Accuracy and precision of non-matrix-matched calibration for lead isotope ratio measurements of lead-poor minerals by LA-MC-ICPMS

Abstract

This study evaluates the necessity of matrix matching for laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) lead isotope ratio measurements of lead-poor (<70 µg g⁻¹ total Pb) feldspar and sulfide using standard–sample–standard bracketing to correct for mass discrimination. Lead isotope ratios of three different feldspar minerals and three different sulfide matrices were measured by LA-MC-ICPMS to determine the quality of data possible when calibrating samples with standards having different physical characteristics and chemical compositions. Lead isotope ratios for the feldspars were calibrated against NIST 612 and BCR2-G. The average ^206,207,208Pb/²⁰⁴Pb and ^207,208Pb/²⁰⁶Pb ratios determined by LA-MC-ICPMS on all three feldspars, independent of standard, are within 0.40% of the average TIMS measurements made in this study on the same grains. External precisions for the LA-MC-ICPMS measurements are better than 0.60% (RSD, 1σ). Lead isotope ratios for 3 sulfides were determined by LA-MC-ICPMS using either NIST 612 or PB-1, a synthetic sulfide glass, as the external calibration standard. For the two sulfides that contain little to no mercury the mean ^206,207,208Pb/²⁰⁴Pb and ^207,208Pb/²⁰⁶Pb ratios are accurate within 0.40% of the average TIMS measurements made in this study on the same grains, with only subtle differences in results between matrix-matched (PB-1) and non-matrix-matched (NIST 612) analyses. Lead isotope ratios determined for MASS-1, a pressed powder sulfide containing high levels of mercury (∼60 µg g⁻¹), are highly variable. This is a result of intrinsic lead isotope heterogeneity in MASS-1 and errors in the isobaric overlap corrections for ²⁰⁴Hg on ²⁰⁴Pb caused by fractionation of Hg/Pb during spot analyses. Despite significant differences in the ablation behavior of silicate glass, feldspar and sulfide, the results demonstrate that precise and accurate lead isotope ratio data can be obtained by LA-MC-ICPMS for feldspar and sulfide containing little to no mercury, using commonly available silicate glasses as calibration standards.