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

This contribution compares 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 the former configuration ion counting is used for the low abundance isotope (204)Pb as well as (200)Hg and (202)Hg isotopes required for correction of isobaric interference of (204)Hg. Accurate measurement of the differences in gain between ion counters and Faraday cups, required for determination of (208)Pb/(204)Pb, (207)Pb/(204)Pb and (206)Pb/(204)Pb 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 (208)Pb/(206)Pb 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 (208)Pb/(206)Pb and (207)Pb/(206)Pb ratios requires Sigma Pb ion beam intensity of > similar to 30-50 mV. At beam intensities < 100 mV Sigma Pb measurement of (208)Pb/(204)Pb, (207)Pb/(204)Pb and (206)Pb/(204)Pb ratios also benefit significantly from the use of ion counting to measure (204)Pb 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 (204)Pb is most appropriate at signal intensities > similar to 200 mV, equivalent to similar to 100 000 counts per second (cps) (204)Pb. Correction for (204)Hg interference does not substantially contribute to uncertainty as long as (204)Hg comprises less than similar to 20% of the total 204 ion beam.