Diffusion:
Multiple paths broadening may be partially offset by ordinary diffusion which results in molecules being transferred from a stream following one pathway to a stream following another. In addition, it seems in which some of the band broadening that used to be attributed to eddy diffusion occurs outside the column say in the tubing that led from the column to the detector and in the detector itself. It could also be the glass wool that is sometimes used to plug the column. The contribution of eddy diffusion has now become very small with well designed gas chromatograph along with good column. However, it is almost negligible compared to other band broadening factors. That factor is independent of mobile phase velocity. If the velocity of mobile phase is low, a huge number of transfers will occur spending a brief time in each. As a conclusion of this, the molecule moves down the column tending to approach the average velocity. Within the case of moderate or high velocities, enough time is not available for diffusion averaging to occur and the band broadening due to the different path length is observed.
From the above discussion, one can conclude that A will depend on irregularity of packing and diameter of the particles, that is.
A = 2 λ dP
λ = is a dimensionless parameter depicting the irregularity of packing, and
dP = is the diameter of particles (cm).