Analysis of nonvolatile lipid oxidation products in vegetable oils by normal-phase high-performance liquid chromatography with mass spectrometric detection

Nonvolatile triacylglyceride (TAC) oxidation products play an important role in the oxidative degradation of lipids. They serve as a reservoir of oxygen-containing species and hence can act as off-flavor precursors or as initiators for further oxidation reactions. Possible nonvolatile lipid oxidation products are TAC with a hydroperoxy, hydroxy, epoxy, or oxo (ketone or aldehyde) group or combination of these groups. The breakdown of TAC hydroperoxides yields nonvolatile glyceride species with two intact fatty acid chains and one shortchain mostly ending in an aldehyde or hydroxy group (21/2 glycerides). By means of normal-phase high-performance liquid chromatography (HPLC) with mass spectrometric (MS) detection, nonvolatile lipid oxidation products can be separated according to polarity. This results in separation into classes of TAC oxidation products, such as epoxy-TAG, ore-TAG, hydroperoxy-TAC, hydroxy-TAG and 21/2 glycerides, which can be identified using selected ion chromatograms. The retention times of TAG oxidation products on the normal-phase HPLC system and the signal intensity of the MS detector are stable enough to enable quantitative analysis based on external calibration. The normal-phase HPLC-MS method is very suitable for the characterization and quantitation of nonvolatile TAG oxidation products in oxidized TAC reference compounds as well as in real oils or oil phases isolated from emulsions, spreads, or other fat- based food products. This method can give detailed information for the study of lipid oxidation mechanisms.