Effect of pigments on the analysis of fatty acids in siccative oils by pyrolysis methylation and silylation

The relative abundance of the dicarboxylic acids azelaic (nonanedioic) and suberic (octanedioic) with respect to monocarboxylic fatty acids is diagnostic for the identification of siccative oils in painting layers. Fatty acids can be produced from the sample by thermal (pyrolysis) and/or chemical (hydrolysis) degradation, and then determined by gas chromatography-mass spectrometry (GC-MS) in the form of methyl or trimethylsilyl (TMS) ester derivatives. However, the presence of matrix components in the layer (e.g. pigments) might interfere with the analytical procedure. In this study, the presence of three inorganic pigments (cinnabar (HgS), lead white (2PbCO3·Pb(OH)2) and zinc white (ZnO)) in linseed oil layers was investigated in relation to the determination of fatty acids by analytical pyrolysis with in situ derivatisation. Two methods were applied: pyrolysis/methylation with tetramethylammonium hydroxide (TMAH) and pyrolysis/silylation with hexamethyldisilazane (HMDS), both in combination with online GC-MS. Pyrolysis/methylation and, to a lesser extent, pyrolysis/silylation were influenced by the presence of the pigments which caused a significant decrease in the relative content of azelaic acid. Discriminating siccative oil from egg tempera might be problematic in the presence of lead white and zinc white. Reactive pyrolysis-GC-MS was compared with the classical wet method (alkaline hydrolysis, silylation, GC-MS analysis) and the effect of sample preparation was considered.