MS analysis of drug samples can lead the law to the growers’ soil.

The latest shipment reaches Miami on a steamy July night. Within hours, it has hit the streets and is spreading through the city. It does not take long before the next dealers down the line cannot possibly know where the powdered poison came from. Moreover, they do not care.

But someone does. Law enforcement agencies want to know the precise route of that summer shipment, from its origins in the foothills of the Andes to the heart of the inner city. “Cocaine is the most widely used narcotic drug,” says James Ehleringer, a researcher at the University of Utah in Salt Lake City, “although identifying it is easy, finding its geographic origin is a forensics nightmare.” He believes he has come up with a solution that will allow the U.S. Drug Enforcement Agency (DEA) to trace the route of a batch of cocaine to the very valley where the coca plants were grown in the vast mountain range. If they can find the source, then they can get to the root of the cocaine problem and chop it down right where it grows.

Building on Ancient History
Ehleringer and his team are not forensic scientists though. They started out trying to prove whether the ancient Anasazi Indians of the southern deserts of Utah used reservoirs to grow corn crops and to find out why the tribe vanished about 700 years ago. It might sound a long way from Utah corn growers to the harvesting of an illicit South American drug, but it is isotopic fingerprints that link the two.

The Anasazi Indians vanished from the Four Corners area around 1300 CE, and many explanations for their disappearance—disease, war, and overuse of farmland—have been disproved. Ehleringer and his colleagues suspect that summer drought was to blame—the people simply could not grow corn and starved. They analyzed oxygen isotopes from rain using MS because warm summer rain contains more heavy oxygen (¹⁸O) than winter rain or snowmelt. By comparing these measurements with annual growth rings in trees from the years before the Anasazi vanished, they hope to be able to tell whether portions of the summer growth had low levels of ¹⁸O, which would indicate that the trees grew in summer by soaking up groundwater rather than rain.

Precision Analysis
In figuring out the history of the Anasazi, the team’s expertise in MS has grown to provide the key to finding those cocaine roots, too. The team has analyzed the ratio of rare ¹⁵N to common ¹⁴N in cocaine samples. Such a precise analysis reveals the types of soil in which the coca plants grew, and so, almost by definition, the location of that soil because the ¹⁴N to ¹⁵N ratio varies from region to region. But it is not only soil that betrays the location of growers. Coca plants growing in a more humid environment tend to open their leaf pores wider and so take up more carbon dioxide than plants in drier conditions. Because plants have an enzyme—ribulose bisphosphate carboxylase—that discriminates against the less common ¹³C isotope, plants growing in humid conditions will have a lower ¹³C to ¹²C ratio. Under drier conditions, however, the amount of carbon dioxide is limited and the enzyme has to work with whatever carbon dioxide it can get (¹³C or ¹²C). Thus, the levels of ¹³C relative to ¹²C increase under drier conditions. The environmental differences between regions strongly influence the coca leaf and cocaine stable isotope ratios. Differences in soils seem to affect ¹⁵N, while humidity levels and the length of the rainy season influence the ¹³C level. Ehleringer believes the technique is so precise in detecting even the slightest differences in humidity levels that it can distinguish between cocaine produced in Colombia, Peru, Ecuador, or Bolivia.

The team couples the isotope ratio information with readily detectable gas chromatograph differences in the patterns of trace alkaloids (truxilline and trimethoxycocaine) found in cocaine, which allows them to correctly identify the source of 96% of 200 cocaine samples that were collected from the Chapare Valley of Bolivia, the Huallaga, Ucayali, and Apurimac Valleys of Peru, and the Putumayo-Caqueta and Guaviare regions of Colombia.

Figure 1. Regional grouping of cocaine samples based on the ratio of 15N to 14N in truxilline (Trux) and 13C to 12C in trimethoxycocaine (TMC). Reprinted with permission from Ehleringer, J. R.; et al. Nature 2000, 408, 311–312.

The team applied a statistical approach to their analysis to demonstrate the probability that a particular sample really did originate in the country they thought it had. Ehleringer believes the technique can accurately identify the country of origin 9 times out of 10. Adding the information gleaned from trace alkaloid analysis helps them narrow the source down with 96% accuracy.

“This is a major advance in determining the source of illicit drugs,” says Bob Klein, research supervisor at the DEA lab. “It will dramatically help the U.S. government in allocating resources to combat drug trafficking.” Previously, the law enforcement agencies relied on the analysis of trace residues of alkaloids to help them track a particular batch of the drug to its source. An analysis of this nature reveals much valuable information so that the DEA can, for instance, identify the processing methods used in making the final narcotic product. However, it does not provide all the information they might want.

Indeed, this approach has met with only limited success because the drug is so often transported from one country to another for processing and ultimate conversion to the nasally active form, cocaine hydrochloride. This long journey from farm to street, explains Ehleringer, makes it much more difficult to identify the exact source. MS isotope analysis brings the location of the growers back into focus by pinpointing them geographically.

“Tracing the country of origin of cocaine is now feasible through automated, routine analysis of both stable isotopes and trace alkaloids, opening up strategic options for identifying source regions and trafficking routes,” Ehleringer notes. The DEA now has its own instrument, and it is playing a significant role in intelligence efforts.

Further Reading

Ehleringer, J. R.; Casale, J. F.; Lott, M. J.; Ford, V. L. Tracing the geographical origin of cocaine. Nature 2000, 408, 311–312.

The Web tutorial "Hitchhiker’s guide to carbon isotope fractionation" can be found at http://icg.harvard.edu/~bio120/Special_Topics/Carbon_isotopes.html.

James Ehleringer’s lab maintains a website at http://ecophys.biology.utah.edu.

The DEA website can be found at www.dea.gov.

David Bradley is a freelance writer living in Cambridge, UK. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office, 1155 16th St N.W., Washington, DC 20036.