Instrumental isotopic fractionation (IIF, also referred to as mass bias) is a major source of error in isotope ratio analysis using multi-collector inductively coupled plasma mass spectrometry (MC ICP-MS). In n(⁸⁷Sr)/n(⁸⁶Sr) isotope ratio measurements the most common strategy to correct for IIF is the application of n(⁸⁸Sr)/n(⁸⁶Sr) as internal - assumingly constant - ratio (internal IIF correction) or the standard-sample-bracketing approach (external IIF correction). In this study, we have investigated if 1) n(⁸⁷Sr)/n(⁸⁶Sr) isotope ratios differ depending on the IIF correction strategy used and 2) if these differences have implication on the outcome of a study on past human mobility.

We obtained tooth enamel from 125 individuals, who were excavated at an Avar age cemetery (7^th-9^th ct. CE) as well as 10 recent environmental samples (soil, water) south of today’s Leobersdorf, Austria. Samples were prepared following standard protocols and measured by MC ICP-MS (Neptune Plus, Thermo Scientific, USA). We performed internal and external IIF correction using the NIST SRM 987 as bracketing standard (SSB-Sr) and Zr as internal standard and standard-sample-bracketing with SRM 987 (SSB-Zr) according to Horsky et al. Uncertainties were calculated following Horsky et al. In addition, SRM 987, SRM 1400 and SRM 1486 were processed in the same way as the samples and serves as quality control materials.

Internal and external IIF correction yielded comparable results for soil and water samples. However, the majority of n(⁸⁷Sr)/n(⁸⁶Sr) isotope ratios in the investigated tooth enamel samples, which were determined by internal IIF correction shifted towards higher values in comparison to data determined by external IIF correction approaches. These shifts most likely resulted from deviation of the theoretical and actual n(⁸⁸Sr)/n(⁸⁶Sr) isotope ratio in these samples e.g., lowest δ^88/86Sr_SRM987 was -1.01‰. The impact of these observations on human provenance will be discussed.

This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement n° 856453 ERC-2019-SyG).