Multipole ion guide for providing an axial electric field whose strength increases with radial position, and a method of operating a multipole ion guide having such an axial electric field

Abstract

A mass spectrometer having an elongated rod set, the rod set having a first end, a second end, a plurality of rods and a central longitudinal axis is described as is a method operating same. Embodiments involve a) admitting ions into the rod set; b) producing an RF field between the plurality of rods to radially confine the ions in the rod set, wherein the RF field varies along at least a portion of a length of the rod set to provide, for each of the ions, a corresponding first axial force acting on the ion to push the ion in a first axial direction; and, c) for each of the ions, providing a corresponding second axial force to push the ion in a second axial direction opposite to the first axial direction; wherein the corresponding first axial force increases relative to the corresponding second axial force with radial displacement of the ion from the central longitudinal axis in any direction orthogonal to the central longitudinal axis such that the first corresponding axial force is less than the corresponding second axial force when the ion is less than a threshold radial distance from the central longitudinal axis and the corresponding first axial force exceeds the corresponding second axial force when the ion is radially displaced from the central longitudinal axis by more than the threshold radial distance in any direction orthogonal to the central longitudinal axis.