GC-MS characterization of hydroxy fatty acids generated from lipid oxidation in vegetable oils

Abstract

Lipid oxidation has long been described as following a radical chain reaction mechanism, where hydrogen abstraction is considered the preferred pathway. Hydroxy compounds are, in theory, major products formed from hydrogen abstraction but their presence is rarely monitored. In this study, we describe a GC-MS technique to characterize hydroxy FA formed during the oxidation of sunflower and canola oils. First, hydroxy FA in oxidized oils were methylated and isolated from non-oxygenated structures using solid phase extraction (SPE). They were then converted into their trimethylsilyl (TMS) derivatives using a N,O-bis (trimethylsilyl) trifluoroacetamide (BSTFA)-pyridine method and identified by their electron ionization (EI) and positive chemical ionization (PCI) spectra. Separation of most isomeric hydroxy FA with very similar structures was accomplished using a DB-23 capillary column with (50%-cyanopropyl)-methylpolysiloxane phase. The fragmentation patterns of the TMS derivatives are discussed in detail and several easily applicable rules for spectral interpretations are presented. The major hydroxy FA arising from oxidation of sunflower and canola oils were allylic and conjugated structures, specifically 8-, 9-, 10- and 11-hydroxyoctadecenoic acid (OH-C18:1) and 9- and 13-hydroxyoctadecadienoic acid (OH-C18:2).

Practical applications: Unsaturated hydroxy FA are potential indicators of hydrogen abstraction reactions by FA alkoxy radicals during lipid oxidation. A prerequisite for the quantification of these hydroxy FA is knowledge of their structures. This work employed a GC-MS method, coupled with SPE and TMS derivatization, to characterize the hydroxy FA derived from oxidation of vegetable oils, which will enable future quantifications of these compounds. In addition, the use of EI and PCI spectra provided clear strategies to interpret mass spectra for both saturated and unsaturated hydroxy FA. The fragmentation patterns in EI and PCI presented in this paper will benefit future studies on identification or quantification of both saturated and unsaturated hydroxy FA in other lipid samples.

http://ift.tt/2xgEYXe