An ion guide or mass analyser 2 is disclosed comprising a plurality of electrodes 2a having apertures through which ions are transmitted in use. A plurality of axial pseudo-potential barriers, wells or corrugations are created along at least a portion of the length of said ion guide or mass analyzer 2 (see Figures 3A, 3B and 4). A pseudopotential barrier is also created at the entrance and/or exit of the ion guide or mass analyser 2 (see Figure 6A, 6B and 7). The amplitude or depth of the pseudo-potential or time-averaged corrugations, wells or barriers is inversely proportional to the mass-to-charge ratio of an ion. One or more transient DC voltages or DC voltage waveforms 4 are applied to the electrodes 2a of the ion guide or mass analyzer 2 in order to urge ions along the length of theion guide or mass analyser 2. The amplitude of the transient DC voltages 4 applied to the electrodes 2a may be increased with time so that ions are caused to be emitted from the ion guide or mass analyzer 2 in reverse order of their mass to charge ratio.