An ion mass spectrometer that has an ion channel shaped to modify the speed of a carrier gas as the carrier gas traverses the ion channel. In one case, the ion channel has segments of constant diameter in which the speed of the flowing gas is constant but different than the speed in other segments of the ion channel. This controlled variation in speed from segment to segment, together with the control of the axial electric field in the ion channel, provide greater control on the separation of ions in the ion channel. A method of analyzing ions based on a variation of at least one of axial electric field and of the speed of the flowing gas in the ion channel is also disclosed.