4.2.2.1: Longitudinal Diffusion

Under normal conditions, diffusion along the capillary tube ( longitudinal diffusion) is one of the more substantial contributors to zone broadening. A solute's propensity to diffuse throughout the solvent depends on the solute and the solvent. A longitudinal diffusion coefficient can be provided for a given system, with a lower coefficient meaning the solute will form narrower zones.

The equation for the number of theoretical plates in a capillary tube is given in Equation 4.8, where N is the number of theoretical plates, µa is the solute mobility, E is the electric field, I is the effective capillary length, and D is the diffusion coefficient of the solute.

Eqn 4.8

This equation only accounts for longitudinal diffusion, although there are often other variables that affect zone broadening.

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