The Use of Diethylamine to Determine the Number of Sulphonate Groups Present Within Polysulphonated Alkali Metal Salts by Electrospray Mass Spectrometry

By using negative-ion diethylamine (DEA)-enhanced electrospray mass spectrometry under mild dissociative conditions, (a low voltage applied across the sampling cone aperture) a number of DEA molecules were observed to associate with the double charged acid ion of polysulphonated salts to form a series of higher-mass peaks. As the cone voltage was increased, these peaks degenerated into a series of lower-mass peaks. Each of the peaks contained within these series were separated by the mass of one DEA molecule. By further increasing the cone voltage, it was possible to dissociate all the DEA adducts from the acid ion. The maximum number of DEA molecules associated with the doubly charged acid ion could be correlated with the number of sulphonate sites present. This technique correctly identified the number of sulphonate groups contained within the series of compounds tested. A mechanism for these processes, based on the ability of protonated DEA to form ion pairs with sulphonate sites, is advanced.