ICP-MS with hexapole collision cell for isotope ratio measurements of Ca, Fe, and Se

To avoid mass interferences on analyte ions caused by argon ions and argon molecular ions via reactions with collision gases, an rf hexapole filled wi th helium and hydrogen has been used in inductively coupled plasma mass spe ctrometry (ICP-MS), and its performance has been studied. Up to tenfold imp rovement in sensitivity was observed for heavy elements (m > 100 u), becaus e of better ion transmission through the hexapole ion guide. A reduction of argon ions Ar+ and the molecular ions of argon ArX+ (X = O, Ar) by up to t hree orders of magnitude was achieved in a hexapole collision cell of an IC P-MS ("Platform ICP", Micromass, Manchester, UK) as a result of gas-phase r eactions with hydrogen when the hexapole bias (HB) was set to 0 V; at an HB of 1.6 V argon, and argon-based ions of masses 40 u, 56 u, and 80 u, were reduced by approximately four, two, and five orders of magnitude, respectiv ely. The signal-to-noise ratio Se-80/ Ar-40(2)+ was improved by more than f ive orders of magnitude under optimized experimental conditions. Dependence of mass discrimination on collision-cell properties was studied in the mas s range 10 u (boron) to 238 u (uranium). Isotopic analysis of the elements affected by mass-spectrometric interference, Ca, Fe, and Se, was performed using, a Meinhard nebulizer and an ultrasonic nebulizer (USN). The measured isotope ratios were comparable with tabulated values from IUPAC. Precision of 0.26%, 0.19%, and 0.12%, respectively, and accuracy of 0.13% 0.25%, and 0.92%, respectively, was achieved for isotope ratios Ca-44/ Ca-40 and Fe-5 6/Fe-57 in 10 mug L-1 solution nebulized by means of a USN and for Se-78/Se -80 in 100 mug L-1 solution nebulized by means of a Meinhard nebulizer.