Chromatographic activity of residual silanols of alkylsilane derivatized silica surfaces

Residual silanols on n-octadecylsilane derivatized silica surfaces have a pronounced effect on the chromatographic performance of reversed-phase high-performance liquid chromatography (RP-HPLC) materials. The present study describes how the results of solid-state NMR investigations on two commercially available reversed-phases for HPLC are verified chromatographically. One phase is a dimethyl-n-octadecylsilane derivatized silica substrate (Rx-C18), while the other is the same substrate derivatized with di-isobutyl-n-octadecylsilane ("stable bond", SB-C18). Four column tests, taken from the literature, are performed in order to assess silanol activity and column hyprophobicity. It is concluded that on the SB-C18 phase, more residual silanols are analyte accessible and that the isobutyl groups contribute significantly to hydrophobicity. Generally, the results of the solid-state NMR method for determining shielded and accessible residual surface silanols on RP-HPLC stationary phases could be confirmed.