Application of LC‐MS and LC‐MS‐MS to the analysis of photo‐decomposed crystal violet in the investigation of cultural heritage materials aging

Abstract

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.

Tracking photo‐degradation of triarylmethane dyes with surface‐enhanced Raman spectroscopy

“…In addition, it saw a widespread diffusion as an artist material starting from the end of the 19th century and was used among the others by Vincent Van Gogh and Paul Gauguin in their writings, drawings, and paintings. [5,6] Tryarylmethane dyes were probably introduced in Asia soon after their invention, as suggested by their ubiquitous use in the woodblock prints of the Meiji periods shown in this article.…”

“…Despite their wide range of applications, triarylmethane dyes are extremely sensitive to light exposure. A series of studies have dealt with the identification of the photo‐degradation processes and the chemical modifications caused by light exposure, under different conditions . It has been noticed that the degradation of CV and MV strongly depends on light fluence …”

“…Up to now, measurement of the photo‐fading level has been conducted through ultraviolet–visible spectrophotometry, while the identification of photo‐degradation products has been performed mainly through separation techniques coupled with different detection methods, in particular liquid chromatography‐mass spectrometry (LC‐MS) and high performance liquid chromatography‐diode array detection …”

“…In forensic science, the individuation of the degradation state of a ballpoint ink entry can help in defining a questioned document . In conservation science, the understanding of the photo‐fading processes helps in the identification of the original materials as well in the definition of proper preservation protocols …”

“…A series of studies have dealt with the identification of the photo-degradation processes and the chemical modifications caused by light exposure, under different conditions. [5][6][7][8][9][10][11][12][13] It has been noticed that the degradation of CV and MV strongly depends on light fluence. [10] Various degradation pathways have been reported for CV and MV [11] :…”