SAN FRANCISCO—The U.S. border has a cash flow problem. Each year, billions of dollars filter into Mexico to pay for Americans' drug habits. Dogs and X-ray machines only catch a fraction of this dirty money, so chemists from a Bay Area company are working with the Department of Homeland Security on a sensor that can detect the smell of money.

Money has a unique smell that comes mostly from the ink, and computerized noses can be trained to sniff it out, said Joseph Stetter, President of KWJ Engineering and co-author of research presented here yesterday at the annual meeting of the American Chemical Society. (Individual bills will accumulate their own aroma as they are passed around, a mixture of sweat, dirt, and yes, drugs.) This signature is made up of common compounds, but in volumes and mixtures unique to money. Stetter wouldn't get into the specifics, however, for fear of tipping off competitors. He adds that they've only captured the chemical signature of U.S. currency so far, but the same principles could be applied to sniff out the currencies of other nations.

The technology is based on mass spectrometry, an established method that's already used to detect the chemical signature of drugs and explosives. KWJ is developing algorithms that would determine whether the cocktail of aromas on each bag, person, or package contains money's chemical fingerprint.

The challenge, Stetter says, is putting all the parts together into a system that can sample, identify, and measure the money smell quickly enough that it doesn't slow traffic through the border. "It also needs to be clever enough to distinguish someone's wad of bills from a backpack full of money," he said. Dollar amounts less that $10,000 are not illegal to bring across the border.

Stetter hopes to have a prototype that border agents could start testing within a year. Consider yourself warned.