Gas analysis in vacuum systems: magnetic, crossed-field and time-of-flight analysers

Abstract

The significance of gas analysis in vacuum systems is becoming more important as increasing demands are made upon the system designer. Some form of mass spectrometer capable of measuring the partial pressure of all the constituent gases and vapours from 10⁻⁴ mbar down to below 10⁻¹⁰ mbar should be attached to the system. Care must be taken to ensure that this instrument does not significantly modify the pressure equilibria. Mass spectrometers suitable for this work are ionization gauges in which positive ions formed by electron bombardment are resolved into a mass spectrum, the intensity of each component being measured separately. None of the instruments so far developed gives a true gas analysis, as all suffer from three fundamental disadvantages:

• (i) They resolve into an ‘ion mass’ spectrum, not a ‘gas composition’ spectrum. For example, the gases nitrogen, carbon monoxide and ethylene all have their most intense signal at mass 28 amu (on the O = 16 atomic scale). Also, many gases, for example large organic molecules, give complex ion patterns which are not readily distinguishable. (See Figure 6.1¹, which presents the ion spectrum of butane, a relatively simple hydrocarbon.)

• (ii) The sensitivity, as in all ionization gauges, is different for each gas, and unfortunately the relative sensitivity between gases is a function of the individual instrument design and operation.

• (iii) The resolution of the mass spectrometer is not perfect; it usually decreases at the high end of the mass scale. In general, for a particular instrument an improvement in resolution is obtained only at the expense of a reduction in sensitivity.