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Mass Spectroscopy Applied to Gas Leak Detection and Fuel Ionization


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MASS SPECTROSCOPY APPLIED TO GAS LEAK DETECTION AND FUEL IONIZATION By JAYANTH POTHIREDDY A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2003

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Copyright 2003 by Jayanth Pothireddy

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To my friends

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iv ACKNOWLEDGMENTS I would like to sincerely thank Dr. Corin Segal for giving me this wonderful opportunity and for his unlimited support, understanding and incredible patience without which I would not have successfully completed my thesis. I would also like to thank Dr. David Mikolaitis and Dr. Martin Vala for their support and Dr. Norman Fitz-Coy for his valuable suggestions. Many thanks go to Mr. Jan Szczepanski for giving me “first lessons” on mass spectrometry, invaluable suggestions, tremendous support and help in developing the mass spectrometry system. My heartfelt thanks go to my friends Venkat, Sujith, Gopi, Anand, “Iranian bros” Amir and Omid, Weizhong, Sampath, Harsha, Adil, Anant, Sasidhar, Charan, Danny, Jonas, Nelson, Zhu Wei, Ron, “Dr” Amit, Errol, Anurag, Priya, Amit “Saale” and Balaji for their support and understanding. I thank Ron Brown for constructing a nice model of the Space Shuttle aft compartment and for his help in moving the mass spectrometer cart to and fro from “chemistry” to “aero.”

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v TABLE OF CONTENTS Page ACKNOWLEDGMENTS..................................................................................................iv LIST OF FIGURES...........................................................................................................vii ABSTRACT....................................................................................................................... ix CHAPTERS 1 INTRODUCTION............................................................................................................1 1.1 Introduction............................................................................................................1 1.2 Cryogenic Fuel Leaks............................................................................................1 1.3 Ionization of Fuels.................................................................................................4 2 EXPERIMENTAL SETUP..............................................................................................9 2.1 Introduction............................................................................................................9 2.2 Aft-Compartment Model........................................................................................9 2.3 Ionization Tube....................................................................................................11 2.4 Mass-Spectrometer...............................................................................................12 2.4.1 Ion Sources................................................................................................14 2.4.2 Mass Analyzers..........................................................................................17 2.4.3 Detectors....................................................................................................18 2.5 The SRS Residual Gas Analyzer.........................................................................19 2.5.1 Ionizer........................................................................................................20 2.5.2 Quadrupole Mass Filter:............................................................................21 2.5.3 Ion Detector...............................................................................................23 2.5.4 Modes of Operation of SRS RGA.............................................................24 2.6 Vacuum System...................................................................................................25 3 RESULTS AND SUMMARY.......................................................................................26 3.1 Introduction..........................................................................................................26 3.2 Data Acquisition and Calibration.........................................................................26 3.2 Leak Detection Tests............................................................................................28 3.3 Ionization Tests....................................................................................................30 3.5 Summary..............................................................................................................33

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vi APPENDIX A AFT COMPARTMENT MODEL................................................................................35 B STANFORD RESEARCH SYSTEMS RESIDUAL GAS ANALYZER 300..............39 B.1 SRS Mass Spectrometer......................................................................................39 B.1.1 Specifications............................................................................................39 B.1.2 Internal Components.................................................................................40 B.2 Electrostatic Ion Lens System.............................................................................41 LIST OF REFERENCES..................................................................................................44 BIOGRAPHICAL SKETCH............................................................................................46

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vii LIST OF FIGURES Figure page 2-1. Aft compartment modelleft side view. .....................................................................11 2-2. Ionization tube........................................................................................................... 12 2-3. Basic components of a mass spectrometer with feedback control carried by a computer..................................................................................................................13 2-4. SRS Mass spectrometer..............................................................................................19 2-5. Ionizer Schematic. Reproduced from Ref. 4..............................................................20 2-6. Quadrupole rod connections. Reproduced from Ref. 4.............................................21 2-7. Ion Detector Components. Reproduced from Ref. 4.................................................24 2-8. Vacuum pumping system...........................................................................................25 3-1. Analog scansmass scale calibration.........................................................................27 3-2. Helium leak detection concentration curve for L =1.3 cm ..........................................29 3-3. Helium leak detection concentration curve for L =2.5 cm. .........................................29 3-4. Butane (C4H10) ionization..........................................................................................31 3-4 (contd.). Butane (C4H10) ionization............................................................................32 3-5. Normalized fragmentation species pressure..............................................................33 A-1. Hull...................................................................................................................... ......35 A-2. Hull-front piece.........................................................................................................3 6 A-3. Hullrear piece.......................................................................................................... 36 A-4. Hullside piece.........................................................................................................3 7 A-5. Hullshoulder piece..................................................................................................37

