Analysis of oxidised diterpenoid acids using thermally assisted methylation with TMAH

An aged intermediate varnish sample from 'The Girl with the Pearl Earring' by J. Vermeer was analysed by thermally assisted methylation with tetramethylammoniumhydroxide (TMAH) followed by gas chromatography in combination with mass spectrometry. Using library search of the available mass spectra, only three diterpenoid acids were identified. In addition to methyldehydroabietate, 7-oxo-dehydroabietic acid methyl ester and 7-oxo-15-hydroxy-dehydroabietic acid methyl ester were identified. There was reason to believe that the remaining unidentified compounds might be related to the oxidised derivatives of dehydroabietic acid and are formed upon methylation with TMAH. In order to investigate the behaviour of pure 7-oxo-dehydroabietic acid and 7-oxo-15-hydroxydehydroabietic acid under these specific derivatisation conditions, these compounds were synthesised and isolated. On derivatisation with TMAH, several methylation products, with varying degrees of methylation were formed from each of the pure compounds. Their structures were elucidated using specific fragment ions present in their mass spectra. Unusual products formed by methylation on a carbon atom under these conditions were observed. Based on these comparative studies, 7-oxo-dehydroabietic acid and 7-oxo-15-hydroxy-dehydroabietic acid were identified as the main components of the varnish.