# Dispersion in Chromatography Columns - Resistance to Mass Transfer Dispersion > Diffusion Controlled Dispersion in the Mobile Phase > Page 34

Summarizing,, the rate theory provides the following equations that describe the variance per unit length (H) for four different columns.

1. The Open Tubular GC Column,

_{} (20)

_{}(21)

This equation is basically that
derived by Van Deemter *et al*. in 1956.

_{} (22)

_{} (23)

It should be
noted that *all the equations assume that the mobile
phase is incompressible* which will not be true for equations (23)
and (24). It follows that equations (23) and (24) will require modification in
order to be applicable to practical situations.