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Chemical Profiling – Tracking Down the Source
By Jay Withgott
Score one for the war on drugs! But was the cocaine really grown in Colombia? Or might it be traced to other
growers, maybe in Bolivia or Peru? If you’re with the Drug Enforcement Agency (DEA), those are important
questions. The answers? That’s where chemistry comes in.
Chemistry detectives look at trace elements, often noting the ratios of their stable isotopes. Many elements possess
multiple stable isotopes – versions of the same element with varying numbers of neutrons and thus, different masses.
As living tissues grow, certain conditions – like temperature and humidity - can favor the inclusion of some isotopes
over others.
Ecologists see these isotopic ratios of carbon, hydrogen, oxygen, and nitrogen as signatures. Using these signatures,
they can reconstruct diets, trace the paths of nutrients in food webs, and track animals that migrate from one region to
another. Now chemists are even using isotopes to help government enforce its laws.
The DEA wanted to know the geographical origin of cocaine it confiscated, so DEA agents collected samples of coca
leaves and purified cocaine from numerous locations throughout South America. Then they analyzed the carbon and
nitrogen in each sample.
As plants grow, they pick up molecules from the soil, water, and air incorporating them into their tissues. As a result,
the chemistry of these tissues reflects the ratio of isotopes present in their home environments. These ratios in plant
tissues are called the “fingerprints of the world.”
It works like this: Whether the plant grew in wet or dry soil might be determined by looking at the nitrogen. How?
Wetter soils host more bacterial activity. Because bacteria prefer nitrogen-14 isotopes, they leave behind more
nitrogen-15 in the soil for plants to pick up. So, plants in wetter climates incorporate more nitrogen-15 than plants
growing in drier climates.
Carbon isotopic ratios in the air vary slightly with humidity. In humid conditions, plants keep their stomata open,
letting in air loaded with more carbon-13 than would be present in drier air. As a result, plants from drier Bolivia have
a different isotopic “fingerprint” than plants from wetter Colombia. By the addition of information about some trace
soil alkaloids that seem to have certain growing locations; these methods for determining the cocaine’s origin have
gained reliability. Combining data on isotopes ratios and trace chemicals, the group was able to produce a chemical
map of the different South America growing regions.
Answer the following questions on a separate sheet of paper. Use complete sentences.
1. What does DEA stand for?
2. Define stable isotopes.
3. What does the ecologist see the isotopic ratios of carbon, hydrogen, oxygen, and nitrogen as?
4. What do ecologist use signatures for (List 3 items)?