Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 84

Comparing equations (61) and (62) it is clear that there is a significant difference between two equations, in that, only the Van Deemter equations should provide an (A) term that is independent of both the linear mobile phase velocity and the solute diffusivity. The fit of the Van Deemter equation to the experimental data confirms the former condition and the plot of the (A) term against solute diffusivity (the data taken from tables 1 and 2 and shown in figure 20 confirms the latter.


Figure 20. Graph of A Term against Solute Diffusivity for Benzyl Acetate

Figure 20 demonstrates that the magnitude of the (A) term appears, within experimental error, independent of the diffusivity of the solute in the mobile phase. Closer examination, however, indicates that there might be a slight residual dependence of (A) on diffusivity. This probably indicates that the velocity range over which the data was taken was not sufficiently high enough to ensure that the first term of the Giddings equation was reduced to a constant value as it is in the simple Van Deemter equation.

This can be examined further by considering the detailed expression for the first term of the Giddings equation. The expanded expression for the first term in the Giddings equation is as follows: