Off-Axially Arranged, Low Noise, High Collection Efficiency Ion Collector for Quadrupole Mass Spectrometer

Abstract

An improved ion detector for a partial pressure measurement at low levels has been developed using the off-axis technique. The ion detector consists of the electrostatic deflection plates and a channel electron multiplier. The arrangement and the configuration design are determined from the results of the map of electric equipotential surfaces and the trajectory analyses. The ion detector of this arrangement can greatly reduce the noise and base line shifts caused by photons and excited neutrals, which are dependent on the emission current and the pressures in the ion source. The noise count rate is on the order of 1-2 cps at a multiplier voltage of 3000 volts. The ion collection efficiency of 100% in this detector arrangement is obtained at a multiplier voltage of 3000 volts or more with the help of electrostatic deflection plates. The ion detector also permits focusing of ions with fairly different ion energies onto the multiplier. The off-axis ion detector gives a factor of 140 increase in effective sensitivity of the mass spectrometer. The minimum detection limit of the mass spectrometer with the off-axis ion detector is also given.