GB1242031A - An improved multipole mass filter - Google Patents

Abstract

1,242,031. Mass spectrometers. HEWLETT-PACKARD CO. 1 Oct., 1969 [26 June, 1968], No. 48318/68. Heading H1D. A multipole mass filter, e.g. a quadrupole, in which the primary electrodes are symmetrically disposed about a central axis Z, has a fringing- field penetrating structure comprising, as shown, two concentric, conical, electricallyconductive and mutually insulated tubes 52, 54 symmetrically disposed about axis Z with their larger ends lying outside the quadrupole adjacent the ion source and their smaller inner ends lying inside the quadrupole. The inner conical electrode 52 is connected via flange 55 to electrode 38 of the ion gun and may be maintained at - 700 volts potential and extends into the quadrupole as far as the focal point f of the ion beam in order to shield it from the defocusing effect of the fringing fields near the quadrupole entrance. The outer electrode 54 is connected to the housing 10 from which the primary electrodes are insulatingly supported and is maintained at the quadrupole ground potential to shield the quadrupole from the effect of the negative potential on inner electrode 52. The quadrupole may be followed, in the beam direction, by a fringing-field compensating structure comprising a first electrode 60 at quadrupole ground potential and a second electrode 62 at a negative D.C. potential which is proportional to the D.C. excitation voltage applied to the quadrupole. The ion beam is formed by electron bombardment of a sample gas and is extracted and focused by repeller electrode 34, e.g. at + 5¢ volts, electrode 36 with curved conductive mesh 40, e.g. at +5 volts, and electrode 38 at - 700 volts. Relative dimensions of the ion gun electrode and the distance of the ion focus f from the centre of curvature C is discussed. For applications other than use in the vacuum of space the mass filter is housed in an evacuated enclosure.