Determination of Phthalic Acid, 2-(3',5'-Dibromo-2',4'-Dihydroxybenzoyl) Benzoic Acid, and Tribromoresorcinol in the Color Additives D&C Red No. 21, D&C Red No. 22 (Eosin Y), and Their Lakes Using UHPLC.

Background: Certain impurities in the color additives drug and cosmetic (D&C) Red No. 21 (R21), D&C Red No. 22 (R22), and their lakes are limited to levels specified in the Code of Federal Regulations (CFR) and are quantified by the U.S. Food and Drug Administration in batches of these color additives submitted for certification. Currently, a lengthy and tedious method based on gravity flow elution column chromatography is used to quantify the following CFR-specified impurities: the intermediate, phthalic acid (PhthAc); the manufacturing by-products, 2-(3',5'-dibromo-2',4'-dihydroxybenzoyl)benzoic acid (Br2BBA); and brominated resorcinol. "Brominated resorcinol" implies the sum of all possible brominated resorcinols, but the current work focused on 2,4,6-tribromoresorcinol (Br3R) as the most probable side-reaction product. Objective: An improved method was needed to quantify PhthAc, Br2BBA, and Br3R in R21, R22, and their lakes. Methods: A rapid ultra-HPLC (UHPLC) method was developed to replace the gravity flow method for quantitative determination of PhthAc, Br2BBA, and Br3R. Results: PhthAc, Br2BBA, and Br3R were quantified by using five-point calibration curves with data point ranges of 0.11-1.55, 0.06-0.77, and 0.04-0.61% by weight, respectively. LODs for the analytes ranged from 0.01 to 0.03%. Recoveries of the analytes ranged from 90.6 to 99.9%. Conclusions: The UHPLC method is accurate and significantly more rapid than the gravity -flow method, requiring approximately 7 min as compared with 6 h to detect PhthAc, Br2BBA, and Br3R in one sample. Highlights: A rapid UHPLC method was developed to determine CFR-specified impurities in the color additives D&C Red Nos. 21 and 22 and their lakes.