GB1371104A - Methods of and apparatus for analysing mixtures - Google Patents

Abstract

1371104 Ion sources APPLIED RESEARCH LABORATORIES Ltd 20 March 1972 [19 March 1971 1 July 1971 10 Sept 1971] 7304/71 30794/71 and 42253/71 Heading H1D In the analysis of a material by a mass analyser, a gaseous medium containing ions of the material is produced by injecting the material in vapour, liquid, or aerosal form, into a high temperature plasma which is at atmospheric pressure, whereafter the gaseous medium containing the ions is fed to the mass analyser through a pressure reducing stage. The high temperature plasma is produced in an argon or helium carrier gas by an r.f. electrodeless discharge, or by a D.C. discharge in gas flowing through a capillary tube. For a massive solid example either an aerosol suitable for injection into the plasma is produced by employing the sample as the cathode for a D.C. arc struck in flowing argon or helium, first compressing a powder mixture of the material with graphite or metal into a solid cathode if the material is an insulator; or a pulsed laser beam is focused on to the sample surface in flowing gas, the sample being moved between successive pulses to vary the point of impact. The plasma containing the ionized material is sampled by extracting a portion through a fine orifice and passing the resulting beam through a succession of low pressure regions. If greater control of ionization is required, ions are removed from the beam by a transverse electric field, and the resulting neutral beam is passed into an electron impact ionizer. As shown in Fig. 1, a D.C. discharge is maintained between the anode 11 and the sample cathode 10, the carrier gas entering at tube 13 and carrying the sample particles into a high temperature plasma formed in the capillary tube 18 by a discharge between electrodes 16 and 17. Most of the plasma passes out through the pumping tube 20, the remainder being drawn off through a fine orifice 21 into the mass analyser entry tube 22, which is maintained at reduced pressure by the pumping tube 23. The apparatus of Fig. 4, in which the plasma is produced by electrodeless discharge in a capillary tube 32 mounted in a microwave cavity 33, is suitable for sample material which must be maintained within a given temperature range: the sample aerosol, e.g. from a chemical reaction and at 300‹ C. is expanded into the pipe 35 at a point of the plasma tail at which it has cooled to that temperature, whereafter a portion is drawn off to the mass analyser 41, thermal lagging 39 preventing further substantial reduction in temperature.