Characterization of reversed-phase liquid chromatographic stationary phases using solvatochromism and multivariate curve resolution

The solvatochromic method has been used to probe thesolid/solution interfac e of bare silica and two modified silica surfaces-phenyl bonded and C-18 bo nded-in mobile-phase mixtures of methanol-water and acetonitrile-water. Spe ctral measurements of solvatochromic dye solutions in different mobile-phas e compositions and of slurries wetted by these same solutions have been rec orded and used to characterize the different solid/ solution interfaces. Mu ltivariate curve resolution has been employed to resolve the spectra collec ted into the contributions due to the different solvated species of the dye , i.e., those related to the dye associated with the stationary phase and t hose related to the dye solvated by the different species present in the mo bile phase. Spectral profiles of the dyes solvated by a methanol-water comp lex in the presence of stationary phase have been resolved for the first ti me. Chromatographic capacity factors (k') have been measured, and they have complementary information about how the retention of the dye changes with the mobile-phase composition on the different silica surfaces. The validity of the spectral studies performed to characterize real chromatographic env ironments, which are usually under much higher pressures, has been investig ated. In a wide range going from atmospheric pressure to values higher than 100 bar, no significant variations of the capacity factors were observed f or the dyes used, and therefore, the information about retention mechanisms and solid/solution interface properties obtained from the spectral studies can be safely extrapolated to the real chromatographic systems. The result s obtained indicate that the phenyl bonded silica shows a dipolarity/ polar izability very similar to that of bare silica. For these two silica surface s, the interactions of the dye and the stationary phase are independent of the mobile-phase composition. The C-18 bonded silica has a significantly lo wer polar character and evidences two retention mechanisms depending upon t he mobile-phase composition.