The Silanophilic Activity

Therefore, the aim of the manufacturer in the development of new reversed phases is primarily the minimization of the silanophilic activity. This is also the most critical parameter for the reproducible production of reversed phases. The silanophilic activity of a reversed phase is determined by the following parameters manufacturing process of the sifica, surface coverage of the stationary phase, and type of silane used (reactive and functional groups of the silane). 

At this point, however, it should be noted that it is not always desirable to use stationary phases with low silanophific activity. For example, when no basic compounds are present in the sample, then silanol groups may well make a positive contribution to the selectivity of the stationary phase. Thus, hydrogen bonds are 

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Figure 3.25. RPLC column selectivity chart. X-axis is an indication of the hybrophobicity and Y-axis is an indication of the silanophilic activity of the bonded phase. This comparative chart is useful for selecting equivalent or dissimilar columns from various vendors. Diagram courtesy of Waters Corporation.

Figure 8.20. Three comparative chromatograms of three different bonded-phase columns using the optimized mobile phase conditions shown in Figure 8.15. The surprise finding of peak shape problems with two specific impurities might be attributed to the higher silanophilic activity of the column. Data presented in parts at Eastern Analytical Symposium, Somerset, New Jersey, November 2003.

Figure 10.9. Comparative chromatograms of a good and a failed silica-base C18 column due to exposure to high pH mobile phase. Severe peak fronting indicated channeling of the packed bed while peak tailing of the basic analyte (amitriptyline) indicated increase of silanophilic activity due to loss of endcapping or bonded groups. Figure courtesy of Waters Corporation.

FIPLC columns packed with high-purity, silica-based bonded phases continue to dominate the market. Modern columns yield more symmetric peaks for basic analytes (less silanophilic activity) and have better batch-to-batch reproducibility and longer lifetimes. Improved bonding chemistries have widened the usable pFI range from 2-8 to 1.5-10 or more. Although C8- and CIS-bonded phases remain the most common, other phases have become quite popular, including phenyl, cyano, and several polar-embedded phases (e.g., amide, carbamate). 

This is the so-called silanophilic activity of the reversed phase. The compounds affine to silanol-groups are usually bases that interact with the acidic silanol groups via ionic interactions. These interactions are energetically much stronger than the usual hydrophobic interactions of reversed phases, which usually lead to wider elution bands for basic analytes on classical reversed phases. 

Residual silanol activity can have a beneficial effect on the separation of neutral polar compounds by supplementing hydrophobic interactions. It should not be constmed that columns with high silanophilicity are undesirable for all chromatographic separations, just mainly for the separation of strong bases. They are the preferred choice for some separations. 

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