Electrochemically-modulated liquid chromatography (EMLC): Column design, retention processes, and applications.

摘要

This dissertation explores a new separation technique, electrochemically modulated liquid chromatography (EMLC), from the column design, retention processes, to the pharmaceutical applications. A literature review, general summary, and perspectives of this technique are also described. Chapter 1 presents the newly designed EMLC column. The principal modification of this design is to connect the porous stainless steel column as counter electrode as opposed to part of working electrode in the previous design. The improvement in performance from this modification results in a shorter response time to changes in applied potential {dollar}(Esb{lcub}rm appl{rcub}){dollar} and a better control of {dollar}Esb{lcub}rm appl{rcub}{dollar} at cathodic values of {dollar}Esb{lcub}rm appl{rcub}.{dollar} The performance of the new design is presented and compared to the previous design.; Chapter 2 describes the study of retention processes of analytes on EMLC. A mixture of substituted aromatic compounds has been investigated to examine the influence of {dollar}Esb{lcub}rm appl{rcub}{dollar} to retention. Results show that donor-acceptor interactions dominate the retention processes and that the analytes with larger submolecular polarity parameters or higher energy levels of highest occupied molecular orbital display larger sensitivities in retention to changes in {dollar}Esb{lcub}rm appl{rcub}.{dollar}; In Chapter 3, EMLC has been applied to the separation of a mixture of structurally similar corticosteroids. Changes in the {dollar}Esb{lcub}rm appl{rcub}{dollar} to the column markedly affected the efficiency as well as the elution order of the separation, with the mixture fully resolved at large negative values of {dollar}Esb{lcub}rm appl{rcub}.{dollar} Mechanistic aspects in terms of the influence of changes in the {dollar}Esb{lcub}rm appl{rcub}{dollar} on the extent of the interactions between these analytes and the stationary phase are briefly discussed.; In Chapter 4, the separation of a mixture of benzodiazepines has been investigated by EMLC. Changes in the {dollar}Esb{lcub}rm appl{rcub}{dollar} to the stationary phase strongly alter the retention of all analytes. The observed dependencies of retention have the unusual effect of stretching both ends of the chromatogram as {dollar}Esb{lcub}rm appl{rcub}{dollar} becomes more negative. That is, the retention for some of the benzodiazepines increases as {dollar}Esb{lcub}rm appl{rcub}{dollar} moves negatively, whereas that for some of the other benzodiazepines decreases. The combined weight of these dependencies results in the ability to achieve a fully resolved separation of the mixture while only marginally increasing the overall elution time.