LC-MS analysis of free fatty acids and their oxidation products

Free fatty acids were analyzed by LC-MS using a Dionex Ultimate 3000 HPLC system coupled online to a Q Exactive hybrid quadrupole-orbitrap mass spectrometer (Thermo Fisher Scientific, San Jose, CA), as previously described (47). Briefly, fatty acids and their oxidative derivatives were separated by a C18 column (Acclaim PepMap RSLC, 300 μm 15 cm, Thermo Fisher Scientific) using gradient solvents A [methanol (20%)/water (80%) (v/v)] and B [methanol (90%)/water (10%) (v/v)], both containing 5 mM ammonium acetate. The column was eluted at a flow rate of 12 μl/min using a linear gradient from 30% solvent B to 95% solvent B over 70 min, held at 95% B from 70 to 80 min, followed by a return to initial conditions by 83 min and re-equilibration for an additional 7 min. Spectra were acquired in negative ion mode. Analytical data were acquired and analyzed using Xcalibur software. A minimum of three technical replicates for each sample was run to increase the reproducibility.