GB730134A - Mass spectrometer - Google Patents

Abstract

730,134. Heavy-ion-beam tubes. BENDIX AVIATION CORPORATION. July 6, 1953 [July 9, 1952], No. 18717/53. Class 39 (1). A mass spectrometer of the time of flight type, in which ions are withdrawn from a storing region in pulses and accelerated towards a detector, has means for decelerating the ions, and then accelerating them again, between the place of withdrawal and the detector. By thus 'increasing the overall time of travel the resolution of the instrument is improved in respect of ions of adjacent mass units, particularly of heavy mass units. Electrons from a filament source 10 can pass through slotted electrodes 12, 16 and a compartment between plates 22, 24 to a collector 20. The slots in 12 and 16 have a collimating effect on the electron stream, and a magnetic field may be provided for the same purpose. In the steady state, however, while electrode 12 has a positive potential, electrode 16 and filament 10 are at ground potential, so that the electrons are not accelerated beyond electrode 12. When negative pulses are applied to electrodes 10 and 12 from a pulse-forming circuit 68, electrode 16 is now relatively positive, and an additional acceleration is imparted to the electrons. Molecules of gases in an unknown mixture in a container 34 are admitted by a conduit 30 into the space between plates 22 and 24, where they are ionized by the accelerated electrons passing through. Plate 24, which has a slot 26, is spaced about 2 mm. from plate 22. A similarly slotted plate 36 is spaced a further 2 mm. away. 10 em. from plate 36 is a mesh electrode 40, with a final collector electrode 46 a further 10 cm. away. When the number of ions in the electron stream between plates 22, 24 approaches saturation, the stream is cut off by removing the negative voltage pulses. At the same time, positive pulses are applied to electrodes 22, 24 and 40, which may be of the respective order of 200, 150, and 150 volts; electrodes 36 and 46 are maintained at zero potential. The resulting fields created cause the ions to be withdrawn from the storage region in a pulse, ions of the same charge being differentially accelerated according to their masses. The electrons are decelerated in passing between electrodes 36 and 40, and accelerated again between 40 and 46, the effect in each case being differential. Since electrodes 36 and 40 have the same voltage difference as electrodes 46 and 40, the electrons regain in the second region the energy they lost in the first region, and impinge on the collector 46 with the same velocity they had in passing electrode'36; but the increase in travel time produces a corresponding increase in time between the collection of ions of adjacent mass units at 40, which enhances the resolution. Specifications 713,993 and 727,683 are referred to.