Identification of linoleic acid free radicals and other breakdown products using spin trapping with liquid chromatography-electrospray tandem mass spectrometry

Linoleic acid radical products formed by radical reaction (Fenton conditions) were trapped using 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) and analysed by reversed-phase liquid chromatography coupled to electrospray mass spectrometry (LC-MS). The linoleic acid radical species detected as DMPO spin adducts comprised oxidized linoleic acid and short-chain radical species that resulted from the breakdown of carbon and oxygen centred radicals. Based on the m/z values, the short-chain products were identified as alkyl and carboxylic acid DMPO radical adducts that exhibited different elution times. The ions identified as DMPO radical adducts were studied by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The LC-MS/MS spectra of linoleic acid DMPO radical adducts exhibited the fragment ion at m/z 114 and/or the loss of neutral molecule of 113 Da (DMPO) or 131 Da (DMPO + H2O), indicated to be DMPO adducts. The short-chain products identified allowed inference of the radical oxidation along the linoleic acid chain by abstraction of hydrogen atoms in carbon atoms ranging from C-8 to C-14. Other ions containing the fragment ion at m/z 114 in the LC-MS/MS spectra were attributed to DMPO adducts of unsaturated aldehydes, hydroxy-aldehydes and oxocarboxylic acids. The identification of aldehydic products formed by radical oxidation of linoleic acid peroxidation products, as short-chain product DMPO adducts, is a means of identifying lipid peroxidation products.