Where A, B and C are constants dependingon the multiple possibilities of flow paths within the column, longitudinaldiffusion and equilibration time, correspondingly. Finally, Uxstands for the linear velocity. 10, 11TheVan Deemter equation describes three terms that influence the efficiency of thecolumn. The first term (A) describes the Eddy (axial) diffusion. This meansthat not all analytes will follow the same path within the column. One path canbe longer than the other one, resulting in band broadening.
Smaller particlesinside the column will result in a lower A term because the space betweenparticles gets smaller. It is importantthat the particles within the column are uniform, meaning same particle shapeand a tight particle size distribution. Shortly put, the A term incorporatesanything on the outside of the stationary particles. The second term (B) describes the longitudinal diffusion, meaning that theanalyte will try to disperse over the axis of the flow. This happens in all systemtubing, but mostly in the column itself or spaces with a respectively largeinternal volume. The best option to reduce this term is to make sure theoptimum flow rate is achieved. The slower the flowrate, the higher the B term gets.
The third and last term (C) describes the mass transfer. This term arises fromthe fact the stationary phase material is porous and the mobile phase withinthese pores is stagnant. As the analyte moves through this stagnant mobilephase within the pores, they do so by diffusion only. The analyte moleculesthat enter the pores, those that won’t enter the pores and those that penetratemore deeply into the pores, will gain different amounts of retention, causing abroadening of the band. These effects can be minimized by reducing the particlesize distribution, this results in pores that are as shallow as possible.
Alower flow rate will also lower the masstransfer effect. The C term incorporates anything on the inside of thestationary particles