Warning to anyone planning a crime: don’t drink the water.
Scientists are reporting this evening that an analysis of hydrogen and oxygen isotopes in human hair reveals where a person drank water, which will allow investigators to figure out if a suspect was in the vicinity of a crime—and also to track past movements of unidentified murder victims.
The new technique analyzes isotopes of hydrogen and oxygen, forms of the elements that have an extra neutron in its atomic nucleus. The ratio of oxygen-18 to oxygen-16 in air is the same everywhere, and ratios of both hydrogen and oxygen ratios in food are also basically the same across the country because the American food supply is so uniform. But water is still like a microbrew, containing different constituents depending on where it comes from.
As a result, geochemist Thure Cerling and ecologist Jim Ehleringer of the University of Utah report in the journal Proceedings of the National Academy of Sciences, a single hair bears clues to where someone has been over the last few weeks or even years, depending on the length of the hair. “You are what you eat and drink—and that is recorded in your hair,” said Cerling in a statement from the university.
Salt Lake County Sheriff's Detective Todd Park is using the hair-isotope method to try to identify a murdered woman whose remains were found by hunters near Interstate-80 along the south end of the Great Salt Lake on October 8, 2000. Detectives recovered 26 bones, some hair, a T-shirt and a necklace. Despite creating a facial reconstruction and publicizing it nationally, the police have been unable to ID the woman, who stood about 5 feet tall and was 17 to 20 years old when she was killed. But when Park arranged with Ehleringer for an isotope analysis of the victim’s hair, it showed that for the last two years of her life she lived in the Northwest, mostly in the Idaho-Montana-Wyoming area, and maybe into Oregon and Washington. Park hopes that examining missing persons records from those areas will lead to an ID.
Ehleringer and Cerling are co-founders of IsoForensics, Inc., which sells isotopic analysis of forensic substances. A method Ehleringer developed to trace the origins of cocaine or heroin is now used by the a method now used by the U.S. Drug Enforcement Administration: variations in the amount of carbon, nitrogen, oxygen and hydrogen isotopes in soil and water show up in coca and poppy plants.
For the hair analysis, says Ehleringer, “You can tell the difference between Utah and Texas,” but probably not Chicago and Kansas City. In general, the amount of the heavy isotopes oxygen-18 and hydrogen-2 levels in drinking water decreases as you move inland from the West Coast: as rainstorms move off the Pacific Ocean, rain drops containing oxygen-18 and hydrogen-2 tends to fall first because it is heavier. But cloud temperatures and season also affect which isotopes rain drops contain, with the result that heavy isotopes are also relatively more common in water inland from the Gulf and southern Atlantic coasts. The lowest concentrations of hydrogen-2 and oxygen-18 are found in water in northern and western Montana, north-central Idaho and northwest Wyoming (because the heavy isotopes drop out of clouds before they reach these inland areas), while the highest amounts of hydrogen-2 and oxygen-18 in drinking water and hair are in southern Oklahoma, north-central Texas, Florida, south Georgia and southern South Carolina (in warm regions with more evaporation, lighter-isotope water evaporates from surface sources first, leaving heavier-isotope water behind to find its way into drinking water.