Gas analysis in vacuum systems: quadrupole mass analysers

Abstract

This instrument has been subject to continuous development over the past. 20 years¹ to the point where it is used successfully in many applications ranging from sophisticated chemical analysis to general vacuum monitoring*. It is, in fact, the most extensively used mass spectrometer in high- and ultra-high vacuum systems for routine gas analysis. In its simple form it has become recognized as a reliable and easy-to-use instrument in the heavy industrial, as well as the research, laboratory. The description of this quadrupole mass spectrometer (or mass filter), often referred to as the residual gas analyser (RGA), and its use in the general vacuum laboratory is emphasized in this chapter. In many applications these RGA mass spectrometers substitute for the ionization gauge, in effect covering the same pressure range but with the advantage of indicating the partial pressure of the main constituents in the vacuum system. The geometry of the quadrupole mass spectrometer is very similar to that of the extractor ionization gauge developed by Pittaway² (described in detail in Chapter 3, section 3.5), with a mass filter interposed between source and collector. The filter is an electrostatic lens consisting of an array of four parallel rods arranged symmetrically around a central axis along which the ions travel. The ‘quadrupole’ structure is illustrated in Figure 7.1 which shows that the inner surfaces of the electrodes have hyperbolic profiles. The principles of using quadrupole magnetic or electric fields for ion beam control were developed before the application to the mass filter³⁻⁵.