Characterization of a new candidate isotopic reference material for natural Pb using primary measurement method

A lead isotopic standard solution with natural abundance has been developed by applying a mixture of a solution of enriched ²⁰⁸Pb and a solution of enriched ²⁰⁴Pb (²⁰⁸Pb-²⁰⁴Pb double spike solution) as bracketing method. The amount-of-substance ratio of ²⁰⁸Pb:²⁰⁴Pb in this solution is accurately measured by applying EDTA titrimetry, which is one of the primary measurement methods, to each enriched Pb isotope solution. Also metal impurities affecting EDTA titration and minor lead isotopes contained in each enriched Pb isotope solution are quantified by ICP-SF-MS. The amount-of-substance ratio of ²⁰⁸Pb:²⁰⁴Pb in the ²⁰⁸Pb-²⁰⁴Pb double spike solution is 0.961959 ± 0.000056 (combined standard uncertainty; k = 1). Both the measurement of lead isotope ratios in a candidate isotopic standard solution and the correction of mass discrimination in MC-ICP-MS are carried out by coupling of a bracketing method with the ²⁰⁸Pb-²⁰⁴Pb double spike solution and a thallium internal addition method, where thallium solution is added to the standard and the sample. The measured lead isotope ratios and their expanded uncertainties (k = 2) in the candidate isotopic standard solution are 18.0900 ± 0.0046 for ²⁰⁶Pb:²⁰⁴Pb, 15.6278 ± 0.0036 for ²⁰⁷Pb:²⁰⁴Pb, 38.0626 ± 0.0089 for ²⁰⁸Pb:²⁰⁴Pb, 2.104406 ± 0.00013 for ²⁰⁸Pb:²⁰⁶Pb, and 0.863888 ± 0.000036 for ²⁰⁷Pb:²⁰⁶Pb. The expanded uncertainties are about one half of the stated uncertainty for NIST SRM 981, for ²⁰⁸Pb:²⁰⁴Pb, ²⁰⁷Pb:²⁰⁴Pb and ²⁰⁶Pb:²⁰⁴Pb, or one eighth, for ²⁰⁸Pb:²⁰⁶Pb and ²⁰⁷Pb:²⁰⁶Pb, The combined uncertainty consists of the uncertainties due to lead isotope ratio measurements and the remaining time-drift effect of mass discrimination in MC-ICP-MS, which is not removed by the coupled correction method. In the measurement of ²⁰⁸Pb:²⁰⁴Pb, ²⁰⁷Pb:²⁰⁴Pb and ²⁰⁶Pb:²⁰⁴Pb, the latter contribution is two or three times larger than the former. When the coupling of a bracketing method with the ²⁰⁸Pb-²⁰⁴Pb double spike solution and a thallium internal addition method is applied to the analysis of NIST SRM 981, the measured lead isotope ratios are in good agreement with its certified values. This proves that the developed method is not only consistent with the conventional one by NIST SRM 981 but also enables measurement of the lead isotope ratios with higher precision.