Characterization of madder and garancine in historic French red materials by liquid chromatography-photodiode array detection

A reverse phase liquid chromatography-photodiode array detection method is developed for the characterization of ancient samples of madder. In the past, modifications of the madder chemical composition were carried out by hydrolysing the plant dye precursors in order to increase its red power, the aim of this transformation being to concentrate the red colour of the matter. After extraction of madder dyes in a water-methanol mixture, different anthraquinonic compounds were identified through the chemical constituents present in a garancine sample and in the two madder species growing in the Mediterranean area: Rubia tinctorum and R. peregrina. These two species comprise alizarin, purpurin, lucidin, rubiadin and pseudopurpurin for aglycones and, lucidin primeveroside, ruberythric acid, galiosin and rubiadin primeveroside for heterosidic precursors. These compounds were identified through retention times and UV spectra in comparison with pure standards. Six ancient materials belonging to the collection of the Roure Museum in Avignon (France) and dating from the nineteenth century are characterized as hydrolysed madder (garancine), synthetic alizarin and R. tinctorum roots. A colorimetric study is performed to compare the colour of each sample according to its chemical composition.