An initial assessment of thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry for the identification of oils from dried paint films

Conservation scientists are frequently asked to identify the materials that are contained in microscopic fragments from works of art. For decades, the only technique successfully used for characterising the type of oil binder used from dried oil paints has been gas chromatography/ mass spectrometry (GCMS). Unfortunately, the process requires a lengthy sample preparation and some of the derivatisation processes used present serious health and safety implications. Although thermally assisted hydrolysis and methylation-GCMS (THM-GCMS) has been used more recently for the analysis of traditional paint binders, its ability to distinguish between different oil types has yet to be ascertained. This paper describes the use of THM-GCMS to examine differences in the fatty acid composition of dried pigmented films of linseed, linseed stand, poppy, safflower and walnut oil. The results were compared for pyrolysis at 610 and 770 °C, two of the most commonly used temperatures in Curie point pyrolysis, in order to verify reproducibility of the results. Based on three runs for each sample, the fatty acid composition (measured as methyl esters) was found to be temperaturedependent, as previously suggested in literature. At the lower pyrolysis temperature (610 °C), a higher degree of reproducibility was seen and the ability to differentiate between each of the dried oil types using palmitate/stearate (P/S) ratios was improved. Interestingly, the P/S observed for each oil at this pyrolysis temperature were found to be close to those obtained by published GCMS procedures. In addition, samples of dried pigmented films of egg yolk were investigated and it was confirmed that this medium could be differentiated from each of the drying oils at this pyrolysis temperature from their azelate/palmitate ratios. These preliminary findings are encouraging for the use of THM-GCMS as an alternative method to GCMS for the identification of oils used as binders in paints.