Pigment Red 57 (Colour Index No. 15850, PR57) and Pigment Red 57:1 (Colour Index No. 15850:1, PR57:1) are certifiable in the USA as the color additives D&C Red No. 6 (R6) and D&C Red No. 7 (R7) for use in drugs and cosmetics. In the EU, PR57:1 is permitted in cosmetics and also as a food additive (E180) for colouring edible cheese rinds. The USFDA batch-certifies R6, R7, and their lakes in accordance with limiting specifications for impurities stated in the Code of Federal Regulations (CFR). In the current work, an impurity not specified in the CFR was studied because of its consistent presence in samples of R6 and R7 submitted for certification. Using spectroscopic methods, the impurity was tentatively identified as 4-[(4-methyl-2-sulfophenyl)azo]-3-naphthalenol (DPR57), the decarboxylated analogue of PR57 and PR57:1. Its identity was confirmed by synthesising DPR57 and determining that the UHPLC retention time, UV/visible spectrum and mass spectrum of the synthetic material were identical to those of the impurity. Using the synthesised DPR57 as a reference material, the impurity was quantified in 43 batches of R6, R7, and lakes produced by eight different manufacturers. Calibration curves ranging from 0.02% to 1.00% (w/w) were prepared by plotting the UHPLC area of DPR57 at 485 nm against its concentration. DPR57 levels ranged from < 0.02% to 0.50%. To facilitate dissolution of the color additive samples for DPR57 analysis, a relatively simple procedure was developed by adapting a previously published method that involves use of a basic solution of the chelating agent EDTA and the organic solvent N,N'-dimethylformamide. A source for DPR57 contamination of the color additives is also proposed.
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