Analysis of anthraquinoid and indigoid dyes used in ancient artistic works by thermally assisted hydrolysis and methylation in the presence of tetramethylammonium hydroxide

Thermally assisted hydrolysis and methylation (THM) in the presence of tetramethylammonium hydroxide (TMAH) was applied to the analysis of red dyes based on the 9,10-anthracenedione skeleton (anthraquinone), namely alizarin, quinizarin, purpurin, carminic acid, and the blue pigment indigo. Alizarin and purpurin are the principal coloured compounds of madder, and carninic acid of cochineal red. Synthetic dyes were dissolved in aqueous TMAH, pyrolysed at 700 degrees C for 10 s and the evolved thermochemolytic products were analysed by gas chromatography/mass spectrometry (GC/MS). The resulting chromatograms of all the madder dyes were characterised by the presence of the methyl esters of 1,2-benzenedicarboxylic acid (dimethyl-o-phthalate) and benzoic acid. In addition, thermochemolysis of alizarin and quinizarin afforded 1,2-dimethoxyanthraquinone and 1,4-dimethoxyanthraquinone, respectively. Carminic acid produced a series of methylated fragments deriving from both the substituted anthraquinoid moiety and the glycosidically-Clinked glucose. Finally, the principal thermochemolysis products of indigo were the N-methylated derivatives of 2-amino benzoic acid methyl ester followed by methylated aniline and indoles. The potential of THM as a rapid technique for the identification of dyes in real samples was illustrated by the analysis of madder lake, cochineal lake, and natural indigo dispersed in a binder. (C) 2000 Elsevier Science B.V. All rights reserved.