Gas Chromatography Mass Spectrometric Analysis of Monohydroperoxide Isomers and Their Secondary Oxidation Products of Methyl Linoleate and Methyl Linolenate

Emulsions of methyl linoleate monohydroperoxides (18:2-monoHP) and methyl linolenate monohydroperoxides (18:3-monoHP)were incubated with ferrous sulfate and ascorbic acid. Gas chromatography mass spectrometric analysis of the trimethylsilyl and ter/-butyldimethylsilyl derivatives of the reaction products showedthat isomerization and secondary oxidation happen competitively during decomposition of 18:2-monoHP, while the secondary oxidation reaction proceeds preferentially and little isomerization is observed in 18:3-monoHP. It is suggested that 18:3-monoHP is more susceptible to secondary oxidation than 18:2-monoHP because of 18:3 specific secondary oxidation resulting in hydroperoxy-cyclic peroxides and dihydroperoxides. Moreover, an experiment using 18O2 has demonstrated that molecular oxygen is scrambled by isomerization and secondary oxidation. It was confirmed that molecular oxygen is attached preferentially to the C-13 position in the 9-monoHPisomer and C-9 position in the 13-monoHP isomer during degradation of 18:2-monoHP.