In our approach, the lipid sample was first separated by RPLC. Using a C18 column with the abovedescribed gradient (see LC-MS/MS experiments), a synthetic standard of 14:0(12Me) eluted ca. 1 min earlier than isomeric 14:0(13Me), as shown in Figure . Importantly, excellent chromatographic resolution of straight chain FA 15:0 from the two branched-chain variants was achieved, as FA 15:0 eluted nearly 3 min after isomeric 14:0(13Me) (Figure ). Although branched-chain FAs are known to elute earlier than their straight-chain isomers on reversed-phase columns, the elution order may not be sufficient for structural assignment in a complex mixture, especially if unknown branched-points are present. As described before, the MS/MS spectra of underivatized (i.e., singly deprotonated FA anions) saturated and branched FA isomers are identical, leading to ambiguous FA identification in the absence of authentic reference standards.