Identification of non-cross-linked compounds in methanolic extracts of cured and aged linseed oil-based paint films using gas chromatography-mass spectrometry

Methanolic extracts of paint samples of different composition and age were qualitatively investigated by GC-MS using an on-column injector after off-line methylation or trimethylsilyl derivatisation, and on-line thermally assisted (trans)methylation with tetramethylammonium hydroxide using Curie-point pyrolysis-GC-MS. The combination of these three analytical strategies led to the identification of typical oxidation products of unsaturated fatty acids by interpretation of their mass spectrum. Some of the identified compounds have not been reported before. Both the off-line and on-line GC-MS strategy show series of short-chain fatty (di)acids and C₁₆ and (oxidised) C₁₈ fatty acids. The major advantage of the on-line pyrolysis-GC-MS approach is that chemical work-up is minimal and very quick. With this technique both the carboxylic acid functionalities, and hydroxyl groups are methylated. Young paint films are shown to contain relatively more oxidised C₁₈ fatty acids and less diacids compared to older paints, which is indicative for the on-going oxidation processes within the paint. After trimethylsilylation, monoacylglycerols are detected indicative for hydrolytic processes, which reflect the relative distribution of the most prominent silylated fatty acids present. Relatively more C₁₆ and C₁₈ monoacylglycerols are found in young paints, whereas older paints contain higher amounts of monoacylglycerols of diacids.