The use of background ions and a multivariate approach to characterise and optimise the dominant H2-based chemistries in a hexapole collision cell used in ICP-MS

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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.

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