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The strontium isotope ratio 87Sr/86Sr is a key tracer with wide-ranging applications in geochemistry, hydrology, paleoclimatology and migration research. To make sound interpretations of 87Sr/86Sr isotope ratios in the context of biogeosciences, researchers need high quality data. In this contribution, we critically evaluate the accuracy of two conceptually different analytical protocols for 87Sr/86Sr determination on the example of a large dataset (n = 135) comprising biogenic and abiogenic materials. Water, soil extracts, and hydroxyapatites (tooth enamel) were prepared according to established procedures and analyzed by solution-based multi-collector inductively coupled plasma mass spectrometry (MC ICP-MS). For the calibration we used two protocols: internal normalization (also termed internal mass bias correction or internal calibration) and standard-sample-bracketing (external calibration). Isotope dilution mass spectrometry was not considered suitable, because this calibration approach becomes very time- and cost-intensive when applying to a large sample set.Analysis of water, soil extracts and hydroxyapatites showed that the majority of 87Sr/86Sr isotope ratios which were determined by internal normalization shifted towards higher values in comparison to data determined by standard-sample-bracketing. Extensive evaluations ruled out sample preparation or measurement errors. Instead, internal normalization yielded biased data, because it is based on the assumption that all samples have the same 88Sr/86Sr isotope ratio, which can be used for normalization. However, in 90% of the investigated samples the 88Sr/86Sr significantly deviated from the assumed invariant value; in particular, the 88Sr/86Sr that is conventionally expressed as δ(88Sr/86Sr)SRM987 ranged from -1.01‰ to 0.20‰. As a consequence, internally normalized 87Sr/86Sr data biased by up to 0.00043, which was 2-times larger than previously predicted by theoretical calculations. These results demonstrate that the choice of calibration method has a much higher impact on the accuracy of 87Sr/86Sr isotope ratios than initially expected. The implication of these findings in biogeoscience applications 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).