Structural determination of four manufacturing impurities of D&C Red No. 33

Four manufacturing impurities of D&C Red No. 33 isolated by counter‐current chromatography were analyzed by NMR and ESI mass spectrometry. Three of these impurities were reported previously with minimal details of structural determination. All four are structurally related to the main component of the dye. The fourth exhibited an unusual discrepancy between the NMR structure and its chemical formula suggested by ESI‐MS results. Structural determination and assignment of the main component and four impurities are discussed as well as resolution of the discrepancy between the NMR and ESI‐MS results of the fourth impurity. Manufacturing impurities of D&C Red No. 33 isolated by counter current chromatography are analyzed by NMR and ESI‐MS. Structural determination, assignment, and resolution of data discrepancy between the NMR and ESI‐MS result are discussed.