The application of beam transport theory to mass spectrometer design

Abstract

The basic contention of this thesis is that the concepts and techniques of beam transport theory ca,n be usefully applied to mass spectrometer design. Most important aimong these concepts is that of the beam emittance, which is the area occupied by the ions of the beam in a phase space defined by the transverse (y or z) displacement and the corresponding conjugate momentum. It can be shown that the emittances in both y and z phase spaces are finite and that, for surface ionization sources, their magnitude is due to the initial Maxwellian velocity distribution of the thermal ions. For maximum transmission these emittances must be matched to the phase space acceptances of the magnetic analyser and collector system. A theoretical study has led to the design of a system, consisting of three quadrupole lenses, which should provide the required matching between ion source and analyser. Such a system can be expected to yield high ion transmission efficiency, of the order of 60 per cent, and to function in a predictable manner. A mass spectrometer incorporation of the above theoretical focussing system has been constructed and tested. The results of these preliminary tests indicate that the instrument is functioning approximately according to expectations.