A mass analyser (2) is provided comprising a plurality of electrodes having apertures through which ions are transmitted in use. A plurality of pseudo-potential corrugations are created along the axis of the mass ((analyser 2). The amplitude or depth of the pseudo-potential corrugations is inversely proportional to the mass to charge ratio of an ion. One or more transient DC voltages are applied to the electrodes of the mass analyser (2) in order to urge ions along the length of the mass analyser (2). The amplitude of the transient DC voltages applied to the electrodes is increased with time and ions are caused to be emitted from the mass analyser (2) in reverse order of their mass to charge ratio. Two AC or RF voltages are applied to the electrodes. The first AC or RF voltage is arranged to provide optimal pseudo-potential corrugations whilst the second AC' or RF voltage is arranged to provide optimal radial confinement of ions within the mass analyser (2).