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

Abstract

To avoid mass interferences on analyte ions caused by argon ions and argon molecular ions via reactions with collision gases, an rf hexapole filled with helium and hydrogen has been used in inductively coupled plasma mass spectrometry (ICP–MS), and its performance has been studied. Up to tenfold improvement in sensitivity was observed for heavy elements (m > 100 u), because 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 three orders of magnitude was achieved in a hexapole collision cell of an ICP–MS (“Platform ICP”, Micromass, Manchester, UK) as a result of gas-phase reactions 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, respectively. The signal-to-noise ratio ⁸⁰Se/ ⁴⁰Ar₂ ⁺ was improved by more than five orders of magnitude under optimized experimental conditions. Dependence of mass discrimination on collision-cell properties was studied in the mass 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 ⁴⁴Ca/ ⁴⁰Ca and ⁵⁶Fe/⁵⁷Fe in 10 μg L^–1 solution nebulized by means of a USN and for ⁷⁸Se/⁸⁰Se in 100 μg L^–1 solution nebulized by means of a Meinhard nebulizer.