Fundamentals of GC–MS Ionisation Techniques

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Electron Impact (EI)
Electrons are emitted from a heated filament (usually made of tungsten or rhenium) and are accelerated across the source by using an appropriate potential (5 - 100 V) to achieve the required electron energy to ionise the molecule (Figure 1).

Figure 1: Electron impact ionisation process (See CHROMacademy.com for Flash video)

The analyte is introduced into the mass spectrometer ion source where it is impacted by the beam of ionising electrons leading to the formation of an analyte radical cation (Figure 2).  The molecular ions produced can fragment by loss of a radical or of a molecule with all its electrons paired (Figure 3).

EI is a relatively harsh form of ionization and as a result the parent often fragments further producing a pattern of fragment ions which can aid structural elucidation, however, the disadvantage of this is the frequent absence of the parent ion.

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