The synthetic opioid fentanyl is so potent that without proper safety procedures, even opening a bag of the powder to test its contents can be dangerous. To give police and other first responders a faster, safer way to screen suspicious packages for opioids like fentanyl, scientists at the US National Institute of Standards and Technology have been designing sensitive analytical tools. Previously, a team at NIST demonstrated that trace amounts of fentanyl, collected using a simple wipe from the outside of a package, could be detected in seconds using thermal desorption direct analysis in real time mass spectrometry. In collaboration with forensic labs in Maryland and Vermont, NIST researchers have now showed that their method allows them to accurately predict the contents of a suspected drug package 92% of the time (Forensic Sci. Int. 2019, DOI:10.1016/j.forsciint.2019.109939). The team analyzed swabs from 191 pieces of drug evidence, mostly plastic bags, and compared the results with extracts from the package contents. Although cross contamination and high levels of a cutting agent led to a few false positives and false negatives, respectively, the technique worked in all cases in which an opioid in powder form was present.