University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Masters Theses Graduate School 8-2003 Trochoidal Electron Impact Time-of-Flight Mass Spectrometry of Chlorodifluoromethane Wesley Daniel Robertson University of Tennessee - Knoxville Follow this and additional works at: https://trace.tennessee.edu/utk_gradthes Part of the Physics Commons Recommended Citation Robertson, Wesley Daniel, "Trochoidal Electron Impact Time-of-Flight Mass Spectrometry of Chlorodifluoromethane. " Master's Thesis, University of Tennessee, 2003. https://trace.tennessee.edu/utk_gradthes/2218 This Thesis is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Masters Theses by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a thesis written by Wesley Daniel Robertson entitled "Trochoidal Electron Impact Time-of-Flight Mass Spectrometry of Chlorodifluoromethane." I have examined the final electronic copy of this thesis for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Master of Science, with a major in Physics. Dr. Robert N. Compton, Major Professor We have read this thesis and recommend its acceptance: Dr. Stuart Elston, Dr. Soren P. Sorensen Accepted for the Council: Carolyn R. Hodges Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) To the Graduate Council: I am submitting herewith a thesis written by Wesley Daniel Robertson entitled “Trochoidal Electron Impact Time-of-Flight Mass Spectrometry of Chlorodifluoromethane.” I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the requirements for the degree of Master of Science, with a major in Physics. Dr. Robert N. Compton ________________________ Major Professor We have read this thesis and recommend its acceptance: Dr. Stuart Elston _____________________________ Dr. Soren P. Sorensen _____________________________ Accepted for the Council: Dr. Anne Mayhew ________________________ Vice Provost and Dean of Graduate Studies (Original signatures are on file with official student records) Trochoidal Electron Impact Time-of-Flight Mass Spectrometry of Chlorodifluoromethane A Thesis Presented for the Master of Science Degree The University of Tennessee, Knoxville Wesley Daniel Robertson August 2003 ii DEDICATION To my family for their love and support throughout this endeavor. iii ACKNOWLEDGMENTS I would first like to express my appreciation to my advisor, Dr. Robert Compton, for his guidance, patience and enthusiasm in helping me become a better scientist. He has always made time for questions and discussion and has provided an excellent learning environment. His enthusiasm for science has and will continue to influence me greatly. I would also like to thank Dr. Jim Parks for the enormous amount of support and opportunity he has provided to me as both a graduate and undergraduate student. His involvement with me has had a great influence on my life and I am much better off for being in his company. A special debt of gratitude is due to Dr. Stuart Elston for the time and energy he put into helping me understand electronics and his help in building some of the electronics discussed in this thesis. He always made time for my questions and provided much needed answers. I would also like to thank the rest of the members of my committee, Dr. Soren Sorensen, and Dr. Breinig for their valuable questions and commits on this work as well as all other friends and colleagues at UT who have always been available for help or discussion of research. I would like to thank Dr. Ray Garrett in particular for taking the time to explain and discus physics with me. I would like extend a special thank you to the members of the Compton research group who have helped make my experience here such an enjoyable one. Their personalities have provided a entertaining atmosphere in which to do research. iv ABSTRACT A Time-of-Fight mass spectrometer (TOF-MS) equipped with both a standard electron gun and a trochoidal electron monochromator was constructed and made operational and used to study ionization processes of the molecule Chlorodifluoromethane (CHF2Cl). + The appearance energies of both the parent ion (CHF2Cl ) and the most intense fragment + ion (CHF2 ) were carefully analyzed and it was observed that the energy for formation of + the parent ion, (CHF2Cl ) at 12.50 eV, was at a higher energy than the fragment ion, + (CHF2 ) at 12.25 eV. This phenomenon has to our knowledge never before been reported in the literature and is of great interest. The trochoidal electron monochromator allowed for the energy distribution of the ionizing electron beam to be within the difference of the appearance energies for the ions, confirming the result. Metastable decay analysis of the parent ion showed