Thermospray, particle beam and electrospray liquid chromatography-mass spectrometry of azo dyes

Abstract

A high-performance liquid chromatographic-mass spectrometric (HPLC-MS) method was used for the analysis and characterization of fourteen commercial azo and diazo dyes. Thermospray analysis of these dyes using the “filament-on” chemical ionization operation mode produced mass spectra consisting primarily of [M + H] + ions with very few fragments. Particle beam electron impact ionization mass spectrometric analysis resulted in molecular ion information and various fragment ions. Characterization of the azo dyes could be achieved by observing typical fragment ions formed by cleavage of the CN bonds on either side of the azo linkage, and cleavage of the NN double bond with transfer of one or two hydrogen atoms to form an imine or amine. Many commercial azo dyes contain precursors, by-products of the synthesis, degradation products or other impurities. Some of these components were tentatively identified by their mass spectra and by review of the chemistry of dye manufacture. Electrospray negative ion operation was suitable for the characterization of a disulfonated azo dye and showed singly and doubly molecular ions and fragment ions at higher capillary-skimmer potential differences. The base peak was attributed to a doubly charged naphthol cleavage product.