title: Cluster ions of diquat and paraquat in electrospray ionization mass spectra and their collision-induced dissociation spectra.
abstract: Cluster ions such as [Cat+X+nM](+) (n = 0-4); [Cat-H+nM](+) (n = 1-3); and [2(Cat-H)+X+nM](+) (n = 0-2), where Cat, X, and M are the dication, anion, and neutral salt (CatX(2)), respectively, are observed in electrospray ionization (ESI) mass spectrometry of relatively concentrated solutions of diquat and paraquat. Collision-induced dissociation (CID) reactions of the clusters were observed by tandem mass spectrometry (MS/MS), including deprotonation to form [Cat-H](+), one-electron reduction of the dication to form Cat(+.), demethylation of the paraquat cation to form [Cat-CH(3)](+), and loss of neutral salt to produce smaller clusters. The difference in acidity and reduction power between diquat and paraquat, evaluated by thermodynamical estimates, can rationalize the different fractional yields of even-electron ([Cat-H](+) and its clusters) and odd-electron (mostly Cat(+)) ions in ESI mass spectra of these pesticides. The [Cat+n. Solv](2+) doubly charged cluster ions, where n <or= 2 and Solv is the solvent molecule (methanol and/or water), are only observed as very weak peaks in precursor ion CID spectra of the Cat(2+) salt cation at low collision energy. The presence of an anion and a solvent molecule in a cluster is assumed to be related to existence of tight and loose ion pairs, respectively, in multiply charged droplets/ions formed by ESI. The results emphasize again the role of solution chemistry concepts such as acidity/basicity, redox power, and ion-pair formation, for ESI.
authors.affiliation: D.I.Mendeleyev Institute for Metrology, 19 Moskovsky pr., 198005 Saint Petersburg, Russia.
authors.forename: Boris L
authors.initials: BL
authors.lastname: Milman
mesh_terms: D004178:Diquat / Q000032:analysis* / Q000737:chemistry*; D007477:Ions / Q000737:chemistry; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010269:Paraquat / Q000032:analysis* / Q000737:chemistry*; D010575:Pesticides / Q000032:analysis / Q000737:chemistry; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D007477:Ions; D010575:Pesticides; D011522:Protons; D004178:Diquat; D010269:Paraquat