Mass spectrometers

ICP-MS apparatus is described with a plasma ion source 1 and a multi-collector mass analyser containing an entrance slit 6, electrostatic analyser 7, analysing magnet 8 and multiple ion collectors 9. In accordance with the invention, an additional magnetic field 10 is applied after the plasma ion source but before the entrance slit of the multi-collector mass analyser. This additional magnetic field deflects ions in the ion beam according to their momentum and so for ions of the same nominal energy acts as a pre mass selector. Such apparatus provides a reduction of the low mass background ions entering the mass analyser in order to enhance the abundance sensitivity of the mass spectrometer for higher mass ions. Line of sight restrictions between the ICP source and the entrance slit also prevents neutrals from entering the analyser.