Use of Zr for mass bias correction in strontium isotope ratio determinations using MC-ICP-MS

Abstract

Isotope abundance ratios and isotopic composition of strontium in a biological sample were determined using MC-ICP-MS whereby zirconium was admixed with solutions of digested NIST SRM 987 and samples and used for mass bias correction with implementation of a combination of standard-sample-standard bracketing and internal normalization. In this manner, the certified value of 8.37861 for ⁸⁸Sr/⁸⁶Sr in SRM 987 was used for mass bias correction of ⁹⁰Zr/⁹¹Zr in two adjacent spiked solutions of SRM 987. Their average was then used to calculate mass bias corrected Sr isotope ratios in the sample. An approximate 2.5-fold improvement in precision of determination of ⁸⁷Sr/⁸⁶Sr and ⁸⁸Sr/⁸⁶Sr was obtained compared to that based on only the standard-sample-standard bracketing technique, although close matching of Sr and Zr concentrations is required in the standard and sample. Absolute isotope ratios of 0.0564240 ± 0.0000042, 0.709362 ± 0.000013 and 8.38034 ± 0.00010 (1SD) and δ^x/86Sr-values of −2.228 ± 0.075‰, −1.377 ± 0.018‰ and 0.207 ± 0.012‰ (1SD) for ⁸⁴Sr/⁸⁶Sr, ⁸⁷Sr/⁸⁶Sr, ⁸⁸Sr/⁸⁶Sr relative to SRM 987, respectively, were obtained characterizing a fish liver sample. In agreement with previous studies, evidence is presented for variation of ⁸⁸Sr/⁸⁶Sr in samples. Estimation of the measurement uncertainty confirmed that the major source of imprecision arises from the uncertainty in the certified value of ⁸⁸Sr/⁸⁶Sr in SRM 987 used for mass bias correction.