A method of mass analysing ions comprising: providing an ion guide comprising a plurality of electrodes, wherein said plurality of electrodes comprises electrodes having apertures through which ions are transmitted in use; applying an AC or RF voltage 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; and 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; wherein said step of driving or urging ions comprises progressively applying one or more transient DC voltages or potentials or 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 along the axial length of said ion guide; and characterised in that the method comprises performing one or more of the following: (i) arranging ions to exit said mass analyser substantially in reverse order of mass to charge ratio; and/or (ii) increasing the amplitude, height or depth of said one or more transient DC voltages or potentials, or DC voltage or potential waveforms, over a cycle of operation of said mass analyser; and/or (iii) increasing the velocity or rate at which said one or more transient DC voltages or potentials or DC potential or voltage waveforms are applied to said electrodes; and/or (iv) increasing a gas flow through said ion guide. providing an ion guide comprising a plurality of electrodes, wherein said plurality of electrodes comprises electrodes having apertures through which ions are transmitted in use; applying an AC or RF voltage 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; and 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; wherein said step of driving or urging ions comprises progressively applying one or more transient DC voltages or potentials or 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 along the axial length of said ion guide; and characterised in that the method comprises performing one or more of the following: (i) arranging ions to exit said mass analyser substantially in reverse order of mass to charge ratio; and/or (ii) increasing the amplitude, height or depth of said one or more transient DC voltages or potentials, or DC voltage or potential waveforms, over a cycle of operation of said mass analyser; and/or (iii) increasing the velocity or rate at which said one or more transient DC voltages or potentials or DC potential or voltage waveforms are applied to said electrodes; and/or (iv) increasing a gas flow through said ion guide. (i) arranging ions to exit said mass analyser substantially in reverse order of mass to charge ratio; and/or (ii) increasing the amplitude, height or depth of said one or more transient DC voltages or potentials, or DC voltage or potential waveforms, over a cycle of operation of said mass analyser; and/or (iii) increasing the velocity or rate at which said one or more transient DC voltages or potentials or DC potential or voltage waveforms are applied to said electrodes; and/or (iv) increasing a gas flow through said ion guide.