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

3-[<i>N</i>-ethyl-<i>N</i>-(4-sulfophenyl)amino]methylbenzene sulphonic acid (<i>m</i>,<i>p</i>-ESBSA) is an impurity commonly contained in Brilliant Blue FCF (Food Blue No.1: <b>B1</b>) and Fast Green FCF (Food Green No.3: <b>G3</b>). However, in the HPLC chromatogram of <b>B1</b> used in this study, several unidentified peaks were observed near the <i>m</i>,<i>p</i>-ESBSA peak. To identify the chemical structures corresponding to these peaks, the <i>m</i>- and <i>o</i>-isomers of the sulfanilic acid motif of ESBSA were synthesised. The <i>m</i>,<i>m</i>-isomer was also synthesised and structurally characterised, similar to <i>m</i>,<i>p</i>-ESBSA. By contrast, the <i>m</i>,<i>o</i>-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 <b>B1</b> and <b>G3</b>. In the analysis of the two <b>B1</b> products, the peak appearing near that of <i>m</i>,<i>p</i>-ESBSA was confirmed to be <i>m</i>,<i>m</i>-ESBSA, whereas <i>m</i>,<i>o-</i>ESBSA was not detected. No isomers were detected in either of the <b>G3</b>.