Controlling the temporal response of mass spectrometers for mass spectrometry

A method and apparatus for operating a mass spectrometer system, having a processing section, provides for the application of both an axial field and periodic application of a flush pulse to the processing section. This gives a reproducible output ion signal from the processing section that is very responsive to changes in operating conditions in the processing section. The mass spectrometer system can comprise: a collision/reaction cell having an input and an output; and a set of elongated rods extending between said input and output, said elongated rods spatially arranged. Separate auxiliary electrodes can be provided to generate the axial field. The invention has particular applicability to ICP-MS, where strong ion currents can result in a collision cell taking some time to reach equilibrium when the operating condition is changed.