Method and Apparatus for Operating a Vacuum Interface of a Mass Spectrometer

A method is disclosed for operating a mass spectrometer vacuum interface, the vacuum interface comprising an evacuated expansion chamber downstream of a plasma ion source wherein the expansion chamber is pumped by an interface vacuum pump to provide an interface pressure in the chamber; the method comprising using a controller to automatically, or according to user input, control the throughput of the interface vacuum pump to control the interface pressure dependent on one or more operating modes of the spectrometer.