In column chromatography, analytes separate into 'bands' as they travel down the column. Brownian motion of the analyte particles in the mobile phase leads the band to broaden and elute from the column with a Gaussian profile.

Another cause of band broadening is longitudinal diffusion, in which the randomly moving analyte molecules in the mobile phase migrate from a high-concentration region to a low-concentration region in the column.

The diffusion coefficient is the constant of proportionality that relates the diffusion speed of an analyte from high-concentration to low-concentration regions to the sharpness of its concentration gradient in the mobile phase.

The diffusion speed is expressed as flux—the amount of analyte crossing a slice of the column per second.

As the diffusion coefficient for a given solvent is higher in a gaseous mobile phase than in a liquid mobile phase, the contribution of longitudinal diffusion to band broadening is more significant in gas chromatography than in liquid chromatography.