Huber equation

Figure 1. H Versus u Curves from the Huber Equation...

The composite curve from the Huber equation is 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 stationary phase and... 

The form of the HETP curve that is produced by the Huber equation is shown in figure (1). 

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. Although the contributions from the resistance to mass transfer in the mobile phase and longitudinal diffusion are common to both equations, the (A) term from the Huber equation increases with mobile phase flow-rate and only becomes a constant value, similar to the multipath term in the Van Deemter equation, when the mobile velocity is sufficiently large. In practice, however, it... 

Examination of the results obtained by Katz et a as given in table 2 show rational fits between the experimental data and the equations of Van Deemter, Giddings and Knox. The fit of the data to both the Huber and Horvarth equations gave alternating positive and negative values for the D constant which is the coefficient of the term involving a fractional power of (u). Furthermore, for the Huber equation, the value of coefficient (E) is consistently zero and for the Horvath equation, is zero for four solvents mixtures out of six, with an extreme value of 97.3 for one solvent. 

Efficiency (N)—A measure of the narrowness of elution bands, the sharpness of peaks, and the performance of a column. Results are in theoretical plates. The Huber equation calculates efficiency versus flow rate, which is plotted on as a Van Deampter plot, which compares column efficiency with flow rate. 

This is known as the Huber equation. Here, u is the mobile-phase linear velocity. The constant C, is the mobile-phase mass transfer term and Cj the stationary-phase term. The B (longitudinal diffusion) term, except at very low mobile-phase velocities, is nearly zero and can be neglected. It is a function of the mobile-phase viscosity and the analyte molecule. The A (eddy diffusion) term turns out to be small compared with diffusion in the liquid phases and almost a constant value and is, therefore, usually neglected. So, H is estimated as ... 

The values of different parameters calculated from DA equation (Equation 2.120), DR equation (Equation 2.121) and Stoeckli-Huber equation (Equation 2.128) obtained from the best fit of the adsorption data are given in Table 2.12. 

It is seen that the first term differs from the Giddings equation and now contains the mobile phase velocity to the power of one-half. However, when E, the first term reduces to a constant similar to the Van Deemter equation. The additional term for the resistance to mass transfer in the mobile phase is an attempt to take into account the turbulent mixing that takes place between the particles. Huber s equation, although not explicitly stated by the authors, implies that the mixing effect between the particles (that reduces the magnitude of the resistance to mass transfer in the mobile phase) only starts when the mobile phase velocity approaches the optimum velocity (as defined by the Van Deemter equation). In addition, the mixing effect is not complete until the mobile phase velocity is well above the optimum velocity. Thus, the shape of the HETP/u curve will be different from that predicted by the Van Deemter equation. The form of the HETP curve that is produced by the Huber equation is shown in Figure (1). 

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