Protocol-dependent variability in ⁸⁷Sr/⁸⁶Sr in human hair: A cautionary note for forensic provenancing

Human hair, primarily composed of the protein keratin, is one of the most frequently recovered forms of trace evidence in forensic investigations. Radiogenic strontium isotope analysis (⁸⁷Sr/⁸⁶Sr) of human hair shows considerable promise for forensic provenancing. However, its application remains challenging because hair contains both biogenic strontium incorporated during growth, and exogenous and diagenetic strontium acquired through environmental exposure. Existing decontamination protocols, largely developed for stable isotope analyses, have not been validated for radiogenic strontium isotope research. This experimental study evaluated the effects of 17 chemical pre-treatment protocols including acetone, methanol:chloroform (2:1), Milli-Q and 0.1 M p.a. hydrochloric acid on the ⁸⁷Sr/⁸⁶Sr and Sr concentrations of modern and experimentally buried human hair. The results demonstrate that chemical pre-treatment itself can alter measured ⁸⁷Sr/⁸⁶Sr, with the magnitude of these shifts depending on hair colour. This effect is likely related to differences in melanin content, as melanin easily binds metal cations, potentially influencing the retention and removal of strontium during chemical pre-treatment. Although the inclusion of a final Milli-Q rinse effectively recovers the biogenic ⁸⁷Sr/⁸⁶Sr in an experimental context, even the most complex and elaborate protocol 13 (AC-MQ-MC2-HCl-MQ) did not remove diagenetic strontium incorporated in pubic, forearm and scalp hair after burial (<14 months). These findings indicate that currently available cleaning procedures cannot reliably recover the original biogenic strontium isotope composition of buried hair and highlight the need for caution when using hair ⁸⁷Sr/⁸⁶Sr data for forensic provenancing.