Fig. 4. The analysis of unsaturated lipids in human serum. (a) Workflow for the identification of the positions of C C bonds via the VACA reaction and LC-MS/MS. (b and c) The extracted ion chromatograms of PC 38:6 (b) before and (c) after the VACA reaction. (d and e) The averaged MS/MS spectra of VACA products of PC 38:6 at m/z 1014.6 at (d) 7.9–9.3 min and (e) 6.2–7.9 min. The fragment ions at m/z 805.6 in panel (d) denote [PC 38:6]⁺, m/z 955.5 denotes [P1/P2–C₃H₉N + H]⁺, and m/z 996.6 denotes [P1/P2–H₂O + H]⁺. The chemical structures show the positions of C C bonds in lipids and the m/z values of the possible diagnostic fragment ions.