# Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 85

_{} (63)

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 2ld_{p}_{, }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 D_{m}. 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.

*J.Chromatogr*.,**270**(1983)62

**Figure 21.
Graph of A Term against Solute Diffusivity for Hexamethylbenzene**