Proposal for a common nomenclature for fragment ions in mass spectra of lipids
A) Negative FTMS2 spectrum of m/z 836.5, corresponding to the acetate adduct of PC 18:3–18:3. The precursor ion is annotated at the lipid species level (i.e. PC 36:6) since that the composition of FA moieties cannot be inferred from the m/z value. Prioritized shorthand notation of fragment m/z values are in boldface and implemented according to the three-step procedure shown in Fig 2. Non-prioritized (redundant) shorthand notation is shown non-boldface and separated from the prioritized shorthand notation by “|”. B) Overview of fragmentation pathways for [PC 18:3–18:3+CH3COO]- with putative structures of neutral and charged fragments. Note that each chemical reaction is mass-balanced (i.e. the total mass of all fragments equal the mass of the intact precursor molecule). Each structure is represented with charge, monoisotopic mass, shorthand notation and fragment type. Note that neutral (shown on the right) and charged (shown on the left) fragments are prefixed with and without a minus sign “-“, respectively. The annotations shown in boldface (prioritized) are based on annotation rules outlined in Fig 2 (step 3). LCF, lipid class-selective fragment; MLF, molecular lipid species-specific fragment, iMLF, intermediate MLF.