Dispersion in Chromatography Columns - Alternative Equations for Peak Dispersion > The Huber Equation > Page 67
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) 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 would seem that the magnitude of the mobile velocity where the (A) term gives a constant value, is quite low, relative to the normal range of operating velocities employed in practical LC. The portion of the composite curve shown at the higher velocities is not quite linear due to the non-linear form of the term for the resistance to mass transfer in the mobile phase and this becomes more apparent at higher mobile phase velocities. At, and around the optimum velocity, however, the form of the two curves differ only slightly.