Can fragment ion intensity be used for fatty acid sn-position determination of food phospholipids?

Fatty acid (FA) regio-position is critical information for the structures of both triglycerides and phospholipids in food products. Although lipase/phospholipase hydrolysis combined with TLC separation and GC analysis can be used to determine the overall sn-2 FA composition of triglycerides/phospholipids in food samples, it is much more challenging to pinpoint FA regio-position at the molecular species level. Currently, the most widely used method to obtain such information is based on the relative intensity of fragment ions generated in tandem mass spectrometry. The reliability of this approach was verified in this study using regio-pure phospholipid standards and by cross-validation with the Paternò–Büchi photochemical reaction. We have demonstrated that for food materials with widespread regioisomers, a standard calibration curve from regio-pure standards is needed to determine the proportion of each isomer, whereas fragment ion intensity can only be used to reveal the sn-position of FA in phospholipid molecules devoid of regioisomers.