nbsp; The HETP curve is the result of a curve fitting procedure to the experimental points shown. The column was 25 cm long, 9 mm in diameter and packed with 8.5 micron (nominal 10 micron) Partisil silica gel. The mobile phase was a solution of 4.8%w/v ethyl acetate in n-decane. The hyperbolic form of the curve is confirmed, the minimum is clearly indicated and the fit of the points to the curve is good. From the curve fitting procedure the values of the Van Deemter constants could be determined and the separate contributions to the curve from the multipath dispersion, longitudinal dispersion and the resistance to mass transfer calculated and included in the figure. It is seen that the major contribution to dispersion at the optimum velocity (where the value of (H) is a minimum) is the multipath effect. Only at much lower velocities, does the longitudinal diffusion effect become significant.

where, A=2ldp, B=2gDm and   Equation (33) is a hyperbolic function which has a minimum value of (H) for a particular value of (u). Thus, a maximum efficiency will obtained at a particular linear mobile phase velocity. An example of an HETP curve obtained in practice showing this hyperbolic relationship is given in figure 11.     Figure 11. HETP Curve for Hexamethylbenzene

;                                                                  (54) Where,   and           The form of the HETP curve for a capillary column is the same as that for a packed column and exhibits a minimum value for (H) at an optimum velocity. Differentiating equation (54) with respect to (u),               Thus, when      H = Hmin,    then,                      and thus,    &

The form of the HETP curve that is produced by the Huber equation is shown in figure 15.      Figure 15.H versus U Curves for the Huber Equation It is seen that the composite curve obtained from the Huber equation is indeed similar to that obtained from that of Van Deemter but the individual contributions to the overall variance are different. The contributions from the resistance to mass transfer in the mobile phase and longitudinal diffusion are common to both equations. However, the (A

Figure 9. HETP Curves for the Same Column and Solute Using the Average Mobile Phase Velocity and the Exit Velocity   The two curves are clearly quite different and, if the results are to be fitted to the HETP equation, only the data obtained using the exit velocity  will give meaningful values for the exclusive dispersion processes. This problem is further emphasized in the graphs shown in figure 10. In figure 10, the individual contributions from the different dispersion processes are obtained by deconvoluting the HETP curve obtained using the average velocity data

takes place between the particles. Huber's equation implies (but, in fact, was not explicitly stated by the authors) that the mixing effect between the particles (that reduces the magnitude of the resistance to mass transfer in the mobile phase) does not commence until the mobile phase velocity approaches the optimum velocity (as defined by the Van Deemter equation). Furthermore, it is not complete until the mobile phase velocity is well above the optimum velocity. Thus, the shape of the HETP/u curve will be a little different from that predicted by the Van Deemter equation