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Optimization of Sample Preparation in Matrix-Assisted Laser Desorption/Ionization (MALDI) Mass Spectrometry of Macromolecules" (2013)

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Optimization of Sample Preparation in Matrix-Assisted Laser Desorption/Ionization (MALDI) Mass Spectrometry of Macromolecules University of Arkansas, Fayetteville ScholarWorks@UARK Theses and Dissertations 8-2013 Optimization of sample preparation in Matrix- Assisted Laser Desorption/Ionization (MALDI) Mass Spectrometry of macromolecules Evgenia Akhmetova University of Arkansas, Fayetteville Follow this and additional works at: http://scholarworks.uark.edu/etd Part of the Analytical Chemistry Commons, and the Physical Chemistry Commons Recommended Citation Akhmetova, Evgenia, "Optimization of sample preparation in Matrix-Assisted Laser Desorption/Ionization (MALDI) Mass Spectrometry of macromolecules" (2013). Theses and Dissertations. 900. http://scholarworks.uark.edu/etd/900 This Dissertation is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of ScholarWorks@UARK. For more information, please contact [email protected], [email protected]. OPTIMIZATION OF SAMPLE PREPARATION IN MATRIX-ASSISTED LASER DESORPTION/IONIZATION (MALDI) MASS SPECTROMETRY OF MACROMOLECULES OPTIMIZATION OF SAMPLE PREPARATION IN MATRIX-ASSISTED LASER DESORPTION/IONIZATION (MALDI) MASS SPECTROMETRY OF MACROMOLECULES A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry by Evgenia Akhmetova Moscow State University Specialist in Chemistry, 2006 August 2013 University of Arkansas This dissertation is approved for recommendation to the Graduate Counsil ________________________________________ Dr. Charles L. Wilkins Dissertation Director ________________________________________ ________________________________________ Dr. David Paul Dr. Ingrid Fritsch Committee Member Committee Member ________________________________________ Dr. Bill Durham Committee Member ABSTRACT New method for the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) and Fourier transform mass spectrometry (MALDI-FTMS) analysis of low molecular weight polyvinyl acetate (PVAc) was developed and then applied to the characterization of commercially available chewing gum. The optimization of MALDI analysis of PVAc was achieved by investigating the influence of sample preparation variables such as the choice of solvent and choice of matrix-analyte ratio. It was demonstrated that the use of ethyl acetate as a solvent and 2,5-Dihydroxybenzoic acid as a matrix yielded the highest signal intensity for the pure polymer sample. The application of TOF technique did not produce accurate molecular structure of PVAc, and so FTMS method was employed as well and allowed to accurately establish the identity of the end groups of the polymer. A sample preparation protocol for the successful MALDI analysis of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was developed by investigating the influence of the matrix, solvent, deposition method and matrix-to-analyte ratio. It was found that the application of dithranol as a matrix, lowest molar matrix:analyte ratio of 250:1, toluene as a solvent for both matrix and analyte and aerospray sample deposition technique for MALDI analysis produced spectra with the highest intensity of PCBM signal, the smallest amount of fragments, the fewest products of gas phase reactions and the best reproducibility. Gas-phase reactions of PCBM in the high vacuum conditions of the FTMS and TOF mass spectrometry experiments were also investigated and a possible mechanism for these reactions was proposed. It was suggested that during or after desorption/ionization step of MALDI process, several kinds of oxidized and reduced PCBM derivatives are formed. The influence of the sample preparation parameters (the choice of the matrix, matrix:analyte ratio, salt:analyte ratio) was investigated and optimal conditions were established for the MALDI time-of-flight mass spectrometry analysis of the poly(styrene-co-pentafluorostyrene) copolymers. These were synthesized by atom transfer radical polymerization. Use of 2,5-Dihydroxybenzoic acid as matrix resulted in spectra with consistently high ion yields for all matrix:analyte:salt ratios tested. The optimized MALDI procedure was successfully applied to the characterization of three copolymers obtained by varying the conditions of polymerization reaction. It was possible to establish the nature of the end groups, calculate molecular weight distributions, and determine the individual length distributions for styrene and pentafluorostyrene monomers, contained in the resulting copolymers. Based on the data obtained, it was concluded that individual styrene chain length distributions are more sensitive to the change in the composition of the catalyst (the addition of small amount of CuBr2) than is the