Artificial photoaging of triterpenes studied by graphite-assisted laser desorption/ionization mass spectrometry

Seven triterpenes were selected for study of their photoaging behavior by g raphite-assisted laser-desorption/ionization mass spectrometry as the prima ry analytical tool. The triterpenes serve as simplified reference systems f or complex triterpenoid resins, which are used as varnishes on paintings. S ome of the triterpenes are main components of such natural resins; others w ere chosen for their particular functional groups or carbon skeleton to eva luate specific influences on the aging process. Progressive aging resulted in incorporation of oxygen and simultaneous loss of hydrogen, recognizable in the mass spectra by the appearance of signals with mass increments of 14 and 16 Da. Degradation of aging products also led to compounds with lower masses than the initial triterpenes. The general aging behavior of the trit erpenes studied was found to be rather similar, although differences in rea ctivity, and, therefore, in the extent of oxidation were found, depending o n the functional groups. Compounds with aliphatic ketone groups were much m ore reactive under the test conditions than others. A mixture of five trite rpenes showed the highest reactivity. Yellowing, an important aspect of agi ng with regard to varnishes on paintings, was found to be related to a gene ral breakdown of the initial triterpenes and was not caused by a single com pound.