| Capillary monolithic columns are widely used in micro-scale separation due to their advantages of easy preparation, good permeability, highly mass-transfer rate as compared to conventional particulate packed columns. Capillary monolithic column is a porous continuous rod prepared by in-situ polymerization or condensation within a capillary. The monolithic columns can be mainly classified into three categories, organic polymer-based and inorganic (silica) and the organic-inorganic hybrid based on the nature of the matrix chemistry. Organic-inorganic hybrid monolithic columns have drawn great attention for the potentially combining the merits of the organic polymer monolith and the inorganic monolith such as good mechanical stability, easy fabrication, wide pH range tolerance.The reversed phase liquid chromatography (RP-LC) for the separation of non-polar and weakly polar compound has been well developed. However, the RP-LC is not suitable for the separation of strong polar and ionic compounds because of the weak retention on the stationary phases. In contrast, the hydrophilic interaction chromatography (HILIC) or HILIC/IX mixed-mode exhibit great performance for these compounds. There are big demands for the separation of polar compounds and anions in bioanalysis and environmental analysis for the interested targets are almost polar or ionic compounds.In this dissertation, we prepared a set of HILIC/AX mixed-mode monolithic columns through the epoxy-amine ring-opening reaction (ROP) and modification in order to achieve effective separation of polar and acidic coumpounds. In the column preparation,2,4,6,8-tetramethyl-2,4,6,8-tetrakis (propyl glycidyl ether) cyclo tetra siloxane (POSS-epoxy) was chosen as a precursor for the introduction of inorganic silica core to the monolith to improve the mechanical stability. A second precursor the branched polyethyleneimine (PEI, Mn=600). as the functional monomer, was used which could provide massive amine groups as the chromatographic interaction sites. After systematic optimization of the preparation conditions including the solvent system, content of porogens, reaction temperature, the PEI-POSS hybrid monolithic columns with 3D skeletal structure were successfully prepared. The morphologies of the obtained monolithic columns were characterized by SEM. The SEM results confirmed that the monolith has a uniform skeleton and cross-linked macro though porous. Because of the existence of rich amines groups (including primary, secondary and tertiary amines), on the matrix surface, the PEI-POSS monolithic column exhibits HILIC/weak anion exchange (WAX) mixed-mode retention mechanisms in chromatographic separation. For the separation of neutral polar compounds, it was found that the peak elution order was based on the polarity from weak to strong, and the retention factor (k) increased with the increase of acetonitrile content in mobile phase. These phenomena were in consistent with the typical HILIC mechanism. To verify the presence of WAX interaction, halogen benzoic acids (HBAs) were chosen for the evaluation of PEI-POSS monolithic columns. The results showed that hydrophobic toluene eluted earlier than HBAs also proved the existence of HILIC mechanism. By changing the pH of the mobile phase, it was found that the retention of HBAs reduced with the decrease of pH. The reason is due to the suppression of the ionization of HBAs at low pH which weaken the ionic interaction between the analytes and the stationary phase. These results proved the PEI-POSS monolithic columns possess the WAX mechanism for acidic compounds. Chromatographic evaluation results confirmed that PEI-POSS monolithic columns with desired mixed-mode retention mechanisms of WAX and HILIC have been successfully preparedthrough the epoxy-amine ROP reaction.In virtue of the rich active amino exist on the matrix surface, the PEI-POSS monolith could be modified via the nucleophilic substitution reaction. Methyl iodide and bromoacetic acid were used to modify the monolithic column. After modification, the HILIC retention of polar compounds was notably increased compared to the unmodified PEI-POSS columns. In addition, the column efficiency also improved and the the highest column efficiency up to 100,000 N/m could be achieved. |
