Where A, B and C are constants depending
on the multiple possibilities of flow paths within the column, longitudinal
diffusion and equilibration time, correspondingly. Finally, Ux
stands for the linear velocity. 10, 11

The
Van Deemter equation describes three terms that influence the efficiency of the
column. The first term (A) describes the Eddy (axial) diffusion. This means
that not all analytes will follow the same path within the column. One path can
be longer than the other one, resulting in band broadening. Smaller particles
inside the column will result in a lower A term because the space between
particles gets smaller. It is important
that the particles within the column are uniform, meaning same particle shape
and a tight particle size distribution. Shortly put, the A term incorporates
anything on the outside of the stationary particles.              
The second term (B) describes the longitudinal diffusion, meaning that the
analyte will try to disperse over the axis of the flow. This happens in all system
tubing, but mostly in the column itself or spaces with a respectively large
internal volume. The best option to reduce this term is to make sure the
optimum flow rate is achieved. The slower the flow
rate, the higher the B term gets.            
The third and last term (C) describes the mass transfer. This term arises from
the fact the stationary phase material is porous and the mobile phase within
these pores is stagnant. As the analyte moves through this stagnant mobile
phase within the pores, they do so by diffusion only. The analyte molecules
that enter the pores, those that won’t enter the pores and those that penetrate
more deeply into the pores, will gain different amounts of retention, causing a
broadening of the band. These effects can be minimized by reducing the particle
size distribution, this results in pores that are as shallow as possible. A
lower flow rate will also lower the mass
transfer effect. The C term incorporates anything on the inside of the
stationary particles

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