Characterization of Subcritical Water as a Chromatographic Mobile Phase in Reversed-Phase Liquid Chromatography

• Other Authors: Allmon, Steven D. (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Chemistry
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-0176

Liquid chromatography, particularly reversed-phase liquid chromatography (RPLC), is a ubiquitous analytical method throughout various industries. It has proven vital to the regulatory framework of agencies worldwide to ensure product quality and safety. With traditional RPLC, organic modifiers (e.g. acetonitrile, methanol, tetrahydrofuran) are generally used in conjunction with water to form a mobile phase with sufficient eluotropic strength to elute analytes of interest from the chromatographic stationary phase to the detector employed for the liquid chromatography system. The organic modifiers used are costly as the analyst must pay not only to procure them but also to safely dispose of them. Additionally, the organic modifiers are usually toxic and flammable. The use of 100% water as a chromatographic mobile phase would present an attractive alternative, however, ambient water is far too polar in most instances to be of much use as a mobile phase for RPLC. Subcritical water, considered in the present work to be water at a temperature greater than 100°C with sufficient pressure applied to keep it within the liquid state (but still below the supercritical point), has been proposed as an alternative to traditional organic modifiers due to the apparent similarities in polarity between subcritical water and traditional organic modifiers. The present work explores the differences between RPLC using subcritical water and traditional organic modifiers. Thermodynamic data presented indicates that there are differences in the analyte retention process between the two systems. Retention in subcritical water is characterized by large, favorable enthalpy of transfer values and unfavorable entropic constributions to retention. Traditional RPLC shows favorable (to a lesser degree) enthalpic contributions to retention with negligible or not as unfavorable entropic contributions to retention. From the thermodynamic data, as well as subsequent linear solvation energy relationship analysis, the differences are attributed primarily to a large disruption in the hydrogen bonding network in water at elevated temperature, as well as the lack of sorbed organic modifier in the stationary phase (increasing dispersive interactions of the analyte with the stationary phase) when using 100% subcritical water as a mobile phase. Selectivity for a shape-constrained analytes is also shown to decrease when using subcritical water mobile phases, likely due to a decrease in conformational ordering of the stationary phase at elevated temperatures. Reduced ordering of the stationary phase coupled with an unfavorable entropy change upon retention also strongly suggests that a significant amount of disordering occurs in the pure water mobile phase at elevated temperatures. Later studies were aimed at estimating the retention factor in pure water, k'w, using a high temperature to low temperature extrapolation. Analysis of the results of this study revealed such an extrapolation is not comparable to the more traditional organic modifier fraction extrapolation to 100% water due to an underestimation of the hydrogen bond donating ability of the subcritical water system during the extrapolation. Finally, subcritical water as an extractions solvent with subsequent analyte focusing is explored. It is shown that it is feasible to re-focus a chromatographic peak using the unique properties of subcritical water with very modest instrumentation. Additionally, differences in effective selectivity were demonstrated resulting from on-column migration of the focused peak. Potential applications of this technique are also discussed. === A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Degree Awarded: Fall Semester, 2010. === Date of Defense: August 17, 2010. === Lipophilicity, Octanol/Water Partitioning, Shape Selectivity, Hot Water Chromatography, Superheated Water, Subcritical Water, Retention Thermodynamics, Liquid Chromatography === Includes bibliographical references. === John G. Dorsey, Professor Directing Dissertation; Shridhar Sathe, University Representative; André M. Striegel, Committee Member; Michael G. Roper, Committee Member; Oliver Steinbock, Committee Member.