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bromide, to form the fluorinated derivatives which are strongly electron capturing in character.

Derivatization can also be carried out after the separation has taken place, and this procedure is called
post-column derivatization. However, post-column derivatization is not performed for chromatographic
purposes but to either increase detector sensitivity or to augment the information provided by the mass
spectra. It is clear that the derivatization must take place in the gas phase, and thus only relatively
volatile derivatizing reagents can be used. The most common derivatizing procedures are bromination,
deuterium exchange and acylation. A number of schemes for post-column derivatization have been
reported. However, recently Ligon and Grade [38] described an instrument developed specifically for
use with GC/MS systems. The principle of their derivatization apparatus is depicted in Figure 5.29.

Figure 5.29
Post Column Derivatization Apparatus
In the original design the apparatus could take eluents from either of two columns, one for normal use,
the other for thermal desorption analysis. The column outlet was connected to the mass spectrometer
using a jet concentrator interface. The reagent was injected into a helium stream using a GC sample
injector, and the helium stream, carrying the reagent