Qualitative analysis of omega-3 fatty acid oxidation by desorption electrospray ionization-mass spectrometry (DESI-MS)

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. (C) 2014 Elsevier B.V. All rights reserved.