Investigating the in-solution photodegradation pathway of Diamond Green G by chromatography and mass spectrometry

Diamond Green G (CI 42040, Basic Green 1) is a triarylmethane dye mostly employed in the industrial dyeing of miscellaneous natural and artificial textile fibres, in the production of printing inks and as an anti-fungal agent for aquaculture products. Diamond Green (DG) tends to fade under light irradiation, similar to several other dyes and pigments belonging to the triarylmethane family. In this paper, an in-solution degradation study was carried out to shed light on DG photostability. The experiments were performed by artificially irradiating DG in water and dimethyl sulphoxide, and analysing aliquots sampled at different time intervals by ultra-high performance liquid chromatography coupled with diode array detector quadrupole and time of flight tandem mass spectrometry. The degradation products formed were characterised and the product-ion spectra discussed and interpreted. On the basis of the structure of the identified compounds and their kinetic trend in relation to the accelerated ageing time, degradation mechanisms were proposed for DG, taking into account the role played by the solvent. Finally, the dyeing formulation of an historical textile sample, collected from a stage costume worn in a Zeffirelli lyric opera, was determined utilising the information collected from the analysis of irradiated DG solutions.