One of the most important characteristics describing the column properties is column permeability. Term permeability refers to the column packed with a stationary phase (particles or monolith) and describes how easy flows the liquid (mobile phase) through the column.

At given pressure, the higher is flow through the column the higher is the permeability of the column. More exact definition of the permeability is described as  the volume flow of fluid per unit time per unit area per unit pressure gradient.

In case of liquid chromatography, there is limiting pressure that has practical uses, so if particle s with small diameter are used (to have as low plate height as possible), then the column with limited length and mobile phase with a low viscosity have to be used.

Column permeability calculation

The column permeability can be easily calculated from the flow and column characteristics using the embeded equation. Here the F_m is the mobile-phase flowrate, η is the mobile-phase viscosity, Δp is the pressure drop across the column, L is the column length, and r is the column inner radius.

One can see that the flow of the mobile phase through the column is directly proportional to the pressure across the column and the fourth power of the particle diameter and inversely proportional to the product of the mobile phase viscosity and the length of the column.

Chromatography is analytical chemistry method which is used (and useful) for the separation of complex mixtures of chemical compounds. The main mechanism of the separation is repeatable distribution of the tested compound in between two different phases.

Usually, one phase is solid, fixed in the separation device and the other is moving and flows through the unit. If gas is a second phase, we are referring to the gas chromatography, in case of liquid as a second phase the name is liquid chromatography.

The device where separation takes place is called chromatographic column. This cylindrical shape column is filled with the different kinds of materials – stationary phases. These materials are usually spherical silica particles with different, but well defined, surface chemistry.

The mobile phase flows through the column together with sample (mixture of compounds). Each compound has various affinity to the surface of stationary phase and therefore is separated form each other. In case of ideal state all compounds are eluted from the column in separated bands.

Various techniques are used to recognize these bands and transform them into the signal. In most of the cases the signal draws chromatographic peak – the “hill like” curve describing Gauss distribution.