Examination of Amino Acid Hydrogen Isotope Measurements of Scalp Hair for Region-of-origin Studies

Rationale: Hydrogen isotope (δ H) analysis of keratinaceous bulk tissues has been used in forensic science to reconstruct an individual's travel history or determine their region-of-origin. Here, we use a compound-specific approach to examine patterns of individual amino acid δ H values in relation to those of local tap water, bulk scalp hair tissues, and region-of-origin.

Methods: We measured δ H values of amino acids in anonymously collected scalp hair (n = 67) and tap water from 28 locations in the United States. Samples were hydrolyzed into their constituent amino acids, derivatized alongside in-house reference materials, and analyzed in triplicate using a GC-C-IRMS system.

Results: Non-essential amino acid (AA ) δ H values and their corresponding tap water samples varied systematically across continental regions. Hydrogen isotope values of alanine, glutamic acid, and glycine were significantly correlated with tap water and an estimated 42%-51% of the hydrogen atoms in these AA were derived from tap water. We used linear discriminate analysis (LDA) to explore regional patterns in scalp hair bulk tissue and amino acid δ H values. For the model that included AA data, 87% of the variance was explained by the first linear discriminant axis (LD1), and was driven by bulk hair tissue, alanine, and proline. This model had an overall 72% successful reclassification with samples from the south and northwest regions reclassifying correctly 92% and 78% of the time, respectively. For the model that included AA data, LD1 explained 81% of the variation and was driven bulk hair, threonine, valine, phenylalanine, and isoleucine. The overall reclassification rate for the model that included AA was 70%.

Conclusions: Our findings suggest that δ H analyses of AA and AA could help improve geolocation models for human and wildlife forensics by simultaneously providing information about both dietary and tap water inputs of hydrogen to tissue synthesis.