A Graphite-Assisted Laser Desorption/Ionization Study of Light-Induced Aging in Triterpene Dammar and Mastic Varnishes

The light-induced aging of natural triterpenes used in varnishes was studied using graphite-assisted laser desorption/ionization mass spectrometry. This method was found to be superior to conventional matrix-assisted laser desorption/ionization for these materials because of higher signal levels and less chemical noise from the matrix. The dammar and mastic raw materials were found to be partially oxidized compared to their nominal composition, with up to six oxygens incorporated. The light-induced aging process leads initially to polymerization, up to trimers. Further aging leads to a decrease in average molecular weight. These observations are supplemented by FTIR and 13C NMR spectra which show complete reaction of keto groups to acids and peroxy species. A general mechanism for the process is proposed, and the consequences for properties of varnish films are investigated.