The combination of RPLC-ESI-FTMS/MS and m-CPBA epoxidation for the location and geometry assignment of double bonds in unsaturated fatty acyl chains of drying oils

Abstract

Abstract Oil painting is the process of painting with drying oils as binders. Such oils mainly consist of triacylglycerols characterized by high levels of unsaturated fatty acyl chains. For instance, poppy seed oils have received much attention for their use as oil paints since very ancient time. Upon exposure to oxygen, not only do drying oils harden to a densely cross-linked polymer network through radical reactions, but their fatty acids also interact with cations of some pigments forming the related carboxylate salts. These so-called metal soaps lead to severe degradation phenomena because they can migrate and aggregate, breaking through the surface of the paint. Since the fatty-acidome of a drying oil is involved in these processes, the current study is chiefly devoted to the identification of both free and total fatty acids in poppy seed oil by reversed-phase liquid chromatography coupled with high-resolution mass spectrometry by an electrospray ionization source (RPLC-ESI-FTMS). Total fatty acids (TFA) were investigated by enzymatic hydrolysis and comparisons were made between the free fatty acid (FFA) profile and the total one resulting from the digestion on the same poppy seed oil. A key component of our approach was the double bond (C=C) epoxidation reaction, which was carried out on unsaturated fatty acids by meta-chloroperoxybenzoic acid (m-CPBA). A chromatographic set up was developed to separate such oxidized lipids, that were identified as deprotonated molecules [epoM-H]− by accurate mass matching in full mass spectra. Their epoxide ring was subsequently cleaved upon tandem mass fragmentation to yield a pair of diagnostic product ions for C=C double bond localization. Interestingly, the double bond geometry (i.e., cis/trans) was also assigned based on the retention times of derivatized-FA. The present approach opens new avenues for the analysis of artworks whether affected by metal soap-related degradation phenomena.