ASSESSMENT OF OVERALL ACCURACY OF LEAD ISOTOPE RATIOS DETERMINED BY INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY USING BATCH QUALITY-CONTROL AND THE YOUDEN 2-SAMPLE METHOD

The determination of lead isotope ratios in environmental samples using a batch quality control protocol is described. Isotope ratios are computed with respect to Pb-204 and are corrected for mass discrimination by the addition of thallium to all solutions. The batch quality control protocol entails measurements of two separate control samples, namely National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 981 Common Lead and a galena sample independently analysed by thermal ionization mass spectrometry. The present study demonstrates results of these two control samples for 43 sample batches analysed over a two-year period of time. Modified Youden-type plots of the two-sample sums and differences are constructed in order to estimate over-all accuracy for individual batches and collectively for the entire dataset. This statistical treatment indicated that 95% of the batches have estimated over-all relative uncertainties for the ratios Pb-206:Pb-204, Pb-207:Pb-204 and Pb-208:Pb-204 of less than 1.23, 1.45 and 1.77%, respectively.