Topics - Multi-Path Effect
The multi-path effect is one factor that contributes to peak dispersion (band spreading) in a packed column. It is a term given to the dispersion that results from the tortuous paths taken by solute molecules as they wind their way through the intersticies of the column packing. It is clear that some molecules will randomly take shorter paths through the packing and move ahead of the average and some molecules will randomly take longer paths through the packing and lag behind the average This process of dispersion was given the term ‘eddy diffusion’ by Van Deemter who first produced and equation that described the variance per length of a column as a function of the linear mobile phase velocity and the various physical properties of the distribution system and the solute being eluted. Van Deemter’s function for eddy diffusion dispersion was a constant depending on the particle diameter of the packing and a ‘packing factor’ only and was independent of the mobile phase linear velocity. Giddings, however, was not satisfied with this function as it implied that, at zero mobile phase velocity, there would still be a multi-path effect. To more accurately account for this anomaly, he introduced a coupling function to the eddy diffusion term so that the dispersion only became constant and independent of mobile phase velocity at relatively high velocities.