Characterisation of proteinaceous binders and drying oils in wall paintingsamples by gas chromatography-mass spectrometry

A reliable analytical procedure has been developed for the characterisation of drying oils and proteinaceous binders in samples of painted artworks. T he method is based on microwave assisted acid hydrolysis followed by the ex traction of the lipid component with diethyl ether. Fatty acids were determ ined after saponification of the organic phase and derivatisation with N-te rt.-Butyl methylsilyl-N-methyltrifluoroacetamide. Amino acids present in th e acid aqueous phase were derivatised with the same silyl agent. The separa tion and quantitative determination of derivatives was performed by gas chr omatography-mass spectrometry. Detection limits of fatty acids (lauric, myr istic, palmitic, stearic, oleic, suberic, azelaic and sebacic acid) and ami no acids were in the range 50-100 pg. The mean recovery of the procedure fo r lipid determination was about 70% and that for proteins 100% with a repro ducibility better than 10% for 1 mg samples. The overall procedure was vali dated by analysing suitable reference wall painting samples. The identifica tion of the proteinaceous binders was achieved by principal component analy sis performed on the fourteen amino acid relative percentages while the ide ntification of the drying oils was based on the values of palmitic to stear ic ratio and azelaic to palmitic ratio. A general pattern recognition schem e based on these values and on the amounts of dicarboxylic acids and the pr esence of cholesterol was developed to distinguish between proteinaceous bi nder, "tempera grassa" and drying oils. The application of the proposed pro cedure to samples from "The Legend of True Cross" (1452) by Piero della Fra ncesca allowed the identification of "tempera grassa" made of egg and linse ed oil. (C) 1999 Elsevier Science B.V. All rights reserved.