Online coupling of high-performance liquid chromatography with surface-enhanced Raman spectroscopy for the identification of historical dyes

High-performance liquid chromatography (HPLC) is still, today, the technique of choice for the analysis of natural dyes in artistic objects and historical textiles, particularly in association with photodiode array (PDA) detection. In the last two decades, surfaceenhanced Raman spectroscopy (SERS) gained also increasing importance for these investigations thanks to its sensitivity and limited requirements in terms of sample quantity. In favor of SERS, its high specificity inmolecular recognition typical of vibrational spectroscopy should be mentioned, whereas this non-separative technique is obviously disadvantaged in the analyses of mixed chromophores, as is often the case of many natural dyes and also of tints obtained by the combined use of different colorants. An optimized experimental setup combining the two techniques, HPLC-PDA and SERS, is proposed in the present work, allowing online SERS detection of different dyeing compounds eluted from the HPLC column. Examples are presented concerning some of the colorants most widely used in history, such as morin and luteolin for yellow dyes, alizarin, purpurin, laccaic, kermesic, and carminic acids for red ones, and indigotin for blue tints.