Abstract

Holistic analysis of ICP-MS spectra modified by the use of H₂, He and mixtures thereof in a hexapole collision cell has enabled the identification of the major chemistries affecting cell performance. Correlation analysis has shown that groups of ions exist that have similar behaviours across a wide range of gas conditions. Examination of these groups indicates that the dominant reactions are related to species derived from argon and water, in particular ¹⁷OH⁺, ¹⁹H₃O⁺, ²¹H₃O⁺, ³⁶Ar⁺, ³⁸Ar⁺ and ⁸⁰Ar₂⁺. The ion signals at these m/z values reflect the concentrations of such species and are ratioed to produce indicators that are highly discriminant of the cell conditions. The objective variable used for optimisation was the S/N, calculated from an equation incorporating terms for the Poisson and flicker noise for both the signal and the background intensities. Detection limits for isotopes such as ²⁸Si, ⁵⁶Fe and ⁸⁰Se are greatly improved at the expense of a general increase in oxide levels and some loss of sensitivity due to ion scattering for isotopes up to approximately m/z 70. It is further demonstrated that, in this largely charge-conservative system, the terminal fate of the charge occurs in parts of the spectrum that are not detrimental to elemental analysis.