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 sodiated 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.

Aqueous solutions of four sulphonated azodyes, Reactive Orange 91, Reactive Red 184, Reactive Blue 182 and Reactive Black 5, containing 0.2 mol/L of electrolyte were treated in a laboratory scale electrolytic cell with a new electrochemical method intended for the textile wastewater treatment. The electrochemical process was monitored by means of absorbance of UV-Vis light and chemical oxygen demand values. Carbon fleece was used as a cathode, and platinated titanium was used as an anode. Reversed-phase high-performance liquid chromatography coupled with photodiode array and mass spectrometric detectors was applied for the analysis of dyes, their impurities and degradation products after the electrochemical treatment. Dihexylammonium and triethylammonium acetate as ion-pairing reagents, or alternatively ammonium acetate as an ion-exclusion suppressing reagent, were used to improve the separation.