Analysis of triglyceride degradation products in drying oils and oil paints using LC-ESI-MS

An LC-ESI-MS method is presented as a novel approach for the study of aged drying oils and oil paints in various stages of oxidation and hydrolysis. The method involves separation and detection of glycerides and fatty acids on a reversed phase column using a polar gradient ranging from methanol/water to methanol/isopropanol with post-column addition of NH4Ac to facilitate electrospray ionisation. This setup allows for a reasonable separation of non-polar triglycerides in drying oil as well as very polar oxidised and hydrolysed tri, di and monoglycerides as well as free fatty acids. Detection is performed by using both positive and negative ionisation mode: positive ions for glycerides, negative ions for carboxylic acid containing degradation products and free fatty acids.In this way, distinction can be made between components in oil and metal stearate mixtures by independently probing the palmitic acid/stearic acid (P/S) ratios of the free fatty acids which mostly derive from the metal stearates, and the glycerides which derive only from the drying oil components.Analyses of 10 year-old titanium white oil paints with medium exudations and 62 year-old paints from Winsor&Newton are presented as examples to show the applicability of the method.