Ion-pairing high-performance liquid chromatography-mass spectrometry of impurities and reduction products of sulphonated azodyes

An HPLC separation method with triethylammonium acetate mobile phase additive developed for the analysis of impurities in polysulphonated azo dyes provides good separation selectivity and compatibility with electrospray ionisation (ESI) mass spectrometry. The negative-ion ESI mass spectra containing only peaks of deprotonated molecules [M-H](-) for monosulphonic acids, [M-xH](x-), and socliated adducts [M(x+y)H+yNa](x-) for polysulphonic acids allow easy molecular mass determination of unknown impurities. Based on the knowledge of the molecular masses and of the fragment ions in the MS/MS spectra, probable structures of trace impurities in commercial dye samples are proposed. To assist in the interpretation of the mass spectra of complex polysulphonated azodyes, additional information can be obtained after chemical reduction of azodyes to aromatic amines. The structures of the non-sulphonated reduction products can be determined by reversed-phase HPLC/MS with positive-ion atmospheric pressure chemical ionisation and of the sulphonated products by ion-pairing HPLC/MS with negative-ion ESI.