Polarizability and inductive effect contributions to solvent-cation binding observed in electrospray ionization mass spectrometry

Electrospray ionization (ESI) mass spectrometry (MS) has been used in conju nction with computer modeling to investigate binding tendencies of alkali m etal cations to low molecular weight solvents. Intensities of peaks in FSI mass spectra corresponding to solvent-bound alkali metal cations were found to decrease with increasing ionic radii (Li+ > Na+ > K+ > Cs+) in either d imethylacetamide (DMAc) or dimethylformamide (DMF), When a lithium or sodiu m salt was added to an equimolar mixture of DMF, DMBc, and dimethylpropiona mide (DMP), the intensities of gas-phase [solvent + alkali cation](+) peaks observed in ESI mass spectra decreased in the order DMP > DMAc much greate r than DMF, A parallel ranking was obtained for alkali metal cation affinit ies in ESI-MS/MS experiments employing the kinetic method. These trends hav e been attributed to a combination of at least three factors. An inductive effect exhibited by the alkyl group adjacent to the carbonyl function on ea ch solvent contributes through-bond electron donation to stabilize the alka li metal cation attached to the carbonyl oxygen. The shift in the partial n egative charge at the oxygen binding site with increasing rt-alkyl chain le ngth (evaluated via computer modeling), however, cannot fully account for t he mass spectrometric data. The increasing polarizability and the augmented ability to dissipate thermal energy with increasing size bf the solvent mo lecule are postulated to act in conjunction with the inductive effect. Furt her evidence of these contributions to solvent-cation binding in ESI-MS is given by the relative intensities of [solvent + Li](+) peaks in mixtures co ntaining equimolar quantities of alcohols, indicating preferential solvatio n of Li+ in the order n-propanol > ethanol > methanol. These experiments su ggest a combined role of polarizability, the inductive effect, and solvent molecule size in determining relative intensities of solvated cation peaks in ESI mass spectra of equimolar mixtures of homologous solvents. (C) 1999 American Society for Mass Spectrometry.