“Welcome to the jungle”: TLC-SERS to wade through real complex mixtures of synthetic dyes

The finding of numerous synthetic strategies between the 19th and 20th century fostered the production of an extensive spectrum of organic dyes for artistic and industrial applications. Shortly after, challenges emerged related to the rapid deterioration of these products. Nowadays, the characterization of coloured pieces of art produced in the first half of the XXth century is further complicated by the absence of reference materials with an unambiguous composition, which are frequently a mixture between the starting dyes and their degradations products. Here we demonstrate as Surface Enhanced Raman Spectroscopy coupled with the simplest separation technique, i.e. Thin Layer Chromatography, is able beyond to recognize several synthetic dyes also to identify their isomers and degradation products. The occasion was the study of a catalogue of dyed textile swatches, produced in 1951 by the well-known Italian industry ACNA (Azienda Coloranti Nazionali e Affini). An apparently easy task turned out to be, indeed, not at all obvious as we found out that the actual composition of the reference samples seldomly corresponded to the declared one. TLC-SERS investigation of historical textile swatches was complemented by the integration of liquid chromatography coupled with a diode array detector and high-resolution mass spectrometry (HPLC-DAD-HRMS). This approach allowed us to validate the findings of TLC-SERS and to distinctively characterise synthetic by-products and degradation markers resulting from the deterioration of the dyes.