Issue Date: June 27, 2011
Isotopes MARK the spot
When Irish criminal investigators couldn’t identify a dismembered body, parts of which they found around a canal near Dublin in the spring of 2005, Wolfram Meier-Augenstein got a phone call.
An analytical chemist at James Hutton Institute, in Dundee, Scotland, Meier-Augenstein has pioneered a way to help determine where unidentified victims like this one lived or traveled. To do so, he measures the stable isotope ratios of carbon, oxygen, hydrogen, nitrogen, and sulfur found in samples of the victim’s hair, teeth, nail, and bone. Checking these elements’ isotope ratios against databases that contain global isotope abundances can provide investigators with the victim’s probable trajectory during his or her last weeks, months, and years.
In the case of the Dublin body, “the Irish investigators had several leads, but they were all equally probable,” Meier-Augenstein says. But his isotope data pointed to a likely home area for the victim. Following that lead, the police eventually identified the body, which led investigators to the murderers, who were dubbed the “Scissor Sisters” during the highly publicized trial.
The use of stable isotope measurements to help identify human bodies is part of an emerging field called isoscapes, a combination of the words isotope and landscape. Identification by isoscapes is based on the idea that a person’s tissue holds an isotope ratio fingerprint that derives from the isotope ratios of food, water, and air, which in turn vary with geography. Researchers have combined global isotope databases, satellite technology, and sophisticated mapping software to create isotope contours on the world. Using such contours, researchers can pinpoint the origin of a wide variety of things including humans, illegal drugs, trafficked endangered animals, and counterfeit scotch. Isoscapes can also help climate scientists follow world geochemical cycles or archaeologists understand migratory routes of ancient civilizations.
The term isoscapes was first coined by Jason West, now an ecologist at Texas A&M University. In 2005, he was a postdoc in James Ehleringer’s laboratory at the University of Utah. Even then, folks from many corners of the university—plant ecologists, microbiologists, geologists—were probing the spatial component of isotope variability. “One day over coffee, we were all chatting about our commonalities, and I came up with isoscapes as a fun word to describe our collective research,” he says. “Then the word escaped into the wild.”
By 2008, the first international meeting for isoscapes was held in Santa Barbara, Calif.; the next such conference will take place this fall in Indiana. “Isoscapes as a term caught on because there were lots of groups doing this sort of research around the world, but the field didn’t have a name,” West adds. “We were all using the same methodology and logic, but just applying it to different systems.”
Isotope analysis itself is not new. The relative abundance of different isotopes in nature has been scrutinized since 1913, when chemist Frederick Soddy first suggested that the number of neutrons in an element’s nucleus can vary. But it wasn’t until the past decade that scientists began to map the spatial distribution of isotopes in nature, study the processes underlying this distribution, and use these isotope landscapes in forensic, environmental, and food applications, West says.
Progress in isoscapes was made possible because in the late 1990s and early 2000s, “there was a growth in the amount of isotope data collected, plus increased accessibility of that data through the Internet. There was also better software to map and interpret it,” says Gabriel Bowen, an earth scientist at Purdue University. All of these technological factors, he says, “came together at the same time” to permit the isoscapes field to develop.
In particular, in the late 1990s, the International Atomic Energy Agency put online “hundreds of thousands” of isotope data points from precipitation that it had collected at 800 meteorological stations around the world since 1961, Bowen says.
In recent years, Bowen and his colleagues have combined the oxygen and hydrogen data from the agency’s database with other precipitation databases into a mega-resource called IsoMAP, or Isoscape Modeling, Analysis & Prediction, one of the most comprehensive, well-stocked databases available to isotope researchers, according to Bowen. It’s been used to confirm—and in some cases debunk—claims on bottled-water labels that the contents originate from a particular region.
Utah’s Ehleringer has used precipitation databases to find the geographical source of marijuana, heroin, and cocaine, on the basis that the plants from which the street drugs are derived rely on rainwater as a source of oxygen and hydrogen. Others have used precipitation databases to ascertain the provenance of wine and naturally derived fragrances, such as lavender extract, which sells for several times more than the synthetic knockoff, Meier-Augenstein says. Lavender plants and grapevines, like poppy flowers, for example, incorporate the isotope ratios of water they consume.
