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9.1.3 Vacuum system

All mass analysers operate under vacuum in order to minimise collisions between ions

and air molecules. Without a high vacuum, the ions produced in the source will not

reach the detector. At atmospheric pressure, the mean free path of a typical ion is

around 52 nm; at 1 mtorr, it is 40 mm; and at 1mtorr, it is 40 m.

In most instruments, two vacuum pump types are used, e.g. a rotary vane pump (to

produce the main reduction in pressure) followed by a turbomolecular pump or diffu-

sion pump to produce the high vacuum.

The rotary vane pump can be an oil pump to provide initial vacuum (approximately

1 torr), while the turbomolecular pump provides working high vacuum (1 mtorr to 1 ntorr).

This is a high-speed gas turbine with interspersed rotors (moving blades) and stators

(i.e. fixed or stationary blades) whose rotation forces molecules through the blade system.

9.2 Ionisation

Ions may be produced from a neutral molecule by removing an electron to produce a

positively chargedcation, or by adding an electron to form ananion. Both positive-

and negative-ion mass spectrometry may be carried out but the methods of analysis in

the following sections will be described mainly for positive-ion MS, since this is more

common and the principles of separation and detection are essentially the same for

both types of ion.

9.2.1 Electron impact ionisation (EI)

Electron impact ionisation(EI) is widely used for the analysis of metabolites, pollu-

tants and pharmaceutical compounds, for example in drug testing programmes.

Electron impact (EI) has major applications as a mass detector for gas chromatography

(GC/MS, Section 11.9.3). A stream of electrons from a heated metal filament is

accelerated to 70 eV potential (the electron volt, eV, is a measure of energy). Sample

ionisation occurs when the electrons stream across a high vacuum chamber into

which molecules of the substance to be analysed (analyte) are allowed to diffuse

(Fig. 9.2). Interaction with the analyte results in either loss of an electron from the

Inlet sourceIon Massfilter Detector systemData

High vacuum system

Fig. 9.1Basic components of mass spectrometers.

354 Mass spectrometric techniques