Dietemann P, Kälin M, Zumbühl S, Knochenmuss R, Wülfert S, Zenobi R
Laboratorium für Organische Chemie, ETH Zentrum, Zürich, Switzerland.
Anal Chem. 2001 May 1;73(9):2087-96. doi: 10.1021/ac000754w.
Photochemical and thermal aging of triterpenoid dammar and mastic resins used as varnishes on paintings were studied using graphite-assisted laser desorption/ionization mass spectrometry. This extends an earlier study on similar materials (Zumbühl et al., Anal. Chem. 1998, 70, 707-715) that focused on photoaging. Progressive aging results in development of groups of signals spaced by 14 and 16 Da, indicating incorporation of oxygen as well as simultaneous loss of hydrogen. Oligomers up to tetramers are formed, while cleavage reactions lead to increased signal intensities in the mass ranges between the oligomers and below the monomers. No major differences were found between the mass spectra of samples aged in light or darkness, except that deterioration was faster in light. Electron paramagnetic resonance spectroscopy revealed similar and significant amounts of radicals in films of dammar stored either in light or in darkness. It is concluded that oxidative radical reactions also take place in darkness and that differences in light and dark aging pathways are minor, although rates may differ. These findings lead to a unified explanation for yellowing of natural resin varnishes, one of the major degenerative changes in the appearance of paintings. It is also shown that the commercially available, nominally fresh resins are already in an advanced stage of oxidation and degradation. Energy-rich substances are formed upon irradiation with sunlight and are believed to restart the autoxidative chain reactions, regardless of storage conditions. As a result, varnishes are oxidized quite quickly (months) even when kept in darkness.
利用石墨辅助激光解吸/电离质谱法研究了用作绘画清漆的三萜类达玛树脂和乳香树脂的光化学老化和热老化。这扩展了早期对类似材料的研究（祖姆布尔等人，《分析化学》，1998年，70卷，707 - 715页），该研究侧重于光老化。渐进老化导致出现间隔为14和16道尔顿的信号组，表明有氧的掺入以及氢的同时损失。形成了直至四聚体的低聚物，而裂解反应导致在低聚物之间以及单体以下的质量范围内信号强度增加。在光照或黑暗中老化的样品的质谱图之间未发现重大差异，只是在光照下劣化更快。电子顺磁共振光谱显示，无论是在光照还是黑暗中储存的达玛树脂薄膜中都存在相似且大量的自由基。得出的结论是，氧化自由基反应在黑暗中也会发生，并且光老化和暗老化途径的差异较小，尽管速率可能不同。这些发现为天然树脂清漆变黄提供了统一的解释，天然树脂清漆变黄是绘画外观的主要退化变化之一。还表明，市售的标称新鲜树脂已经处于氧化和降解的高级阶段。在阳光照射下会形成富含能量的物质，并且据信无论储存条件如何，这些物质都会重新引发自氧化链反应。因此，即使保存在黑暗中，清漆也会很快（几个月）被氧化。
