Amino Acid Specific Stable Isotope Ratios in Human Keratin Tissue for Region-of-Origin and Residency Determination

Stable isotope analysis of keratin proteins (e.g., hair or fingernails) have been used for over a decade as a diagnostic forensic tool to reconstruct an individual’s travel history or residency. Hydrogen isotope (𝛿2H) values of scalp hair have been particularly useful because they grow at predictable rates, are metabolically inert, and reflect sources of hydrogen derived from food and drinking water. The 𝛿2H value of hair is strongly correlated with drinking water and has been used to create predictive geolocation models. While these models have been informative, the prediction areas are broad and may cover sub-continental regions, limiting their value

We propose to develop a new analytical tool that could increase the accuracy of predictions for residency determination by examining the influence of food and drinking water on 𝛿2H and carbon isotope (𝛿13C) values of essential (AAESS) and non-essential (AANESS) amino acids in scalp hair collected across North America. Based on our pilot data and recently published experiments by our research group, we expect that AANESS will be more faithful proxies for drinking water inputs, while AAESS will more closely track dietary (protein) inputs. Our proposal will investigate these initial findings over two years during which will analyze scalp hair, tap water, and foods from a total of 52 cities and towns in 23 states to examine how tap water as well as regional/local dietary inputs influence 𝛿2H and 𝛿13C values of individual AAs in scalp hair. We will compare spatial assignment predictions based on bulk scalp hair 𝛿2H data to those derived from 𝛿2H analysis of individual AA to assess if a compound-specific approach improves estimates of location of residency. Our overarching goal is to identify candidate AA that have increased fidelity to local drinking water and food that could be critical to improving residency studies of human remains. We will disseminate our findings through journal articles, conference presentations, and lecture/outreach opportunities.

The application of our findings has the potential to improve the geospatial resolution of isotope analysis as a tool for law enforcement agencies involved in murder investigations, human trafficking, and immigration. Our results will not only further forensic science research, but they will also aid in our understanding of 𝛿2H patterns of specific AA that may be critical to developing this tool to study forensic wildlife conservation, specifically the characterization of animal migration patterns and the determination of the origin of animal products seized by customs agents.