Oleanolic and ursolic acid in dammar and mastic resin: Isomer discrimination by using ToF-SIMS and multivariate statistics

Triterpenoid resins, such as dammar resin and mastic resin, are natural products used as protective varnishes on paintings. Dammar resin consists of a large amount of triterpenoid molecules including two triterpenic acids: oleanolic acid and ursolic acid. Similarly, mastic resin is a complex blend of terpenoids but containing oleanolic acid. In this paper, high-mass-resolution time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been used to characterize the fragmentation patterns of oleanolic acid and ursolic acid. The hypothesized fragmentation pathways for these triterpenic acids have also been discussed. In order to distinguish between these two structural isomers, principal component analysis (PCA) has been adopted. This multivariate statistical analysis technique permits to identify the characteristic fragments and discriminates between very similar mass spectra. In addition, dammar resin and mastic resin have been characterized. A selection of characteristic peaks from ursolic and oleanolic acids mass spectra has been used to build a dataset for a multivariate analysis to compare triterpenoid resins and triterpenic acids. The results of PCA have revealed that mastic resin is strictly related to oleanolic acid, whereas dammar resin is differentiated by ursolic acid fragments. This study confirms that, in the cultural heritage field, the time-of-flight secondary ion mass spectrometry technique combined with PCA can be a powerful tool to investigate the chemical composition of organic compounds with very complex composition.