Page 194 - Tandem Techniques

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The heavier ions having greater momentum suffer less dispersion than the lighter elements, thus causing
a preferential loss of the lighter elements. Sandra et al. [5] introduced a simple modification to the ion
lens system that increases the number of ions reaching the analyzer and this modification is shown in
Figure 5.9.

The modification consists of the insertion of a short length of stainless steel rod 2.2 cm long and 2 mm
in diameter along the axis of the existing lens system. The rod is mounted axially through the photon
stop and isolated from it by a ceramic insulator. The optimum position of the photon stop was found to
be about 1.5 cm from the front end of the rod and 0.7 cm from the rear end. It was found that a potential
of 5 V applied to the rod improved the ion collection efficiency by about a factor of five (500%), with a
corresponding reduction in the minimum detectable ion concentration.

Examples of Some Gas Chromatography/Mass Spectrometry Applications

One of the first gas chromatography/mass spectrometer systems to be developed was that described by
Banner et al. (6) and, for historical interest, a diagram of their original apparatus is shown in Figure 5.
10. The mass spectrometer used by Banner et al. was a rapid scanning magnetic sector instrument that
easily provided a resolution of one mass unit. Nowadays, mass spectrometers (giving vastly improved
resolution) are mostly used with capillary columns, and operated in a very similar manner, with the
column eluent passing directly into the ionization source of the spectrometer. Today the single, or triple
quadrupole mass spectrometer are the most commonly used mass spectrometers in GC/MS tandem
systems, and have been shown to give extremely impressive in-line sensitivity, extended mass range
and a respectable resolution. GC/MS tandem systems have a number of attributes that make them
particularly useful for certain types of applications. The chromatograph provides a high separating
capability that can handle exceedingly complex mixtures, and the mass spectrometer can contribute
both high resolution and very