where γ is an obstruction factor, typically equal to 0.6-0.8, and DM is the diffusion coefficient.
The only way to change B or B/u is by varying the type, pressure, and flow rate of the mobile
phase. High flow rates reduce molecular diffusion.
In LC, molecular diffusion in the stationary phase is very small compared to that in gases. In
GC, it dominates only at flow rates less than uopt. We usually operate at flow rates greater
than uopt since H does not increase very much and separations are faster.