The use of background ions and a multivariate approach to characterise andoptimise the dominant H-2-based chemistries in a hexapole collision cell used in ICP-MS

Holistic analysis of ICP-MS spectra modified by the use of H-2, He and mixt ures 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 do minant reactions are related to species derived from argon and water, in pa rticular (OH+)-O-17, (H3O+)-H-19, (H3O+)-H-21, Ar-36(+). Ar-38(+) and Ar-80 (2)+. The ion signals at these m/z values reflect the concentrations or suc h species and are ratioed to produce indicators that ire highly discriminan t of the cell conditions. The objective variable used for optimisation was the S/N, calculated from an equation incorporating terms for the Poisson an d flicker noise for both the signal and the background intensities. Detecti on limits for isotopes such as Si-28, Fe-56 and Se-80 are greatly improved at the expense of a general increase in oxide levels and some loss of sensi tivity due to ion scattering for isotopes LIP to approximately m/z 70. It i s further demonstrated that, in this largely charge-conservative system, th e terminal fate of the charge occurs in parts of the spectrum that are not detrimental to elemental analysis.