Hydroorganic Elution Mode Superimposition of RP or HILIC Mechanisms

3 Hydroorganic Elution Mode Superimposition of RP or HILIC Mechanisms 

The cinchona alkaloid-based CSPs are actually mixed-mode RP/weak anion-exchange phases and HILICAVAX phases, respectively. The surface layer of these 

CSPs has, overall, a hydrophobic character (very similar to RP phases with C4-C8 ligands) which stems from contributions of the chiral selectors itself and (capped) linker groups (only a portion of the linkers are utilized for selector attachment) which constitutes a kind of hydrophobic basic layer on the support surface. Hence under typical RP-conditions, hydrophobic interactions between lipophilic residues of the solute and hydrophobic patches of the sorbent may be active and thus a reversed-phase like partition mechanism may be superimposed upon the primary ion-exchange process k = A rp -I- A ix). This A Rp-retention contribution may be especially important for eluents with high aqueous content. 

Therefore, it is not further surprising that the organic modifier type and content is an effective variable to adjust retention and to optimize enantioselectivities. Methanol and acetonitrile have been frequently found to be complementary in their selectivity profiles and these two organic modifiers are advised to be tested in a preliminary screening experiment. 

Moreover, in various experiments it was found that at a constant total counterion concentration in the eluent the dependence of the retention factors on the organic modifier content tp largely follows the linear solvent strength theory (LSS) (Equation 1.5) 

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