Collections > Electronic Theses and Dissertations > Characterization and Control of Band Broadening in Ultra-High Pressure Liquid Chromatography Columns
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Improving column performance remains paramount to advancing liquid chromatography (LC) technology. To that end, a series of experiments was designed to both measure and reduce band broadening in LC columns. The main broadening mechanisms (multiple flow path dispersion, longitudinal molecular diffusion, and resistance to mass transfer) were investigated. Dispersion due to multiple flow paths within a packed bed were studied with a series of columns prepared using different packing conditions. Packed column microstructure was analyzed by confocal laser scanning microscopy (CLSM) to determine bed morphology. Column efficiency was correlated to the bed morphology and the radial particle size distribution. Research on longitudinal molecular diffusion focused on the validity of different stationary phase diffusion models. Evidence was found supporting the recently proposed surface-restricted model of surface diffusion. This result has implications affecting both gradient separations and the calculation of the van Deemter B-term. An attempt to reduce the resistance to mass transfer term in the stagnant mobile phase focused on the implementation of a novel sub-2 μm macroporous silica stationary phase support. Unfortunately, the raw packing material contained a significant number of small mesoporous fines that limited the theoretical benefits of these particles, so a hydrodynamic chromatography (HDC) method was developed to remove the fines and reduce the particle size distribution (PSD). In addition to the classical on-column broadening mechanisms described above, broadening effects due to LC operation were studied, including frictional heating and extracolumn band broadening. When mobile phase flows over particles, frictional heating occurs. This heating can decrease column efficiency in sub-2 μm particle-packed columns larger than 1.0 mm in diameter. Studies of frictional heating effects on columns with different dimensions, particle types, and thermal environments were conducted to determine the impact these effects on performance. The LC instrument can diminish efficiency due to extra-column volumes that increase the measured peak variance. The variance contributions of the injector and connecting tubing on a capillary UHPLC system were measured. A possible coupling effect between the components was found that increases the tau-type (exponential decay) broadening contributions from the injector more than theory predicts.