The composition of Erythrosins, Fluorescein, Phloxine and Rose Bengal: a study using thin-layer chromatography and solvent extraction

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Abstract
Commercial samples of Erythrosin B (CI 45430), Erythrosin Y (CI 45425), Fluorescein (CI 45350), Phloxine (CI 45410) and Rose Bengal (CI 45440) have been analysed by thin-layer chromatography. The Erythrosins were found to be mixtures consisting in the main of 4′-iodofluorescein, 4′,5′-di-iodofluorescein, 2′,4′,5′-triiodofluorescein and 2′,4′,5′,7′-tetraiodofluorescein, in some instances together with 2′,4′,5′-tri-iodo-4,5,6,7-tetrachlorofluorescein and 2′,4′,5′,7′-tetraiodo-4,5,6,7-tetrachlorofluorescein. Samples of Fluorescein were mixtures of the nominal dye usually with traces of several unidentified, fluorescent components. Those of Phloxine consisted mainly of mixtures of 4′-bromo-4,5,6,7-tetrachlorofluorescein, 4′,5′-dibromo-4,5,6,7-tetrachlorofluorescein, 2′,4′,5′-tribromo-4,5,6,7-tetrachlorofluorescein and 2′,4′,5′,7′-tetrabromo-4,5,6,7-tetrachlorofluorescein, often with 4,5,6,7-tetrachlorofluorescein Samples of Rose Bengal were mixtures of 4′-iodo-4,5,6,7-tetrachlorofluorescein, 4′,5′-di-iodo-4,5,6,7-tetrachlorofluorescein, 2′,4′,5′-tri-iodo-4,5,6,7-tetrachlorofluorescein and 2′,4′,5′,7′-tetraiodo-4,5,6,7-tetrachlorofluorescein together with some unidentified components. Most of the commercial dye samples gave an insoluble residue when extracted with methanol. This residue was usually inorganic carbonate or halide. Some possible practical consequences of the various impurities are discussed.