Type of Document Dissertation Author Quach, Van URN etd-09012009-164008 Title Evaluation of Polar-embedded Reversed-phase Liquid Chromatography Columns and the Temperature Dependence of the Phase Ratio Degree Doctor of Philosophy Department Chemistry and Biochemistry, Department of Advisory Committee
Advisor Name Title John G. Dorsey Committee Chair Michael G. Roper Committee Member Sanford A. Safron Committee Member William T. Cooper Committee Member Michael Blaber Outside Committee Member Keywords
- Van't Hoff
- Phase Ratio
Date of Defense 2009-08-06 Availability unrestricted AbstractThe most commonly used analytical separation technique, reversed-phase liquid chromatography (RPLC), can suffer from irreproducible retention times and asymmetric peaks when analyzing basic compounds. This has been of considerable concern especially in the pharmaceutical industry where many drug analytes contain basic amine groups. Though the fundamental basis for peak asymmetry is debatable, the conventional thought is that it arises from residual silanols on the stationary phase surface that interact with the analyte through hydrogen bonding and electrostatic interactions. Column manufacturers have invested heavily into column technologies to eliminate this insidious effect. The introduction of polar-embedded columns has offered one possible solution to improving the chromatography. However, due to their novelty, little is known about these types of columns.
This study investigated the character of seven polar-embedded columns along with one conventional column as a comparison. Using a variety of test analytes including acids, bases and neutral analytes, selectivity studies demonstrated that these column offer unique chromatographic properties. The polar-embedded columns showed improved peak shape for basic compounds, especially at low pH. However, one column exhibited decreased efficiency for 4-n-butylbenzoic acid which was attributed to the manufacturing process. Different manufacturing processes were also responsible for the differences seen in columns with identical polar-embedded groups. In addition, log-log selectivity plots revealed similarities between columns with different bonded phases. Interestingly, the two column pairs that demonstrated the most similarity based on log-log plots were from the same manufacturer.
vanít Hoff analysis was used to calculate the transfer enthalpies for four classes of compounds. Enthalpy values for the columns were compared to values reported for conventional alkyl columns and were in agreement. The selection of homologous analyte pairs also allowed for the calculation of and of methylene transfer. The thermodynamic values for methylene selectivity were found to depend significantly on the class of analytes used.
The last portion of this research investigated the role of the phase ratio in vanít Hoff plots. Typically, plots of vs result in a linear relationship. However, the existence of non-linear vanít Hoff plots raises the possibility that a change in the phase ratio with respect to temperature may be the reason. Historically, non-linear vanít Hoff plots were attributed to a change in the thermodynamics of retention, i.e., a change in the retention mechanism. Assessments of experimental vanít Hoff plots using a C18 and C1 phase confirmed the conventional interpretation and showed that the phase ratio is temperature independent. Specifically, non-linear vanít Hoff plots using a C1 phase were observed. Since C1 phases cannot experience changes in the stationary phase volume, the resulting non-linear plot was attributed to a change in the thermodynamics of retention. Further corroboration of this conclusion was established when non-linear methylene selectivity plots were obtained.
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