Instrumentation for Gas Chromatography - Forensic Applications of Gas Chromatography - page 27

Sample inlet

Ionisation chamber

Heated cathode
filament Anode

Lenses

Mass analyser

Figure 2.18 Electron impact ionisation.

CH 4 + (g) + CH 4(g) → CH 5 + (g) + CH 3 * (g) (2.7)

X (g) + CH 5 + (g) → XH + (g) + CH 4(g) (2.8)

where CH 4 + is the molecular ion and CH 5 + is the reactant ion.
( Note: As a result of electron impact ionisation, the ion generated is
representative of the molecular weight (X + ) of that compound; in chemical
ionisation, the ion generated has a molecular weight plus 1 (XH + ) of that
compound.)
The generated ions, of specific m/z ratios, are then separated by a mass
spectrometer. A range of different mass spectrometers is available for GC.
he most common are

• Quadrupole MS
• Ion trap MS
• Time-of-light MS

2.6.5.1 Quadrupole MS
In a quadrupole MS, four stainless steel rods are located horizontally to each
other (Figure 2.19) such that the same combination of direct current (DC)
and radio frequency (RF) voltages can be applied to opposite rods at the
same time. Based on a specific combination of DC/RF voltages, an ion with a
selected mass to charge (i.e., m/z) ratio will pass through the quadrupole MS
and be detected; at that moment, all other ions of different m/z ratios are lost.
Rapidly altering the combined DC/RF voltages allows ions of different m/z
ratios to pass through the mass spectrometer and be detected. For GC-MS,