The legal problems of convicting someone on the basis of trace amount of exotic designer drugs require telling a jury that a defendant can be convicted only by importing enough of a synthetic drug to make it valuable, a federal judge has ruled.

Eastern District Judge Jack Weinstein noted that trace amounts of other drugs, such as cocaine, can be found on most paper currency in the United States, and that a felony conviction must be supported by more than “de minimis” amounts of a synthetic drug. Weinstein made that ruling in last week’s trial of Chin Chong, a defendant in a scheme to bring the controlled substance methylone, commonly known as “Molly,” into the United States from China.