A multipole ion optical device comprises a first plurality of electrodes distributed along a first axis; and a second plurality of electrodes distributed along a second axis, generally parallel to the first axis, to define an ion channel between the first and second pluralities of electrodes. Each of the first plurality of electrodes and the second plurality of electrodes is configured to receive a respective RF voltage having an asymmetric waveform and such that adjacent electrodes of the first and second pluralities of electrodes receive RF voltages having different phases. The first and second plurality of electrodes and the plurality of RF voltages are configured such that a strength of an electric field in the ion channel is sufficient for ions to experience mobility variation.