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viii A-6. Hulltop piece........................................................................................................... 38 A-7. Hullbottom piece ...................................................................................................38 B-1. Ionizer Components..................................................................................................40 B-2. Quadrupole Mass Filter Components........................................................................40 B-3. Electrostatic ion lens system.....................................................................................41 B-4. ICP, ion sampling interface, and vacuum system. Reproduced from Ref. 21..........42 B-5. Schematic of SRS mass spectrometer with the electrostatic ion lens assembly for ionization studies.....................................................................................................43

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ix Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science MASS SPECTROSCOPY APPLIED TO GAS LEAK DETECTION AND FUEL IONIZATION By Jayanth Pothireddy August 2003 Chair: Corin Segal Major Department: Mechanical and Aerospace Engineering A quadrupole mass spectrometry system was evaluated for two distinct applications: gas leak detection in a scaled model of the space shuttle aft compartment and to characterize the effect of fuel ionization. Stanford Research Systems Residual Gas Analyzer 300 was evaluated in this study. The leak detection experiments were conducted by simulating He gas leaks for cryogenic fuel leaks, inside a model of the space shuttle aft compartment. For this study, He gas leak at a pressure of 241 kPa was initiated through a 1.6 mm tygon tube at six predetermined locations inside the aft compartment model. The air purge rate into the model was 0.012 SCMS. The mass spectrometer did not detect the presence of He in the model even after an hour of leak. The non-detection of leak by the SRS mass spectrometer might be that the amount of He leak initiated into the model was low at the conditions tested.

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x In the ionization studies, butane (C4H10) gas was introduced into the SRS mass spectrometer bypassing the ionization tube. The effects of internal ionization on butane were studied at 25, 40 and 70 eV ionization potential (IP). The results showed increasing intensity and fragmentation of butane into smaller species with increasing IP. The study indicated the absence of methyl radical (CH3 +) for butane ionization at 25 eV Also the study showed the inherent limitation of SRS mass spectrometer, that it cannot be operated without switching the filament ON. Hence, for the ionization studies using the SRS mass spectrometer system, the filament has to be physically isolated from the system to characterize the effect of external ionization in the ionization tube.

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1 CHAPTER 1 INTRODUCTION 1.1 Introduction A gas analysis mass spectrometry system has been evaluated for two distinct applications: (1) a model of the space shuttle engine compartment to detect the presence of potentially explosive mixtures, and (2) high-speed gaseous hydrocarbons ionized to provide accelerated chemical reactions in a model scramjet. Gas composition detection with application to these issues are described, generally, in this section. 1.2 Cryogenic Fuel Leaks The danger of a fire on the space shuttle can be generated by leak of fuel or oxidizer. The space shuttle main engines use liquid hydrogen 2(LH) and liquid oxygen 2(LO)as propellants. The cryogenic fuel, 2LHand the oxidizer, 2LOare stored in the external tank and fed to the main engines located in the aft compartment of the space shuttle during launch. A scaled model of the space shuttle aft compartment has been build and fuel leaks have been modeled at several locations. The current mass-spectrometer system is used to detect the spreading of this fuel throughout the compartment. Mass-spectrometers have been used in the past to detect gas leaks in several applications. Griffin et al .1 have noted that mass-spectrometry is the only technology proven to monitor all necessary gas constituents at the required limits of detection, as low as 25 parts-per-million by volume (ppmv). Traditionally, NASA used four massspectrometer based leak detection systems for each launch support: Prime Hazardous Gas

