Mass spectrometry as a tool to study ageing processes of diterpenoid resins in works of art: GC- and LC-MS studies

Products of diterpenoid resins present in aged works of art were studied by Pyrolysis-TMAH-GCMS and reverse phase HPLC-APCI-MSMS. A number of oxidation products of abietic acids were identified and different stages of oxidation of these acids can be distinguished with both methods. Reverse phase HPLC does not resolve abietic acid and its isomers. A large number of oxidation products present in very low amounts in a relatively fresh pine resin (colophony) could be traced in single ion chromatograms of the HPLC-APCI-MS. Only four compounds were observed in the diterpenoid resin fraction of a naturally light aged varnish by HPLC. Two of the compounds were oxidation products not previously seen by GC methods. No indication of products more highly oxidized than the 15-hydroxy-7-oxo-dehydroabietic acid was found. LC-APCI-MS/MS spectra of three abundant oxidized diterpenoid acids (DHA, 7-oxo-DHA and 15-OH-7-oxo-DHA) show a loss of 128 a.m.u. corresponding to CID fragmentation which involves the A ring.