Development of Green Chromatographic Techniques and Stimuli-Responsive Materials Based on CO2-Switchable Chemistry

Abstract

Developing alternatives to organic solvents and salts in chromatographic separation is highly desired. In this thesis, original studies were performed to demonstrate the feasibility of using CO2-modified aqueous solvents as an environmentally friendly mobile phase. Porous polymer monoliths were considered as a straightforward approach for the preparation of capillary columns with various functionality. A copolymer column containing dimethylaminoethyl methacrylate (DMAEMA) was investigated for the effect of CO2 on separation. Although a slight decrease of retention time of aromatic compounds was initially observed using acetic acid-modified solvent, the chromatographic separation using CO2-modified solvent was not reproducible, presumably resulting from the difficulty of reliably introducing gaseous CO2 into the nano LC system. Because different pH and temperature conditions can be easily applied, the pH and thermo-responsive behaviour of the copolymer column was also investigated. It showed the capability of pH and temperature for manipulating retention time and selectivity for various compounds. Because of the presence of ionizable groups, the column was also demonstrated for ion exchange separation of proteins. Following the initial work, a conventional HPLC system was used instead. A custom CO2 delivery system (1 bar CO2) was assembled to provide CO2-modified aqueous solvent with pH 3.9 ~ 6.5. A significant hydrophobicity switch of the stationary phase was observed by a reduction in retention time, when using CO2-modified solvents for the diethylaminoethyl (DEAE) and polyethylenimine (PEI) functionalized columns. In particular, the polyethylenimine column can be used to perform separation of organic molecules using 100% water without any organic solvent added. Another study was also conducted utilizing primary, secondary and tertiary amine functionalized silica particles (3.5 µm). A pH-/CO2-dependent ion exchange separation was demonstrated considering the protonation / deprotonation of both stationary phase and analytes. Carboxylic acid compounds were effectively separated using only carbonated water as the mobile phase. Despite the development of green chromatographic separations, this thesis also demonstrated the pH-/CO2-responsive surface wettability / adhesion of a polymer monolith surface grafted with functional polymers. Preliminary results indicate significant potential for applications such as drug screening and cell culture by introducing stimuli-responsive domains in droplet microarrays.