Weyermann Céline, Kirsch Dieter, Vera César Costa, Spengler Bernhard
Institute of Inorganic and Analytical Chemistry, Justus-Liebig University Giessen, Schuberstr. 60, Bldg. 16, D-35392 Giessen, Germany.
J Forensic Sci. 2009 Mar;54(2):339-45. doi: 10.1111/j.1556-4029.2008.00975.x. Epub 2009 Feb 6.
Crystal violet is a very common dye in ballpoint ink. Recent research suggests that the degradation of triarylmethane dyes gives an indication of the age of a ballpoint pen entry on a document. The main problem for the quantitative evaluation of the degradation is that it is highly dependent on the exposure to light. Moreover additional factors, such as additives and substrate play an important role in this process. The aim of this work is to compare the degradation pathways of the pure dye in water and ethanol upon exposure to xenon light by UV/VIS spectrophotometry and laser desorption ionization. Significant differences have been observed in the products and the kinetics of the degradation. N-demethylation, an expected decomposition process, was found to take place only in aqueous solution and kinetics calculations showed that the degradation occurred 2.5 times faster in ethanol compared to water. The degradation of crystal violet in inks from four ballpoint pens on paper was also studied for entries made over 2-3 years. It was observed that degradation reactions were quenched by the presence of another dye due to competitive absorption. It was also observed that the thickness of a stroke (concentration of ink) influenced the degradation process. In the absence of light only one ballpoint pen showed slight degradation. A better understanding of the influence of the paper, ink composition, and storage conditions is necessary to interpret correctly the age of an ink based on the degradation of dyes.
结晶紫是圆珠笔油墨中一种非常常见的染料。最近的研究表明，三芳基甲烷染料的降解情况能指示文件上圆珠笔字迹的书写时间。对降解进行定量评估的主要问题在于，它高度依赖于光照。此外，添加剂和底物等其他因素在这个过程中也起着重要作用。这项工作的目的是通过紫外/可见分光光度法和激光解吸电离，比较纯染料在水和乙醇中暴露于氙灯下的降解途径。在降解产物和动力学方面已观察到显著差异。N-去甲基化是一种预期的分解过程，结果发现它仅在水溶液中发生，动力学计算表明，与水相比，在乙醇中降解速度快2.5倍。还研究了四支圆珠笔在纸上书写2 - 3年以上的油墨中结晶紫的降解情况。观察到由于竞争性吸收，另一种染料的存在会使降解反应受到抑制。还观察到笔划厚度（油墨浓度）会影响降解过程。在无光条件下，只有一支圆珠笔显示出轻微降解。为了基于染料降解情况正确解读油墨的书写时间，有必要更好地了解纸张、油墨成分和储存条件的影响。
