Improved DFT-Based Interpretation of ESI-MS of Aqueous Metal Cations

We present results showing that our recently developed density functional theory (DFT)-based speciation model of the aqueous Al³⁺ system has the potential to improve the interpretations of ESI-MS studies of aqueous metal cation hydrolytic speciation. The main advantages of our method are that (1) it allows for the calculation of the relative abundance of a given species which may be directly assigned to the signal intensity in a mass spectrum; (2) in cases where species with identical m?z ratios may coexist, the assignment can be unambiguously assigned based on their theoretical relative abundances. As a demonstration of its application, we study four pairs of monomer and dimer aqueous Al³⁺ species, each with identical m/z ratio. For some of these pairs our method predicts that the dominant species changes from the monomer to the dimer species under varying pH conditions.