pentafluorostyrene component distribution. ACKNOWLEDGEMENTS I would like to express deep gratitude to my advisor Dr. Charles L. Wilkins for his guidance and help throughout my time in graduate school. His mentoring helped me to grow as a scientist and learn how to be an independent researcher, thus making the time I’ve spent at the University of Arkansas the most important time of my life. I also would like to thank Dr. Ingrid Fritsch, Dr. Bill Durham and Dr. David Paul for their help and input during my graduate career – their comments and recommendations improved my understanding of the subject of this dissertation and were very helpful to me. I would like to acknowledge the Chemistry and Biochemistry Department of the University of Arkansas for providing financial support for my research. In addition, I want to thank the undergraduate students whom I was privileged to mentor and who contributed to the research described in this dissertation. I’m thankful to the friends I made during my stay in graduate school, and especially past members of Wilkins group for their support, friendship and productive conversations about mass spectrometry. I’m very grateful to my wonderful new American family and my parents for their love and support, and to my husband Lucian who is always there for me when things get rough. TABLE OF CONTENTS I. MATRIX-ASSISTED LASER DESORPTION IONIZATION MASS SPECTROMETRY AS APPLIED TO THE ANALYSIS OF SYNTHETIC POLYMERS – AN OVERVIEW…………............1 A. Introduction………………………………………………………………………………………..1 B. Fundamental aspects of Matrix-Assisted Laser Desorption/Ionization …………………….2 C. Important parameters in sample preparation for MALDI analysis of polymers…………….4 1. Selection of the matrix………………………………………………………………….4 2. Influence of the solvent…………………………………………………………………6 3. Choice of cationizing agent…………………………………………………………….7 4. Importance of a sample deposition method………………………………………….8 D. Time-of-flight mass spectrometry: an overview……………………………………………….9 E. The fundamentals of Fourier transform mass spectrometry………………………………..11 II. SYNTHETIC POLYMERS..………………………………………….…………………………………..15 A. Introduction………………………………………………………………………………………15 B. Classification of polymers.………………………………………………………..…………….17 1. Molecular structure…..………………………………………………………………..17 2. Molecular geometry……………………………………………………………………17 3. Structural isomerism…………………………………………………………….…….18 4. Response to the environment………………………………………………………..19 5. Physical state………………………………………………………………………..…19 6. Chemical composition…………………………………………………………………21 7. Application……………………………………………………………………………...21 C. Main types of polymerization reactions……………………………………………….………22 D. Molecular weight distributions of synthetic polymers …………………………...…………..24 III. METHOD DEVELOPMENT FOR COMPOSITIONAL ANALYSIS OF LOW MOLECULAR WEIGHT POLYVINYL ACETATE (PVAc) BY MALDI-MASS SPECTROMETRY AND ITS APPLICATION TO THE ANALYSIS OF CHEWING GUM……….……...…………………………..26 A. Introduction………………………………………………………………………………………26 B. Experimental.………………………………………………………..……………………….….29 1. Materials…..…..………………………………………………………………………..29 2. Instrumentation and analysis…………………………………………………………30 3. Sample preparation…………………………………………………..………….…….31 C. Results and Discussion.……………..……………………………..……………………….….32 1. Analysis of low molecular weight polyvinyl acetate used as a chewing gum base …..…..……………………………………………………………….……..32 2. Method development for the analysis of chewing gum……………………………39 D. Conclusions……………………………………………………...……………………….……....41 IV. MALDI MASS SPECTROMETRY ANALYSIS OF [6,6]-PHENYL-C61-BUTYRIC ACID METHYL ESTER (PCBM)….……………………………………………………………………..……..52 A. Introduction………………………………………………………………………………………52 B. Experimental.………………………………………………………..……………………….….54 1. Materials…..…..………………………………………………………………………..54 2. Instrumentation and analysis…………………………………………………………54 3. Sample preparation…………………………………………………..………….…….55 C. Results and Discussion.……………..……………………………..……………………….….56 1. Sample preparation optimization – preliminary investigation…..…..……………..56 2. New sample preparation protocol development, analysis with TOF mass spectrometry………………………………………….…………………………..……57 3. Further sample preparation optimization and FTMS analysis ….....……………..58 D. Conclusions……………………………………………………...……………………….……....61 V. STRUCTURAL CHARACTERIZATION OF FLUORINATED POLYSTYRENES BY MALDI MASS SPECTROMETRY………………………………………………………………………………..73 A. Introduction………………………………………………………………………………………73 B. Experimental.………………………………………………………..……………………….….76 1. Materials…..…..………………………………………………………………………..76 2. Instrumentation and analysis…………………………………………………………77 3. Sample preparation…………………………………………………..………….…….77 C. Results and Discussion.……………..……………………………..……………………….….78 1. Sample preparation optimization for MALDI-TOF mass spectrometry analysis …..…..……………………………………………………………….………..78 2. Structural
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