A plasma source ion trap (PSIT) mass spectrometer has been modified to incorporate a radiofrequency octopole ion guide/collision cell between the ion source and the mass spectrometer. This modification allows ions sampled from the plasma to undergo reactions prior to mass spectrometric analysis. This capability can obviate the need for chemical or chromatographic separation of the sample, remove mass spectral intereferences and enable formation of analytically useful molecular ions. Distinct reaction chemistries can be utilized in the octopole and/or ion trap. Performance of the instrument is dramatically improved if hydrogen gas is introduced into the octopole; molecular hydrogen reacts with Ar + and certain other plasma ions, greatly reduces their intensities, and cools and focuses the ion beam prior to its injection into the ion trap. Selective reactions of atomic ions that are reported include the neutralization of Ar + by reaction with H 2 and elimination of Y + , Zr + and Xe + by reaction with O 2 . Thus, 40 Ca + can be detected without interference from 40 Ar + . Applications to the detection of non-naturally occurring isotopes include a 200-fold faster reaction of 90 Y + and 90 Zr + with O 2 compared with 90 Sr + and a 10 4 -fold faster reaction of 129 Xe + with O 2 compared with 129 I + . Improvements in absolute sensitivity and linear dynamic range of the PSIT-octopole combination are also presented. Benefits for conventional inductively coupled plasma mass spectrometry (without ion traps) from suitably implemented ion/molecule chemistry are also discussed.