In practice, it is more convenient to express (H) in terms of (t_d) as opposed to (t_a). The ratio of the mean phase residence times is the time the solute spends in the mobile phase divided by the time spent in the stationary phase,

            Thus,      

             and          (11)

Equation (11), is derived from the approximate random walk theory, however, it is rigorously correct and applies to heterogeneous surfaces containing wide variations in properties as well as to perfectly uniform surfaces. It can also be used as the starting point for the random walk treatment of diffusion controlled mass transfer similar to that which takes place in the stationary phase in GC and LC columns.