Abstract
The oxidation of omega-3 fatty acids, docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and alpha-linolenic acid (ALA), was studied by MS and tandem mass spectrometry (MS/MS) using a custom-designed DESI source, which was coupled to an ion trap (IT)-mass spectrometer. DESI-MS showed that oxidation due to aging occurred most readily by peroxidations and keto formations of the omega-3 fatty acids, which are the expected oxidation pathways for these molecules. The oxidized products detected were compared between DESI-MS and ESI-MS after a three-week period of sample aging at the room temperature and ambient conditions. The mass spectra of the omega-3 fatty acids obtained by these two techniques in negative ion mode were similar, while oxidation of omega-3 fatty acids was observable with DESI-MS in positive ion mode only after prolonged aging of the samples. The peroxidation products of fatty acids, similar to those observed by DESI and ESI-MS, were also detected by MALDI-MS. Additionally, a commercial dog kibble was analyzed by DESI-MS. DHA, EPA, and some possible oxidation products were identified from the surface of this food sample using DESI-MS and MS/MS. The results demonstrate that DESI-MS can be used efficiently in negative ion mode to monitor oxidation of fatty acids due to their aging both in pure forms and on the surfaces of solid food samples.
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