Isomer synthesis focusing on sulfanilic acid moiety of ESBSAs: possible impurities in Brilliant Blue FCF and Fast Green FCF

3-[N-ethyl-N-(4-sulfophenyl)amino]methylbenzene sulphonic acid (m,p-ESBSA) is an impurity commonly contained in Brilliant Blue FCF (Food Blue No.1: B1) and Fast Green FCF (Food Green No.3: G3). However, in the HPLC chromatogram of B1 used in this study, several unidentified peaks were observed near the m,p-ESBSA peak. To identify the chemical structures corresponding to these peaks, the m- and o-isomers of the sulfanilic acid motif of ESBSA were synthesised. The m,m-isomer was also synthesised and structurally characterised, similar to m,p-ESBSA. By contrast, the m,o-isomer obtained under similar conditions yielded indistinct 1H-NMR signals. Cation-exchange was found to resolve this issue, enabling accurate structural analysis. The retention times and photodiode array spectra of the two isomers were clearly identified under analytical conditions for B1 and G3. In the analysis of the two B1 products, the peak appearing near that of m,p-ESBSA was confirmed to be m,m-ESBSA, whereas m,o-ESBSA was not detected. No isomers were detected in either of the G3.