An Electron Transfer Dissociation or Proton Transfer Reaction device comprising: an ion guide comprising a plurality of electrodes having at least one aperture, wherein ions are transmitted in use through said apertures; a first device arranged and adapted to apply one or more first transient DC voltages or potentials or one or more first transient DC voltage or potential waveforms to at least some of said plurality of electrodes in order to drive or urge at least some first ions along and/or through at least a portion of the axial length of said ion guide in a first direction; wherein said first device is arranged and adapted to reduce the amplitude, height or depth of said one or more first transient DC voltages or potentials or said one or more first transient DC voltage or potential waveforms applied to said plurality of electrodes along the length of said ion guide from a first end of said ion guide to a central or other region of said ion guide; and a second device arranged and adapted to apply one or more second transient DC voltages or potentials or one or more second transient DC voltage or potential waveforms to at least some of said plurality of electrodes in order to drive or urge at least some second ions along and/or through at least a portion of the axial length of said ion guide in a second different direction; wherein said second device is arranged and adapted to reduce the amplitude, height or depth of said one or more second transient DC voltages or potentials or said one or more second transient DC voltage or potential waveforms applied to said plurality of electrodes along the length of said ion guide from a second end of said ion guide to a central or other region of said ion guide. an ion guide comprising a plurality of electrodes having at least one aperture, wherein ions are transmitted in use through said apertures; a first device arranged and adapted to apply one or more first transient DC voltages or potentials or one or more first transient DC voltage or potential waveforms to at least some of said plurality of electrodes in order to drive or urge at least some first ions along and/or through at least a portion of the axial length of said ion guide in a first direction; wherein said first device is arranged and adapted to reduce the amplitude, height or depth of said one or more first transient DC voltages or potentials or said one or more first transient DC voltage or potential waveforms applied to said plurality of electrodes along the length of said ion guide from a first end of said ion guide to a central or other region of said ion guide; and a second device arranged and adapted to apply one or more second transient DC voltages or potentials or one or more second transient DC voltage or potential waveforms to at least some of said plurality of electrodes in order to drive or urge at least some second ions along and/or through at least a portion of the axial length of said ion guide in a second different direction; wherein said second device is arranged and adapted to reduce the amplitude, height or depth of said one or more second transient DC voltages or potentials or said one or more second transient DC voltage or potential waveforms applied to said plurality of electrodes along the length of said ion guide from a second end of said ion guide to a central or other region of said ion guide.