Scientists have developed a handheld probe capable of non-destructively distinguishing between tumors and healthy tissue within 10 seconds, which could enable rapid cancer diagnoses and help surgeons remove all traces of malignant masses during operations. Residual cancer after surgery puts patients at risk for relapses, but most pathology labs require several days to evaluate if tumor cells remain in samples taken during operations. Seeking a more effective way to analyze tissues in real-time, Jialing Zhang et al. created the MasSpec Pen, a handheld device that gently extracts molecules from tissue using tiny volumes of water (10 microliters, or roughly one-fifth the size of a single drop), that then transfers the samples through flexible tubing to a mass spectrometer (an instrument that calculates the masses of molecules contained in a sample). After analyzing 253 human patient samples from lung, ovary, thyroid, and breast tumors as well as healthy tissues, the scientists developed a "molecular profile" that could identify cancers with 96.4% sensitivity, 96.2% specificity, and 96.3% accuracy. The researchers went on to show that the MasSpec Pen reliably identified tumors in living mice and importantly, did not cause any damage to healthy tissues. Unlike existing mass spectrometry tools that require harsh solvents, pressurized gasses, or high voltages, the MasSpec Pen gathers molecules for analysis using only water. Additionally, the MasSpec Pen's tip was 3-D printed with a safe and biocompatible material called PDMS. The authors say investigating larger sample sets may make the MasSpec Pen even more accurate, and allow it to diagnose a wider range of tumors from different types of tissues.