Separation efficiency mobile phase velocity

It should be noted that at a flow rate of 2 ml/min., the mobile phase velocity will be well above that of the optimum and so the maximum efficiency will not be realized. Generally, when there are more theoretical plates available than required, the flow rate is increased until the separation required is just realized. This procedure trades efficiency for time and allows the separation to be achieved in the minimum time given the column and phase system that has been chosen. It must be emphasized that the minimum analysis time can... 

In practice, because of the slow diffusion rates in liquids, dispersion due to longitudinal diffusion becomes important only at very low velocities. Because the dispersion increases only slowly with increasing mobile phase velocity, flow rates used in hplc are considerably higher than the value corresponding to minimum dispersion. This gives us fast separations without too much loss of efficiency. 

The quantity N is approximately constant for different bands or peaks in a chromatogram for a given set of operating conditions (a particular column and mobile phase, with fixed mobile-phase velocity, and temperature). Hence N is a useful measure of column efficiency the relative ability of a given column to provide narrow bands (small values of tw) and improved separations. 

The separation efficiency of a column for liquid chromatography and the relation with the mobile-phase velocity (u) can be described by the Van Deemter equation, which in lumped terms reads [2]... 

In addition to the enhanced diffusivity effect, another issue needs to be taken into account when considering stationary-phase mass transfer in CEC with porous particles. The velocity difference between the pore and interstitial space may be small in CEC. Under such conditions the rate of mass transfer between the interstitial and pore space cannot be very important for the total separation efficiency, as the driving mechanism for peak broadening, i.e., the difference in mobile-phase velocity within and outside the particles, is absent. This effect on the plate height contribution II, s has been termed the equilibrium effect [35], How to account for this effect in the plate height equation is still open to debate. Using a modified mass balance equation and Laplace transformation, we first arrived at the following expression for Hc,s, which accounts for both the effective diffusivity and the equilibrium effect [18] ... 

Effect of Mobile Phase Velocity on Terms in Equation 30-27 Figure 30-15 shows the variation of the three terms in Equation 30-27 as a function of mobile phase velocity. The top curve is the summation of these various effeets. Note that an optimum flow rate exists at which the plate height is at a minimum and the separation efficiency is at a maximum. 

---