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2 Detection System (HGDS), Backup HGDS, Portable Aft Mass Spectrometer2 (PAMS), and Hydrogen Umbilical Mass Spectrometer (HUMS). NASA has introduced a new replacement system called the Hazardous Gas Detection System (HGDS) 2000 with new features including Multiple Detector Sample Delivery system, control of inlet pressure, use of an inexpensive quadrupole mass spectrometer, and new pumping technology. This system is capable of monitoring gas in helium, nitrogen, or air background and also enables near simultaneous monitoring of all gas constituents of interest. The system can detect the leaks of hydrogen and oxygen as low as 25 ppmv and argon concentrations as low as 10 ppmv, with a response time of less than 10 seconds. Griffin et al .3 reported the results of performance tests done to evaluate the Stanford Research Systems (SRS) RGA 100 quadrupole mass spectrometer for using as a detector subsystem on the HGDS 2000. It is mentioned that while the SRS mass spectrometer meets the needs for this system, no commercial mass spectrometer system was found to give the performance required. Accuracy, limits-of-detection, drift, response time (the time for the concentration reading to reach 95% of the actual value was taken as the response time), and recovery time (the time required for the concentration reading to measure within 5% of actual) of the system have been evaluated. For all the tests conducted, the SRS Residual Gas Analyzer4 (RGA) was set to Noise Floor (NF) = 2 and mass-to-charges ( m/z ) monitored were 2, 4, 32, and 40; and detector was Faraday Cup. Their tests showed that the RGA maintained good linearity and all the values fell within 10% of the readings. The response time was measured as 8 sec for each component and the recovery time was measured to be less than 20 sec except for H2, which was 2 min. It

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3 was concluded that the mass spectrometer was the best method of leak detection for HGDS 2000. The flammability limits of an oxygen and hydrogen mixture are an oxygen volume fraction < 4% or a hydrogen volume fraction < 4%. Altitude has little effect on the flammability limits of hydrogen and oxygen mixtures until altitudes above 80,000 feet.5, 6 Leak detection has a particular role in the evaluation of new composite materials based fuel tanks. New hydrogen tanks made of polymer-matrix composite (PMC) material are under consideration as an enabling technology for reducing the weight of reusable launch vehicles and increasing their payload. A key developmental issue for these lightweight structures has been the 2LH leakage through the composite material. Rivers et al .7 have developed a method to calibrate and measure the leakage through the X-33 liquid hydrogen tank at cryogenic temperatures and under mechanical loads. The Flexible Microleak Detection System (FMLDS) developed for the test uses a mass spectrometer to determine the leaks and rate of permeation in the event of low leak rates. The results reported indicate that the errors are less than 10% for leak rates ranging from 0.3 to 200 cm3/min. A miniature quadrupole mass spectrometer array and gas chromatograph have been designed by Chutjian et al .8 and built for NASA flight missions. The system is used for detection, by astronauts in EVA, of 22N, O, the hydrazines, and 3NHleaks in the hull of the ISS, and of absorbed hydrazines on the astronauts’ suits. Also the system will find use in long-duration human flight to monitor in the spacecraft the atmosphere, the quality of drinking water, and the microbial content of the air and surfaces.

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4 Hunter et al .9 discuss the needs of aeronautical and space applications and the point-contact sensor technology to address these needs. The method of leak detection currently used for the space shuttle is a mass-spectrometer connected to an array of sampling tubes placed throughout the region of interest. The device was able to detect hydrogen in a variety of ambient environments, but it had its drawbacks, delay time with leak detection and pinpointing the exact location of the leak. 1.3 Ionization of Fuels In many practical devices such as ramjets experimental investigations have shown that there are serious difficulties concerning ignition of hydrocarbons due to relatively long times to initiate chemical reactions relative to the residence times. These problems are more predominant in time-limited combustion systems, in particular, supersonic combustion-based propulsion devices, where the fluid residence times may approach the chemical reaction rates. Therefore, methods are needed to accelerate the reactions to achieve stable and efficient energy deposition in the flow. A possibility to increase the production of free radicals from hydrocarbon fuel is to ionize it. The ions are expected to contribute to the acceleration of chemical reactions. The degree of ionization and the ions production are evaluated via mass spectrometry. The main difference among the methods for plasma ionization lies in the form in which energy is delivered to the sample: electric current (glow discharge, spark), microwaveinduced plasma (MIP), pulsed laser.10 Duncan indicated the recent developments in ionic molecular species production and to study their spectroscopy. Also new ion production schemes and/or modified ion sources are discussed, as are various mass spectrometry configurations used for mass selection.11

