Identification of PC and PC- O species by MFQL queries relying on complementary signature ions. (a) MS/MS spectrum of the precursor ion of the acetate adduct of PC 36:1 (m/z 846.6224), in which four signature ions are recognized: molecular ion (MS); fragment of neutral loss of acetate and methyl group (Δm/z = 74.0 (NL)); acyl anions of the two fatty acid moieties (FA 281.3 and FA 283.2; both boxed in the chemical structure at the top). (b) MS/MS spectrum of the acetate adduct of PC-O 34:3 (m/z 800.5808). Signature ions are the same as in (a), except m/z 464.4 representing the fragment produced by neutral loss of the sn-2 fatty acid moiety. (c) Quantitative profiles of PC and PC-O species reported from abundances of different signature ions. MS, precursor ions in MS spectra; NL 74, neutral loss Δm/z 74 in MS/MS spectra; FA/FAO, acyl anions of fatty acid moieties and (for PC-O) neutral loss of sn-2 fatty acid moiety. The relative abundance of species was normalized to the total abundance of species within each (PC or PC-O) class.