Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1997 Electrochemically-modulated liquid chromatography (EMLC): Column design, retention processes, and applications En-Yi Ting Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Analytical Chemistry Commons, Medicinal and Pharmaceutical Chemistry Commons, Medicinal Chemistry and Pharmaceutics Commons, and the Medicinal-Pharmaceutical Chemistry Commons Recommended Citation Ting, En-Yi, "Electrochemically-modulated liquid chromatography (EMLC): Column design, retention processes, and applications " (1997). Retrospective Theses and Dissertations. 12253. https://lib.dr.iastate.edu/rtd/12253 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. 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UMI A Bell & Ifowell Infomiation Conq)a]qr 300 Noith Zsdb Road, Ann Aibor MI 48106-1346 USA 313/761-4700 800/521-0600 Electrochemicaiiy-modulated liquid chromatography (EMLC): Column design, retention processes, and applications by En-Yi Ting A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Major: Analytical Chemistry Major Professor: Marc D. Porter Iowa State University Ames, Iowa 1997 DMI Number: 9737766 UM Microform 9737766 Copyright 1997, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 ii Graduate College Iowa State University This is to certify that the Doctoral dissertation of En-Yi Ting has met the dissertation requirements of Iowa State University Signature was redacted for privacy. Major Professor Signature was redacted for privacy. For the Major Program Signature was redacted for privacy. qjxthe Graduate College iii TABLE OF CONTENTS ACKNOWLEDGMENTS vi GENERAL INTRODUCTION 1 Dissertation Organization 1 Literature Review 1 Dissertation Overview 5 CHAPTER 1. MODIFICATION COLUMN FOR 7 ELECTROCHEMICALLY MODULATED LIQUID CHROMATOGRAPHY (EMLC) ABSTRACT 7 INTRODUCTION 7 EXPERIMENTAL SECTION 9 RESULTS AND DISCUSSION 14 CONCLUSIONS 33 ACKNOWLEDGMENT 34 REFERENCES AND NOTES 34 CHAPTER 2. RETENTION MECHANISM OF NEUTRAL 36 AROMATIC COMPOUNDS ON ELECTROCHEMICALLY MODULATED LIQUID CHROMATOGRAPHY (EMLC) AT A POROUS GRAPHITIC CARBON STATIONARY PHASE ABSTRACT 36 INTRODUCTION 37 iv EXPERIMENTAL SECTION 38 RESULTS AND DISCUSSION 42 CONCLUSIONS 52 ACKNOWLEDGMENT 52 REFERENCES AND NOTES 53 CHAPTER 3. SEPARATIONS OF CORTICOSTEROIDS USING 55 ELECTROCHEMICALLY-MODULATED LIQUID CHROMATOGRAPHY (EMLC): SELECTIVITY ENHANCEMENTS AT A POROUS GRAPHITIC CARBON STATIONARY PHASE ABSTRACT 55 INTRODUCTION 56 EXPERIMENTAL SECTION 57 RESULTS AND DISCUSSION 60 CONCLUSIONS 71 ACKNOWLEDGMENT 71 REFERENCES 72 CHAPTER 4. SEPARATIONS OF BENZODIAZEPINES USING 74 ELECTROCHEMICALLY-MODULATED LIQUID CHROMATOGRAPHY (EMLC): EFFICIENT SEPARATIONS FROM CHANGES IN THE VOLTAGE APPLIED TO A POROUS GRAPHITIC CARBON STATIONARY PHASE ABSTRACT 74 INTRODUCTION 75 V EXPERIMENTAL SECTION 76 RESULTS AND DISCUSSION 80 CONCLUSIONS 90 ACKNOWLEDGMENT 90 REFERENCES 91 SUMMARY AND PROSPECTUS 93 Summary 93 Prospectus 95 REFERENCES 99 vi ACKNOWLEDGMENTS The author gratefully acknowledges his major professor Marc D. Porter for his guidance, encouragement, and patience during the past five years of study. Discussions with the members of the Porter group have been invaluable throughout some of the research projects and their contributions are greatly appreciated. The author would like to express the deepest appreciation to his wife, Ya-Fang, for her patience, trust and understanding through these years. This work was partially supported by the National Science Foundation and the Ames Laboratory, and was perfonned at Ames Laboratory under Contract No. W-7405-Eng-82 with the U.S. Department of Energy. The United States govemment has assigned the DOE Report number IS-T1817 to this thesis. 1 GENERAL INTRODUCTION Dissertation Organization This work describes the continued development of a new separation technique, electrochemically-modulated liquid chromatography (EMLC), from column design, retention mechanisms to phamnaceutical applications. The introduction section provides a literature review of the technique as well as a brief overview of the research in each of the chapters. This section is followed by four chapters which investigate the issues of EMLC column design, the retention mechanism of monosubstituted aromatic compounds, and the EMLC-based applications to two important classes of phamnaceutical compounds (i.e., corticosteroids and benzodiazepines). The dissertation concludes with a general summary, a prospectus, and a list of references cited in the General Introduction. Literature Review High performance liquid chromatography (HPLC) has been and continues to be an important separation tool in the operation of many analytical laboratories [1-3]. A large number of columns with different stationary phases have been developed for various modes of separations, including normal-phase [4], reversed-phase [5], affinity [6], and ion chromatography [7]. Limitations and disadvantages of these columns lie in the fixed property of the stationary phases which cannot be manipulated to improve separation performance. For example, the conventional 2 way to solve co-elution problems is to optimize the mobile phase by changing the percentage of organic component or, for certain applications, the pH buffer or other additives. In general, however, the stationary phase plays a subordinate role in the optimization strategy. Thus, the analytical chemist Is judicious in the choice of a stationary phase, and furthennore, once that choice has been made for a certain separation, the usual route pursued to achieve the necessary resolution is to vary the composition of the mobile phase. As one consequence, a large amount of waste is generated during mobile phase optimization, which is becoming an ever increasing portion of operational costs. In order to overcome these limitations, many altematives have been presented, such as: transfomiable stationary phases and dynamic coating techniques [8-9]. EMLC is one of the new altemative techniques, combining electrochemistry and chromatography. This technique originates from electrosorption and electrodesorption concepts in electrochemistry, taking advantage of the effect of changes in applied potential (Eapp,) on the interactions between an electrode surface and analytes and, more interestingly, transforms a chromatographic column into a three-electrode electrochemical cell, with the stationary phase being the working electrode. In 1960, Blair and Murphy [10] described a process for desalting water based on periodic sorption and desorption of ions on the surface of porous carbon materials. In 1963, Fujinaga [11] first presented a design from the union of a thin-layer electrochemical cell and liquid chromatographic column to perform separations. Strohl [12] and Roe [13] in 1964 3 also proposed a liquid chromatographic column with a potential-controlled stationary phase. One of the similarities among these early works was the use of conductive material for stationary phases (e.g., glassy carbon and amalgamated metal particles). The conductive nature of the stationary phase allowed alterations in Eapp, to be used as a convenient means for changing the surface charge of the stationary phase. Fujinaga further demonstrated through improvements in cell design the feasibility of a voltage gradient throughout the stationary phase to enhance separations [14-15]. In 1972 [16], Strohl presented the manipulation of the retention of electroactive organic species (i.e., quinones) by changing their redox states on carbonaceous particles; this was followed by an investigation of the separation of inorganic cations by changes of