The effect of ion energy on reactivity and species selectivity in hexapole collision/reaction cell ICP-MS

Abstract

This paper describes experiments aimed at discriminating the different effects of ion energy in collision/reaction cell ICP-MS. It is demonstrated that the input ion energy, as determined by the plasma offset potential and ion energy distribution, is a key determinant of cell reactivity and this is termed the ion kinetic energy effect (IKEE). The ion kinetic energy is varied by alteration of the potential difference between the plasma and the hexapole cell. The plasma offset potential and ion energy distribution are not accurately known but are inferred from “stopping curves” produced by varying the pole bias of the quadrupole analyser. Kinetic energy discrimination (KED), where the difference in bias potentials between the quadrupole mass analyser and the hexapole cell is exploited to reject slow cell-formed ions, is shown to be a different effect. It can be used to change the relative levels of polyatomic ions arriving at the detector. The influence of IKEE and KED on the levels of plasma and analyte oxide (MO⁺) ions and on the ²¹(H₃O)⁺/³⁶Ar⁺ reactivity indicator ratio are considered. It is shown that IKEE can be used to influence the reactive attenuation of argide ions and the production of MO⁺ in the cell. KED is shown to preferentially reject cell-formed MO⁺ from the mass analyser.