A mass spectrometer is disclosed comprising an ion beam attenuator which attenuates an ion beam repeatedly switching between a zero transmission mode of operation during a time period ΔT1 and a non-zero transmission mode of operation during a time period ΔT2. The degree of attenuation of the ion beam can be varied by varying the mark space ratio ΔT2/ΔT1. The ion beam attenuator may release ions in packets or pulses but the packets or pulses of ions may be converted into a continuous ion beam by a relatively high pressure ion guide or gas collision cell arranged downstream of the ion beam attenuator.