Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 85
From the equation for (A) it is seen that when the value of (u) is sufficiently large the right hand function in the denominator becomes negligible and the term simplifies to 2ldp, the same as the first term in the Van Deemter equation. However, if (u) takes values, where the right hand side of the denominator was not quite zero, then the value of the (A) term would show a slight decrease with increasing values of Dm. This effect may well be substantiated by the slight slope of the straight line in figure 20.
The curves in figure 21, which is the same plot of the value of the (A) term against solute diffusivity but this time for hexamethylbenzene (eluted at the dead volume) gives even stronger confirmation of this possibility. This indicates that it might be useful to examine the (A) term more closely.
Figure 21. Graph of A Term against Solute Diffusivity for Hexamethylbenzene