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.