Synthesis and Quantitation of m,p-ESBSA: An Impurity in Brilliant Blue FCF and Fast Green FCF

3-[N-Ethyl-N-(4-sulfophenyl)amino]methylbenzenesulfonic acid (m,p-ESBSA) is a common impurity in Brilliant Blue FCF (Food Blue No.1: B1) and Fast Green FCF (Food Green No.3: G3). For the quality control of B1 and G3, quantitative analysis of m,p-ESBSA is required. However, no analytical-grade m,p-ESBSA suitable as a quantitative standard is readily available. In this study, we established an efficient synthesis that provides m,p-ESBSA with a quality that can be used for quantitative analysis. The synthetic route to m,p-ESBSA involves inexpensive starting materials, and the product was structurally characterized by detailed NMR analysis. In addition, the suitability of the synthesized samples for quantitative analysis was confirmed by ¹H-qNMR analysis and quantitation of food dye products. The results showed that the purity of the synthesized m,p-ESBSA was adequate for use as an analytical standard in quantitative analyses.