Page 185 - Tandem Techniques

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such columns and peak concentrations are usually adequately high. Consequently, the column flow can
be passed directly into the mass spectrometer and the total sample will enter the ionization source.

Ion Sources Used in GC/MS

In order to discriminate between molecules of different masses by electrostatic or magnetic deflection
systems, the molecules must be ionized and/or they must be fragmented into charged ions. There are
two types of ion source in general use with GC/MS tandem systems and they are electron impact
ionization (El) and chemical ionization (CI). More recently, the inductively coupled plasma ionization
process (ICPI), which is usually employed for liquid or solid samples, has also been introduced to
GC/MS systems and has been used for certain types of inorganic analysis.

Electron Impact Ionization

The design and function of the electron impact ionization source has been discussed in Chapter 2. It is a
relatively hard ionization procedure, and thus produces many fragments and often a relatively small
parent ion. In fact, in some cases no parent ion may be produced at all. A diagram of the electron impact
ion source similar to that given in Figure 2. 30 in Chapter 1 is shown in Figure 5.3.

Figure 5.3
The Electron Impact Ion Source