Page 150 - Tandem Techniques

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Chapter 4
Gas Chromatography IR Spectroscopy (Gc/Ir) Tandem Systems

The gas chromatography/infrared spectrometer tandem system was the second type of tandem
instrument to be developed, the first being the GC/MS combination. The GC/MS instrument, although
very useful, was expensive, complex, and the early models were somewhat difficult to operate. In
contrast, the IR spectrometer was less expensive, easy to operate and the IR spectrum was usually quite
adequate to confirm the identity of an eluted component. In addition, the resolution obtainable from the
mass spectrometers available in the early days of gas chromatography was inadequate for the
elucidation of a completely unknown structure, so the need for supporting evidence from an IR
spectrum was usually necessary.

Initially IR spectra were obtained off-line, by condensing the eluted solute in a cooled trap. The solute
was then either dissolved in a suitable IR solvent, made into a 'mull' with 'Nujol' (a very pure form of a
high boiling normal alkanes), or pressed into an alkali halide pellet and the spectrum obtained using
standard techniques. The efficient collection of the solute by condensation, however, could be difficult
as, due to the very low concentrations at which each solute was eluted, the partial pressure of the
condensed material was often similar to its partial pressure as it left the GC column. The most efficient
method of collecting the solute was to use argon as the carrier gas, and condense the argon and the
solute simultaneously in a tube immersed in a liquid nitrogen bath. Subsequently,