A kinetic parameter concerning mass transfer in silica monolithic and particulate stationary phases measured by the peak-parking and slow-elution methods.

Mass transfer in monolithic C18-silica stationary phases and C18-silica gel particles was studied. A traditional kinetic parameter, gamma(s)D(s), which is a diffusion coefficient of solute molecules in the stationary phase, was measured by two unusual approaches, i.e., peak-parking and slow-elution methods. The correlation between the ratio of gamma(s)D(s) to molecular diffusivity (Dm) and the retention factor (k) was represented by one common curve, irrespective of the RPLC conditions. A similar curved profile was also observed between another kinetic parameter (D(Ls)), which is related to the axial diffusive molecular migration in the stationary phase, and the retention equilibrium constant (Ka). The values of D(Ls) and Ka were calculated from those of gamma(s)D(s) and k, respectively. The ratio of D(Ls)/Dm increases with decreasing Ka and seems to approach around unity when Ka is infinitely small. The dependence of D(Ls) on Ka was also studied from extra-thermodynamic points of view. The linear correlation between In D(Ls) and In Ka suggests the existence of a kind of linear free energy relationship between the mass transfer in the stationary phase and the retention equilibrium. Because these characteristics of D(Ls) are similar to those of the surface diffusion coefficient (D(sur)), D(Ls) seems to correspond to D(sur).