Simultaneous detection of a wide range of synthetic and natural dyes in artworks using UHPLC-PDA-HRMS

This paper presents a validated method using ultra-high pressure liquid chromatography with photodiode array detection coupled to high-resolution mass spectrometry (UHPLC-PDA-HRMS) for the simultaneous analysis of a wide range of natural and synthetic organic colourants, including neutral, acidic and basic dyes. In total, 30 natural and 62 synthetic organic dye reference samples (which contain 118 compounds because some of the dyes are composed of mixtures) were analysed. The method demonstrated good linearity for the 12 dyes selected for method validation achieving correlation coefficients (R²) exceeding 0.99 for both PDA and MS detectors. Detection limits (LOD) varied from 0.23 ppm (Crystal violet) to 4.12 ppm (Amido naphthol red G) based on the UHPLC-PDA signal, and from 0.024 ppm (Diamond green G) to 0.65 ppm (Cochineal red A) based on the UHPLC-MS signal. Precision, measured by intra-day relative standard deviation (RSD) values was consistently under 5 % for UHPLC-PDA and under 9 % for UHPLC-MS, while inter-day RSD values were below 13 % for UHPLC-PDA and below 8 % for UHPLC-MS. The method's applicability was further tested on two historical artworks: a tapestry by Lanckaert (1587-1589) from the collection of the Museum De Lakenhal, Leiden, The Netherlands, and a 19th-century hip cloth from Aceh, Indonesia. Results from the case studies underscore the method's effectiveness in identifying complex mixtures of both synthetic and natural organic colourants in a single run using minute sample sizes. Beyond offering critical insights into historical dyeing procedures that support conservation work, the method also enhances the understanding of technological advancements in textile production.