Provenance studies also often rely on databases that chart the geographic variation of nitrogen, carbon, strontium, and sulfur isotope ratios. Some of these databases are public and online but many more are on “hard drives in research labs around the world,” having been developed for a specific purpose, West says.
For example, Meier-Augenstein’s team developed one such database when police asked him whether stable isotope forensics could be used to connect burnt matchsticks found at the scene of a crime with matchsticks in a box at a suspected arsonist’s home.
Although online databases for tree-ring carbon isotopes exist, Meier-Augenstein says, he didn’t know whether aspen—the wood commonly used for matchsticks—had a distinctive enough isotope distribution around the world to give an answer that could stand up in court. And he also didn’t know whether burning of the matchstick or the ignition chemicals at the matchstick’s tip would change the isotope ratios of the wood. His then-graduate student Nicola Farmer, now at the U.K.’s Forensic Explosives Laboratory, collected matchboxes from various places in the world and did the experiments to show that isotope ratios could be used to tell whether the matchsticks did in fact match. (They didn’t.) She has since explored using the same isotope-based technique to trace the origin of other materials found at crime scenes, such as duct tape used to silence victims.
Isotope databases can also be used to track individuals’ geographical trajectory over time, such as animals’ migratory paths. The isotope ratios in hair or feathers hark back to the place where the animal consumed water and food as the tissue grew. Thus, isotope ratios at different lengths of hair or feathers will also vary if the individual traveled during the time the tissue sample grew.
For example, British police went to Meier-Augenstein with a long lock of hair from a Vietnamese man who had died after being dumped at a hospital emergency room in England. The police knew his identity from a worldwide fingerprint database, but they did not know how the murdered man, an illegal alien, came to the U.K.
Meier-Augenstein’s team chopped the 14-cm-long hair, which represented about 14 months of growth, into 5-mm increments. From isotope ratios in the different sections of hair, the team proposed that the man had been in the Ukraine a year before his death, had then traveled to Germany, and finally ended up in the U.K. How and when he traveled from Vietnam to the Ukraine could not be determined from isotope ratios because the hair sample did not include that period of his life.
The police later discovered that the man had hired human traffickers to get him illegally into the U.K. through Eastern Europe. Unfortunately, the murdered man had to repay his debt by working in a gang-run marijuana-growing operation, Meier-Augenstein says. When a rival gang stole a large crop harvest from the victim, he was killed by his own gang for failing to protect the valuable product. The isotope-based map was just one of many forensic tools that helped convict two men for the murder, but it was essential for piecing together the murdered man’s life trajectory.
Mapping these trajectories is neither simple nor foolproof. It requires that databases be stocked with quality isotope data. “We have good data for North America and Europe, where there are more monitoring stations,” West says. In other parts of the world, the data are not as dense. But efforts are under way to rectify this, he adds. Furthermore, interpretation of any isotope ratios measured in a human tissue—for example, hair—requires a sophisticated understanding of the biological processes by which humans incorporate different isotopes into that tissue. For example, humans acquire oxygen primarily by breathing air and consuming water, either directly or through food. A given geographic region may have different oxygen isotope ratios in air and water, and isoscape researchers must figure out the relative contribution of these two sources to oxygen in hair.
As increasing amounts of good-quality data make their way into isotope databases, researchers are improving their ability to understand the underlying physical, chemical, and biological reasons for the different distributions of elemental isotopes in everything from plant cells to rock faces.
Furthermore, technological advances are making it easy to measure isotope data in forensic or ecological samples, as well as facilitating preparation of the standards needed to develop the global databases. Isotope-ratio mass spectrometry—which distinguishes isotopes on the basis of mass—is the workhorse analytical tool and current gold standard for measuring isotope ratios. A newer technology based on diode laser absorption spectroscopy discriminates isotopes according to the laser energy they absorb. This laser method may provide an inexpensive tool that researchers can take into the field for on-site measurements, which are not possible with isotope-ratio mass spec, West says.
As researchers improve their ability to determine isotope ratios in samples of hair, heroin, or honey, the difference of just a few neutrons will continue to bring them closer to their sample’s geographic origin. ◾
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