Topics - Van Deemter Equation
Van Deemter Equation
The Van Deemter equation gives the variance per unit length of a packed
column in terms of the physical properties of the solute and phase system and the linear mobile phase velocity. In liquid chromatography the mobile phase velocity is taken as the exit velocity, that is the ratio of the flow rate in ml/second to the cross-sectional area of the ‘column-exit flow path’. For a packed column the cross-sectional area of the column exit flow path is usually taken as the 0.6 times the cross-sectional area of the column.Alternatively, the linear velocity can be taken as the ratio of the column length to the dead time. If the mobile phase is a gas then the pressure correction
must be applied. The variance per unit length of the column is taken as the ratio of the column length to the column efficiency in theoretical plates. The Van Deemter equation is a hyperbolic function that predicts that there is an optimum velocity at which there will be the minimum variance per unit column length and, thence, a maximum efficiency. The Van Deemter equation was the result of the first application of the rate theory to the chromatography elution process.