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Reversed-phase liquid chromatography (RPLC) is the most popular analytical technique for separating complex mixtures. The most common stationary phases used are octadecyldimethyl (C18) phases with silica as the solid support. Although silica is the most widely used support, it is not without problems. Silica-based stationary phases have been under investigation since they were first produced in the late 1970s, and studies still continue to try to improve the phases. Silica has a small pH range (3-8) where mixtures can be separated without degradation of the column performance. Above a pH of 8, silica supports dissolve and destroy the column. Below pH 3, the silicon-carbon bond is cleaved, and the column is destroyed. Also, the silica surface has 8 μmol/m2 of reactive silanols for covalent bonding with an alkylsilane. Unfortunately, due to steric hindrance, only about 45% of the silanols can be covalently bound. The remaining silanols left on the surface after derivatization are deleterious to the separation of basic solutes. This dissertation describes the investigation of improving silica-based stationary phases for reversed-phase liquid chromatography. This work focuses on the synthetic methods used to decrease silanol activity and increase pH stability through the removal of silanols or by increasing the bonding density of reversed-phase stationary phases. The use of dehydroxylation to remove silanols was investigated. Dehydroxylation is the removal of silanols to form stable siloxane bonds, which happens thermally above ~400ºC. The useful temperature range is from ~400- 800ºC. Above 800ºC, the silica surface sinters (melts) to reduce the surface area and becomes chromatographically useless. These phases were characterized using 29Si cross-polarization magic angle spinning solid-state NMR (29Si CP-MAS) and diffusereflectance infrared Fourier Transform Spectroscopy (DRIFTS), along with liquid chromatography. Dehydroxylation was shown to decrease silanol activity and increase pH stability.
A Dissertation Submitted to the Department of Chemistry and Biochemistry in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy.
Includes bibliographical references.
Florida State University
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