GB1338213A - Dual mode chemical or electron impact ionization source for mass spectrometer - Google Patents

Abstract

1338213 Ion sources SCIENTIFIC RESEARCH INSTRUMENTS CORP 13 June 1972 [18 June 1971] 27555/72 Heading H1D In an ion source which is operable in either one of two modes, an electron beam is fired through an aperture into a gas which is either at low pressure, to produce electron impact ionization, or at a higher pressure, to produce chemical ionization, operation in the second mode being effected by changing from the use of a relatively large aperture, e.g. 1À5 mm.<SP>2</SP> or more-to the use of an aperture having sufficiently small dimensions-e.g. 0À15 mm.<SP>2</SP> or less-to provide an impedance to gas flow sufficient to permit the maintenance of the required higher gas pressure in a substantially closed container. In one embodiment (Figs. 2 and 3, not shown) two apertures (20, 22) of appropriately different size are formed in a movable plate (24) which may be adjusted to align either aperture with an electron-emitting filament. Alternatively, the aperture may be in the form of a continuously adjustable slit (Figs. 4 and 5, not shown), in which two plates (42, 44) spring-urged together and having a pair of inclined ends are forced apart to a desired extent by the thrust of a wedge member. In a further embodiment, Fig. 7, a switch 90 permits the energization of either one of a pair of electron-emitting filaments 72 and 82, the electron beams from which pass through corresponding small and large apertures 74, 86 into a high pressure region 70, and a low pressure region communicating therewith through a small aperture 78, the resulting ion beam in either case being accelerated by the plates 80.