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A Different Kind of CSI: Crime and Stable Isotopes

Can two samples be chemically identical, yet not the same? Juries need to know

Sandra J. Ackerman

A jar of exotic honey, a bottle of olive oil, a dish of delicately flavored sea bass—most people would probably consider these objects out of place in a lab or a courtroom. To a certain kind of analytical chemist, though, they invite forensic investigation. Well-informed consumers often scan the labels on the food they buy to determine its ingredients and origin, but the stable-isotope analyst takes the investigation much further: all the way to the isotope ratio mass spectrometer.

2013-01SciObsAckermanFA.jpgClick to Enlarge ImageThis is not a matter of mere inquisitiveness. From time to time, both civil and criminal courts have to contend with cases of fraud in food and other substances: synthetic vanilla flavoring sold as the natural extract, real maple syrup cut with molasses, ersatz shampoo marketed as a well-known brand. Sometimes it’s not the identity of the product but the source that is misrepresented: Was this wood logged legally, or was it poached from a protected area? How can I be sure this honey is the rare treat made from the nectar of the sourwood tree, which blooms only when the circumstances are just right, rather than an ordinary table honey with an expensive-looking label? If a shipment of lumber is seized by agents of the Department of Agriculture, or if a disgruntled beekeeper brings suit against a commercial food manufacturer for false advertising, the charges can lead to a trial, with a stable-isotope analyst called to give evidence.

“Food authenticity” was the focus of a colloquium held recently in the historic university town of Leipzig, Germany. The session formed part of a meeting that brought together more than 250 stable-isotope analysts from two dozen countries to discuss the ever-increasing applications of their field. In many cases of adulterated or falsely sourced foods, “Stable isotope analysis may be able to make a difference for the benefit of both industry and consumers,” says Wolfram Meier-Augenstein of the James Hutton Institute for environmental, crop and food science, in Dundee, Scotland, who introduced the session.

News headlines and science fiction stories rarely feature the stable (non-radioactive) isotopes, but these are what make up the bulk of the world and everything in it. Of all the chemical elements found in nature, most exist in multiple isotopic forms—that is, their number of protons and electrons is always fixed, but the number of neutrons may vary by one or more. Of course, every additional neutron brings an infinitesimal gain in atomic mass. This distinction, between an element’s atomic number (the number of protons) and its atomic mass (the number of protons plus neutrons), has tripped up generations of chemistry students.

A well-known example of multiple isotopes is the element carbon, which usually takes the form of carbon-12 but can also occur in two other isotopic forms. Carbon-13, with one extra neutron, is a stable isotope; carbon-14, with two extra neutrons, is radioactive. (It is because carbon-14 decays at a known rate that archaeologists can estimate the age of their finds by means of carbon-14 dating.)

When it comes to the forensic use of stable-isotope analysis, the underlying principles are straightforward. Chemists have known for a long time that beet sugar, for example, always displays a characteristic ratio of carbon-13 to carbon-12 isotopes, clearly distinguishable from the 13C/12C ratio of cane sugar. The different ratios arise from the two plants’ differing approaches to photosynthesis. Sugar beets get their carbon fix by means of the so-called Calvin-Benson cycle, which leaves the plant with relatively few carbon-13 atoms amidst an abundance of carbon-12. Sugarcane, which grows in more adverse conditions, carries out a more complex pathway of photosynthesis (known as the Hatch-Slack cycle) that allows the C4 plants using it to make especially efficient use of water; this process also causes C4 plants to take in a much higher proportion of carbon-13 atoms than do C3 plants. Under stable-isotope analysis, the two kinds of sugar give unmistakably different readings.

Even hydrogen, the simplest element of all, exists in two stable isotopic forms. Stable-isotope analysis can discriminate between the hydrogen found in a sample of rainwater, with its characteristic ratio of hydrogen-2 to hydrogen-1 isotopes, and the hydrogen in a sample of water taken from an aquifer, which bears a different 2H/1H ratio—and between both of these and the hydrogen in seawater, for that matter. Altitude and latitude, temperature and degree of mineralization in the water are all traits that play a role in determining the stable-isotope “signature” of a given sample. This much is old news.

What is new in just the past few years—what has brought stable-isotope analysis out from the shadow of its more glamorous radioisotopic counterpart—is the development of a special kind of mass spectrometer (MS) equipped with dedicated collector channels for each isotope of a given element. The multicollector isotope ratio MS can sort the various isotopes of an element according to their minute differences in atomic mass, and then determine the proportion of each isotope present in a sample, with far greater precision than a conventional desktop mass spectrometer. Provided there is enough background information available with which to compare their readings, stable-isotope analysts can now help to answer questions about the composition and origin of explosive materials; they can determine whether an individual animal was bred in captivity or illegally caught or poached; they can even determine the provenance, and hence help to identify, victims of human trafficking or of deadly violence.

By the way, that last requirement—a basis for comparison—is as important as the lab instrument itself. As with the forensic use of DNA, a lab read-out from stable-isotope analysis can tell us very little unless it is placed in context. The technique, says Meier-Augenstein, “is at its most powerful when the sample in question and its isotopic composition can be compared to that of a sample of known provenance.” In other words, does the dietary history encoded in this victim’s hair and fingernails match that of a person reported missing? Can the fresh, wild-caught dishes at this seafood restaurant pass a test of spectrometry as well as of gastronomy? Would the bees foraging in the sourwood grove be glad to claim this honey as their own? In matters of identification both large and small, the stable isotopes do not lie.—Sandra J. Ackerman

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