Peak parking-moment analysis. A strategy for the study of the mass-transfer kinetics in the stationary phase

The combination of peak parking and moment analysis is proved to be a powerful strategy for studying the molecular diffusivity of solute molecules in stationary phases and for determining the corresponding kinetic parameters. The physical meaning of the traditional kinetic parameter, gamma D-s(s), the diffusion coefficient of solute molecules in stationary phases, is first clarified. A second kinetic parameter, D-Ls, is then derived from gamma D-s(s), and the correspondence between D-Ls and the surface diffusion coefficient, D-sur, is discussed. Then, the results of peak parking experiments carried out with RPLC systems using aqueous methanol solutions and C-18-silica columns are reported. The moments of pulse responses were measured and are analyzed. The values and some characteristics of D-Ls measured by the peak parking-moment analysis method are compared with those of D-sur determined using the pulse response-moment analysis method. The comparison of the numerical values of D-Ls and D-sur suggests that D-Ls is nearly equal to D-sur and that it exhibits the same dependence on the retention strength as does D-sur with regard to the mass-transfer kinetics and extrathermodynamics. This comparison allows us to conclude that D-Ls corresponds to D-sur. The advantages and disadvantages of the peak parking-moment analysis method were compared with those of the pulse response-moment analysis method. The combination of these two methods provides a comprehensive strategy for measuring the mass-transfer kinetics in the stationary phase.