Calibration of boron isotope ratio measurements by MC-ICP-MS using normalisation to admixed internal standards

Abstract

Online normalisation to isotope ratio of admixed internal standards is an important technique in isotopic analysis by multicollector ICP-MS (MC-ICP-MS). It is widely used for minimising drift in measurements of δ-values and calibrating isotope amount ratios for medium and heavy mass elements. However, its potential has been underexplored for light elements, largely due to the greater magnitude of instrumental isotope fractionation (IIF) observed in the low mass region and the need to conduct measurements in dynamic mode. This work reports for the first time a comprehensive assessment of the performance of Li, C, and Mg as internal standards for both relative and absolute ¹¹B/¹⁰B isotope ratio measurements by MC-ICP-MS. The data show that normalisation to any of the tested elements allowed correcting for drift in IIF and obtaining more reproducible results. The regression model has been used to determine n(¹¹B)/n(¹⁰B) isotope ratios in the NIST SRM 951a boric acid isotopic material with a certified boron isotope amount ratio. The n(¹¹B)/n(¹⁰B) values and expanded uncertainties (k = 2) determined for this material by using isotopically certified Li, C, and Mg internal standards were 4.0688(569), 4.0439(264) and 4.0294(706), respectively, and agree well with the NIST SRM 951a certified value of 4.0437(033). Carbon has shown the best internal standard performance in the measurement of both δ¹¹B and n(¹¹B)/n(¹⁰B) values. Calibration of n(¹¹B)/n(¹⁰B) isotope ratios using internal standards with SI-traceable isotope ratios offers a low-cost alternative method for establishing traceability of results to the SI. The ease of implementation of the method makes it a useful tool for fostering quality control procedures in laboratories seeking the ISO/IEC 17025:2017 accreditation.