# Principles and Practice of Chromatography - Peak Dispersion in a Chromatographic Column > Longitudinal Diffusion > Page 44

** **

Van Deemter *et
al.* derived the following function for the multi-path variance contribution
(_{}) to the
overall variance per unit length of the column,

_{} (4)

where (d_{p})
is the particle diameter of the packing,

** ** and
(l) is a constant that depended on the quality of the packing.

## Longitudinal Diffusion

Solutes when contained in a fluid naturally diffuse and spread driven by their concentration gradient. Thus, in a chromatographic column a discrete solute band will diffuse in the gas or liquid mobile phase. It also follows, that because the diffusion process is random in nature, a concentration curve that is Gaussian in form will be produced. This diffusion effect occurs in the mobile phase of both GC and LC columns. The diffusion process is depicted in figure 21.

**Figure 21 Peak Dispersion by
Longitudinal Diffusion**