Improved in situ measurements of lead isotopes in silicate glasses by LA-MC-ICPMS using multiple ion counters

A new technique that improves the spatial resolution and quantification limits of the measurement of lead isotope ratios in silicate glasses with < 15 mu g g(-1) total Pb by LA-MC-ICPMS is presented. The new method provides the capability of making quantitative, in situ lead isotope measurements on tiny objects of geologic interest such as mineral growth bands, melt inclusions, and accessory minerals, even where they are lead poor. The method allows for the concurrent, static measurement of Pb-204, Pb-206, Pb-207, Pb-208 along with Hg-202 in five Channeltron ion counters. Standard-sample-standard bracketing using USGS BCR2-G as the calibrant is used to correct for instrumental mass bias. Accuracy and precision of the method was evaluated by replicate analyses of various MPI-DING reference glasses with low lead concentrations (similar to 1-11 mu g g(-1)) and well-determined isotopic ratios. Spot sizes for in situ analyses were as small as 40-69 mm, providing better spatial resolution than previous LA-MC-ICPMS results reporting 204 Pb. Measured lead isotope ratios for the MPI-DING reference glasses T1-G (11.6 mu g g(-1) total Pb) and ATHO-G (5.67 mu g g(-1) total Pb) agree within 0.10% and 0.15% respectively of the preferred values using 40 mm spots. For MPI-DING KL2-G (2.07 mu g g(-1) total Pb) and ML3B-G (1.38 mu g g(-1) total Pb) measured lead ratios agree within 0.75% of the accepted values with typical precisions of < 1.9% (2RSD) using 69 mm spots; measured Pb-207/Pb-206 and Pb-208/Pb-206 are within 0.45% of preferred values with precisions of < 0.50% (2RSD). These results demonstrate improvement over previous LA-MC-ICPMS data in terms of both quantification limits and spatial resolution, while retaining similar levels of accuracy and precision.