two parent ion states present. One state decayed with a lifetime of 2 ms and another parent ion state appeared to be stable. It was also shown that the parent + ion readily dissociated into the CHF2 fragment ion and possible mechanisms for this effect is discussed. The results show the need for future theoretical calculations of the ionic potential energy surfaces for this molecular cation in order to assess the various possibilities for this mechanism. The atmospheric relevance of the instability of the parent ion of CHF2Cl and its short lifetime to dissociation, in which an ozone destroying chlorine atom is released, should be further considered. v TABLE OF CONTENTS CHAPTER PAGE I. INTRODUCTION 1 Historical Perspective and Relevance of Halogenated Methanes 1 Investigative Techniques 4 II. HISTORY OF THE TOF-MS 7 Early TOF-MS 7 Modern TOF-MS 14 III. TOF-MS EXPERIMENTAL APPARATUS 18 Introduction 18 Principles of TOF-MS and Experimental Apparatus 19 Wiley-McLaren Space Focusing 26 Gas Introduction System and Supersonic Nozzle Jet Expansion 30 Micro-Channel Plate Detector 37 Data Acquisition: Labview Computer Program 39 Testing and Initial Measurements 40 IV. TROCHOIDAL ELECTRON MONOCHROMATOR 44 AND STANDARD ELECTRON GUN SYSTEMS Standard Electron Gun 44 Electron Gun Power Supply 47 Electron Beam Characteristics 51 Trochoidal Electron Monochromator 54 TEM Resolution Measurements Using 64 SF6 Negative Ion Resonance V. TEM-EI-TOF-MS OF CHLORODIFLOUROMETHANE 70 Introduction 70 Experimental 71 CHF2Cl Mass Spectrum 72 Parent and Fragment Ion Appearance Energy 74 Conclusion 83 REFERENCES 85 APPENDICES 88 Appendix A: Circuit for Adding Negative DC Offset to Extraction 89 Pulse Appendix B: Circuit for Micro-Channel Plate Detector in Negative 91 vi Ion Mode Appendix C: Construction Details for the Electron Gun Power 93 Supply (EGPS) VITA 97 vii LIST OF FIGURES FIGURE PAGE 2.1 Redrawn Schematic of first working TOFMS reported by Cameron 8 and Eggers as the “Velocitron”. 2.2 Wolff and Stephens’5 non-magnetic TOF-MS with its ten-stage 10 copper-beryllium dynode electron multiplier. 2.3 Top and side views of the helical path taken by ions in Goudsmit’s 11 “Chronotron”. 2.4 Rubidium (top) and Xenon (bottom) spectrums collected with 13 Goudsmit’s “Chronotron”. 2.5 Lincoln Smith’s magnetic period mass spectrometer, the 15 “Synchrometer”. 3.1 Schematic of Time of Flight with electron guns, x-y deflector 20 plates, Einzel lens, and micro-channel plate detector. 3.2 TOF-MS with labeled voltages and lengths. 21 3.3 Schematic of Time of Flight Vacuum system, chamber and pumps. 25 3.4 Photo of TEM-TOF-MS System. 27 3.5 (a) Time of Flight interaction region with first order focusing (top) and 29 (b) second order focusing (bottom) as incorporated in this instrument. 3.6 Plot of optimal ratio of flight tube to extraction voltages vs flight 30 tube length for Wiley-Mclaren focusing. 3.7 TOF-MS Gas Introduction System. 33 3.8 Set up for collecting ion current from backing plate. 36 3.9 Detector assembly for positive ion detection. 38 3.10 Recording of Ar+ ions as a function of electron energy. 41 3.11 EI-TOF positive ion spectrum of Xenon (top) and Iso-pro calculated 42 Xenon isotope distribution at resolution 600. 4.1 Schematic of Standard electron gun filament and focusing plates. 45 viii 4.2 Energy Diagram for electrons in electron gun. 48 4.3 Electron beam energy curve with no magnetic field or aerodag. 52 4.4 Electron energy curve with magnetic field and aerodag. 55 4.5 Displacement of electrons in TEM. 58 4.6 Schematic of Trochoidal Electron Monochromator. 60 4.7 Displacement plot for 3.0 volts across the deflection plates. 63 4.8 Displacement plot with 1.5 volts across the deflection plates. 65 4.9 Sulfurhexafluoride negative ion zero energy resonance (SF6-) 67 curve taken with standard low resolution electron gun. 4.10 Sulfurhexafluoride negative ion zero energy resonance (SF6-) 68 curve taken with TEM high resolution electron gun. 5.1 Electron impact positive ion spectrum, at an impact energy of 73 75 eV, of CHClF 2. 5.2 EI-TOF-MS Spectrum of CHF2Cl at electron impact energy 75 (top) 12.3 eV and (bottom) 12.7 eV. 5.3 Parent ion signal from TOF-MS with excellent resolution. 77 + 5.4 Ionization curve for Parent Ion CHF2Cl . 78 + 5.5 Ionization curve for Fragment Ion CHF2 . 79 5.6 Metastable decay data showing the ions detected after the parent ion 81 + CHF2Cl (86) was selected and focused into collision chamber with argon. 5.7 Metastable decay data showing the ions detected after the parent ion 82 + CHF2Cl (86) was selected and focused into empty collision chamber. A.1 Circuit used to add Negative DC offset to extraction pulse.