Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 87

Figure 22 discloses, in more detail, the factors that control the magnitude of the (A) term and the effect of particle diameter on the mobile phase velocity at which the Giddings equation simplifies to the Van Deemter equation. For very small particles (e.g. 3 m) the Giddings equation simplifies to the Van Deemter at a velocity of about 0.2 cm/sec but for the larger particles (e.g., 10 m) it occurs at about 1 cm/sec. However, at the optimum velocity, irrespective of the particle diameter, the contribution from the coupling term is very small and so the Van Deemter equation can be used with confidence in column design.



 Figure 23. Graph of the (B) Term against Diffusivity

In summary, the Data of Katz et al. shows some slight dependence of the (A) term on Dm, (which can be explained on the basis of the calculations given above). However, as a result of the curve fitting procedure to the equation   it is shown not to be dependent on (u) and thus, supports the Van Deemter equation as opposed to the Knox equation. It does, however, also support the idea that the Van Deemter equation is a special case of the Giddings equation.