An Investigation of Ion Production and Complementary Electrostatic Ion Extraction and Focusing Systems for High Sensitivity Mass Spectrometer Ion Sources

Abstract

Over the past six years, one of the major developments in the analytical applications of the mass spectrometer has been its use when connected directly to a gas chromatograph. However,the quantities of organic compound released from the chromatograph column are frequently too small to be analysed in the conventional mass spectrometer. It is therefore essential to increase sensitivity so that smaller sample quantities will give a useful spectrum. Hitherto, the sensitivity has been extremely small; typically for every million molecules entering the ion source only one ion eventually reaches the mass spectrometer collector. In the present study it is shown that the most significant limiting factor is the electron bombardment source. In spite of its importance, there has been little previous quantitative investigation. In the first part of the thesis a mathematical treatment of the ion optics of the source is given. New computational techniques have been developed which enable the focal properties of the source to be computed rapidly and accurately. The theoretical predictions have been confirmed by experiments carried out on a commercial mass spectrometer. In the mathematical analysis, the effects of electron space-charge were neglected. However, in the second part of the thesis it is shown experimentally that under certain circumstances, electron beam space-charge can cause the ion current emitted from the source to increase appreciably. The experimental evidence, supported by a further theoretical analysis, suggests that this arises because the space-charge forces in the electron beam restrict the emergence of ions from the source to within a small angular wedge. When the pressure in the source is increased above a certain critical value, the ion current increases with pressure at a much reduced rate. It is shown that at this critical pressure,there is complete neutralisation of the electron space-charge by the ions produced. A new type of source is proposed which minimises this effect in organic mass spectrometry.