Stable isotopes address a chronic problem

I eased into the annual American Geophysical Union meeting today at a poster session devoted to the dazzling secrets you can learn from isotopes. (Which was pretty much where I left off last year.) Right off the bat, I fell in with the drug crowd from Utah.

Picture the scene of a pot bust: a garage filled with product and a couple of flannel-shirted (and severely bummed) perps making up stories about where it came from. A new analysis may one day bypass that tedious step and let police ask the marijuana directly.

Jason West, from the University of Utah, is tracing seized marijuana back to where it grew by studying the isotopic signature recorded inside the plant’s tissues. His work so far is good enough to tell whether a stash came from Humboldt County or was ferried in from Mexico. But he’s aiming for a county-by-county level, or better.

The analysis works because hydrogen and its heavier brother, deuterium, occur in differing amounts in rainwater across the country. Plants suck up this water and use it to make molecules like sugars and proteins (not to mention tetrahydrocannabinol), preserving the ratio of deuterium to hydrogen in the process. By measuring that ratio in seized pot (a very small sample that has been very carefully loaned out by law enforcement, West stresses) and comparing the result to a map of rainfall values across California, you can narrow down where the pot grew.

Of course there’s some scatter in the data, in part because some cellular machinery uses deuterium preferentially over hydrogen. But even these complications can be unraveled. For instance, West found, plants making more THC had lower deuterium ratios. Once he measured that relationship, some fairly standard statistics let him factor out its effect to further narrow down the growing location. By employing another isotopic marker – a good choice would be strontium, West said – authorities could be even more precise. (Once I found out about West’s research, I learned that a lab in Alaska is doing similar work, trying to learn how much cannabis is homegrown vs. smuggled in from the south)

If all that sounds a little far-fetched, it’s been working for European wines for a decade or so. That industry has the advantage of being both legal and ferociously proud of its geographic origin, or terroir. Some European Union scientists make a career of detecting labels that fudge their place of origin. (Science News has a story on it.)