Laser Ablation MC-ICP-MS measurements of Pb isotope composition: Controls on Precision and Accuracy and Comparison between Faraday and Faraday-Ion Counter detection systems

In this contribution I discuss controls on precision and accuracy and compare in-situ laser 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 this 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. Mass bias is effectively controlled by external normalization to 208Pb/206Pb ratios measured in 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. 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, can also be made by analysis of the same standard glasses to within ± 0.3 % 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 total beam intensities <100 mV measurement ratio to 204Pb 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 only 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 < ~20% of the total 204 ion beam.