Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 91

In figure 23 the values of (Hmin.) are plotted against solute diffusivity and it is seen that the independence of (Hmin.) on diffusivity is largely confirmed. Close examination, however, shows that neither of the lines for the two solutes are completely horizontal with the baseline, but the dependence of (Hmin) on diffusivity is extremely small for the solute benzyl acetate. The slight slope of the line for the solute hexamethylbenzene might well result from the fact that either the (A) term is not completely independent of the diffusivity (Dm) as shown by the results in figure  21, or the resistance to mass transfer in the stationary phase does make a small but significant contribution to the value of (H).

The curves relating the optimum velocity with solute diffusivity are shown in figure 24 and it is seen that the straight lines predicted by the Van Deemter equation are realized for both solutes. It should be noted that similar treatment of the Knox equation does not predict that values of Hmin should be independent of the solute diffusivity neither does it predict that (uopt) should vary linearly with solute diffusivity. This is strong evidence, supporting the validity of the Van Deemter equation, as opposed to the Knox equation.



Figure 24. Graph of Optimum Velocity against Solute Diffusivity