Dispersion in Chromatography Columns - Resistance to Mass Transfer Dispersion > Diffusion Controlled Dispersion in the Mobile Phase > Page 30

For a GC capillary column,

                                                       (14)

and a close approximation for an LC or GC packed column, (HMTS) would be given by

                           .                        (15)                                 

Diffusion Controlled Dispersion in the Mobile Phase

Dispersion processes in the mobile phase will also be diffusion controlled, thus, again reiterating equation (7),

                                      

While in the mobile phase a solute molecule must traverse from localities of high velocity (near the center of a capillary column or the center of an inter-particle channel) to that of low velocity (the interface between the two phases at the capillary column walls or, in the packed column, the surface of the particles). Let the "exchange time' between the two extreme velocities be (tm). Now, the distance between the extremes of velocity will depend on the geometry of the column system, but in general it can be assumed a molecule must diffuse a distance (wmdm) to move from one velocity extreme to the other. Depending on the column (tubular or packed), and on the homogeneity of the packing, (particle size and shape) the value of (wm) may range widely from much less than unity to significantly greater than unity. Now, from equation (7),