Quadrupole devices

A method of operating a quadrupole device (10) is disclosed. The quadrupole device (10) is operated in a mode of operation by applying a repeating voltage waveform comprising a main drive voltage and at least one auxiliary drive voltage is applied to the quadrupole device to the quadrupole device (10). The intensity of ions passing into the quadrupole device is varied with time in synchronisation with the repeating voltage waveform. This may be done such that the number of ions per unit phase which initially experience a phase within a first range of phases of the repeating voltage waveform is greater than the number of ions per unit phase which initially experience a phase within a second range of phases of the repeating voltage waveform.