Am. Striegel et al., Polarizability and inductive effect contributions to solvent-cation binding observed in electrospray ionization mass spectrometry, J AM SOC M, 10(3), 1999, pp. 254-260
Citations number
27
Categorie Soggetti
Spectroscopy /Instrumentation/Analytical Sciences
Journal title
JOURNAL OF THE AMERICAN SOCIETY FOR 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.