# Dispersion in Chromatography Columns - Resistance to Mass Transfer Dispersion > Page 26

If (k_{d})
is the *desorption rate* *constant* then the mean desorption time (t_{d})
for the adsorbed molecule will be _{}. Correspondingly, if the *adsorption
rate constant* is (k_{a}), then the mean adsorption time for a free
molecule in the mobile phase will be _{}.

Consider a
peak moving down a column. During this migration process, adsorption and
desorption steps will constantly and frequently occur and each occurrence will
be a random event. Now a *desorption step*
will be a random movement forward as it releases
a molecule into the mobile phase. Conversely, an adsorption
step will be a random movementbackward, as
it is a period of immobility for the molecule while it resides in the
stationary phase and the rest of the zone moves forward. The total number of
random steps, taken as the solute mean position moves a distance _{} along
the column, is the number of forward steps *plus* the number of backward
steps. Now the distribution of the solute is dynamic and is also an equilibrium
system, consequently, each desorption step must be followed by an adsorption
step. It follows that the total number of steps will be twice the number of
adsorption steps that take place in the migration period.