Analysis of unsaturated fatty acids by supercritical fluid chromatography tandem mass spectrometry coupled with online Paternò-Büchi reaction

•Online Paternò-Büchi (PB) reaction coupled with SFC-MS was established.•Benzaldehyde was optimized as the PB reagent with a high reaction yield.•The method for pointing C C locations in fatty acids was comprehensively evaluated.•Fatty acid isomers in human plasma were qualitatively and quantitatively analyzed.