# Plate Theory and Extensions - The Plate Theory > Page 3

Said (1) developed the Martin concept (2) to derive the elution curve equation in the following way.

As equilibrium must exist in each plate by definition, then

Xs = KXm                                        (1)

 where (Xm) is the concentration of solute in the mobile phase, (Xs) is the concentration of solute in the stationary phase, and (K) is the distribution coefficient of the solute between the two phases and is defined with reference to the stationary phase, i.e., K = Xs/Xm

The larger the value of (K), the more the solute will be distributed in the stationary phase under equilibrium conditions. (K) is a dimensionless constant and, in gas/liquid and liquid/liquid systems, (Xs) and (Xm) can be measured as mass of solute per unit volume of phase. In gas/solid and liquid/solid systems, (Xs) and (Xm) can be measured as mass of solute per unit mass of phase.

Equation (1) reiterates the general distribution law and presumes the adsorption isotherm as linear. In both gas/solid chromatography (GSC) and liquid/solid chromatography (LSC), virtually all the solutes exhibit Langmuir type isotherms between the two phases which, over a wide concentration range, is certainly not linear. However, at the extremely low solute concentrations employed in chromatography, (i.e., that portion of the isotherm that is pertinent) the isotherm can be considered as linear.