Preservation of hair stable isotope signatures during freezing and law enforcement evidence packaging

Stable isotope signatures of bioelements are utilized for geolocation of unknown human remains. Hair in particular can generate a high-temporal resolution record of recent travel history, providing critical investigative leads. However, systematic studies of law enforcement packaging materials and evidence packaging protocols are needed, including the full range of sample types and conditions anticipated in casework. Arm 1 of this study examined the impact of freezing storage on hair samples using the FBI's recommended storage materials (paper, plastic) and Mesa Police Department's evidence packaging guidelines for varying periods of freezing storage (three weeks, five months). Hair studied was from individuals of different ancestry, including cosmetic treatments (relaxer, dyes), and exposure to decomposition fluids outdoors for up to 10 months. Arm 2 evaluated longer-term storage, comparing hair stored in a desiccator to hair frozen at −20 °C for up to four years. Samples and certified standards were anonymized and randomized during sample preparation. To prevent cognitive bias from influencing interpretations, unblinding of samples only occurred after data correction and reduction were complete. Both the experimental and longer-term storage studies demonstrated δ¹³C, δ¹⁵N, and δ¹⁸O values had no significant offsets between frozen samples and those stored at room temperature. However, there were small systematic offsets (+2 to +3‰) in δ²H values, with frozen samples being enriched in ²H compared to controls. In a minority of samples frozen for >six months, δ²H offsets of >9‰ were observed, an amount that could impact the interpretation of an individual's geographic travel history.