Electrospray ionization mass spectrometry investigation of the blocking effect of sulfate on the formation of aluminum tridecamer

Aluminum containing aqueous solutions (AlCl3 and Al2(SO4)3) with several pH values were investigated by electrospray ionization mass spectrometry (ESI-MS) in the positive ion mode. Several aluminum complexes containing sulfate ions were observed, whereas aluminum complexes with chloride ions were absent. The aluminum sulfate complexes formed were considered to be outer-spherical. In AlCl3 solution, aluminum tridecamer (Al13), whose existence has been confirmed by several methods, was observed by ESI-MS as divalent and trivalent ions. However, in Al2(SO4)3 solution, no peaks corresponding to Al13 were observed. This indicates that the presence of sulfate ions prevented the formation of Al13. The hydrolysis equilibria of aluminum ions were perturbed by sulfate ions, and different polymerization/precipitation pathways of aluminum in the presence of sulfate ions were suggested. Since each aluminum species in the solution can be separately identified by ESI-MS from their m/z (mass to charge ratio), it is expected that this method will provide new information on the solution chemistry of aluminum ions.