This talk sketches recent applications in medicine of ambient ionization mass spectrometry – an experiment done to analyze specific compounds in complex samples (tissue, biofluids) in situ without any sample workup and often with miniature mass spectrometers. The particular method used, desorption electrospray ionization (DESI), is a very simple technique in which droplets of solvent impact a surface and extract components. The resulting secondary droplets are analyzed, on-line, by mass spectrometry. The medical example emphasized is intrasurgical diagnosis of the type and stage of brain cancer using measurements of the pattern of phospholipid ions. The experiment is done in the context of the standard of care with small biopsies being analyzed by mass spectrometry in parallel with conventional histochemical analysis. The MS data allow the disease state (healthy white matter, healthy grey matter, glioma) to be assessed, including an estimate of the percentage tumor cell concentration and hence indicating tumor margins. In addition to information from the lipid pattern, oncometabolites associated with an isocitrate dehydrogenase (IDH) mutation are detected during surgery and shown to be reliable indicators of the mutation. Data from some 50 brain cancer patients is reported. Data on other cancers as well as other medical applications of mass spectrometry are also discussed.
 Takats et al. Science, 306 (2004) 471-473.
 Pirro et al., PNAS, 114 (2017) 6700-6705.