Topics - Resistance to Mass Transfer

Resistance to Mass Transfer Resistance to mass transfer is one of the dispersion processes that cause the solute peak to broaden during its passage along the chromatographic column. The dispersion process results from the exchange kinetics of the solute between the two phases. During passage through the column, the solute continually transfers from the mobile phase to the stationary phase and from the stationary back to the mobile phase. This process is not instantaneous because a finite time is required for the solute to transfer by diffusion through the mobile phase in order to enter the stationary phase. Thus, the molecules close to the stationary phase will enter it immediately, whereas those some distance from the stationary phase will enter it sometime later. However, as the mobile phase is moving, during the time the molecules are diffusing towards the stationary phase they will be swept along the column and, thus, away from those molecules that were close and entered it rapidly. This causes the solute peak to be dispersed (spread) and the process is called resistance to mass transfer in the mobile phase. Dispersion due to resistance to mass transfer in the stationary phase is exactly analogous to that in the mobile phase. Those molecules close to the surface of the stationary phase will leave the surface and enter the mobile before those that have diffused further into the stationary phase and require a longer period to diffuse back to the surface. Thus, those molecules that quickly enter the mobile phase because they were close to the surface will be swept away from those molecules still diffusing to the surface. This process also causes the solute peak to be dispersed and this process is called resistance to mass transfer in the stationary phase.