Multi-isotopes in human hair: A tool to initiate cross-border collaboration in international cold-cases

Abstract

Unidentified human remains have historically been investigated nationally by law enforcement authorities. However, this approach is outdated in a globalized world with rapid transportation means, where humans easily move long distances across borders. Cross-border cooperation in solving cold-cases is rare due to political, administrative or technical challenges. It is fundamental to develop new tools to provide rapid and cost-effective leads for international cooperation. In this work, we demonstrate that isotopic measurements are effective screening tools to help identify cold-cases with potential international ramifications. We first complete existing databases of hydrogen and sulfur isotopes in human hair from residents across North America by compiling or analyzing hair from Canada, the United States (US) and Mexico. Using these databases, we develop maps predicting isotope variations in human hair across North America. We demonstrate that both δ2H and δ34S values of human hair are highly predictable and display strong spatial patterns. Multi-isotope analysis combined with dual δ2H and δ34S geographic probability maps provide evidence for international travel in two case studies. In the first, we demonstrate that multi-isotope analysis in bulk hair of deceased border crossers found in the US, close to the Mexico-US border, help trace their last place of residence or travel back to specific regions of Mexico. These findings were validated by the subsequent identification of these individuals through the Pima County Office of the Medical Examiner in Tucson, Arizona. In the second case study, we demonstrate that sequential multi-isotope analysis along the hair strands of an unidentified individual found in Canada provides detailed insights into the international mobility of this individual during the last year of life. In both cases, isotope data provide strong leads towards international travel.

Procedures from samples to sulfur isotopic data: A review

“…47,279 Hair samples are washed in a bath of a 2:1 solution of CHCl 3 :CH 3 OH for 2 h, rinsed three times with MQ, and then airdried. 280 A baited barbed wire hair trap is used to sample the hair of wild animals. 281 The residue is dried for organic sulfur extraction by the Eschka method 282 or directly packaged for EA-IRMS measurements.…”

“…Hair samples from tame animals and humans are taken by cutting from each subject at the nape of the neck about 2.5–5 cm from the scalp 47,279 . Hair samples are washed in a bath of a 2:1 solution of CHCl 3 :CH 3 OH for 2 h, rinsed three times with MQ, and then air‐dried 280 . A baited barbed wire hair trap is used to sample the hair of wild animals 281 …”

Spatial variations in tap water isotopes across Canada: Tracing water from precipitation to distribution and assess regional water resources

“…The dataset generated in this study provides a baseline to track forensically relevant materials across Canada. Recent studies in Canada have already demonstrated how this database could provide key information to investigate cold cases [69,70] and reconstruct individual travel history [71].…”