The development of a generic analysis method for natural and synthetic dyes by ultra-high-pressure liquid chromatography with photo-diode-array detection and triethylamine as an ion-pairing agent.

In cultural heritage the characterization of organic colorants is a challenging task. Currently, different chromatographic techniques are used to analyze natural and synthetic dyes separately, since the classes differ significantly in chemical properties and, therefore, chromatographic behavior. To save time, costs and sample material, we developed a method suitable for a wide variety of organic colorants using ultra-high-performance liquid chromatography coupled to a photo-diode-array detector. Gradient elution was performed with a mobile phase consisting of water and methanol with 5 mM triethylamine added as an ion-pairing agent at a pH of 3. Both linear and step gradients were optimized using the 'Program for Interpretive Optimization of Two-dimensional Resolution' (PIOTR) Pirok et al. [22]. Two optimized linear gradients and two step gradients were evaluated experimentally. The method was applied on a complex dye mixture containing nearly 130 natural- and synthetic-dye reference compounds. More than 100 of these compounds could be identified in a single experiment. The feasibility of the method was demonstrated by analyzing samples of several precious objects that were found in the Texel shipwreck Vos et al. [2] and of two embroideries of Emile Bernard, the results of which are described in this paper.