A mass analyser comprising: an ion guide comprising a plurality of electrodes; means for applying a first AC or RF voltage having a first frequency and a first amplitude to at least some of said plurality of electrodes such that, in use, a plurality of axial time averaged or pseudo-potential barriers, corrugations or wells are created along at least a portion of the axial length of said ion guide; means for applying a second AC or RF voltage having a second frequency and a second amplitude to a plurality of said electrodes in order to confine ions radially in use within said ion guide; wherein said first frequency is substantially different from said second frequency; and means for driving or urging ions along and/or through at least a portion of the axial length of said ion guide so that in a mode of operation ions having mass to charge ratios within a first range exit said ion guide whilst ions having mass to charge ratios within a second different range are axially trapped or confined within said ion guide by said plurality of axial time averaged or pseudo-potential barriers, corrugations or wells; characterised in that said means for driving or urging ions comprises means for applying one or more transient DC voltages or potentials or one or more DC voltage or potential waveforms to at least 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or 100% of said electrodes such that, in use, a plurality of axial DC potential hills, barriers or wells are translated along the length of said ion guide; and further comprising means arranged and adapted to progressively increase the amplitude or depth of said one or more transient DC voltages or potentials or said one or more DC voltage or potential waveforms such that in operation ions are arranged to exit said mass analyser substantially in reverse order of mass to charge ratio so that ions having a relatively high mass to charge ratio exit said mass analyser prior to ions having a relatively low mass to charge ratio. an ion guide comprising a plurality of electrodes; means for applying a first AC or RF voltage having a first frequency and a first amplitude to at least some of said plurality of electrodes such that, in use, a plurality of axial time averaged or pseudo-potential barriers, corrugations or wells are created along at least a portion of the axial length of said ion guide; means for applying a second AC or RF voltage having a second frequency and a second amplitude to a plurality of said electrodes in order to confine ions radially in use within said ion guide; wherein said first frequency is substantially different from said second frequency; and means for driving or urging ions along and/or through at least a portion of the axial length of said ion guide so that in a mode of operation ions having mass to charge ratios within a first range exit said ion guide whilst ions having mass to charge ratios within a second different range are axially trapped or confined within said ion guide by said plurality of axial time averaged or pseudo-potential barriers, corrugations or wells; characterised in that said means for driving or urging ions comprises means for applying one or more transient DC voltages or potentials or one or more DC voltage or potential waveforms to at least 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or 100% of said electrodes such that, in use, a plurality of axial DC potential hills, barriers or wells are translated along the length of said ion guide; and further comprising means arranged and adapted to progressively increase the amplitude or depth of said one or more transient DC voltages or potentials or said one or more DC voltage or potential waveforms such that in operation ions are arranged to exit said mass analyser substantially in reverse order of mass to charge ratio so that ions having a relatively high mass to charge ratio exit said mass analyser prior to ions having a relatively low mass to charge ratio.