MOST of the world’s supply of cocaine comes from just three South American countries: Colombia, Peru and Bolivia. Much of it is headed for the United States and Europe. Law-enforcement officials from America patrol international waters in the Caribbean and eastern Pacific, hoping to seize cocaine shipments before they reach their intended destinations. When they succeed in nabbing any smugglers, contraband samples are sent to chemists to help determine the source.

The drug’s origins can be identified from telltale “fingerprints” formed by the chemical composition of the coca plant, from which cocaine is derived. These compounds vary naturally. The amount of nitrogen-containing compounds, known as alkaloids, differs between coca cultivars. And ratios of stable isotopes (non-radioactive atoms of the same element that contain different numbers of neutrons) are indicative of different regions. Typically, ratios of carbon-13 to carbon-12, which change according to temperature and altitude, and ratios of nitrogen-15 to nitrogen-14, which vary based on precipitation and soil conditions, are examined.

Previously, these data could identify from which of five regions a particular batch of cocaine was produced. But no longer. Over the past 15 years or so coca cultivation has expanded to at least 19 regions, making accurate geographic identification more difficult. Now a team of scientists led by Jennifer Mallette and Paul Beyer of the US Drug Enforcement Administration and their colleagues have come up with a more comprehensive chemical analysis to fill in the gaps. Their results, published in Scientific Reports, added two more isotope ratios: hydrogen-2 to hydrogen-1, and oxygen-18 to oxygen-16. Plants incorporate different ratios of these isotopes based upon ambient conditions such as precipitation and humidity, thus providing yet another clue to their geographic origin.

The researchers applied their enhanced technique to build a database of chemical fingerprints of cocaine samples from 572 specimens of coca leaves taken from all 19 known cocaine-growing regions. The cocaine was produced in the laboratory using the same processing methods employed by criminal enterprises.

The final database, combined with statistical models, allowed the team to determine that nearly two-thirds of seized shipments originated in south-west Colombia. But then they were sent another sample that matched nothing in the database. The isotope analysis suggested that this cocaine originated from an area north of the Chapare Valley in Bolivia, a region not previously suspected of coca cultivation. Subsequent intelligence gained from a pilot involved in trafficking the shipment showed the team had fingered the right location, thereby confirming the value of their new drug-hunting technique.