In-situ analysis of Pb isotope ratios using laser ablationMC-ICP-MS: Controls on precision and accuracy and comparison between Faraday cup and ion counting systems

Abstract

This contribution compares in-situlaser ablation multicollector ICP-MS (LA-MC-ICP-MS) measurements of Pb isotope composition using parallel Faraday cup and ion counter detector array to those made using solely Faraday cups. In the former configuration ion counting is used for the low abundance isotope ²⁰⁴Pb as well as ²⁰⁰Hg and ²⁰²Hg isotopes required for correction of isobaric interference of ²⁰⁴Hg. Accurate measurement of the differences in gain between ion counters and Faraday cups, required for determination of ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb ratios using ion counting, can be made by analysis of standard glasses to within ±0.3% (2s). For analyses made using Faraday cups or combined Faraday cup and ion counting detector arrays mass bias is effectively controlled by external normalization to ²⁰⁸Pb/²⁰⁶Pb ratios measured in the same standard glasses. Provided that He sweep gas flow rates are kept constant and that adequate time is allowed for stabilization after changes in He flow, mass bias can be controlled to within ±0.05% (2s). The precision of measurements of Pb isotope ratios is strongly dependent on ion beam intensities. Internal precision of ≤ 0.02% (2se) on ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios requires ∑Pb ion beam intensity of > ∼30–50 mV. At beam intensities < 100 mV ∑Pb measurement of ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb ratios also benefit significantly from the use of ion counting to measure ²⁰⁴Pb intensity, with an overall improvement of 1–2 orders of magnitude in internal standard error. However, the additional uncertainty associated with calibration of the Faraday cup–ion counter relative gain suggest that use of Faraday cup to measure ²⁰⁴Pb is most appropriate at signal intensities > ∼200 mV, equivalent to ∼100 000 counts per second (cps) ²⁰⁴Pb. Correction for ²⁰⁴Hg interference does not substantially contribute to uncertainty as long as ²⁰⁴Hg comprises less than ∼20% of the total 204 ion beam.