In column chromatography a mixture of substances is dissolved in a mobile phase and passed over a stationary phase in a column. A selectivity factor is defined as the ratio of distribution coefficients, which describe the equilibrium distribution of an analyte between the stationary phase and the mobile phase. The selectivity factor is equal to the selectivity coefficient with the added assumption that the activity of the stationary phase, the substrate in this case, is equal to 1, the standard assumption for a pure phase. The resolution of a chromatographic column, R is related to the selectivity factor by:
where α is selectivity factor, N is the number of theoretical plates k and k are the retention factors of the two analytes. Retention factors are proportional to distribution coefficients. In practice substances with a selectivity factor very close to 1 can be separated. This is particularly true in gas-liquid chromatography where column lengths up to 60 m are possible, providing a very large number of theoretical plates.
In ion-exchange chromatography the selectivity coefficient is defined in a slightly different way