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The microscopic isotopes within a fingernail can pinpoint travel—down to relatively short trips, or even certain medical treatments, according to a new study in the Journal of Forensic Sciences.

The caveat: the oxygen and hydrogen signatures from local drinking water are a much better indicator than the carbon and nitrogen from food habits, according to the paper.

“Fingernail tissues are useful for understanding human movement and travel reconstruction that could aid anthropological and forensic studies in determining travel histories of human remains,” write the University of Utah authors. “We can see changes related to individual trips within the length of a fingernail.”

In “Traveling There and Back Again: A Fingernail’s Tale,” the scientists looked at six participants from Salt Lake City—all of whom were traveling to exotic locales as far-flung as India, Panama, Ecuador, Indonesia, Australia and New Zealand.

The participants collected their fingernail clippings they would normally have produced for a period stretching anywhere from nine to 24 months, and stored them in labeled coin envelopes.

The samples were cleaned by the investigators, by being washed in deionized water and washed in a chloroform and methanol mixture, and finally rinsed in an ultrasonic bath. Then the stable isotopes were measured at the school’s SIRFER facility, using an isotope ratio mass spectrometer operated in continuous flow mode. (The statistical significance of the layers of keratin were assessed by the software program PRISM v. 5.0.)

The drinking water oxygen and hydrogen isotopes showed changes in locale, they conclude.

“Isotope profiles were consistent with their travel histories. In addition, when we compared observational data and modeled predictions, we saw that stable isotope observations for both individuals general fit the modeled values well,” they write. “However, during the turnover period and convergence of the resident population mean the model occasionally overestimated the observed values.”

Some other observations:

  • One subject who came down with dengue fever and was hospitalized for a week in Indonesia showed a less-marked oxygen isotope signature. The reason: intravenous fluids were administered to re-hydrate the person.
  • Two of the subjects went to the Galapagos Islands together. Their water drinking showed the 4.5-month trip—but one of them showed a sharp decline in oxygen values. It was learned that that individual traveled to Quito, Ecuador for two weeks right in the middle of their Galapagos stay. “It is remarkable that short-term travels of participant 6 were detectable,” the paper concludes. “The oxygen isotope ratios of this individual’s fingernails revealed the brief Galapagos-Quito travel event, highlighting the ability of the fingernail stable isotope ratio record to capture brief trips or movements.”
  • However, dietary isotopes of carbon and nitrogen could show differences in peoples’ food choices—but little variability showing geographical travel, they conclude. “The differences in carbon isotope ratios could be influenced by the proportion of fast food and marine seafood that they regularly reported eating as part of their diet, which would result in more positive carbon isotope ratios. However, the exact explanation is unknown.”
  • Oxygen isotope traces could especially show important forensic markers establishing relatively short timelines, according to the authors. “The rapid turnover time for oxygen isotope ratios in these tissues is linked to the full exchange of oxygen isotopes in protein sources with body water sources,” they conclude.

Isotopes have been a huge niche field within forensic science, also including teeth, bone and hair, as well as other organic material, like matchstick wood. The author of Stable Isotope Forensics, Wolfram Meier-Augenstein, spoke to Forensic Magazine earlier this year about some of the promise and limitations of the analytical methods.

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