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Elimination of Electrochemical Oxidation During Sample Ionization

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Elimination of Electrochemical Oxidation During Sample Ionization Elimination of Electrochemical Oxidation during Sample Ionization Using Liquid Sample Desorption Electrospray Ionization (DESI) A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree of Master of Science Najah K. Almowalad August 2016 © 2016 Najah K. Almowalad. All rights Reserved 2 This thesis titled Elimination of Electrochemical Oxidation during Sample Ionization Using Liquid Sample Desorption Electrospray Ionization (DESI) by NAJAH K. ALMOWALAD has been approved for the Department of Chemistry and Biochemistry and the College of Art and Science by Hao Chen Associate Professor of Chemistry and Biochemistry Robert Frank Dean, Collage of Art and Science 3 ABSTRACT ALMOWALAD, NAJAH K., M.S., August 2016, Chemistry Elimination of Electrochemical Oxidation during Sample Ionization Using Liquid Sample Desorption Electrospray Ionization (DESI) Director of Thesis: Hao Chen This thesis introduces a method to eliminate the electrochemical oxidation that takes place during sample ionization by using liquid sample desorption electrospray ionization (DESI) method. Several organic compounds including N, N, N', N'-tetramethyl-p- phenylenediamine (TMPD), N-phenyl-p-phenylenediamine (PPD), phenothiazine (PT), andtriphenylamine (TPA)were observed to produce significant amount of oxidized product ions when electrospray ionization (ESI) in the positive mode ion was used for their ionization due to the inherent oxidation by ESI. Interestingly, the oxidation of these organic compounds can be avoided by using liquid sample desorption electrospray ionization (DESI). This phenomenon could be explained by the fact that no high voltage is directly applied to the sample solution during DESI analysis. Master of Analytical Chemistry. Elimination of Electrochemical Oxidation during Sample Ionization Using Liquid Sample Desorption Electrospray Ionization (DESI). 4 DEDICATION I dedicate this thesis to my parents, my king, the US and brothers. 5 ACKNOWLEDGMENTS First, I would like to express my thankfulness and appreciation to my advisor, Dr. Hao Chen, for his great support in science and wonderful advice for three years. He gave me all the encouragements and eager to do experiments and conduct research. He also guided me to work hard and to enjoy research at Ohio University. Second, I would like to thank Dr. Peter Harrington and Dr. Mark McMills for cooperation as committee members to give me suggestions about my thesis. Third, I would like to thank Dr. Hao Chen‘s group members, Dr. Mei Lu, Dr. Pengyuan Liu, Dr. Qiuhua Wu, Dr. Zhi Li, Dr. Hetong Qi, Si Cheng, Dr. Qiuling Zheng, Yi Cai, Li Xiyang, Chang Xu, and Yuexiang Zhang for their support, cooperation and friendship. Fourth, I really want to thank my father for his support and for being with me in the U.S for the period of my study. Also, I want to thank my mother for her emotional support and encouragements from my home country (Saudi Arabia). 6 TABLE OF CONTENTS Abstract……………………………………………………………..………….………….3 Dedication…………………………..…………………………………..…………………4 Acknowledgments………………………………………………………..………………..5 List of Figures………………………………………………………………..…….……...8 Chapter 1: Introduction………………………………………………..…………………10 1.1 Mass Spectrometry………………………………………………..…………..10 1.2 Ionization Methods……………………………………………..…………….10 1.2.1 Chemical Ionization (CI) and Electron Ionization (EI)…..………..10 1.2.2 Electrospray Ionization (ESI)………….…………………..………11 1.2.3 Ambient Desorption Mass Spectrometry……………….……........12 1.2.4 Desorption Electrospray Ionization (DESI)…………….…..……..15 1.2.5 Inherent Oxidation by ESI………..…………………….…………17 1.3 Mass Analyzers………………………………………………….……………18 1.3.1 Quadrupole Mass Analyzer……………...…….........……...…..….18 1.3.2 Ion Trap Mass Analyzer………………...………...……..….……..19 1.3.3 Time of Flight Mass Analyzer…………….........……………….…20 1.4 Tandem Mass Spectrometry…………………...…………..………………….21 Chapter 2: Comparison of ESI and DESI for the Ionization of Several Organic Compounds…...….............................................................................................................23 2.1 Chemicals and Materials …………………………………….………………23 2.2 Experimental Conditions …………....…………………………………..…..24 2.3 Results and Discussion…………………………………………………...….25 7 2.3.1 N,N,N',N'-tetramethyl-p-phenylenediamine(TMPD)………...…....25 2.3.2 N-phenyl-p-phenylenediamine(PPD)………………......…….…...28 2.3.3 Phenothiazine (PT)………...………..……………...…………..…31 2.3.4 Triphenylamine(TPA)………...….....…….………..…...…......….34 Chapter 3: Summary and Future Work…………….………...……..……………..……..36 References.…………..................……..……………..………………..……………….…37 8 LIST OF FIGURES Figure 1-1: Schematic showing the process of ESI[1]…………..