Gas Chromatography - Tandem Techniques - The Basic Principles of Mass Spectroscopy > The Sector Mass Spectrometer > Chemical Ionization > Page 69


Methane produces more active reagent ions than isobutane, and thus, although methane ions produce a number of fragments by protonation, isobutane, by similar protonation processes, produce almost exclusively the protonated molecular ion. This is shown by the mass spectrum of methyl stearate in figure 56. Spectrum (A) was produced using methane as the reagent gas and shows significant fragmentation together with the protonated parent ion. Spectrum (B), however, obtained with butane gas exhibits the protonated molecular ion only.


The chemical ionization source and simple ion impact source are very similar and most electron impact sources can also work as chemical ionization sources. When used in the electron impact mode dual-action sources are not as efficient as the dedicated electron impact sources, but the ionization efficiency is not reduced by more than 50%. Continuous use of a chemical ionization source causes significant contamination which results in the build-up of residues from the ionization process As a result, the source requires baking-out fairly frequently.


A diagram of a typical gas inlet system for chemical ionization is shown in figure 57,





Figure 57. A Gas Inlet System for Chemical Ionization