Abstract: An improved apparatus and method for plasma source mass spectrometry, the apparatus comprising: means (16) for generating a plasma (15) at substantially atmospheric pressure in a gas; means (4) for introducing a sample to the plasma wherein the sample is ionized to form sample ions (17); means (19, 23) for transmitting the ions from the plasma into an evacuated chamber (24); a mass filter (26) disposed within the evacuated chamber; a substantially non-multiplying ion detector (58) comprising an ion collector (59), the detector being responsive to the charge of at least some of the sample ions which pass through the mass filter; and means for inhibiting the response of the detector to electrically neutral particles. Typically the detector comprises a suppressor (63) and means (64) for negatively biassing the suppressor with respect to the collector, and also a shield (61) disposed to shield the suppressor from the neutral particles.