…..….....…..….………11 Figure 1-2: Some direct ionization techniques [8]…………………………......………….14 Figure 1-3: Some direct desorption/ionization techniques[8]……………….………........14 Figure 1-4: Some two-step ionization techniques[8]…………….……………….........….15 Figure 1-5: DESI for analyzing solid sample on surface[7]……………….……......….…16 Figure 1-6: DESI for analyzing liquid sample[9] …………………………………....…...17 Figure 1-7: Schematic showing quadrupole mass analyzer[1]….…....…….........…....…..19 Figure 1-8: Schematic showing ion trap mass analyzer………..…….........………….….20 Figure 1-9: Schematic showing TOF mass analyzer[10]……………..…..........…...……..21 Figure 1-10: Diagram of a triple quadrupole mass spectrometer[13]…………...........…...22 Figure 2-1: Structures of organic compounds TMPD, PPD, PT and TPA...…........…….24 Figure 2-2: a) ESI-MS spectrum of 30 µM of TMPD dissolved in solvent ACN/H2O/HOAc (v/v/v, 50:50:1). b) DESI-MS spectrum of 30 µM of TMPD dissolved in solvent ACN/H2O/HOAc (v/v/v, 50:50:1)………………….........................………....25 Figure 2-3: a) MS/MS spectrum of the protonated TMPD [M+H]+ (m/z 165)generated by DESI. b) MS/MS of the protonated TMPD [M+H]+ (m/z 165) generated by DESI; c) MS/MS of the radical cation TMPD (m/z164) [M+.] generated by ESI….…..........……..26 Figure 2-4: a) ESI-MS spectrum of 50 µM of PPD dissolved in solvent ACN/H2O/HOAc (v/v/v, 50:50:1). b) DESI spectrum of 50 µM of PPD dissolved in solvent ACN/H2O/HOAc (v/v/v, 50:50:1)……………………………..................……………...28 9 Figure 2-5: a) MS/MS spectrum of the protonated PPD [M+H]+(m/z 185) generated by ESI. b) MS/MS spectrum of the protonated PPD[M+H]+of (m/z 185) generated by DESI………………………………………...………...…………......………..………….29 Figure 2-6: a) MS/MS spectrumof the radical cation PPD M+. (m/z 184) generated by ESI; b) MS/MS of [M-H]+(m/z 183) generated by ESI………………...............….……...…..30 Figure 2-7: a) ESI-MS spectrum of 50 µM of PT dissolved in solvent ACN/H2O/HOAc (v/v/v, 50:50:1). b) DESI-MSspectrum of 50 µM of PT dissolved in solvent ACN/H2O/HOAc (v/v/v, 50:50:1)………………………......................…….………..…31 Figure 2-8: a) MS/MS spectrum of the protonated PT (m/z 200) generated by ESI. b) MS/MS spectrum of the protonated PT (m/z200) generated by ESI. c)MS/MS spectrum of the PT radical cation (m /z 199) generated by ESI……….....................………...……32 Figure 2-9: a) ESI-MS spectrum of 50 µM of TPA dissolved in solvent ACN/H2O/HOAc (v/v/v, 50:50:1). b) DESI-MS spectrum of 50 µM of TPA dissolved in solvent ACN/H2O/HOAc (v/v/v, 50:50:1)………….…………......................................…....…..34 Figure 2-10: a) MS/MS spectrum of the protonated TPA (m/z 246) generated by ESI. b) MS/MS spectrum of the protonated TPA (m/z 246) generated by DESI……...........…....35 10 CHAPTER 1: INTRODUCTION 1.1 Mass Spectrometry Mass spectrometry (MS) is a sensitive tool for chemical analysis. Since 1912, MS was used to differentiate the isotopes of Neon (masses: 20,22).[2] After that, MS was widely used for isotopic and elemental measurements especially for the analysis of nuclear weapons such as Uranium (U235, U238).[2] Also, MS was used for analyzing complexes mixture such as petroleum products and organic compounds. In the last 20 years, MS has been established as a method for the analysis for large and bio-molecules such as proteins and nucleic acids.[3] Nowadays, MS has become a useful analytical tool that provides information about qualitative and quantitative analysis.[1] 1.2 Ionization Methods 1.2.1 Chemical Ionization (CI) and Electron Ionization (EI) There are many ionization methods that produce ions during the mass spectrometric analysis such as electron impact (EI) and chemical ionization (CI). EI is commonly used for organic mass spectrometry.[4] By using an electron with kinetic energy of 70 eV to ionize a molecule in the gas phase, an electron could be expelled from the target molecule to generate the corresponding radical cation (i.e., the molecular ion). In this EI process, several extensive fragments could be produced. The fragmentation could prevent the molecular ion from being observed. Despite EI, the ionization by CI for a reagent gas such as hydrocarbons, amines or alcohols by the electron impact occurs +. first. For example, methane can be used as a reagent gas, which leads to form [CH4] + that reacts subsequently with another methane molecule. A mix of ions such as [C2H5] , 11 + + [C3H5] , and [CH5] are subsequently yielded. In the end, a proton transfers from these reagentions to the analyte M and produces the protonated analyte [M+H]+.[4] 1.2.2 Electrospray Ionization (ESI) Nowadays, ESI is the most commonly used ionization technique. Because ESI can be used to produce ions from both small and large chemical substances such as peptides, proteins, and non-covalent complexes by using
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