Ion Chromatography - Dispersion by Resistance to Mass Transfer in the Mobile Phase
Dispersion by Resistance to Mass Transfer in the Mobile Phase
During chromatographic elution the solute molecules continually transfer from the mobile phase to the stationary phase and then back again to the mobile phase. The transfer process is not instantaneous as a finite time is required for the solute to diffuse through each phase to the boundary surface in order to enter the other phase. Molecules that are close to the phase boundary will transfer quickly to the other phase whereas those molecules that have diffused deeper into the phase will will find their way to the the interface a significant interval of time later. As the mobile phase is moving, it is obvious that during this interval of time, those molecules remaining in the mobile phase will be swept along the column and dispersed away from from those molecules that were close and entered the phase immediately. The dispersion process involved in the transfer of solute molecules from the mobile phase to the stationary phase is diagramatically represented in figure 10.
Figure 10. Dispersion by the Process of Mass Transfer in the Mobile Phase.
The diagram depicts 6 solute molecules in the mobile phase and molecules 1 and 2 (closest to the interface) immediately enter the stationary phase. During the period while molecules 3 and 4 diffuse to the interface the mobile phase moves on. When molecules 3 and 4 transfer to the stationary phase they will enter at a point some distance ahead of molecules 1 and 2. Finally when molecules 5 and 6 diffuse to the interface the mobile phase has moved even further along the column ahead of molecules 3 and 4. The six molecules originally close together are now spread out in the stationary phase. Van Deemter derived an expression for the contribution to solute band dispersion from resistance to mass transfer in the mobile phase () as follows,
where (k') is the capacity ratio of the solute.
and the other symbols have the meaning previously ascribed to them.