Page 152 - Tandem Techniques

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Consequently, besides being the first tandem system to be described, the device was also the first triplet
instrument to be reported (GC/IR/MS) and for this reason will be described in some detail. The layout
of the pneumatic system of the Tandem Instrument is shown in Figure 4.1.

The procedure involves sensing the peak as it starts to elute and then diverting it through the IR cell to
the packed trap. When the elution of the peak is complete, the carrier gas is arrested, the trap heated and
the solute regenerated into the IR cell by a stream of nitrogen. The IR vapor spectrum is then obtained
in the usual manner. After the IR spectrum is completed, a sample of the solute vapor is drawn from the
IR cell into the mass spectrometer source, and the mass spectrum obtained. When the mass spectrum
had been run, the carrier gas is turned on again and the chromatographic development continued. On the
arrival of the next peak the procedure is repeated. Due to the nature of the stop/start technique, the
process was given the term interrupted elution development.

In order to ensure that the quality of the separation was not impaired by the stop/start procedure, a
carefully designed pneumatic system was used. When the elution of the peak was complete and the
solute resided in the packed trap, a solenoid valve at the end of the column closed, and the pressure in
the column was released by actuating a valve at the front of the column. This caused some slight back
development of the solutes remaining in the column which was accompanied by some peak sharpening.
The trap was then heated and a separate supply of nitrogen, controlled by another solenoid valve, was
used to regenerate the solute into the heated IR cell. When the spectra had been obtained, the trap was
cooled by a stream of cold water controlled by another solenoid valve and the column terminal valve
opened. In order to prevent a sudden increase in the column pressure as the carrier gas was turned on
again, a flow programmer was used that brought the column up to pressure over a period of about 10
seconds. A diagram of the IR cell is shown in Figure 4.2.
In the 1960s, there was a limited number of materials available that were transparent to infrared light
and that could be used as cell windows for IR