Raman and surface-enhanced Raman spectroscopy for the analysis of Mexican yellow dyestuff

In pre-Hispanic Mesoamerica, pigments and dyes were used in the elaboration of a large variety of colored objects. Obtaining information regarding the objects' material composition is useful in restoration and preservation processes, as well as for recovering knowledge of its production technology and the context and history of the object. Yellow colors have been obtained from a large variety of Mexican natural resources. Although mineral yellow pigments, such as orpiment and oxides, are relatively easy to identify by nondestructive and noninvasive techniques (X-ray fluorescence, Raman spectroscopy, fiber optics reflectance spectroscopy, and Fourier transform infrared spectroscopy), organic yellow colorants are difficult to analyze by these techniques. Therefore, most works dealing with the identification of yellow dyes use destructive methods such as liquid chromatography-mass spectrometry. The aim of this work is to elaborate a methodology based on Raman spectroscopy and surface-enhanced Raman spectroscopy for the study of yellow colorants used in pre-Hispanic and colonial Mexico, in order to aid in their identification by spectroscopic techniques. The yellow colorants selected for this work were extracted from local plants: zacatlaxcalli (Cuscuta tinctoria), old fustic (Maclura tinctoria), weld (Reseda luteola), marigold (Tagetes erecta), and xochipalli (Cosmos sulphureus).