Selective ionization of oxidized versus non-oxidized lipid species using different solvent additives in direct infusion MS

Lipid oxidation in food products is a crucial problem that causes undesirable changes in the food’s flavor, texture, and nutritional value and consequently reduces shelf life. Even though lipid oxidation has been examined extensively and is rather well understood in bulk oils and fats, the processes behind it in more complex systems like emulsified foods are still largely unresolved. Oxidation reactions are believed to progress from the oil/water interface to the core of the oil drop-lets, making it important to understand the contribution of interfacial lipids (i.e., MAG, DAG, and PL) to the lipid oxidation process. To study this, novel analytical tools are needed that allow the characterization of the highly complex mixture of oxidized species encountered in aged emulsified foods. In this study, a direct infusion mass spectrometry (MS) approach was set up to selectively ionize oxidized lipid species versus their non-oxidized precursors (DAG and TAG). Three mobile phase additives were investigated (NH4HCO2,C2H3NaO2, and NaI) at three different concentrations,and three ion source parameters (i.e., sheath gas temperature, nozzle, and capillary voltage) were optimized. A fractional factorial design was conducted to examine not only the direct effect of the operating parameters on selective ionization of oxidized lipid species, but also assess their combined effect. A three-level process was chosen to examine the effect of the selected parameters: (1) on the whole mass range of oxidized versus non-oxidized lipid species, (2) on selected lipid species and their different oxidized forms, and (3) on the fragments of the lipid species investigated in the previous step. Selective ionization of oxidized versus non-oxidized lipid species was favored more by the use of sodium containing solvent additives. These findings will contribute to future studies on the influence of interfacial composition on lipid oxidation in complex emulsified food systems.