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 closest to the surface, (1 and 2), enter the stationary phase immediately. While molecules 3 and 4 diffuse through the mobile phase to the interface, the mobile phase moves on. As a consequence, when molecules 3 and 4 reach the interface, they

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 stationaryphase 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

the solute by polar interactions. The mobile phase must be chosen to complement the stationary phase so that the selected interactions are concentrated in the stationary phase. Thus, a reversed phase having strong dispersive interactions would be used with a strongly polar mobile phase (e.g., mixtures of methanol and water acetonitrile and water or tetrahydrofuran and water). In contrast, if the strongly polar silica gel is selected for the stationary phase then a strongly dispersive mobile phase would be appropriate (e.g., n-heptane, n-heptane/methylene chloride or n-heptane with a small quantity of n-propanol or ethanol). In general the mobile phase must be chosen so that the selected interactions strongly dominate in the stationary phase and are minimized in the mobile phase

50% of that mobile phase is actually moving. In addition, about 18% of the mobile phase is interstitial, but static, and about 31% of the mobile phase is contained within the pores and is also static. Just over 6% of the mobile phase in the pores has a different composition to that of the mobile phase proper and, thus, constitutes a second stationary phase. Good agreement was obtained between the different methods of measuring the thermodynamic dead volume. Weighing the column filled with mobile phase and then weighing dry, measuring the retention volume of one pure component of the mobile phase and measuring the retention volume of a completely permeating solute by extrapolation, all give very closely similar values.  About 12% of the column volume is occupied by the stationary phase, which is equivalent to about 17% of the mobile phase content of the column.  The values given in Table 2 are probably representative of most reverse phase columns but will differ

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 two closest to the surface) will enter the mobile phase and begin moving with the mobile phase along the column. This process will continue while molecules 3 and 4 diffuse to the interface at which time they, also, will enter the mobile phase and start following molecules 1 and 2 down the

Molecular Interactions in the Mobile Phase The 'elutive' capacity of the mobile phase (as opposed to the 'retentive' capacity of the stationary phase) depends on the strength of the different interactions that can take place in the mobile phase and the probability of a particular interaction occurring. Purnel and Laub (34) experimentally demonstrated in GC, that for a stationary phase consisting of a pair of non-associating liquids, the distribution coefficient of a solute was linearly related to the volume fraction of either liquid. This relationship indicated that the