Don't try this at home: method relies on chloroform
In response to requests for assisting in tracing drugs, the DEA lab developed a method for detecting traces of cocaine as small as .25mg.
So what's the problem with this method? A study of Chicago Federal Reserve Bank cash found a third of randomly selected money samples of $50 and $100 bills in general circulation failed the test. Ultimately, the DEA recommended "the project be terminated."
Here are the full docs:
Image via FBI.