Improving Separation by Changing the Selectivity

The selectivity of a separation is based on differences in the physicochemical interactions of the analytes with the stationary and mobile phases. This difference is measured as AtR on a chromatogram. 

To increase Vs, the chromatographer can increase the surface area of the stationary phase materials in normal-phase liquid chromatography, increase the stationary phase volume in reversed-phase or partition liquid chromatography, or increase the ion-exchange capacity in ion-exchange liquid chromatography. In general, if the internal diameter of a column is constant, the retention time 

There is no molecular interaction in size-exclusion liquid chromatography, and therefore the resolution can only be improved by increasing the column length. 

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