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

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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 ∼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.