# Principles and Practice of Chromatography - Peak Dispersion in a Chromatographic Column > The Resistance to Mass Transfer in the Stationary Phase > Page 48

where ** ** (k')
is the capacity ratio of the solute,

** ** ** **
(d_{f}) is the effective film thickness of the
stationary

phase,

** **
(D_{S}) is the
diffusivity of the solute in the stationary

and the other symbols have the meaning previously ascribed to them.

Now, applying the law of Summation of Variances,

_{}
(8)

where (_{}) is the
total variance/unit length of the column.

Thus substituting for (_{}), (_{}),
(_{}) and (_{}) from equations
(4), (5), (6) and (7), respectively,

_{}
(9)