Gas Chromatography - Tandem Techniques - Gas Chromatography IR Spectroscopy (GC/IR) Systems > Page 42

 

The IR vapor spectrum was then obtained in the usual manner. After completing the IR spectrum some of the solute vapor was drawn from the IR cell directly into the mass spectrometer source using the high vacuum of the mass spectrometer, and a mass spectrum obtained. The carrier gas was then turned on again and the chromatographic development continued. When the next peak arrived the same procedure was used to obtain IR and MS spectra of the next peak. This development process was termed interrupted elution development.

 

To ensure that the quality of the separation was not impaired by the stop/start procedure, a carefully designed pneumatic system was employed. When the elution of the peak was complete (as determined by the detector) and the solute was collected in the packed trap, a solenoid valve at the end of the column closed, and the column released by opening a valve at the front of the column. This caused slight back-development of the solutes remaining in the column accompanied by some peak sharpening. After the trap was heated, a separate supply of nitrogen, (controlled by another solenoid valve) was used to drive the solute back into the heated IR cell. After the spectra had been taken, the trap was then cooled with cold water by actuating another solenoid valve and finally the column terminal valve opened. The column inlet-pressure was brought up to the operating pressure over a period of about 10 seconds using a flow programmer. A diagram of the IR cell is shown in figure 31.

At the time of instrument there were a limited number of IR transparent materials available for cell window construction. The material used by Scott et al. was silver chloride (horn silver) and the cell was gold plated internally to reduce light absorption losses.