During the shotgun experiment all peaks of plausible PE precursors will be fragmented.
Other species of PE class will fragment similarly. The latter was dominated by abundant acyl anion fragments (m/z 255.2 and 267.2) originating from 16∶0 and 17∶1 fatty acid moieties, respectively. In this example, PE molecular anion having m/z 702.5 was detected in the survey MS spectrum (here termed as MS1-) and then its MS/MS spectrum (here termed as MS2-) was acquired. Usually the location of fatty acid moieties at the glycerol backbone (sn-1 or sn-2) could be inferred from the relative abundance of corresponding acyl anions. However, by identifying both fatty moieties the analysis may recognize the individual molecular species (here PE 16∶0/17∶1). If identified by precursor m/z and/or lipid-class specific fragment (for example, originating from the phosphoethanolamine head group), lipids are annotated by their class and total number of carbon atoms and double bonds in both fatty acid moieties (in this case, PE 33∶1). The chemical structure of PE 16∶0/17∶1 in its zwitterion form is shown at the bottom panel with two fatty acid moieties (16∶0 and 17∶1) boxed other PE species may differ by their fatty acid moieties. PE molecules consist of a glycerol backbone to which the phosphoethanolamine head group and two fatty acid moieties are attached via phosphoether and ester bonds, respectively.
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