Abstract: A mass spectrometer system and a method of operating a mass spectrometer are provided. An RF field is produced between the plurality of rods to radially confine the ions in the rod set. The RF field has a resolving DC component field. The resolving DC component field is varied along at least a portion of a length of the rod set to provide a DC axial force acting on the ions.

Abstract: A method of operating a mass spectrometer having an elongated rod set and a set of auxiliary electrodes is provided, the rod set having an entrance end and an exit end and a longitudinal axis. The method comprises a) admitting ions into the entrance end of the rod set; b) trapping at least some of the ions in the rod set by producing a barrier field at an exit member adjacent to the exit end of the rod set and by producing an RF field between the rods of the rod set; and, c) providing an auxiliary AC excitement voltage to the set of auxiliary electrodes to energize a first group of ions of a selected mass to charge.