The present invention is concerned with improving the focusing of ions having a particular mass to charge ratio by optimising the electric field for the focusing of ions having that particular mass to charge ratio. In particular, the stigmatic focusing of ions can be improved by adjusting a voltage applied to an ion-optical lens 50 for ions of different mass to charge ratios as those ions pass through the lens. In one embodiment, a digital to analogue converter 32 and amplifier 34 are AC coupled to a high voltage DC power supply unit 38 by high voltage capacitor 36. The digital to analogue converter 32 generates a low voltage wave form which is amplified by the amplifier 34 and then added to the output of the high voltage supply 38 so that the desired voltage waveform is generated and applied to ion-optical lens 50 as ions pass through the lens.