The Separation Ratio Selectivity of a Solute

The separation ratio of two solutes (A) and (B), (aA/B) is taken as the ratio of their corrected retention volumes,/.. . 

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It is clear that the separation ratio is simply the ratio of the distribution coefficients of the two solutes, which only depend on the operating temperature and the nature of the two phases. More importantly, they are independent of the mobile phase flow rate and the phase ratio of the column. This means, for example, that the same separation ratios will be obtained for two solutes chromatographed on either a packed column or a capillary column, providing the temperature is the same and the same phase system is employed. This does, however, assume that there are no exclusion effects from the support or stationary phase. If the support or stationary phase is porous, as, for example, silica gel or silica gel based materials, and a pair of solutes differ in size, then the stationary phase available to one solute may not be available to the other. In which case, unless both stationary phases have exactly the same pore distribution, if separated on another column, the separation ratios may not be the same, even if the same phase system and temperature are employed. This will become more evident when the measurement of dead volume is discussed and the importance of pore distribution is considered. 

To aid in solute identification, a standard substance is usually added to a mixture and the separation ratio of the solutes of interest to the standard is used for identification purposes. In practice, separation ratios are calculated as the ratio of the distances in centimeters between the dead point and the maximum of each peak. If the flow rate is sufficiently constant and data processing is employed, then the corresponding retention times can be used. 

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