Principles and Practice of Chromatography - Peak Dispersion in a Chromatographic Column > The Resistance to Mass Transfer in the Stationary Phase > Page 49

Equation (9) is the Van Deemter equation that describes the variance per unit length of a column in terms of the physical properties of the column contents, the distribution system and the linear velocity of the mobile phase. Alternatively the Van Deemter equation can be expressed in the form,

 

(10)

 

where (H) is the Height of the Theoretical Plate. The relationship between 0 and (sx) is explained in The Plate Theory and Extensions .

Hence the term "HETP equation" for equation (10). This form of the Van Deemter equation is very nearly correct for LC but, due to the compressibility of the gaseous mobile phase in GC, neither the linear velocity nor the pressure is constant along the column. Furthermore, as the diffusivity, (Dm), is a function of pressure, the above form of the equation can only be approximate. However, equation (10) generally gives the correct form of the relationship between (H) and the linear velocity (u). It also predicts that there will be an optimum velocity that gives a minimum value for (H) and thus, a maximum efficiency. Pressure corrections for retention volume and the height of the theoretical plate are derived in Plate Theory and Extensions and The Mechanism of Chromatographic Retention .