Spectroscopic and chromatographic characterization of chemically modified silica sorbents

Abstract

The most commonly used chromatographic and solid phase extraction sorbents are silica-based. Usually trifunctional silanes are used to modify silica sorbents because they can form polymer networks that provide increased stability as compared to sorbents prepared with monofunctional silanes. Unfortunately, the reactions of trifunctional silanes with a silica surface, and with each other, are difficult to control and the resulting polymerized bonded phase is very complex. Although these sorbents have been used extensively for various applications, studies of bonded phase chemistry continue to this day. The importance of such studies lies in the fact that an understanding of chemically modified silica sorbents will lead to the development of better chromatographic and solid phase extraction methods. This study focuses on the influence of silica topography on the structure and surface coverage of bonded phases, as well as the differences between hydrocarbon and ion exchange bonded phases. Solid-state NMR spectra, recorded for alkyl sorbents, showed that the porosity of the base silica determined the surface coverages of the different bonded phases. Large silanes and siloxane oligomers were excluded from micropores, limiting their access to a significant fraction of the silica surface area. These conclusions were made after analyzing the spectra of endcapped sorbents, which contained peaks that represented polymer and surface bound endcapping reagent. These experiments were supplemented with normal and reversed phase chromatographic data that indicated that shorter alkyl silanes were not dispersed evenly on the heterogeneous silica surface, as opposed to octyl and octadecyl silanes. In addition, bonded phases that contained excessive vertical polymerization were found to exhibit non-uniform retention behavior. Spectroscopic characterization of anion exchange sorbents supported the hypothesis that ionic silanes affect the structure of a bonded phase. It was discovered that the post-modification of cation exchange precursor sorbents also produced bonded phases without vertical polymerization. The uniformity of both anion and cation exchange sorbents was confirmed by studying the effects of endcapping. However, it was shown that an octyl thioacetate phase hindered bond breakage during postmodification oxidation. The resulting octyl sulfonic acid sorbent was shown to exhibit increased selectivity for analytes with both hydrophobic and ionic character.