MASS SPECTROMETER WITH AXIAL EJECTION AND WITH ROD GEOMETRY FOR GENERATING A TWO-DIMENSIONAL QUADRUPOLE FIELD WITH ADDED OCTOPOLE COMPONENT AND METHOD OF OPERATING THE SAME

摘要

A mass spectrometer (210) having an elongated rod set (238), and a method of operating same. The rod set has an entrance end, an exit end and a longitudinal axis. Ions are admitted into the entrance end of the rod set. At least some of the ions are trapped in the rod set by producing a barrier field at an exit member (242) adjacent to the exit end of the rod set and by producing an RF field between the rods of the rod set adjacent at least the exit end of the rod set. The RF and barrier fields interact in an extraction region adjacent to the exit end of the rod set to produce a fringing field. Ions in the extraction region are energized to mass selectively eject at least some ions of a selected mass to charge ratio axially from the rod set past the barrier field. The RF field is a two-dimensional substantially quadrupole field having a quadrupole harmonic with amplitude A2, an octopole harmonic with amplitude A2, and a hexadecapole harmonic with amplitude A8. A8 is less than A4, and A4 is greater than 0.1% of A2.