Dispersion in Chromatography Columns - The Van Deemter Equation > Page 46

Note, this equation ignores the second order effect of any longitudinal diffusion that might be present in the stationary phase.In fact, in the original form, the equation was introduced by Van Deemter for packed GC columns and consequently, the longitudinal diffusion term for the liquid phase was not included and the function , was replaced by, . This was because  the diffusivity of the solute in a gas is four to five orders of magnitude greater than in a liquid.

Thus, in the expression for longitudinal diffusion,

                   

the second function would not be significant.

 

In addition, Van Deemter considered the resistance to mass transfer in the mobile phase to be negligible. This was because the eddies that form between the particles causes a turbulence or secondary flow that greatly increases the diffusivity of the solute between the pores. As a result, the function,, was also not included. Consequently, the equation actually developed by Van Deemter took the form,