Accuracy Improvement in the LA-MC-ICP-MS Measurement of 87Sr/86Sr in Bioapatite Using Dentin of Carcharinus leucas (Bull shark) to Estimate 87Rb/85Rb Instrument Induced Fractionation

Abstract

In situ ⁸⁷Sr/⁸⁶Sr microanalysis in bioapatite using laser ablation (LA-)MC-ICP-MS is an essential tool for provenance studies, as enamel tissue build-up records the regional Sr isotopic composition of ingested food and water sources. Several factors hamper the acquisition of reliable and precise ⁸⁷Sr/⁸⁶Sr data: isobaric interferences, elemental and isotopic fractionation of Rb and Sr, and the lack of certified reference materials. Here we thoroughly characterise several teeth from Carcharinus leucas (Bull shark) for the spatial distribution of Sr and Rb mass fraction by LA-ICP-Q-MS, and their ⁸⁷Sr/⁸⁶Sr ratio by solution MC-ICP-MS, MC-TIMS and LA-MC-ICP-MS, and which were used subsequently as a reference material during laser ablation measurement of Sr isotope ratios in bioapatite. We establish a protocol to estimate the ⁸⁵Rb/⁸⁷Rb mass bias from the analysis of shark teeth that demonstrates that the common assumption of equal Sr and Rb instrument induced fractionation can lead to a systematic bias in ⁸⁷Sr/⁸⁶Sr isotope ratios. Using the shark teeth as an "external" reference material to measure β_Rb yielded ⁸⁷Sr/⁸⁶Sr compositions in unknown samples that are within approximately -42 ppm to +66 ppm of expected values, instead of approximately -50 ppm to +190 ppm if assuming β_Rb = β_Sr. Finally, this methodology was tested on fossil gomphotherid (Rhynchotherium sp.) enamel, allowing us to make preliminary inferences about its palaeobiogeography.