The sensitive surfaces of many unvarnished 20th century oil paintings are of great concern for conservators and collection keepers. They may show degradation problems such as paint delamination, dripping, and soft and sticky paint and pose challenges for cleaning due to solvent sensitivity. We report for the first time the use of an innovative ambient ionization technique, surface acoustic wave nebulization-mass spectrometry (SAWN-MS), for the identification and characterization of fatty acids, dicarboxylic species and glycerides in water-sensitive modern oil paints. The composition of 10 relevant Winsor and Newton 1964-1965 paint swatches that present different degrees of water sensitivity and two paint samples from a painting by the British artist Francis Bacon were studied. Principal component analysis was used for SAWN-MS data classification. Electrospray ionization (ESI)-MS was used as control method, specifically to compare the obtained ratios of markers of interest by the two ionization techniques. The results obtained by both ESI-MS and SAWN-MS are correlated and discussed in a broader context including the information on the oil media obtained by gas chromatography (GC-MS) and also on the inorganic materials and salts characterized using a combination of methods in previous reports on samples from the same manufacturer. SAWN-MS was found to be a suitable tool for the determination of soluble organic constituents present in the paints. The method provides an indication of the level of oxidation and hydrolysis of the paint film by monitoring specific markers such as free palmitic and stearic acids, azelaic acid, monoacylglycerols, and diacylglycerols. The data showed that a higher level of water sensitivity coupled with a high level of oxidation and hydrolysis is linked to higher dicarboxylic acid, diacyl- and triacylglyceride content and lower levels of short chain fatty acids. The data obtained by SAWN-MS provided a good correlation between the monitored species and the degree of water sensitivity.
Keywords: case studies; mass spectrometry; surface acoustic wave nebulization; water sensitive modern oil paints.