GB969943A - Improvements in or relating to transit-time mass spectrometers - Google Patents

Abstract

969,943. Mass spectrometers. MAXPLANCK-GESELLSCHAFT ZUR FORDERUNG DER WISSENSCHAFTEN e.V., and INSTITUT FUR PLASMAPHYSIK G.m.b.H. Dec. 21, 1962 [Dec. 22, 1961], No. 48501/62. Heading H1D. In a mass spectrometer including a longitudinally extending vacuum tube, the ions produced from a sample to be analysed are caused to oscillate along a portion of the length of the tube; during oscillation the ions become separated into groups in accordance with their charge/mass ratio and these groups are detected a predetermined time after production of the ions. As shown, the vacuum tube of a mass spectrometer contains an arrangement 1 of electrodes having holes on a common axis and connected to a voltage divider and voltage sources so that a trough-shaped, preferably parabolic, potential distribution with a relatively negative minimum point 2a is produced along the tube axis. A pulse generating source 3a is connected to an electron gun system 3 for producing bursts of ions which are accelerated along the axis of the tube. When the ions reach the end of the electrode arrangement 1, they are stopped by the positive potential barrier 9 existing there, and are returned to their production point where they are again reflected by the other positive peak 8 of the potential trough. The ions thus oscillate in the potential trough between the ends of the electrode arrangement 1 and separate into groups in accordance with their charge/mass ratio. At a specific time after production of the ions, a pulse from a source 7 is applied to the end electrode 1a which is thus made more negative. The groups of ions are then able to surmount the lower potential barrier 9<SP>1</SP> for entry into an electron multiplier 5, the output signals from which can be detected and analysed in known manner. The disturbing influence of the electric fields associated with the electron gun system may be reduced by the provision of at least one compensating electrode 3b, e.g. of the iris type, also connected to the pulse source 3a. The detection of the ions may be made by an inductive collector arranged at one end of the tube, or by a signal induced into one of the electrodes in the centre of the electrode arrangement 1. In a second embodiment (Fig. 2, not shown) the electron gun system and the electron multiplier are arranged at the same end of the tube, and the electron beam for producing the ions is injected perpendicularly to the tube axis; also, a shielding electrode situated between the ion source and the electrode arrangement 1 is arranged to be supplied for a short period with a positive potential of such magnitude that the ions created cannot pass into the potential trough. This enables the electron gun system to produce a relatively large number of ions which can be released all together by removing the potential from the shielding electrode, so that the operating time of the ion source can be made relatively long without undesirable widening of the output signal.