separation techniques: van deemter equation

Van Deemter Equation and Peak Broadening in
Chromatography

The Van Deemter Equation:
H = A + B/u + C·u
Where: H = plate height (measure of band broadening), u = linear velocity of mobile phase.



Terms in the Equation:
A-term (Eddy Diffusion)
• Caused by multiple flow paths around packing particles.
• Independent of flow rate.
• Minimized by uniform, well-packed columns.
B-term (Longitudinal Diffusion)
• Spreading of solute molecules along column axis due to natural diffusion.
• More significant at low flow rates (∝ 1/u).
• Causes broad peaks when solutes remain longer in the column.
C-term (Mass Transfer Resistance)
• Caused by finite equilibration time between stationary and mobile phases.
• Increases at high flow rates (∝ u).
• Leads to broadening when solutes cannot equilibrate fast enough.



Application: Explaining Peak Broadening
• At low flow rates → longitudinal diffusion dominates (broad peaks).
• At high flow rates → mass transfer resistance dominates (broad peaks).
• At intermediate flow rates → balance of effects, minimum plate height (sharp peaks).
• Produces U-shaped Van Deemter curve with optimum flow rate for maximum efficiency.