Figure 22 Resistance to Mass Transfer in the Mobile Phase Van Deemter derived the following expression for the variance contribution by the resistance to mass transfer in the mobile phase, (), (6)   where (k') is the capacity ratio of the solute, and the other symbols have the meaning previously ascribed to them. The Resistance to Mass Transfer in the Stationary Phase Dispersion due to resistance to mass transfer in the stationary phase is exactly analogous to that in the mobile phase. Solute molecules close to the interface will leave the stationary phase and enter the mobile phase before those that have diffused further into the stationary phase and have a longer distance to diffuse back. Thus, as those molecules that were close to the surface will be swept along in the moving phase, they will be dispersed from those molecules still diffusing to the surface. The

The Resistance to Mass Transfer in the stationary Phase The resistance to mass transfer in the Stationary phase is depicted in figure 8. Figure 8. Resistance toMassTransferintheStationary Phase The dispersion resulting from the resistance to mass transfer in the stationary phase can be described in the same way as that in the mobile phase. Molecules close to the surface of the stationary phase, will leave and enter the mobile phase before those that have diffused farther into the stationary phase and, thus, have further to diffuse back to the surface. Consequently, during the period required for the solute molecules to diffuse to the stationary phase surface, those molecules that were close to the surface will be swept along by the moving phase and dispersed from those molecules still diffusing to the surface. In figure 6,molecules 1 and 2, (the

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

and (g) is a constant that depended on the quality of the packing. The Resistance to Mass Transfer in the Mobile Phase During passage through a chromatographic column, the solute molecules are constantly and reversibly transferring from the mobile phase to the stationary phase. This transfer is not instantaneous; time is required for the molecules to pass (by diffusion) through the mobile phase to reach the interface and enter the stationary phase. Those molecules close to the stationary phase enter it immediately, whereas those molecules some distance away will find their way to it some time later. Since the mobile phase is continually moving, during this time interval, those molecules that remain in the mobile phase will be swept along the column and dispersed away from those molecules that were close and entered the stationary phase immediately. This process is depicted in figure 22. The diagram shows 6 solute molecules in the mobile phase and the pair

only describe the resistance to mass transfer in the mobile phase contained in the pores of the particles, and thus, would constitute an additional resistance to mass transfer in the stationary (static mobile) phase. This concept has some indirect experimental support in the development of the form of f1(k') from experimental data which will be discussed later. The form of f1(k') is shown to be closer to the original form given by Van Deemter for f2(k') that is appropriate for the resistance to mass transfer in the stationary phase. It is not known for certain, but it is possible and likely, that this was the reason why Van Deemter et al. did not include a resistance to mass transfer term for the mobile phase in their original form of the equation. The Huber Equation The next HETP equation to be developed was that of Huber and Hulsman in 1967 (17). These authors introduced a modified multipath term somewhat similar in form to that of Giddings and a separate term describing the

nbsp; Consequently, the contribution from the resistance to mass transfer in the stationary phase, for all practical purposes, can be ignored. The resistance to Mass Transfer Ratio for a larger column is shown in figure 16.   It is seen from figure 16 that the larger diameter column exhibits an even grater resistance to mass transfer ratio and at a (k') of unity the resistance to mass transfer in the stationary phase contributes to less than 2% of the total resistance to mass transfer and at practical exit velocities (i.e., 20-30 cm/sec) the fraction is reduced to less than 1.5 % Column Length 15 m Column Diameter 300 mm Figure 16. Graph of Resistance to Mass Transfer Ratio against Mobile Phase Exit Velocity.   It is clear, that for all practical purposes the resistance to mass transfer in the stationary phase can be ignored and equation (2) can be reduced to