9780521516358book.pdf

substance (to produce a cation) or electron capture (to produce an anion). The analyte

must be in the vapour state in the electron impact source, which limits the applicabil-

ity to biological materials below ca. 400 Da. Before the advent of electrospray and

MALDI, the method did have some applicability to peptides, for example, whose

volatility could be increased by chemical modification. A large amount of fragmen-

tation of the sample is common, which may or may not be desirable depending on the

information required.

Chemical bonds in organic molecules are formed by the pairing of electrons.

Ionisation resulting in a cation requires loss of an electron from one of these bonds

(effectively knocked out by the bombarding electrons), but it leaves a bond with a

single unpaired electron. This is a radical as well as being a cation and hence the

representation as M.þ, the (þ) sign indicating the ionic state and the (.) a radical.

Conversely, electron capture results both in an anion but also the addition of an

unpaired electron and therefore a negatively charged radical, hence the symbol M..

Such radical ions are termedmolecular ions, parent ionsorprecursor ionsand under

the conditions of electron bombardment are relatively unstable. Their energy in excess

of that required for ionisation has to be dissipated. This latter process results in the

Mass analyser

and detector

Ion

Ions

Electron beam

Filament

Source chamber

Heater/sample vaporiser

Sample insertion

+ 70 eV

Ion repeller,

+4 kV

focussing

Fig. 9.2Electron impact source. Electrons are produced by thermionic emission from a filament of tungsten

or rhenium. The filament current is typically 0.1 mA. Electrons are accelerated toward the ion source chamber

(held at a positive potential equal to the accelerating voltage) and acquire an energy equal to the voltage

between the filament and the source chamber, typically 70 eV. The electron trap is held at a fixed positive

potential with respect to the source chamber. Gaseous analyte molecules are introduced into the path of the

electron beam where they are ionised. Owing to the positive ion repeller voltage and the negative excitation

voltage that produce an electric field in the source chamber, the ions leave the source through the ion exit

slit and are analysed.

355 9.2 Ionisation