Below is a van Deemter plot of the separation of a mixture of neutral molecules

Question

Below is a van Deemter plot of the separation of a mixture of neutral molecules by micellar electrokinetic chromatography.

Map 5.5 5.0 4.5 4.0 3.5 3.0 Scroll down to answer both 2.5 parts of this question 1.5 0.5 1.0 Electroosmotic velocity (mm/s) Why does the plate height increase at low and high velocities? The micelles have not yet imparted a charge to the neutral molecules at low velocities, thus the equilibration time increases, which leads to an increase in the C term in the van Deemter equation. At high velocities, the equilibration time is even longer since the flow of the solution in the capillary makes it harder for the micelles and analyte to equilibrate At low velocities, the pressure in the capillary tube is lower, leading to less resolved O separations. At high velocities, the pressure in the tube is too high, leading to analyte compression and lower resolution At low velocities, the micelles move slowly and pack the column, leading to an increase in the A O term in the van Deemter equation. At high velocities, the band broadens due to diffusion as the analyte moves quickly through the capillary, leading to an increased plate height. The plate height increases at lower velocities due to diffusion of the band as it spends more time in the capillary. At high velocities, band broadening increases due to the finite equilibration time with the micelles

Explanation / Answer

The plate height increases at lower velocities due to diffusion of the band as it spends more time in the capillary. At high velocities, band broadening increses due to the finite equilibration time with the miscelles.

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