Nonose Naoko, Suzuki Toshihiro, Shin Ki-Cheol, Miura Tsutomu, Hioki Akiharu
National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8563, Ibaraki, Japan.
National Metrology Institute of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8563, Ibaraki, Japan.
Anal Chim Acta. 2017 Jun 29;974:27-42. doi: 10.1016/j.aca.2017.04.019. Epub 2017 Apr 26.
A lead isotopic standard solution with natural abundance has been developed by applying a mixture of a solution of enriched Pb and a solution of enriched Pb (Pb-Pb double spike solution) as bracketing method. The amount-of-substance ratio of Pb:Pb 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 Pb:Pb in the Pb-Pb 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 Pb-Pb 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 Pb:Pb, 15.6278 ± 0.0036 for Pb:Pb, 38.0626 ± 0.0089 for Pb:Pb, 2.104406 ± 0.00013 for Pb:Pb, and 0.863888 ± 0.000036 for Pb:Pb. The expanded uncertainties are about one half of the stated uncertainty for NIST SRM 981, for Pb:Pb, Pb:Pb and Pb:Pb, or one eighth, for Pb:Pb and Pb:Pb, 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 Pb:Pb, Pb:Pb and Pb:Pb, the latter contribution is two or three times larger than the former. When the coupling of a bracketing method with the Pb-Pb 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.
通过采用富集铅溶液和富集铅溶液的混合物（铅 - 铅双稀释剂溶液）作为括值法，研制出了具有天然丰度的铅同位素标准溶液。通过将主要测量方法之一的EDTA滴定法应用于每种富集铅同位素溶液，精确测量该溶液中Pb:Pb的物质的量比。此外，通过ICP - SF - MS对影响EDTA滴定的金属杂质以及每种富集铅同位素溶液中所含的次要铅同位素进行定量。铅 - 铅双稀释剂溶液中Pb:Pb的物质的量比为0.961959 ± 0.000056（合成标准不确定度；k = 1）。候选同位素标准溶液中铅同位素比值的测量以及MC - ICP - MS中质量歧视的校正，是通过括值法与铅 - 铅双稀释剂溶液以及铊内标法相结合来进行的，即将铊溶液添加到标准溶液和样品中。候选同位素标准溶液中测得的铅同位素比值及其扩展不确定度（k = 2）分别为：Pb:Pb为18.0900 ± 0.0046，Pb:Pb为15.6278 ± 0.0036，Pb:Pb为38.0626 ± 0.0089，Pb:Pb为2.104406 ± 0.00013，Pb:Pb为0.863888 ± 0.000036。对于Pb:Pb、Pb:Pb和Pb:Pb，扩展不确定度约为NIST SRM 981规定不确定度的一半；对于Pb:Pb和Pb:Pb，扩展不确定度约为NIST SRM 981规定不确定度的八分之一。合成不确定度由铅同位素比值测量的不确定度以及MC - ICP - MS中质量歧视的剩余时间漂移效应组成，而耦合校正方法无法消除该效应。在Pb:Pb、Pb:Pb和Pb:Pb的测量中，后者的贡献比前者大两到三倍。当将括值法与铅 - 铅双稀释剂溶液以及铊内标法相结合应用于NIST SRM 981的分析时，测得的铅同位素比值与其认定值吻合良好。这证明所研制的方法不仅与NIST SRM 981的传统方法一致，而且能够以更高的精度测量铅同位素比值。
