Characterization of the interactions on anion-exchange modified silica sorbents

Abstract

Understanding the interactions at the modified silica interface used as a stationary phase in various chromatographic techniques is of great importance in elucidating the mechanism of solute retention. Investigating the factors that control the selectivity and efficiency for retention of a solute is also important as it can lead to the manipulation of the interfacial properties to give improved separations. In this research, solid phase extraction was used to obtain information about silica based anion-exchange stationary phases and their interaction with acidic analytes. Solid Phase Extraction (SPE) experiments using strong anion-exchange sorbents containing a fixed positive charge illustrated the importance of the counter-ion present at the surface. From these studies, it was determined that different counter-anions have different affinities for the ion-exchange site. Lower selectivity counter-anions (i.e. acetate) are more easily displaced from the sorbent by acidic analytes than a higher selectivity counter-anion (i.e. citrate). The general trend amongst counter-ions beginning with the counter-ion with the greatest affinity for the ion-exchange site is shown here: citrate > maleate > sulfate > formate > phosphate > chloride > hydroxide > nitrate > propionate > acetate. Overall, selectivity was not only determined to be a function of ionic interactions, but was also found to be a function of the extent of hydration of both the counter-ion and the surface. Weak anion-exchange sorbents consisting of primary and secondary amines were also investigated. In order for weak sites to contain ion-exchange sites, the pH needs to be selected so that the surface is ionized. Due to the pH dependence of weak anion exchangers, studies were undertaken to determine the effect that pH has on the extraction of acidic analytes. It was determined that the pH at the surface is not necessarily that of the bulk solution. It was also concluded that ionic, hydrophobic, strong dipole, and charge-induced dipole interactions contribute to the extraction of acidic compounds. SPE was also applied to the isolation and purification of acidic compounds. With a better understanding of the surface/solvent environment, a generic approach was developed for the extraction of toxicologically relevant compounds from biological matrices. By understanding the influence of pH, counter-anion, and degree of hydrophobic character of both analyte and surface, an enhancement in extraction efficiency and selectivity was achieved.