Tests for the Comparison of Columns and Their Expressiveness

The chemical character of a phase, i.e., if it appears more polar or more apolar, is regarded as the most important criterion for the comparison of RP colunms. Therefore, the aim of more or less aU tests reported in the literature is to check the silanophilicity or the hydrophobidty. In the 1980s and early 1990s, Walters, Sander, and Tanaka also took steric aspects into consideration. The efficiency was thereby further checked. In the following, the expressiveness of such tests of silanophihdty / hydrophobicity is discussed in short form and some results are presented. Finally, of some recently developed tests [5], one is described that yields information about the homogeneity of the surface. 

In recent years, several tests have been developed to characterize the silanophilic properties of RP phases. These tests are usually based on the behavior of the phases towards basic substances, as, for example, in the case of the EngeUiardt test, whereby acidic, neutral, and basic compounds are used. As a measure of the silanophilicity, one uses either the elution of a base compared to phenol (an earlier elution than phenol indicates a low silanophilicity of the phase) or the asymmetry factor of p-ethylaniline in a methanol/water eluent Recent results show that a classification by these criteria can only be quite rough silanophilic / hydro-phobic . 

The lack of differentiation between acidic and mild silanol groups, the neglecting of (almost invariably fotmd) mixed mechanisms, and the use of quite weak bases restdts in a doubtfijl similarity of the phases. Some phases, denoted 

C oncentration of acidic silanol groups. These are groups which are dissociated even in an acidic environment and which are able to interact with protonated bases (ion exchange). 

Total concentration of available silanol groups i.e. good shielding of the surface, activity of total silanol groups (hydrogen bonding, dipole-dipole interactions, etc.). 

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