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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

30 cm/s. and at such velocities, the resistance to mass transfer in the stationary phase is less than 2% of the total resistance to mass transfer and this is still at a (k') value of unity. It follows that for this column, the dominant resistance to mass transfer dispersion effect occurs in the mobile phase, except possibly at (k') values close to zero (when the solute is eluted close to the dead volume of the column). Consequently, under most conditions used in practice, the resistance to mass transfer in the mobile phase will be the controlling effect on peak dispersion.   Column Length 30 m Column Diameter 100 mm   Figure 15. Graph of Resistance to Mass Transfer Ratio against Mobile Phase Exit Velocity

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

to mass transfer in the stationary phase, the curves will show a positive intercept.   In figure 25. the Resistance to Mass Transfer term (the (C) term from the Van Deemter curve fit) is plotted against the reciprocal of the diffusivity for both solutes. It is seen that the expected linear curves are obtained and that there is a small, but significant, intercept for both solutes. This indicates that there is a small but, nevertheless, significant contribution from the resistance to masstransfer in the stationary phase for these two particular solvent/stationary phase/solute systems. Overall, however, all the results in figures 23, 24 and 25 support the Van Deemter equation extremely well. J.Chromatogr.,270(1983)65. Figure 25. Graph of (C) Term against the Reciprocal of the Solute Diffusivity. Katz et al. (12) also examined the effect of particle diameter on the value of the overall resistance to mass transfer constant (C). They employed columns packed with 3.2 m, 4.4

, measured at atmospheric pressure and (g) is the inlet/outlet pressure ratio of the column.   A description of the various dispersion processes that take place in a column are given in book 9 of this series. In the case of the capillary column the first expression in equation (2) describes the effect of longitudinal diffusion, the second expression described the effect of the resistance to mass transfer in the mobile phase and the last expression the resistance to mass transfer in the stationary phase.   It is interesting to estimate the relative magnitude of the two components of the resistance to mass transfer by examining the ratio of the resistance to mass transfer in the mobile phase to that in the stationary phase, i.e., (3)   In order to evaluate equation (3) for any given column and phase system the relationship between (h) and (uo) needs to be identified.