Preparation And Evaluation Of The Novel C16 Monolithic Silica Columns For Capillary Electrochromatography

Capillary electrochromatography (CEC) is an exciting miniaturized separation technique, which combines the advantages of high efficiency of capillary electrophoresis (CE) with high selectivity of high performance liquid chromatography (HPLC). Therefore, CEC has a double retention mechanism of both CE and HPLC, which is not only suitable for separation of charged compounds but also neutral compounds.Column is the core of chromatographic separation. Monolithic columns for capillary electrochromatography have attracted increasing interests due to its advantages of easy preparation, excellent permeability and high efficiency. Generally, monolithic columns can be classified into two categories: polymer-based and silica-based monoliths. The latter, which posseses high mechanical strength, excellent solvent resistance and temperature resistance, is widely used in the field of chromatographic separation science. However, most of silica-based monoliths used for CEC are traditional C18 columns in reversed phase(RP) mode, which obtain relatively satisfactory results for nonpolar analytes separation. Analysis of acidic and basic compounds is still difficult with traditional silica-based bonded stationary phases in RP-CEC. To solve this problem, novel C16 monolithic silica columns were prepared in this thesis. Details are followed as:1. A C16 silica monolith was prepared by organic-inorganic hybrid sol-gel technique. Exactly, the hybrid matrix was in-suit prepared in capillary firstly, then chemically modified with hexadecyltrimethoxysilane(HDTMS). Characterization of the prepared silica monolith was performed. And the RP chromatographic behavior of hybrid matrix C16 silica monolith was evaluated with alkylbenzenes. Moreover, four anilines were successfully separated with symmetrical peaks. The results showed that the prepared monolithic silica column possesses potential application for separation of basic compounds.2. A silica-based monolithic stationary phase with RP and weak anion-exchange(WAX) was prepared. The mixed-mode monolithic silica column was prepared using the sol-gel technique and followed by a post-modification with HDTMS and aminopropyltrimethoxysilane(APTMS). The amino groups on the surface of the stationary phase were used to generate a substantial anodic EOF as well as to provide electrostatic interaction sites for charged compounds at low pH. A variety of analytes were used to evaluate the electrochromatographic characterization and column performance. The monolithic stationary phase exhibited RP chromatographic behavior toward alkylbenzenes and phenols. The model anionic solutes were separated by the mixed-mode mechanism, which comprised RP interaction, WAX, and electrophoresis. Symmetrical peaks can be obtained for basic solutes because positively charged amino groups can effectively minimize the adsorption of positively charged analytes to the stationary phase.3. A silica-based monolithic stationary phase with enhanced electroosmotic flow was prepared. First, a hybrid silica-based matrix was prepared via sol-gel process, and then chemically modified with HDTMS and (3-mercaptopropyl) trimethoxysilane(MPTMS). Finally, the thiol groups on the surface of the stationary phase were oxidized to sulfonic acids by hydrogen peroxide at room temperature. Characterization of the prepared silica monolith was performed. The results showed that the introduced sulfonic acid moieties on the monoliths could produce a strong and relatively stable EOF in a broad pH range. The prepared column exhibited RP chromatographic mechanism for neutral compounds. Basic solutes were successfully separated without peak tailing because the dissociations of silanols were suppressed at low pH.