Doctoral Dissertations

Date of Award

5-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Georges A. Guiochon

Committee Members

Frank Vogt, Michael D. Best, Rebecca A. Prosser

Abstract

The radial heterogeneity of some columns used in high performance liquid chromatography (HPLC) was investigated using an on-column microelectrochemical amperometric detector. Such a detector allowed the recording of the elution profiles at different spatial positions throughout the column exit cross-section. From this, we obtain information about the radial distribution of the mobile phase velocity, column efficiency, and analyte concentration. In all cases, the results obtained show that the spatial distribution of the mobile phase velocity does not follow a piston-flow behavior but exhibits radial heterogeneity with differences not exceeding 5% between the center and wall regions of any column. The efficiency was found to be lower in the wall region of the column than in its core region (the central core with a radius of 1/3 the column inner radius) by up to 40-50% in some columns. The radial distribution of the maximum concentration of the peaks varies throughout the column exit section, partially due to the radial variations of the column efficiency. The technology used in constructing the microelectrochemical detectors was further exploited to fabricate and incorporate an online detector array for a pressurized flat wide column measuring 10x10x0.1 cm in dimensions. Thus, unlike traditional thin layer chromatography, samples in this pressurized flat bed are completely eluted and detected in a time-based mode just like they are in HPLC. Also, a lateral arrangement of the detector array allows for an easy monitoring of the homogeneity of the flat wide column. Also, information on the surface properties of three novel chemically bonded phase packing materials for HPLC was obtained using solid state cross-polarization (CP) magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic experiments for the 29Si, and 13C nuclei. These packing materials were: Cogent bidentate C18 bonded to type-C silica, hybrid packing materials XTerra MS C18, and XBridge Prep. C18. The spectra obtained using cross-polarization magic angle spinning (CP-MAS) on the Cogent bidentate C18 bonded to type-C silica show the surface to be densely populated with hydride groups (Si-H), with a relative surface coverage exceeding 80%. The hybrid packing materials XTerra and XBridge gave spectra that reveal the silicon atoms to be bonded to alkyl moieties embedded in the molecular structure of these materials with over 90% of the alkyl silicon atoms found within the completely condensed silicon environments.

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