Optimization of sample treatment for the identification of anthraquinone dyes by surface-enhanced Raman spectroscopy
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The study and characterization of old artifacts such as pigments requires the use of techniques that need a small amount of sample to perform the analysis because of the high value of these samples. In recent years, organic molecules such as anthraquinone dyes have been identified by surface-enhanced Raman spectroscopy (SERS). However, different sample treatments must be applied to isolate the organic dye from the mordant, which produces great fluorescence in the Raman measurements. In this work, optimization of sample treatment for the SERS analysis of anthraquinone dyes was performed. Sample mass, the organic solvent, and its volume were optimized and different slide materials and excitation lasers were compared to choose the best conditions for the identification of the dyes. The optimization of sample treatment resulted in 5 mg of sample as the optimum amount. Further, two consecutive extractions with 0.5 mL of ethyl acetate was the best option for the extraction of the dye. A quartz slide was used instead of a glass slide to reduce background signal, and an excitation laser of 532 nm offers better results than one of 785 nm. The optimized method was applied to the characterization of the dyes used in Pompeian pink and purple lake pigments. Alizarin and purpurin anthraquinone dyes, obtained from madder lake, were identified as the organic colorants. The SERS results were confirmed by those obtained by means of UV–visible spectroscopy.
KeywordsSurface-enhanced Raman spectroscopy Liquid–liquid extraction Lake pigment Anthraquinone Madder lake
The authors thank the Naples National Archaeological Museum for the permissions given to extract samples from the bowls containing the raw lake pigments considered in this work. This work was funded by the Ministry of Economy and Competitiveness (MINECO) and the European Regional Development Fund (FEDER) through the project DISILICA-1930 (reference BIA2014-59124-P). I.M. is grateful to the Basque Government, which funded his predoctoral fellowship. M.C.-V. acknowledges project 1140524 from Fondecyt Chile.
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Conflict of interest
The authors declare that they have no conflict of interest.
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