Patent ID: 7709785

Claim:
A method of operating a mass spectrometer having an elongated rod set, the rod set having an entrance end, an exit end spaced from the entrance end, a plurality of rods and a central longitudinal axis, the method comprising: a) admitting ions into the entrance end of the rod set; b) producing an RF field between the plurality of rods to radially confine the ions in the rod set, the RF field having a resolving DC component field; and, c) varying the resolving DC component field along at least a portion of a length of the rod set to provide a DC axial force acting on the ions, wherein at any point along the portion of the length of the rod set, the DC axial force acting on the ions increases with radial displacement of the ions from the central longitudinal axis in a first selected radial direction, such that at that point the DC axial force: i) moves the ions towards the exit end when displacement of the ions from the central longitudinal axis is in the first selected radial direction; and ii) does not move the ions toward the exit end when displacement of the ions from the central longitudinal axis is in at least one other radial direction different from the first selected radial direction.