A study on kinetics of thin-layer chromatography of amino acids on silica gel The contribution of different mechanisms to the spreading of chromatographic zones is investigated in the separation of amino acids on silica layers. For that purpose the flow pattern of the solvent in the thin layer and the two-dimensional spreading of zones have to be considered. Molecular diffusion in the mobile phase and a retarded mass transfer to the stationary phase have been found to be predominant spreading mechanisms. The procedure offers a method to select suitable separation conditions.