Page 27 - Tandem Techniques

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from the point of view of tandem techniques, the important characteristics are defined by the four
classifications given above.

GSC is used largely for the separation of permanent gases and very low boiling liquids and is rarely
used in direct association with identification techniques, except occasionally with the quadruple mass
spectrometer. GLC, on the other hand, is frequently used in conjunction with both the IR spectrometer
and the mass spectrometer for the separation and identification of virtually all classes of volatile
compounds. LLC systems can be extremely difficult to maintain in sufficiently stable conditions to
permit an associated spectrometer or other identifying instrument to function in a consistent and reliable
manner. This is due to the labile nature of the stationary liquid phase in contact with the flowing mobile
phase. It follows that LLC is only rarely employed in tandem systems but, should some means of
stabilizing the stationary phase be developed, this situation could rapidly change. On the other hand,
LSC is the most popular form of liquid chromatography and is now being employed more extensively
than even GLC; in fact LSC is now used in many applications that were originally in the realm of GLC.
The stationary phases employed in LSC are either silica gel or, more commonly, a bonded phase and
more recently certain polymeric materials. A bonded phase is formed when silica gel is treated with an
appropriate silanizing reagent to link a hydrocarbon chain or other organic moiety on each of the
hydroxyl groups situated on the silica surface. Such stationary phases are fairly stable, unless exposed
to mobile phases of extreme pH, and thus are very satisfactory for the operation of instruments in
tandem. The polymeric phases are mostly made from micro-reticulated polystyrene resin cross-linked
with divinyl benzene. The two basic chromatographic techniques will now be discussed in some detail
to allow the pertinent factors that effect the interfacing between the chromatograph and spectrometer or
other identifying technique to be clearly understood.

Gas Chromatography

A block diagram of the basic gas chromatograph is shown Figure 1.1. It consists of a gas supply, the
nature and complexity of which depends