Many lipids contain carbon-carbon double bonds. Sometimes the only difference between two lipids is the location of those double bonds. But mass spectrometry—the go-to tool for analysis of lipid mixtures—on its own can’t distinguish between isomers with double bonds in different locations on otherwise identical molecules. A team led by Yu Xia and Zheng Ouyang of Purdue University now reports that they can use the Paternò-Büchi photochemical reaction with complex lipid mixtures to modify lipid C=C bonds in a way that pinpoints their locations (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1523356113). In the reaction, acetone is activated with 250-nm ultraviolet light and undergoes a [2+2] cycloaddition across lipid double bonds. The researchers analyze the products by mass spectrometry. During fragmentation, the products cleave across the ring to form pairs of diagnostic ions, one with an aldehyde moiety and the other with an isopropene moiety at the location of the original double bond. By using the method, the researchers were able to identify and quantify unsaturated lipids in various animal tissues and found differences in the lipid isomer compositions of normal and cancerous mouse breast tissues.