New approaches towards the identification of yellow dyes in ancient textiles

Flavonoid dyes were analysed by negative ion mode electrospray ionisation quadrupole ion trap mass spectrometry (ESI QIT MS). New structure-dependent breakdown pathways were identified and the mechanisms were supported by deuterium labelling experiments. The results were extended to provide important structural information on unidentified flavonoids in natural yellow dye extracts. Samples of alum mordanted wool dyed with pure flavonoids or with natural dyes were exposed to accelerated light ageing. Under these conditions photooxidation was found to be the main degradation process. The photodegradation products were analysed by photodiode array high performance liquid chromatography (PDA HPLC) and by liquid chromatography electrospray ionisation quadrupole ion trap mass spectrometry (LC-ESIMS). In general the photodegradation products of flavonols retain information on the nature of substituents of the B-ring and therefore provide partial structural information related to the original dye molecule. The nature of the light source was found not to influence the nature of the photodegradation products detected. No photodegradation products could be identified from flavone or flavonol 3-O-glycosides dyed textiles under the light ageing conditions used. The acid hydrolysis dye extraction process was found to influence the chemical profile of the dye extracts. The identification of the degradation produces allowed their quantification and hence the kinetics of the photooxidation process of flavonoid dyes on wool fibres could be studied. It was found that experimental data did not fit a consecutive first order reaction model. A rapid initial rate of decay of the flavonol was followed by a slower process. The results suggest that a model where the dye population is divided into two groups with different decay rates fits the experimental data adequately. The rates of decay were found to be influenced by the nature of the light source.