A glycerophospholipid with two different fatty acyl chains can adopt either of two isomers, depending on where the chains are attached to the glycerol backbone. Such positional isomers can be tough to distinguish by current analytical methods. Stephen J. Blanksby of Queensland University of Technology and Todd W. Mitchell and Rachel L. Kozlowski of the University of Wollongong, in Australia, have found that desorption electrospray ionization mass spectrometry in combination with collision-induced dissociation and ozone-induced dissociation can tell glycerophospholipid isomers apart (Sci. Rep. 2015, DOI: 10.1038/srep09243). When used in series, the dissociation methods break the isomers into characteristic sets of fragment ions that can be used to identify them in complex mixtures. The researchers analyzed synthetic standards of one such pair of isomers, both of which turned out to be mixtures. They then analyzed the proportions of isomer pairs in several biological extracts and tissues. The isomer ratios varied significantly among extracts from different organisms and even among adjacent tissues from the same organism.