Cocaine on currency: Higher levels associated with criminality

Currency contamination with cocaine

In the early 1990s, it was discovered that the majority of banknotes in the USA and Canada were contaminated with cocaine and this picture was soon mirrored in Europe. It was widely believed that they got this way by contact with each other, the initial contamination occurring during drug transactions and by using banknotes for snorting the drug. Another proposed route that does not include the criminal fraternity involves the transfer of cocaine to notes from currency counters in banks.

The widespread tainting of banknotes led the US Appeals Court to rule that large sums of money could not be seized as the proceeds of drug trafficking when a sniffer dog sensed cocaine. They all have cocaine on them. However, there might still be different “degrees of contamination” which could distinguish criminal cash from that in general circulation. Intuitively, you would expect notes involved directly in illegal drug transactions to be more heavily contaminated.

This avenue has been explored by scientists in the USA. Thomas Jourdan and colleagues from the University of Central Oklahoma and Oklahoma State University-Center for Health Sciences combined data acquired by the FBI from 1993-2002 with their own data collected from 2003-2009. This made up measurements of cocaine on US bills over a 17-year period.

Thousands of bills tested

Over the whole period, more than 4100 bills from 66 cities in 43 states, plus DC, were analysed. They included banknotes submitted to the FBI during criminal investigations and control bills acquired from a bank orf post office to set background levels for “normal” currency in general circulation.

The criminal notes were sampled in the field by a household canister vacuum fitted with a filter and screened by a portable ion mobility spectrometer. The same instrument was used to test that the collection areas were free of cocaine contamination.

The early samples tested by the FBI were analysed by GC/MS before an LC/MS procedure was introduced. The Oklahoma samples were analysed by LC-tandem-MS using electrospray ionisation in multiple reaction monitoring mode. In all cases, deuterated cocaine was added as an internal standard.

An administrative detection limit was established at 0.5 ng/sample, so any samples containing less cocaine than this were declared to be "not detected." The detection limit determined from multiple calibration curves was 0.0036 ng cocaine/bill.

General vs. criminal contamination

As expected, more than 97% of the tested bills were contaminated with measurable levels of cocaine. At this stage, even though it is tempting to do so, the team recognised that they could not extrapolate this to the entire currency in circulation within the US because the sampling scheme was not representative of the country as a whole.

Between 600 and 900 bills of each denomination were tested and the average cocaine levels were 2.15, 3.62, 3.18, 1.61, 2.26 and 1.49 ng/bill for the $1, $5, $10, $20, $50 and $100 notes. The national weighted average for any note was 2.34 ng/bill.

The next step involved the development of a mathematical model to predict the likelihood that a banknote has a cocaine content that is statistically higher than the background levels. As Jourdan put it, a member of the jury will want to know "How different from the currency in my wallet is the money recovered in this case?"

The recorded data were fitted to a power curve which was revised so that the area under the curve had a value of one when it was integrated for cocaine levels. Formulae were derived to identify the probability that a note has a specific concentration of cocaine. The likelihoods of a bill having up to 1, 5, 10 or 20 ng of cocaine were all above 0.6 but these values fell sharply for bills contaminated at higher levels. The likelihood for 40 ng was 0.0111 and the values fell steadily to 0.0009 for 300 ng/bill.

This information will be helpful to law enforcement agencies and prosecutors when they are trying to prove that banknotes contaminated with a certain level of cocaine are more likely to be associated with criminal activities.

How cocaine is spread over currency

The distribution of cocaine was fairly uniform across all denominations which is in line with the theory that they became contaminated in the same way. To test one theory, the researchers acquired pristine $20 and $50 bills from the Federal Reserve Bank and checked that they were not contaminated with cocaine.

Next, they passed the notes through a mechanical currency counter from a bank and retested them. The fronts and the backs of the bill were contaminated to the same level, averaging 2.67 ng/ face of the bill. This is very close to the average level found for all of the other 4100 notes tested and was reached after just one pass through the machine.

So, one of the principal methods by which cocaine first reaches a banknote is not by contact with other banknotes, but by passing through currency counters in banks or retail outlets.