Application of HPLC–DAD–QTOF to the Analysis of Natural and Synthetic Organic Pigments in Paint Layers

Abstract

In the field of diagnostics for cultural heritage the identification of natural lakes in paint micro samples is a real analytical challenge. The combination of efficient chromatographic separation procedures coupled with the use of a high resolution detector, allows obtaining the selectivity and sensitivity required for such research task, consuming a micro—scaled sample. In this article we describe the methodology and the results of the analytical characterization of a large set of natural organic lakes both synthesized in laboratory according to ancient recipes and obtained from real case studies. The study, conducted by High Performance Liquid Chromatography (HPLC) coupled to UV–VIS diode array detector (DAD) and Tandem Mass Spectrometry detectors as quadrupole—time of flight (QTOF), had the purpose to obtain first a fingerprint of the lakes by identification and quantification of all the organic coloured substances present in the extracts. The application of a micro sample pre-treatment procedure which combines a soft decomplexation method with an effective extraction of the organic component has allowed identifying both the aglycones (colouring compounds) and the corresponding glycosides characterizing the different dying species to obtain a more complex and reliable profile to identify the dye source. The procedure can also be applied to modern synthetic dyestuffs, present in many contemporary art manifestations, its behaviour and preservation condition.