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5 Combustion systems benefit from the acceleration of chemical reactions to reduce the ignition delay time and to enhance the recombination reaction rates. Buriko et al .12 in their studies have found that air-fuel reactivity can be increased by injecting additives that contain radicals in large concentration, or by generation of free radicals prior to injection through ionization. Also Buriko et al. indicated that the ignition delay time can be decreased by as much as 20% due to free radicals resulting from ionization of hydrocarbons:3237CO,H,CH,HCO,CHO,CH etc., and over 50% when radicals containing atomic oxygen anion(O)are present. These results coincide with other recent studies conducted by Williams et al .13 which indicate that reactions can be accelerated when ions found in air plasmas react with aromatic fuel compounds typically found in aviation (JP) fuels. Using experimental techniques and theoretical analyses, Arnold et al .14 have conducted studies on gas-phase ion molecule reactions of the primary atmospheric cations 22(NO, O, O, N, and N) with two isomers of octane, n818CH and iso818CH ( 2,2,4 -trimethylpentane). Reaction rate constants and branching ratios for the reactions were measured from 300 to 500 K It is reported that the ion-molecule reaction rates are significantly faster and initiate radical formation at much lower temperatures. Arnold et al .15 have done extensive studies of the reactions of air plasma ions with a variety of alkanes. Especially the larger alkanes commonly found in fuels such as gasoline or kerosene, to explore the possibility that air plasma ions could be used as additives to enhance the rate of combustion of hydrocarbon fuels or to reduce the ignition delay time. They have shown that the positive ions 3HO and NO,which appear through the ionization of air (air plasma ions), have a significant effect on accelerating

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6 the decomposition reactions of large alkane compounds, beginning with 6C groups. This result is of particular interest for practical devices, such as aerospace vehicle engines, that employ mixtures of hydrocarbons of large C groups. Also the effects of these ions are influenced in different ways depending on the temperature at which these reactions take place. For example, the lower effectiveness of 3HO, which decomposes thermally between 300-400 K Arnold et al .16 has studied reactions of atomic oxygen anion(O)with a large number of alkanes. The efficiencies for the reactions of O with ethane, propane, and butane increase with increasing size, becoming collisional for butane at room temperature. For pentane and larger alkanes, the reactions are collisional at all temperatures. Extending the studies, Arnold et al .17 reported temperature dependence of reaction rates of the atmospheric plasma cations obtained from air ionization with benzene, a typical aromatic compound over a broad temperature range of 250 to 1400 K and the reactions were found to proceed at collisional rates. In the past, gas-phase ion-molecule reactions up to 5 Torr were studied by flow tube experiments that were based on laminar flow dynamics. Arnold et al .18 have developed a new instrument, the turbulent ion flow tube (TIFT), for studying ionmolecule reactions in the very high pressure range. It is designed to operate at room temperature and over a pressure range from approximately 20-750 Torr. Williams et al .13 reported the effect of air ions on other aromatic compounds, such as JP-10 (a synthetic high density compound), ethyl benzene and isooctane. The studies indicated a significant reduction in the ignition delay time, with stronger effects noted at low temperatures and large ionization levels. Possible explanations for the observed

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7 effects are: (i) the ion-molecule reactions discussed are associated with high exothermicities, thus, the reduction of ignition delay time could be due to additional heat added to the system leading to an effective rise in temperature and thereby reducing the ignition delay time; (ii) the production of radicals initiated by the air-plasma chemistry followed by dissociative recombination of the resulting molecular ions could be responsible; (iii) reactions of the air-plasma ions with the fuel molecules result in the rapid decomposition of the initial constituents to smaller fuel fragments with lower ignition temperature. Munson and Field presented a new technique in mass spectrometry called chemical ionization mass spectrometry.19 This was based on the formation of the ions of an unknown material by chemical reactions in the gas phase. A reaction gas was introduced into the ionization chamber of a mass spectrometer that can produce a set of ions, which are either non-reactive or only very slightly reactive with the reaction gas itself at pressures of 1 Torr When a small amount of another material is present in the mixture at these high pressures, the stable ions of the reaction gas will react with the second material to produce a spectrum of ions characteristic of the second material. It is reported that the spectra produced by this method are often more useful for determining the structure of compounds and identifying compounds and mixtures than electron impact spectra. Also the fragmentation patterns of chemical ionization mass spectrometry correspond closely to the structures of the molecules and appear to result from localized attack at reactive centers in the molecule. Palmer et al .20 gives a good description on the development and application of mass spectrometry instrumentation to support the goals of U.S. space program. The main

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8 focus of the study is the application of MS in studying the composition of planetary atmospheres and monitoring air quality on manned space missions.

