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

Abstract

Dedicated to Prof. Albert Eschenmoser on the occasion of his 75th birthday Seven triterpenes were selected for study of their photoaging behavior by graphite-assisted laserdesorption/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.

Aging and yellowing of triterpenoid resin varnishes – Influence of aging conditions and resin composition

“…In GALDI-MS, analytes are desorbed without fragmentation and detected as sodium adducts, due to added sodium chloride. Thus, each analyte only results in one signal at an m/z value corresponding to (M þ Na) þ [26,27], see Table 1. Oxidation of a pure triterpenoid compound results in a wide variety of products with mass increments of 14 and 16 Da [26].…”

“…Thus, each analyte only results in one signal at an m/z value corresponding to (M þ Na) þ [26,27], see Table 1. Oxidation of a pure triterpenoid compound results in a wide variety of products with mass increments of 14 and 16 Da [26]. These increments are explained by incorporation of oxygen atoms (16 Da) and simultaneous loss of hydrogen atoms.…”

“…This new method allowed sensitive detection of a very wide variety of oxidation products, especially in the early stage of aging. It was demonstrated that aging of a single triterpene compound results in literally hundreds of oxidation and aging products, and up to seven oxygen atoms can be incorporated into a single triterpenoid molecule [26]. Additionally, it was demonstrated that oxidation also takes place in darkness, and on a relatively short timescale, within weeks and months, rather than years and decades [10,27,28].…”

Analysis of dammar resin with MALDI‐FT‐ICR‐MS and APCI‐FT‐ICR‐MS

“…that oxidation in dammar resin (and dammar varnish) takes place over a short period (about 6 months) and is extensive even if the resin is kept in darkness. The oxidation is accompanied by partial polymerization and partial decomposition of the original constituents . From every individual original constituent of the resin, numerous new compounds can be formed.…”

“…In particular, the high molecular mass region (up from approximately 600) has remained almost unexplored. In the case of mass‐spectrometric methods, the analysis has been based only on nominal m/z ratios, which does not enable us to distinguish between isobaric ions.…”