Comparing different light-degradation approaches for the degradation of crystal violet and eosin Y

Organic colourants have important applications in many fields. Their photostability is an important characteristic. Several methods to study photodegradation were compared in this work. Eosin Y (C.I. Generic name: Acid Red 87, EY) and crystal violet (C.I. Generic Name: Basic Violet 3, CV) were used as test compounds, both in solution and dyed on silk. Commonly applied methods were included, viz. Xenotest, Microfading-Tester, and light-box (Spectrolinker) experiments. A novel method was based on a liquid-core-waveguide (LCW) cell. After photodegradation on textile, extraction was performed using dimethyl sulfoxide (DMSO). The degraded solutions and extracts were analysed with liquid chromatography combined with diode-array detection and mass spectrometry. The degradation products were compared between techniques. Degradation in the LCW cell progressed much faster than in standard tests (Xenotest and Spectrolinker) and could be performed online, without a need for extraction or sample transfer. The degradation of CV in the LCW was comparable to its degradation in standard tests. For EY, there was a clear difference in degradation mechanisms between in-solution and on-textile samples. This could be due to the matrix or to incomplete extraction. Because the light sources used in the different experiments differed in energy and spectral emission, the results could not be quantitatively compared. However, the degradation products formed were shown to be independent of the light source. Therefore, the LCW is an attractive method for rapid and efficient studies into the chemistry of photodegradation.