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    Tracing Cocaine And Heroin With Carbon Isotopes
    By Enrico Uva | January 4th 2012 03:23 PM | 2 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    Isotopes were discovered 100 years ago. Without knowledge of them, we would have no nuclear accidents, weapons, or waste, but we would also be without a medical probe, a weapon against cancer and a way of generating electricity. We would have no clue about the age of the earth or about the intricacies of photosynthesis and respiration. There would be no isotope signatures that reveal diets of the past and sources of illegal drugs.

    Combine the two Greek words for equal and place, isos and topos, respectively, and you get the word isotope, so-chosen because isotopes of a given element have an equal number of protons and therefore occupy the same place in the periodic table.

    What differs for isotopes is the number of neutrons. Sometimes that leads to different nuclear properties. Always, it leads to different masses, which often leads to varying rates of chemical reactions.

    The most common isotopes of carbon are 12C, 13C, and 14C. The latter is the radioactive friend of archeology, constantly replenished by the mostly high-altitude interaction of cosmic ray products(neutrons) and atmospheric nitrogen. The elemental isotope gets oxidized and mixes evenly with the rest of the air. As a result, a small percentage of carbon dioxide is radioactive and every 5690 to 5770 years, half the carbon atoms of those molecules degenerate back to 14N and beta rays. Since 14CO2 finds its way into plants, it's all over the food chain, and the lower the ratio of 14C/12C, the older the organic material is.

    12C and 13C both stable have nuclei. Unlike 14C neither undergoes alchemy-like transformations into a neighboring element on the periodic table. Either atom from the pair of isotopes, like 14C,  sandwiches itself in the CO2 molecule between a pair of double bonds with oxygen. But the different masses of the different carbon isotopes means that no matter how you view the bond, as a classical mechanics-type of spring or in terms of quantum mechanical energy states, the result is the same: more energy is needed to break the bond, resulting in a higher activation energy for bond cleavage. In nature, it spells out a lower rate of photosynthesis for carbon dioxide carrying 13C or 14C compared to 12C.

    What makes matters more interesting is that how much faster 12C uptake occurs is not constant in the plant world. One difference can be caused by the type of photosynthetic pathway. C4(corn, grasses)and CAM(desert and succulents)plants, which use a more efficient pathway have a different ratio of 13C/12C when compared to C3 plants. Technically what's measured is the depletion of 13C compared to a standard according to the formula:



    The standard used was a marine fossil with an unusually high 13C/12C ratio. Now that they've run out, they use NBS-21 graphite. Other factors that can change δ13C include drought, humidity and genotypes.

    What this all leads to is an isotopic signature or fingerprint of organic material. Across the entire geographical distribution of coca leaves(the source of cocaine), the δ13C  varies from -32.4  to -25.3 parts per thousand.  Investigators have thus been able to identify whether a cocaine sample originated from the Putumayo or Caqueta regions of Colombia or from the Huallaga and Ucayali Valleys from the Apurimac Valley of Peru. Since mass  spectrometry is used, only a 1 to 2 mg sample is needed, and a similar δ15N ratio can be used for corroboration purposes.

    It is similarly possible to identify the source of heroin. There is only a limited overlap in the
    δ13C and δ15N values of heroin samples from Mexico and South America, and from the same type of data one can also distinguish whether it originated from Southeast or Southwest Asia. Furthermore, since heroin is acetylated morphine, one can use δ13C to find the source of the reagent acetic anhydride. In one investigation of heroin manufacturers in New Delhi, one distributor's anhydride's δ13C was almost triple that of his peers, not surprisingly since he was known to import his raw materials. The imported acetic anhydride's δ13C was also similar to the δ13C of the same product of a major North American pharmaceutical company, but as far as I know there was no other evidence of a direct link between them.


    SOURCES: 

    Sourcing drugs with stable isotopes www.psla.umd.edu/courses/plsc685/2010/isotopesdrugs.pdf

    Farquhar, G.D., Ehleringer, J.R., Hubick, K.T., Carbon isotope discrimination and photosynthesis, Annu. Rev. Plant Physiol. Mol. Biol., vol. 40, 1989, pp. 503-537.

    Wikipedia: various isotope
    articles

    Comments

    Fossil Huntress
    A great well-balanced piece and a nice intro to isotopes. Muchos! 
    UvaE
    Glad you enjoyed it!