A Different Kind of CSI

October 9, 2014  |  By  | 

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 13 C/ 12 C 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 C 4 plants using it to make especially efficient use of water; this process also causes C 4 plants to take in a much higher proportion of carbon-13 atoms than do C 3 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 2 H/ 1 H 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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