“I Know Where You Spent Last Summer”: Isotopes in Hair and Teeth to Trace the Regions-of-Origin of Unidentified Murder Victims

EHLERINGER, JAMES R; CERLING, THURE E; CHESSON, LESLEY A; BOWEN, GABRIEL; VALENZUELA, LUCIANO O; PODLESAK, DAVE

Washington

Conferencia; Forensics Isotope Ratio Mass Spectrometry Conference; 2010

We developed and tested two models to predict the geographic regions-of-origin of humans based on the stable isotope composition of their scalp hair and teeth. The “hair” model incorporated exchangeable and nonexchangeable hydrogen and oxygen atoms in amino acids to predict the hydrogen and oxygen isotope ratio values of scalp hair. We evaluated “hair” model predictions with stable isotope analyses of human hair from 65 cities across the USA. This model, which predicts hair isotopic composition as a function of drinking water and dietary carbohydrate and protein isotope ratios, explained more than 85% of the observed variation and reproduced the observed slopes relating the isotopic composition of hair samples to that of local drinking water. The “tooth” model was simpler and was based on the relationship between tap water at a location and the oxygen isotope ratio of carbonate in five authentic tooth enamel samples; here a linear model also explained 85% of the observed variations. We constructed isoscape maps of the expected average H and O isotope ratios in human hair and O isotopes in tooth enamel carbonate across the contiguous 48 states of the USA. We then applied these models to two cases where the identity of the murder victim was unknown. In case 1, the unidentified victim was referred to as “Saltair Sally”. The hair isotope results indicated that over the last two years of her life she moved between three isotopically distinct regions, one of which was consistent with the region where her remains were recovered. Tooth analyses indicated that she likely grew up in the same region where her remains were found. In the second murder case, which has now been resolved, hair isotopes faithfully recorded the lack of movement of the individual prior to death and accurately predicted the isotopic region consistent with where Ms. Mary Alice Willey had lived over the last year of her life (San Francisco). Tooth isotope analyses indicated that she had grown up in a different region. The region predicted was consistent with the Los Angeles area where the individual is now known to have grown up.