An ion guide includes a plurality of rods, arranged about an axis that extends lengthwise from one end to the other of the guide. The rods guide ions in a guide region along and about the axis. A conductive casing surrounds the rods. The casing and the rods are geometrically arranged to produce an axial electric field along the axis. Specifically, the geometry is such that a first constant applied DC voltage (UDC), applied to the rods, and a second constant applied DC voltage (UCASE) applied to the casing, produce a voltage gradient between said casing and said axis that has a different magnitude at different positions along said axis.