Electron spin resonance studies under dynamic mobile phase conditions on chemically modified silica.
The electron-spin resonance (ESR) techniques of spin labeling and probing are shown to be powerful methods for the study of bonded silicas used in chromatographic separations under dynamic solvent flow conditions. Microscopic information gained from these studies includes solvation composition of the bonded phase at various locales in the bonded phase, configuration of the bonded phase and solvent-flow effects. In the spin-label study, 2-(2,2-dimethyloxazolidine-N-oxy)hexylsilane (DOXYL hexyl siloxane) was used as the label. ESR spectra show changes in the motional freedom of the bonded surface as a function of mobile phase composition and elution time and demonstrate chain stiffening with the introduction of water, as inferred by other studies. In the spin-probe work, various nitroxide compounds of the 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) family were introduced under flow conditions as a 0.1 mM solution in various water-methanol mobile phases. C18 sample columns were equilibrated with this mixture. Interpretation of the spectra yielded information about the solvated environment on the surface that was sampled by the probes as a function of both the mobile phase and the polarity of the probe.
Miller
C
C
Department of Chemistry, Knox College, Galesburg, IL 61401.
Dadoo
R
R
Kooser
R G
RG
Gorse
J
J
D004578:Electron Spin Resonance Spectroscopy; D012822:Silicon Dioxide
D012822:Silicon Dioxide