Selectivity properties of the mobile phase

Eluent selectivity is the ability of different mobile phases to change the separation factor a of two or more compounds present in the sample. It has nothing to do with eluent strength but is another means that allows a separation to be influenced. Adsorption chromatographic selectivity has two different aspects, localization and basicity. 

Of the solvents listed in Section 5.1, the upper half of the table from fluoroalkane to dichloroethane represents nonlocalizing solvents with the exception of diethyl ether. This solvent as well as the lower half of the table from triethylamine to water are the localizing eluents. Therefore the two solvents dichloromethane (nonlocalizing) and diethyl ether (localizing) differ in this important selectivity property although they have almost identical strength ( = 0.30 and 0.29, respectively). 

Basicity is one of the axes of the solvent selectivity traingle of Fig. 5.1. The most basic of the common HPLC solvents are the ethers. 

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