Moment Analysis of Mass-Transfer Kinetics in C18-Silica Monolithic Columns

The moment analysis of elution peak profiles based on new moment equations provides information on the mass-transfer characteristics of C18-silica monolithic columns. The flow rate dependence of the HETP data was analyzed using the generalized van Deemter equation, after correction of these data by subtraction of the external mass-transfer contribution to band broadening. Kinetic parameters and diffusion coefficients related to the mass-transfer processes in monolithic columns were derived by taking advantage of the different flow velocity dependence of their contributions to band broadening. At high flow rates, axial dispersion and diffusive migration across the monolithic C18-silica skeleton contribute much to band broadening, suggesting that it remains important to reduce the influence of eddy diffusion and the mass-transfer resistance in the stationary phase to achieve fast separations and a high efficiency. Surface diffusion plays a predominant role for molecular migration in the monolithic stationary phase. Although the value of the surface diffusion coefficient (Ds) depends on an estimate of the external mass-transfer coefficient, Ds values of the order of 10^-7 cm^2 s^-1 were calculated for the first time for the C18-silica monolithic skeleton. The value of Ds decreases with increasing retention of sample compounds. Analysis of a kind of time constant calculated from Ds suggests that the "chromatographic corresponding particle size" is ~4 (mu)m for the C18-silica monolithic stationary phase used in this study. The accuracy of the Ds values determined was discussed.