Page 190 - Tandem Techniques

Page 173

Methane produces more aggressive reagent ions than isobutane, and thus whereas methane ions produce
a number of fragments by protonation, isobutane, by a similar protonation process, will produce almost
exclusively the protonated molecular ion. This is clearly demonstrated by the mass spectrum of methyl
stearate shown in Figure 5.6. Spectrum (A) was produced by chemical ionization using methane as the
reagent gas and exhibits fragments other than the protonated parent ion. In contrast, spectrum (B)
obtained with butane as the reagent gas exhibits the protonated molecular ion only.

The chemical ionization source is very similar to the simple ion impact source; in fact most mass
spectrometer electron impact sources can perform the dual role, and in addition, act as a chemical
ionization source. Dual-action sources do not perform quite as well as dedicated electron impact sources
when used in the electron impact mode, but the loss of ionization efficiency is certainly no more than
50%. Continuous use of a chemical ionization source results in significant source contamination which
impairs the performance of the spectrometer. This results from the build-up of residues from the
ionization process and thus the source requires cleaning by baking-out fairly frequently. A diagram of a
typical gas inlet system for a chemical ionization source is shown in Figure 5.7,

Figure 5.7
A Gas Inlet System for Chemical Ionization

The diagram illustrates the use of three different reagent gases but any number could be incorporated if
desired. The source pressure is usually held at 0.1-0.5 torr and a low pressure regulator is incorporated
to