Application of Analytical Methods for Analysis of Dyes Employed in Works of Art

Natural organic dyes suffer from degradation during the time. Therefore, the works of art need to be restored and the identification of dyes by analytical methods is important here. The possibilities of using three separation (thin-layer chromatography - TLC, capillary zone electrophoresis - CZE, and micellar electrokinetic chromatography - MEKC) and two spectroscopic (infrared microspectroscopy - IR and Raman microspectroscopy) methods for the identification of red dyes (acaroid, brazil, dragon blood, kamala, logwood, cochineal, madder, lac, and sandalwood) were studied and critically discussed. In TLC, under optimised conditions (silicagel with octadecyl, 80/20 (v/v) MeOH/1% TEA in 0.1 mol·dm-3 acetate buffer, pH 5, and 80/20 (v/v) MeOH/1% TEA in 0.025 mol·dm-3 tetraborate buffer, pH 7 and 9), the stain broadening of sandalwood, logwood, acaroid, dragon blood, and brazil was observed. Carminic acid, haematein and purpurin were not detected. A comparison of CZE and MEKC showed that MEKC was superior as it permits separation of all analytes, using 0.015 mol·dm-3 sodium dodecyl sulfate in 0.01 mol·dm-3 tetraborate buffer, pH 8.5, at a voltage of 20 kV. The developed method was validated and applied to the identification of dyes in Rubia tinctorum, Rubia cordifolia, and in the mycelium of fungi..