Artificial Photoaging of Triterpenes Studied by Graphite-Assisted Laser Desorption/Ionization Mass Spectrometry

Abstract

Seven triterpenes were selected for study of their photoaging behavior by graphite-assisted laser-desorption/ionization mass spectrometry as the primary analytical tool. The triterpenes serve as simplified reference systems for complex triterpenoid resins, which are used as varnishes on paintings. Some of the triterpenes are main components of such natural resins; others were chosen for their particular functional groups or carbon skeleton to evaluate 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 triterpenes studied was found to be rather similar, although differences in reactivity, and, therefore, in the extent of oxidation were found, depending on the functional groups. Compounds with aliphatic ketone groups were much more reactive under the test conditions than others. A mixture of five triterpenes showed the highest reactivity. Yellowing, an important aspect of aging with regard to varnishes on paintings, was found to be related to a general breakdown of the initial triterpenes and was not caused by a single compound.

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