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vapor, joined the column eluent between the column and the jet interface. Reagent samples were
injected though the system and the mass spectra taken, to ensure that no artifacts were produced. The
reagent reached the mixing T in about 1 sec and the reaction time was also about 1 sec, so that peaks as
little as 5 sec peak width at half height could be efficiently derivatized. The distinctive advantage of this
system was that specific peaks could be chosen exclusively for derivatization. The sample was run
normally, and the retention times noted. The separation was repeated and reagent injected at the
appropriate times to derivatize the chosen peaks.

Some Special Applications of GC/MS

The presence of dispersive (nonpolar) toxicants in the many and various sources of aqueous effluents, is
an ever present challenge for environmental chemists. Such materials are often in very low
concentration and usually need extraction, and concentration, to bring them to a level that is convenient
for assay. Some techniques that can be used for this purpose have already been discussed, but some
further examples will be given that are of general use. Burkhard and Durham [33] developed a
satisfactory method based on a GC/MS tandem system. In fact the sample was prepared for analysis by
a preliminary liquid chromatography separation process, and so the method might be considered to be
another triplet system. An appropriate volume of the sample, for example river water is passed through
a preconditioned, C18 solid phase extraction tube, which selectively removes all the dispersive
constituents of the water, on the front of the packing. The absorbed materials are then displaced from
the packing using 25, 50, 75, 80, 85, 90, 95, and 100% methanol in water, providing 8 aliquots. The
fractions are then concentrated, and each is then separated on an appropriate LC column, employing a
suitable gradient. However, the gradient will need to be varied somewhat for each aliquot. LC fractions
are collected at minute intervals and concentrated before injection onto the GC column. Due to
preliminary LC separation, the fractions give very simple chromatograms to interpret, and, with the aid
of the mass spectrometer, the sample can not only be quantitatively assayed, but the identity of the
toxicants can usually be unambiguously confirmed.