Since the identification of organic colorants in works of art (such as dyes on textiles or organic pigments) by Raman spectroscopy is generally limited by the presence of a strong fluorescence background, the current study evaluated and compared the effectiveness of minimizing fluorescence in the analysis of Cape Jasmine (Gardenia augusta L.) by dispersive Raman spectroscopy at three different excitation wavelengths (633, 785 and 1064 nm) and by surface-enhanced Raman spectroscopy (SERS) with and without acid hydrolysis.
The study showed that these vibrational techniques offer an alternative analytical approach, when, as is particularly the case of Cape Jasmine, sample preparation procedures that are routinely applied for natural organic dyes and pigments cause alterations that lead to low sensitivity in the more classical high-performance liquid chromatography-photodiode array (HPLC-PDA) analytical protocols. Samples of the yellow dye G. augusta L. in the following forms were analyzed: dyed on alum mordanted wool, dyed on nonmordanted and alum mordanted silk, pigment precipitated on hydrated aluminum oxide, extract mixed with a protein binder and painted on glass, and as a water-based glaze applied on a mock-up of a typical Chinese wall-painting. Raman bands at 1537, 1209 and 1165 cm−1 are identified as discriminating markers for the carotenoid colorant components crocetin and crocin. (publisher abstract modified)