All chromatographic separations are based on the differences in the extent to which solutes are partitioned (distributed) between the stationary phase and the mobile phase.

For the equilibrium distribution of a solute between the two phases, the partition coefficient or the distribution coefficient K is given by, $$K = \frac { { C }_{ S } }{ { C }_{ M } } $$ where, \({ C }_{ S }\) = Concentration of the solute in the stationary phase.

\({ C }_{ M }\) = Concentration in the mobile phase at equilibrium.

According to the plate theory developed by Martin and Synge, a chromatographic column consists of a series of discrete yet continuous horizontal layers which are called the theoretical plates.

An equilibration of the solute between the stationary and the mobile phases takes place at each of these plates. Migration of the solute is then assumed to take place by a series of stepwise transfers between one plate to the other immediately below.

The efficiency of separation in a chromatographic column increases as the number of theoretical plates increase. This is because the number of equilibrations will also correspondingly increase.

If the length of the column is \(l\) and the height equivalent of a theoretical plate is \(h\), then number of theoretical layers \(n\) is given by, $$n=\frac { l }{ h } $$

Thus, The Height Equivalent of a Theoretical Plate (HETP) is defined as **The height of a layer of the column, such that the solution leaving the layer is in equilibrium with the average concentration of the solute in the stationary phase throughout the layer.**