Dispersion in Chromatography Columns - Resistance to Mass Transfer Dispersion > Diffusion Controlled Dispersion in the Stationary Phase > Page 29

Diffusion Controlled Dispersion in the Stationary Phase

The difference between diffusion controlled dispersion and dispersion resulting from adsorption and desorption processes is that the transfer process is concentration controlled. Reiterating equation (7),

Thus, during solute transfer between the phases, (t) is now the average diffusion time (tD) and (s) is the mean distance through which the solute diffuses, i.e., the depth or thickness of the film of stationary phase (df).

          Thus,            or                        (12)

   where (DS) is the Diffusivity of the solute in the stationary phase.

Substituting (tD) for (td) from (12) in (11), 

                                                       (13)

                 where (q) is a configuration factor.

The constant (q) accounts for the precise shape of the 'pool' of stationary phase which, for a uniformly coated GC capillary column, (q = 2/3). Diffusion in 'rod' shaped and 'sphere' shaped bodies, (e.g., paper chromatography and LC) (q=1/2 and 2/15), respectively (7).