Electrospray ionization of alkali and alkaline earth metal species. Electrochemical oxidation and pH effects

The utility of electrospray ionization mass spectrometry (ESI-MS) for chara cterizing dissolved metal species has generated considerable interest in th e use of this technique for metal speciation, However, the development of a ccurate speciation methods based on ESI-R;IS requires a detailed understand ing of the mechanisms by which dissolved metal species are ionized during e lectrospray. We report how the analysis of alkali and alkaline earth metal species pro,ides new information about some of the processes that affect el ectrospray ion yield. Selected metal ions and organic ligands were combined in 50:50 water-acetonitrile buffered with acetic acid or ammonium acetate and analyzed by flow injection ESI-MS using mild electrospray conditions. S pecies formed by alkali metal ions with thiol and oxygen-donating ligands w ere detected in acidic and neutral pH solutions. Electrochemical oxidation of N,N-diethyldithiocarbamate and glutathione during electrospray was indic ated by detection of the corresponding disulfides as protonated or alkali m etal species, The extent of ligand oxidation depended on solution pH and th e dissociation constant of the thiol group, Tandem mass spectrometric exper iments suggested that radical cations such as [NaL](+.) (where L = N,N -die thyldithiocarbamate) can be generated by in-source fragmentation of disulfi de species. Greater complexation of alkali metals at neutral pH tvas indica ted by a corresponding decrease in the relative abundance of the free metal ion, The number of alkali metal ions bound by glutathione and phthalic aci d also increased with increasing pH, in accordance with thermodynamic equil ibrium theory. Alkaline earth metal species were detected only in acidic so lutions, the absence of 8-hydroxyquinoline complexes being attributed to th eir relative instability and subsequent dissociation during electrospray, H ence, accurate speciation by ESI-MS depends on experimental conditions and the intrinsic properties of each analyte, Copyright (C) 2000 John Wiley &; Sons, Ltd.