A mass spectrometer is disclosed wherein an ion beam attenuator is arranged upstream of an ion trap mass analyser. An ion tunnel ion trap comprising an upstream ion accumulation section and a downstream ion accumulation section is arranged upstream of the ion beam attenuator. Ions are released from the ion tunnel ion trap and the intensity of the ion beam which is transmitted to the ion trap analyser is controlled by the ion beam attenuator. The fill time during which ions are admitted into the ion trap mass analyser remains substantially constant and is substantially independent of the intensity of the ion beam.