Band broadening:

In the above context, it may be important to visualize the exact situation leading to band broadening from mass transfer effects. There are many flowing streams of the mobile phase and the layer making up the stationary phase. Both of these have finite widths. As a result of this, there is some time required for the solute molecules to diffuse from the interior of these phases to their interface for the mass transfer to occur. This condition prevails throughout the column. If the rates of mass transfer are infinite, there will not be any band broadening.

In order to quantify the role of different factors in mass transfer, we will have to primarily consider three parameters: carrier gas velocity (u), stationary phase mass transfer coefficient (C_S) and mobile phase mass transfer coefficient (C_M). We can see that both longitudinal broadening and mass transfer broadening depend upon the velocity of the carrier gas. Therefore, here is a marked difference in the direction of the diffusion in the two cases. Within the case of longitudinal diffusion, the direction of movement of solute molecules tends to be parallel to the flow of the gas whereas in mass transfer it tends to be perpendicular to the flow. As a result of this, the effect of carrier gas velocity in mass transfer broadening is in contrast to that in longitudinal broadening. The faster is the rate of mobile phase movement, the lesser the time available for equilibrium to be attained. Thus, as the velocity rise, the plate height increases. We will have to examine the condition in terms of characteristics of stationary phase. The Mass transfer term that also denoted by (C_S) is a complex function f_S(k´) of retention factor k´ and directly proportional to the square of the thickness of the film on the support particles d²f and inversely proportional to the diffusion coefficient D_S of the solute in the film.