Apparatus comprising: a first device for separating ions according to their rate of change of ion mobility with electric field strength, said first device comprising a Field Asymmetric Ion Mobility Spectrometry ("FAIMS") device; and a second separate device (2; 12) for separating ions according to their ion mobility, said second device comprising an ion mobility separator or ion mobility spectrometer, and said second device being arranged downstream of said first device; said second device (9;12) comprises an upstream entrance region, a down stream exit region and an intermediate region; wherein said second device comprises a plurality of electrodes (8; 17); characterised in that: in use, one or more transient DC voltages or one or more transient DC voltage waveforms are progressively applied to said electrodes (8; 17) so that at least some ions having a first ion mobility are separated from other ions having a second different ion mobility; in said entrance region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a first amplitude; in said intermediate region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a second amplitude; and in said exit region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a third amplitude. a first device for separating ions according to their rate of change of ion mobility with electric field strength, said first device comprising a Field Asymmetric Ion Mobility Spectrometry ("FAIMS") device; and a second separate device (2; 12) for separating ions according to their ion mobility, said second device comprising an ion mobility separator or ion mobility spectrometer, and said second device being arranged downstream of said first device; said second device (9;12) comprises an upstream entrance region, a down stream exit region and an intermediate region; wherein said second device comprises a plurality of electrodes (8; 17); characterised in that: in use, one or more transient DC voltages or one or more transient DC voltage waveforms are progressively applied to said electrodes (8; 17) so that at least some ions having a first ion mobility are separated from other ions having a second different ion mobility; in said entrance region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a first amplitude; in said intermediate region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a second amplitude; and in said exit region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a third amplitude. in use, one or more transient DC voltages or one or more transient DC voltage waveforms are progressively applied to said electrodes (8; 17) so that at least some ions having a first ion mobility are separated from other ions having a second different ion mobility; in said entrance region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a first amplitude; in said intermediate region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a second amplitude; and in said exit region the amplitude of said one or more transient DC voltages or said one or more transient DC voltage waveforms has a third amplitude.