Analysis of sulphonated dyes and intermediates by electrospray mass spectrometry

Molecular weights of dyes and intermediates containing one to four sulphonic acid groups were determined by negative-ion electrospray mass spectrometry. Mass spectra of monosulphonated dyes exhibit an important ion that arises from the loss of a cation from a sulphonic group. Sulphonated dyes form molecular ions with different charges either by subsequent loss of protons, i.e. [M¯xH]x-, or from adducts with sodium ions, i.e. [M¯(x+y) H+yNa]x-. The maximum values of x or (x+y) are equal to the total number of sulphonic and carboxylic acid groups, where x is higher than y. This method is applicable to various types of poly-sulphonated dyes, including anthraquinone, azo or metal complex dyes. The HPLC/MS technique can be used for the analysis of mixtures of dyes and intermediates also.