Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 92
Returning to equation (49), it is seen that the Van Deemter equation predicts that the total resistance to mass transfer term must also be linearly related to the reciprocal of the solute diffusivity. Furthermore, it is seen from equation (49), that, if there is a significant contribution from the resistance 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 mass transfer 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.
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 m, 7.8 m, and 17.5 m, and obtained HETP curves for the solute benzyl acetate in 4.3%w/w of ethyl acetate in n-heptane on each column. The data was curve fitted to the Van Deemter equation and the values for the A, B and C terms for all four columns calculated. According to the Van Deemter equation the (C) term should be linearly related to the square of the particle diameter. A graph relating the value of the (C) term with the square of the particle diameter is shown in figure 26.