Photodegradation of Azo and Hydrazone Colourants on Cellulose-Based Materials: A Mass Spectrometric Screening

Abstract

The preservation of coloured paper objects poses significant challenges to heritage science, as both cellulose-based supports and organic pigments are extremely unstable. In particular, azo dyes and hydrazone pigments remain comparatively under-investigated in degradation studies. In this work, Metanil Yellow (AY36), Orange II (AO7), and Amido Naphthol Red G (AR1) were selected to investigate the behaviour of β-naphthol, substituted β-naphthol, and diphenylamine azo dyes. Standard dyes cast on paper mock-ups were used as simplified model systems to evaluate possible differences in their behaviour upon light exposure through colourimetry, and to determine candidate molecular markers for identifying dyes in historical and severely degraded samples by chromatographic and mass spectrometric analysis. After 233 days of natural light exposure, substantial colour variations were observed (ΔE > 25), together with a 10%–20% decrease in the chromatographic peak areas of the parent dyes. Unreported degradation markers were detected and tentatively characterised, allowing us to hypothesise different photodegradation pathways involving oxidation, formaldehyde addition, azo bond cleavage, and phenylation. Although most of these degradation mechanisms have been reported in solution under strongly oxidative conditions, this work provides the first evidence of their occurrence in paper mock-ups. Some degradation products were also detected in a 20^th century stage costume thread.