The effects of H2O, He, N2 and H2 on ion kinetic energies in inductively coupled plasma mass spectrometry

This study compares measurements of ion energy and the polyatomic species abundance in conventional solution inductively-coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS for multiple quadrupole instruments. Ion energy measurements in plasma sources have been used as a diagnostic tool for understanding processes occurring within the plasma during extraction of ions. Ion energy measurements have also allowed for determination of the nature of polyatomic species formation. In this study, ion energy measurements show that dry plasma conditions (laser ablation) result in a different ion energy distribution vs. mass compared to wet plasma (solution nebulization), suggesting a lower apparent temperature for dry plasma. The effect of helium and nitrogen or hydrogen additions to an argon plasma shows that helium is unlikely to ionize in the plasma or participate in charge transfer reactions, but primarily affects the interface pressure between the cones and the temperature of the plasma. Hydrogen addition to a dry plasma shows a slight increase in atomic ion energy, while nitrogen addition does not, and this suggests the two diatomic gases behave differently in the plasma. Investigation of ion energies with and without electrically shielding the torch shows no net benefit to using the shield for dry plasma. This behaviour, as well as differences in ion lens settings required for optimal ion beam properties, suggests different capacitive coupling between wet and dry plasma (un-shielded torch). This has implications for the optimal configurations of interface design of ICP-MS, which until now have been optimized primarily for wet plasma conditions.