SELECTIVE REMOVAL OF PLASMA MATRIX IONS IN PLASMA SOURCE-MASS SPECTROMETRY

We report a new method for selective removal of argon ions and other p lasma matrix ions in plasma source MS. The method consists of sampling the plasma and reacting the sampled plasma and analyte ions with hydr ogen gas. Reactions have been studied in three instruments: in the ion trap of a plasma source ion trap (PSIT) mass spectrometer and in the post-skimmer region of both a conventional ICP mass spectrometer and a second PSIT. In the ion trap, the reaction between Ar+ and H-2 procee ds at nearly the collisional rate whereas reaction of most other atomi c ions is four to five orders of magnitude slower. For modest H-2 pres sures and reaction times in the ion trap [10(-4) Torr (1 Torr = 133.32 2 Pa) and 10 ms], the Ar+ signal is reduced by six orders of magnitude . We have examined reactions of H-2 with 33 different atomic ions; the only ions for which a reaction was evident were N+, O+, Cl+, and Ar+. The decrease in Ar+ occurs by a sequence of fast reactions resulting in charge transfer from Ar+ to form the low m/z ions H-2(+) and H-3(+) , which are rapidly ejected from the ion trap. The net effect is the s elective removal of Ar+ chemically, not by virtue of its mass-to-charg e ratio only, as in resonant ion ejection methods. In the conventional ICP-MS experiments the reaction time is short, limiting the decrease in Ar+ to about 40-fold in preliminary and unoptimized experiments. Ho wever, the reaction is still selective: simple scattering by H-2 reduc es the Sc-45(+) signal at only 5% of the rate of reactive loss of Ar+. Production of H-2(+) and H-3(+) is observed directly in the conventio nal ICP-MS experiments, indicating that the chemistry in the post-skim mer region is consistent with that observed in the ion trap. We discus s methods by which the magnitude of Ar+ reduction observed in the ion trap might be realized in conventional ICP-MS, thus possibly allowing a greater analyte ion transmission efficiency and reduced space-charge effects.