Abstract

This dissertation focuses on improving the chromatographic efficiency of polymeric organic monoliths by characterizing and optimizing the bed morphology. In-situ characterization techniques such as capillary flow porometry (CFP), 3-dimensional scanning electron microscopy (3D SEM) and conductivity measurements were developed and implemented to quantitatively characterize the morphology of poly(ethylene glycol) diacrylate (PEGDA) monoliths. The CFP measurements for monoliths prepared by the same procedure in capillaries with different diameters (i.e., 75, 150, and 250 μm) clearly showed a change in average through-pore size with capillary diameter, thus, certifying the need for in-situ measurement techniques. Serial sectioning and imaging of PEGDA monoliths using 3D SEM gave quantitative information about the average pore size, porosity, radial heterogeneity and tortuosity of the monolith. Chromatographic efficiency was better for a monolith with smaller average pore size (i.e., 5.23 μm), porosity (i.e., 0.49), radial heterogeneity (i.e., 0.20) and tortuosity (i.e., 1.50) compared to another monolith with values of 5.90 μm, 0.59, 0.50 and 2.34, respectively. Other than providing information about monolith morphology, these techniques also aided in identifying factors governing morphological changes, such as capillary diameter, polymerization method, physical/chemical properties of the pre-polymer constituents and weight proportion of the same. A statistical model was developed for optimizing the weight proportion of pre-polymer constituents from their physical/chemical properties for improved chromatographic efficiency. Fabricated PEGDA columns were used for liquid chromatography of small molecules such as phenols, hydroxyl benzoic acids, and alkyl parabens. The chromatographic retention mechanism was determined to be principally reversed-phase (RP) with additional hydrogen bonding between the polar groups of the analytes and the ethylene oxide groups embedded in the monolith structure. The chromatographic efficiency measured for a non-retained compound (uracil) was 186,000 plates/m when corrected for injector dead volume. High resolution gradient separations of selected pharmaceutical compounds and phenylurea herbicides were achieved in less than 18 min. Column preparation was highly reproducible, with relative standard deviation (RSD) values less than 2.1%, based on retention times of the phenol standards (3 different columns). A further improvement in chromatographic performance was achieved for monoliths fabricated using a different polymerization method, i.e., living free-radical polymerization (LFRP). The columns gave an unprecedented column performance of 238, 000 plates/m for a non-retained compound under RP conditions.

Degree

PhD

College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry

Rights

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