Identification of unlawful food dyes by thin-layer chromatography-fast atom bombardment mass spectrometry

A thin-layer chromatographic-fast atom bombardment mass spectrometric (TLC-FAB-MS) method incorporating an analyte condensation technique was established for the identification of the 27 food dyes consisting the twelve dyes permitted for use in foods and the fifteen unlawful dyes in Japan. The use of magic bullet [1,4-dithiothreitol-1,4-dithioerythritol (3:1)] as a matrix allowed the measurement of the FAB mass spectra of the food dyes except for Food Blue No. 2 (Indigo Carmine). The separation was performed on a C18-modified silica gel TLC plate using the following two solvent systems: methanol-acetonitrile-5% aqueous sodium sulphate solution (3:3:10) and methyl ethyl ketone-methanol-5% aqueous sodium sulphate solution (1:1:1). The condensation technique for concentration of a diffuse sample spot on the TLC plate improved the detection limit 4-20-fold with good reproducibility. The method was successfully applied to the identification of unlawful dyes in imported foods.