DETERMINATION OF DISSOLVED METAL SPECIES BY ELECTROSPRAY-IONIZATION MASS-SPECTROMETRY

The distribution of metal species in solution was determined using now injection electrospray ionization mass spectrometry. Complexes formed by selected metal ions with added organic ligands in 50:50 water/acet onitrile and 50:50 water/methanol under acidic, neutral, and basic con ditions were detected using electrospray ionization conditions optimiz ed to best represent solution-phase interactions. Metal species contai ning acetate, nitrate, and solvent molecules predominated in acidic so lution but became less abundant at higher pH. Interactions between met al ions and added organic ligands became more selective with increasin g pH, showing the expected preference of hard and soft ligands for met al ions:of the corresponding type. Species distributions also tended t oward larger complexes as pH increased. Overall ion yield was greater for aqueous acetonitrile than for aqueous methanol solutions; however, reduction of copper(II) in aqueous acetonitrile resulted in the detec tion of copper(I) complexes for certain ligands. Experimental results for copper(II) and 8-hydroxyquinoline in 50:50 water/methanol showed g ood agreement with aqueous speciation predicted using the thermodynami c equilibrium model MINEQL. Detection of neutral complexes was achieve d by protonation, deprotonation, or electrochemical oxidation during e lectrospray.