Communication. Selective removal of plasma matrix ions in plasma source mass spectrometry

Abstract

We report a new method for selective removal of argon ions and other plasma matrix ions in plasma source MS. The method consists of sampling the plasma and reacting the sampled plasma and analyte ions with hydrogen 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 H₂ proceeds at nearly the collisional rate whereas reaction of most other atomic ions is four to five orders of magnitude slower. For modest H₂ pressures and reaction times in the ion trap [10^–4 Torr (1 Torr = 133.322 Pa) and 10 ms], the Ar⁺ signal is reduced by six orders of magnitude. We have examined reactions of H₂ 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 H₂⁺ and H₃⁺, which are rapidly ejected from the ion trap. The net effect is the selective removal of Ar⁺ chemically, not by virtue of its mass-to-charge 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. However, the reaction is still selective: simple scattering by H₂ reduces the ⁴⁵Sc⁺ signal at only 5% of the rate of reactive loss of Ar⁺. Production of H₂⁺ and H₃⁺ is observed directly in the conventional ICP-MS experiments, indicating that the chemistry in the post-skimmer region is consistent with that observed in the ion trap. We discuss 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.