Dispersion in Chromatography Columns - Dispersion Processes that take Place in an LC Column > The Resistance to Mass Transfer in The Mobile Phase > Page 23

The Resistance to Mass Transfer in The Mobile Phase

As a solute band progresses along a column, the solute molecules are continually transferring from the mobile phase into the stationary phase and back from the stationary phase into the mobile phase. This transfer process is not instantaneous, because a finite time is required for the molecules to traverse (by diffusion) through the mobile phase in order to reach, and enter the stationary phase. Thus, those molecules close to the stationary phase will enter it almost immediately, whereas those molecules some distance away from the stationary phase will find their way  to it a significant interval of time later. However, as the mobile phase is moving, during this time interval while they are diffusing towards the stationary phase boundary, they will be swept along the column and thus dispersed away from those molecules that were close and entered it rapidly. The dispersion resulting from the resistance to mass transfer in the mobile phase is depicted in figure 7.The diagram shows 6 solute molecules in the mobile phase and those closest to the surface (1 and 2) enter the stationary phase immediately. During the period, while molecules 3 and 4 diffuse through the mobile phase to the interface, the mobile phase moves on. Thus, when molecules 3 and 4 reach the interface they will enter the stationary phase some distance ahead of the first two. Finally, while molecules 5 and 6 diffuse to the interface the mobile phase moves even further down the column until molecules 5 and 6 enter the stationary phase further ahead of molecules 3 and 4. Thus, the 6 molecules, originally relatively close together, are now spread out in the stationary phase. This explanation is a little over-simplified, but gives a correct description of the mechanism of mass transfer dispersion.


Figure 7. Resistance to Mas Transfer in the Mobile Phase