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spectrometer. Nevertheless, as one form accepts the sample as a vapor into a gas stream that transports
the sample to the mass spectrometer, it will be discussed in this section dealing with GC tandem
systems.

The affinity membrane has been described by a number of workers, for trace analysis [44-46] and
analysis of the chemical constituents of biological systems [47]. A system that describes a type of
membrane that is very specific is that of Chen Xu et al. [48], who used it to selectively extract and
detect aldehydes. A diagram of the selective membrane interface is shown in figure 5.32. The
preparation of an affinity membrane to render it specific can be very complex, and the particular
procedure used by the authors for forming an aldehyde-specific membrane will be described below. The
membrane is supported diametrically across the center of a flat cylindrical chamber and held in place by
two fluid-tight PTFE gaskets. The sample, which can be a gas or a liquid, and in this particular case is a
liquid, passes across one side of the chamber that has been bisected by the membrane. The selected
solute diffuses across the membrane and evaporates into a stream of helium gas that passes through the
other half of the transfer chamber.

Figure 5.32
The Membrane Interface

The transfer film was prepared from a square sheet of cellulose dialysismembrane (2 cm x 2 cm, 0.28
µm thick), having a molecular weight cut-