Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 86

Rearranging equation (63)

                                                         (64)

                  Thus, if wudp >>>2lDm,  then,  A = 2ldp

However, it is now seen that the product of (udp) now determines the condition for the Giddings equation to simplify to the Van Deemter equation and, thus, it will depend not only on the mobile phase velocity but also the particle diameter of the packing.

Figure 22. Graph of A Term against Mobile Phase Velocity for Different Particle Sizes

Equation (64) is similar in form to the Langmuir Adsorption isotherm and it would be interesting to see the effect of the particle diameter and mobile phase velocity on the magnitude of A. Employing equation (64) curves relating (A) and (u) for different particle diameters were constructed and are shown in figure 22. For convenience (l) is taken as 0.5 (as recommended by Giddings for a well packed column), (w) is taken as unity and Dm taken as 2 x 10-5 cm2/sec. (the diffusivity of benzyl acetate in n-heptane).