The photodegradation of indigo has been studied with emphasis on the degradation products. Ultraviolet–visible spectrometry, high performance liquid chromatography and direct temperature‐resolved mass spectrometry were employed for the characterisation of aged samples. Indigo, isatin, isatoic anhydride, anthranilic acid and indirubin were detected, together with other unidentified compounds. A new degradation product, tryptanthrin, was identified by both high performance liquid chromatography and mass spectrometry techniques, and two other possible degradation products have also been suggested. The degradation of synthetic and natural indigo has been compared and an explanation for the higher rate of degradation of natural indigo proposed.

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