Characterisation of proteinaceous binders and drying oils in wall painting samples by gas chromatography–mass spectrometry

Abstract

A reliable analytical procedure has been developed for the characterisation of drying oils and proteinaceous binders in samples of painted artworks. The method is based on microwave assisted acid hydrolysis followed by the extraction of the lipid component with diethyl ether. Fatty acids were determined after saponification of the organic phase and derivatisation with N- tert.-Butyl methylsilyl- N-methyltrifluoroacetamide. Amino acids present in the acid aqueous phase were derivatised with the same silyl agent. The separation and quantitative determination of derivatives was performed by gas chromatography–mass spectrometry. Detection limits of fatty acids (lauric, myristic, palmitic, stearic, oleic, suberic, azelaic and sebacic acid) and amino acids were in the range 50–100 pg. The mean recovery of the procedure for lipid determination was about 70% and that for proteins 100% with a reproducibility better than 10% for 1 mg samples. The overall procedure was validated by analysing suitable reference wall painting samples. The identification of the proteinaceous binders was achieved by principal component analysis performed on the fourteen amino acid relative percentages while the identification of the drying oils was based on the values of palmitic to stearic ratio and azelaic to palmitic ratio. A general pattern recognition scheme based on these values and on the amounts of dicarboxylic acids and the presence of cholesterol was developed to distinguish between proteinaceous binder, “tempera grassa” and drying oils. The application of the proposed procedure to samples from “ The Legend of True Cross” (1452) by Piero della Francesca allowed the identification of “tempera grassa” made of egg and linseed oil.