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Advanced System Components for the Development of a Handheld Ion Trap Mass Spectrometer

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Advanced System Components for the Development of a Handheld Ion Trap Mass Spectrometer ADVANCED SYSTEM COMPONENTS FOR THE DEVELOPMENT OF A HANDHELD ION TRAP MASS SPECTROMETER Kevin Philip Schultze A dissertation submitted to the faculty at the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry. Chapel Hill 2014 Approved by: J. Michael Ramsey R. Mark Wightman James W. Jorgenson Michel R. Gagné Bo Li © 2014 Kevin Philip Schultze ALL RIGHTS RESERVED ii ABSTRACT Kevin Philip Schultze: Advanced System Components for the Development of a Handheld Ion Trap Mass Spectrometer (Under the direction of J. Michael Ramsey) This work describes the development and implementation of system components utilized in a handheld ion trap mass spectrometer (MS). Current state-of-the-art portable mass spectrometers fall in the 30 to 40 lb. range due to their large and heavy vacuum systems. The strategy used in this work to create lighter, handheld instruments is to eliminate much of the vacuum pumping requirements by operating the entire instrument under 1 Torr of background pressure, i.e. 3 orders of magnitude higher than conventional instruments. Reducing the ion trap size by a factor of 20 and operating these miniature traps at increased RF drive frequencies compensates for the ions’ reduced mean free path, making mass analysis possible at these elevated pressures. The first component developed was a novel ion trap geometry. The design is a modification of the cylindrical ion trap (CIT), using three electrodes to produce an elongated two-dimensional trapping region capped by planar mesh electrode features on each end. The SLIT’s increased dimensionality led to a tenfold increase in sensitivity versus a CIT with the same critical dimensions while maintaining identical resolution. This 5 mm long SLIT with critical dimensions xo = 500 µm and zo = 650 µm represents the smallest linear type ion trap reported to date and maintains higher resolution than their larger counterparts. SLIT iii operation in pressures exceeding 1 Torr of helium, nitrogen, and air buffer gasses also represents the highest pressure operation reported for linear ion traps. Ion detection at elevated pressures was achieved using a Faraday cup and charge sensitive amplifier. Mass spectra were collected using both a commercial amplifier and two custom made amplifiers. The smaller of the two custom Faraday cup amplifiers was incorporated into a miniature vacuum chamber along with the SLIT mass analyzer and glow discharge ion source. The operation of this MS system is the first reported miniature instrument operated without the use of turbomolecular vacuum pumps, and can in fact be operated with only a simple miniaturized roughing pump. Incorporating the miniature chamber, pump, and a custom miniature RF system, a prototype MS was developed weighing 2 lbs. and drawing 8.2 W. Finally, gas chromatography - mass spectrometry (GC-MS) experiments were carried out utilizing the newly developed miniature MS instrumentation with a full size GC to demonstrate the feasibility of a future handheld GC-MS device. Detection of a variety of organic analytes during 2.5 minute separations was demonstrated in both helium and nitrogen carrier gasses with limits of detection of 0.056 ng injected on-column. iv TABLE OF CONTENTS LIST OF FIGURES ............................................................................................................... ix LIST OF ABBREVIATIONS AND SYMBOLS ............................................................... xiv CHAPTER 1: MINIATURIZING MASS SPECTROMETERS AND THE QUADRUPOLE ION TRAP ................................................................1 Introduction ..................................................................................................................1 1.1 Miniaturization of Common Mass Analyzers .....................................................2 Sector Analyzers ................................................................................................3 Time-of-Flight....................................................................................................4 Ion Cyclotron Resonance ...................................................................................5 Linear Quadruopoles ..........................................................................................5 Quadrupole Ion Traps ........................................................................................7 1.2 The Quadrupole Ion Trap: Theory and Operation ............................................8 Qualitative Description of QIT Trapping ..........................................................8 Mathematical Treatment of QIT Operation .......................................................9 Motion of Trapped Ions in the QIT..................................................................13 Higher Order Fields .........................................................................................14 The QIT as a Mass Spectrometer .....................................................................15 1.3 Miniature Ion Traps and High Pressure Operation .........................................17 The Cylindrical Ion Trap .................................................................................17 Microscale Cylindrical Ion Traps ....................................................................19 v High Pressure Operation of CITs .....................................................................21 1.4 Figures ...................................................................................................................23 1.5 References .............................................................................................................28 CHAPTER 2: THE STRETCHED LENGTH ION TRAP ................................................33 Introduction ................................................................................................................33 2.1 Concept .................................................................................................................35 2.2 General Operation ...............................................................................................37 Instrumentation ................................................................................................37 Initial SLIT Construction and Performance .....................................................39 Performance Improvements and Comparison to CIT ......................................42 High Pressure SLIT Operation .........................................................................44 2.3 Alternative SLIT Configurations and Tolerance Studies ................................48 SLIT Arrays and Serpentine Traps ..................................................................48 SLIT Tolerance Considerations .......................................................................50 Ion Ejection Profiling .......................................................................................52 2.4 Conclusions ...........................................................................................................56 2.5 Figures ...................................................................................................................58 2.6 References .............................................................................................................82 CHAPTER 3: FARADAY CUP SYSTEMS FOR ION DETECTION AT HIGH PRESSURES ...............................................................................84 Introduction ................................................................................................................84 3.1 Feasibility of Faraday Cups as Ion Trap Detectors ..........................................85 Custom CTIA Faraday Cup Amplifier ............................................................86 Commercial FC Amplifier Testing ..................................................................88 vi 3.2 High Pressure Faraday Cup Operation .............................................................90 Performance of a Miniaturized Faraday Cup Amplifier ..................................90 Signal Dependence on FC Distance and Bias ..................................................92 3.3 Faraday Cup Application: Study of Convective Flow Through Ion Traps ..............................................................................................93 3.4 Conclusions ...........................................................................................................95 3.5 Figures ...................................................................................................................97 3.6 References ...........................................................................................................108 CHAPTER 4: DEVELOPMENT OF A MINIATURIZED PROTOTYPE MASS SPECTROMETER .........................................................................109 Introduction ..............................................................................................................109 4.1 Description of Miniaturized Vacuum Chamber .............................................110 Glow Discharge Ion Source ...........................................................................111 Sample and Buffer Gas Inlet ..........................................................................112 Ion Trap Related Hardware ............................................................................113 Detector Modules ...........................................................................................114
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