Stationary Phase Characterization with Empirical LFER Parameters

5 Stationary Phase Characterization with Empirical LFER Parameters 

In a first follow-up publication [18], general investigations into this subject are described. In three later publications, 87 stationary phases based on ultra-pure (type B) silica were studied with regard to these correlations [19], then a further 87 phases based on a metal-rich (type A) silica [20], and finally 21 reversed phases with embedded polar groups or polar endcapping [21]. 

The steric parameter S reflects the resistance of the stationary phase to undergo insertion of solute molectdes with a higher thickness . It exhibits very similar correlations with stationary phase properties such as the hydrophobicity parameter H. However, the S parameter cannot be related to the so alled shape-selectivity parameter defined by Sander and Wise [22-24], which describes the ability of phases to differentiate between planar and twisted polyaromatic hydrocarbons (PAH). These two parameters do not correlate at all and obviously characterize very different properties. 

The most difficult parameter for a chemical interpretation in the case of a simple silica-based reversed-phase without additional polar groups is the B parameter, which characterizes the stationary phase H-acceptor basicity. With type B silicas, it is mainly related to adsorbed water at the stationary phase surface. With some of the type A phases, a correlation with the metal content may be seen, but this is not a general rule. Some polar modified phases exhibit pronounced Bparameters, but a imiversal dependence on the chemical identity of the polar group cannot be found. 

The last parameter to discuss is the pH-dependent C-parameter, which describes the cation-exchange activity of the reversed phase. As might be expected, the size of C as well as the magnitude of its pH dependence is more pronoimced with 

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