Topics - Diffusion
Diffusion is the process whereby solute is transferred in a fluid from a point of high concentration to a point of lower concentration. Diffusion is a concentration driven mass transfer process. The diffusivity of a substance is defined as the mass transferred per unit area per unit time in a fluid, under unit concentration gradient. Solute diffusivity affects the quality of a chromatographic separation in two ways. If the diffusivity of the solute is high in the two phases of a chromatographic system, solute exchange is rapid (the resistance to mass transfer is low) and peak dispersion is restrained and the column efficiency will be increased. Unfortunately, if the diffusivity of the solute is high in the two phases of a chromatographic system, the longitudinal diffusion in the two phases is also high and this increases the peak dispersion and reduces the column efficiency. Thus, in a chromatographic column there is a rather complicated competition between the two diffusion affects. The competitive nature of the two processes is quantitatively described by the equation for the variance per unit length of a column, or, as it is more commonly known, the HETP equation.