# 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 (s_{x}) 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, (D_{m}),
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
.