Page 166 - Tandem Techniques

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The interferometer's modulated IR beam focuses on the narrow argon 'stripe' that contains the sample.
In practice, the IR sensitivity with this system of isolation is commensurate with that of the mass
spectrometer. The improved IR sensitivity is partly due to the detector element being approximately the
same size as the sample 'stripe' and partly due to the sample being in an inert argon matrix, which
causes the absorption bands to be much sharper and thus much higher. An example of the relative
absorption peak heights for a liquid sample, solid sample and a matrix isolated sample according to
Mattson is shown in Figure 4.15.

Figure 4.15.
Absorption Peaks for Liquid, Solid
and Matrix Isolated Samples
Courtesy of Mattson Instruments Inc.

It is seen that matrix isolation sharpens the peak considerably and will provide a very significant
increase in sensitivity. The interferometer is contained in a vacuum chamber to keep the background
free of spectral contaminants, which results in a remarkably stable output. The system is cleaned very
rapidly by merely warming the drum and pumping out the argon and sample vapor, leaving the disk
with absolutely no residue.

An interesting example of the use of a GC/IR tandem system, that can provide both high sensitivity and
good spectroscopic resolution, is in studying the influence of molecular structure on physiological
activity. Structural isomers exhibit widely different biological activity and traditionally such structures
were identified by mass spectrometers employing GC/MS tandem instruments. However, the mass
spectrometer does not differentiate well between certain important isomers, such as 1,4-