 2.11. Collimated light from a broad band infrared source passes into the optical system and
impinges on a beam splitter that comprises a very thin film of germanium. Approximately 50% of the
light passes through the film and is reflected back along its path by a fixed mirror, where half of the
light intensity (25% of the original light intensity) is reflected by the same beam splitter, through the
sample cell, to the infrared sensor. The other 50% fraction of the incident light is reflected at right
angles to its incident path onto a moving mirror. Light from the moving mirror returns along its original
path and half of the light intensity is transmitted through the beam splitter, through the sample cell, to
the infrared sensor. Thus 25% of the incident