A plasma mass spectrometer comprises a plasma torch (1) for generating ions from a sample introduced into a plasma (2), a nozzle-skimmer interface (3, 5) for transmitting said ions into a first evacuated chamber (11), ion guiding means (12), an apertured diaphragm (18) dividing said first evacuated chamber (11) from a second evacuated chamber, and an ion mass-to-charge ratio analyzer in the second chamber for producing a mass spectrum. The ion guiding means comprises a multipole rod-set (13, 14, 15), means for applying an AC voltage between rods in the set, and means (22) for introducing into said ion guiding means an inert gas selected from the group comprising helium, neon, argon, krypton, xenon and nitrogen so that the partial pressure of said inert gas inside said rod-set is at least 10null3 torr. Interfering peaks in the spectrum, such as ARnull, are thereby reduced