Department of Chemical Sciences, University of Padova, via Marzolo, 1, 35131, Padova, Italy.
J Mass Spectrom. 2012 Dec;47(12):1660-70. doi: 10.1002/jms.3110.
In this work, the accurate liquid chromatography-ultraviolet-visible (LC-UV-Vis), LC-mass spectrometry (MS) and LC-MS-MS analysis of the photo-degradation products of crystal violet (CV) is reported. CV is a light fugitive early synthetic dye which had a widespread diffusion into the market starting from the end of the XIX century and was used among others by V. Van Gogh and P. Gauguin in their writings, drawings or paintings. On-line photodiode array detector enabled simultaneous UV-Vis spectra acquisition. Many degradation compounds were identified through their exact mass (2 ppm accuracy) and MS-MS technique. In particular, all CV demethylated products, demethylated Michler's ketone and particularly some compounds that most likely contain oxygen, such as N-oxides, were found. Fragmentation products are all justified by the proposed fragmentation scheme, in term of precursor exact mass and isotopic profile, characteristic losses in fragmentation and rebuilt structure formula. In particular, we hypothesized the presence of N-imido oxides and hydroxylamine derivates, never reported before, together with the demethylated derivatives of the studied dyes. All these compounds, although at trace level in our samples, contribute to the discoloration and fading of works of arts made with CV. In particular, demethylation of CV by UV light leads to formation of compounds absorbing at shorter wavelengths than CV (blue shift) or no-absorbing in visible range (yellow-colourless) with an overall effect that may appear reddish-brown. This phenomenon justifies drawings appearing grey or brown on aged yellowed paper, when CV-based inks or paints were used. The final aim was to better characterize the photo-degradation of early synthetic dyes (in particular of CV) and to gain a better insight into the discoloration and fading of purple ink strokes made of CV.
本工作报道了结晶紫（CV）光降解产物的精确液相色谱-紫外可见（LC-UV-Vis）、LC-质谱（MS）和 LC-MS-MS 分析。CV 是一种光褪色的早期合成染料，自 19 世纪末开始广泛扩散到市场，并被 V.梵高和 P.高更等艺术家用于他们的著作、绘画或画作中。在线光电二极管阵列检测器能够同时采集 UV-Vis 光谱。通过它们的精确质量（2 ppm 精度）和 MS-MS 技术，鉴定出许多降解化合物。特别是，发现了所有 CV 去甲基产物、去甲基米氏酮，特别是一些可能含有氧的化合物，如 N-氧化物。通过提出的碎裂方案，根据前体精确质量和同位素分布、碎裂过程中的特征损失以及重建的结构公式，对碎片产物进行了合理的解释。特别是，我们假设存在 N-亚胺氧化物和羟胺衍生物，这是以前从未报道过的，与研究染料的去甲基衍生物一起存在。所有这些化合物，尽管在我们的样品中含量很少，但它们会导致用 CV 制成的艺术品褪色和褪色。特别是，CV 在紫外光下的去甲基化会导致形成比 CV 吸收波长更短的化合物（蓝移）或在可见光范围内不吸收（黄色无色）的化合物，整体效果可能呈红棕色。这种现象解释了为什么当使用基于 CV 的油墨或涂料时， aged 泛黄纸上的 CV 基笔画会出现灰色或棕色。最终目的是更好地描述早期合成染料（特别是 CV）的光降解，并更深入地了解 CV 紫色墨迹的褪色和褪色。
