Identification of a novel compound, 1-carboxy-7-bromo-6-hydroxy-2,3,4,5-tetrachloroxanthone, as an impurity in the colour additives D&C red nos. 27 and 28 (phloxine B)

The focus of this work was the characterisation of an impurity sometimes found in batches of the hydroxyxanthene colour additives D&C Red No. 27 (R27) and D&C Red No. 28 (R28, phloxine B). Countercurrent chromatography (CCC) was used to separate and concentrate the impurity (named here pHXCA) to ∼48% from a dye sample that contained ∼0.74% pHXCA. Pure pHXCA was obtained by collecting analytical HPLC-separated fractions from multiple injections of the CCC-concentrated sample. Various NMR and mass spectral techniques were used to determine the structure of pHXCA. In particular, the presence of a 35/37Cl isotope shift in the 13C NMR spectrum pointed to the location of the C-Cl bond specific to pHXCA and facilitated identification of its structure as 1-carboxy-7-bromo-6-hydroxy-2,3,4,5-tetrachloroxanthone. Significantly, that structure was different from the one erroneously reported by other researchers. A rapid screening of 18 dye batches (R27, n = 13; R28, n = 5) representing nine manufacturers, revealed that 12 batches contained pHXCA, with levels that ranged from 0.0009 to 0.182%. A side reaction during the manufacture of R27/28 in which a bromine atom is replaced with a chlorine atom in a closely related contaminant is suggested as the potential basis of pHXCA formation.