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Couple weeks ago we saw how stable isotope analysis was used in a paleoanthropological context, with a Neandertal tooth. A new study published in this week’s issue of PNAS, extends stable isotope analysis from bones to hair — and this time instead of looking at strontium, a metal, hydrogen and oxygen isotopes were screened. Here’s the title and link to the paper, “Hydrogen and oxygen isotope ratios in human hair are related to geography.”

This study has some important applications to those that work forensic cases. Unfortunately, people don’t always die with their ID cards on them. Investigators often resort to extracting data from the remains in creative ways to solve where the deceased came from, ultimately in order to pin down who the deceased was. DNA analysis can be one of the most definitive ways of identification, only if the probable next of kin also agrees to compare their DNA to the deceased. Other forms rely on morphological comparison, such as the dead person’s teeth to dental records. I’ve discussed how stable isotope analysis can also be applied, to give us a general understand on where in the world this person may have come from.

In this new study hair was screened. Hair’s principle component is a protein called keratin. The amino acids that make up the keratin are synthesized from many different building blocks. Two of these building blocks are hydrogen and oxygen molecules that originally come from dietary and environmental sources. Different versions, or isotopes of hydrogen and oxygen exist in our water and food sources. And their relative amounts get incorporated into our bodies, such as our hair, during biosynthesis. In order to see if different people’s hair relate to regional variation of hydrogen and oxygen isotope levels, the authors of this study, collected discarded hair samples from 65 different barbershops in the United States. They limited their barbershops to those from small towns, just to limit the probability that outsiders would be getting their hair cut.

They also took water samples from the areas they collected the hair, and first correlated if isotope composition of the hair matched up with isotope composition in the regional water samples. They were able to confirm this… there is a linear relationship of hydrogen and oxygen isotopes in hair to hydrogran and oxygen isotopes in water. I’ve included a graph of the relationships between hydrogen isotope ratios and oxygen isotope ratios of human scalp hair and tap water for samples randomly acquired in cities representing 18 states across the United States for your visual entertainment:

Plots of the relationships between mean H isotope ratios and mean O isotope ratios of human scalp hair and tap water for samples randomly acquired in cities representing 18 states across the United States.

Since there exist a relationship of naturally occurring environmental isotopes to those represented in hair, the authors Distribution of Hydrogen and Oxygen Isotope Levels in the USwere able to construct a geographical distribution of different isotope levels. They used a tool many archaeologist out there may know of, ArcGIS, to construct their distribution map. This is a really informative map, not only does it show where in the US, the levels of two different hydrogen and oxygen levels, but one could use this map for their own research.

Say, hypothetically, I, a forensic anthropologist, have hair samples from a murder victim and I would like to know where in the United States they may have come from. Sampling the levels of deuterium in the hair, an isotope of hydrogen that has a neutron, I quantify a minus -89 level of deuterium. That amount of deuterium is pretty widespread throughout all of the Midwest. So sampling the oxygen-18 isotope, also represented in the hair sample, would give me another data point to narrow down the results. If my sample comes back at a -13.1 level of oxygen-18, I can have pretty good confidence the hair came from a person who spent some time in the south-eastern Illinois. If I can also correlate the hair, say in color, to missing persons from south-eastern Illinois, I maybe able to identify the person.

I think I went a bit overboard with this explanation of the research but I consider this sorta work really creative. I also really commend the authors because they went above and beyond what most would do. Sampling 65 different barbershops throughout the US is no easy task! I also really commend that they released their map, because I’m sure some missing persons investigator, and the family of missing persons, will be really be grateful to have access to this distribution.

    Ehleringer, J.R., Bowen, G.J., Chesson, L.A., West, A.G., Podlesak, D.W., Cerling, T.E. (2008). From the Cover: Hydrogen and oxygen isotope ratios in human hair are related to geography. Proceedings of the National Academy of Sciences, 105(8), 2788-2793. DOI: 10.1073/pnas.0712228105
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