Variation of the dispersion, resolution and inclination of the focal plane of a single-focussing mass spectrometer by use of two quadrupoles

Abstract

A description is given of the combination of a single-focussing magnetic-sector type mass spectrometer with two quadrupole lenses: an electrostatic quadrupole at the detector side and a magnetic one at the source side of the sector magnet. The electrostatic lens changes the mass dispersion, whilst the magnetic lens is used as an image-corrector lens which moves and rotates the focal plane to the desired position producing a sharp line spectrum on the detector plane perpendicular to the main ion beam. The ion detector consists of a (flat) channeltron electron multiplier array with a phosphor screen and a Vidicon-multichannel-analyser combination for simultaneous read-out. A computer program is discussed for the design of the lens system and the calculation of the ion trajectories of the different masses. Experiments (performed with 1,4-diisopropylbenzene) show a maximum ratio of 1.2 of simultaneously detectable masses when both quadrupoles are inoperative; 1.6 with the quadrupoles in operation and 1.01 when both quadrupoles have reversed polarities, with a mass resolution of 350, 200 and 2000 respectively. Deviations between theory and experiment, expansion and limitations of the lens system, and extension of the mass range are discussed.