Topics - Column Permeability
In chromatography, the term permeability usually refers to a packed bed (as in a packed chromatography column) and describes how ‘free’ the bed is to permit fluid-flow. The greater the fluid flow through a packed bed, driven by a given differential pressure, the greater the permeability of the bed. The classic definition of permeability
, when referred to a packed bed, is the volume flow of fluid per unit time per unit area per unit pressure gradient. The expression for the volume flow of fluid through a packed bed is given by D’Arcy’s law. D’Arcy’s law states that the volume flow of fluid through a packed bed is directly proportional
to the differential pressure across the bed and the fourth power of the particle diameter and inversely proportional
to the product of the fluid viscosity and the column length. In liquid chromatography, there is a maximum pressure that is practical to use, and so, if small diameter particles are to be employed to obtain short plate heights, then the column must have a limited practical length and solvents of the lowest viscosity should be used.