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9 CHAPTER 2 EXPERIMENTAL SETUP 2.1 Introduction This chapter describes the experimental model for the leak detection, the ionization tube for ionization studies, the mass spectrometry theory, the description and working of SRS mass spectrometer and finally the vacuum pumping system. 2.2 Aft-Compartment Model The Space Shuttle aft-compartment houses the main propulsion system: main engines, LO2 and LH2 feed lines, avionics bays, tanks, etc. A scaled, 1:4, Plexiglass (outer construction) geometric model of the space shuttle aft-compartment is constructed to include the following components inside the hull ( see Figure 2-1): 1. The main LH2 supply line connecting the interface with the external tank to hydrogen manifold. This is a 10.2 cm diameter line. 2. The 3 LH2 lines connecting hydrogen manifold to the 3 main engines. These are 7.6 cm diameter lines. 3. The main LO2 supply line connecting the interface with the external tank to the oxygen manifold which is a 10.2 cm diameter line. 4. The 3 LO2 lines connecting the oxygen manifold to the 3 main engines, which are 7.6 cm diameter lines. 5. The hydrogen drain and fill lines. These are 5.1 cm diameter lines. 6. The oxygen drain and fill lines which are 5.1 cm diameter lines. 7. Three spherical hydrazine tanks which have a diameter of 17.8 cm. 8. Four spherical helium tanks which have a diameter of 15.3 cm.

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10 9. Three boxes representing the avionics bay, two with 35.630.526.6 and one with 35.528.020.3 dimensions (cm). 10. Three cylinders with a length of 38.0 cm and diameter of 50.8 cm represent the main engines. Table 2-1. Check Valve placement on the hull-front plate. Co-ordinates [cm] Check Valves Y Z 1 -23.8 134.2 2 23.8 134.2 3 -38.5 119.0 4 38.5 119.0 5 -10.9 99.5 6 18.3 76.4 7 -33.0 47.8 8 33.0 47.8 9 -47.3 8.4 10 -31.3 3.6 11 31.3 3.6 12 47.3 8.4 13 -26.7 17.9 14 26.7 17.9 The hull consists of flat-sided smooth outline planar figures and modeled without any structural frame members such as beams, trusses, etc. The purging of aftcompartment is done with air through the check valves placed on the hull front piece. The total purge flow rate was 0.012 SCMS. Table 2-1 gives the co-ordinates for the checkvalve placement in the model co-ordinates. The hull ventilation is provided through four airtight hatches carved out on the hullside and shoulder pieces. The leak was initiated from a 16000 kPa industrial grade He gas cylinder at six predetermined locations inside the hull and sampling is done through six predetermined sample locations. He is used to simulate hydrogen gas leaks. The location of leak and sampling points are at manifold left (L1, S1), manifold right (L4, S4), bottom left (L2, S2), bottom right (L5, S5), side left (L3, S3) and side right (L6, S6). The leak and

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11 sampling is done through a 1.3 mm tygon tube. The detailed model drawings are included in the Appendix A. Figure 2-1. Aft compartment modelleft side view. 2.3 Ionization Tube The selected hydrocarbon gases have been ionized in the device shown in Figure 22. It has a glass tube with two circular electrodes at each end. The potential will be varied to obtain various degrees of ionization. The gases are ionized by the high dc potential of about 900 V applied between the two electrodes. The ionized gases are then directed into the mass spectrometer for measurements of ion production as a function of the ionization potential applied to the electrodes. Air purge supply Manifold Air purge Engine Leak ports Propellant feed lines

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12 Figure 2-2. Ionization tube. 2.4 Mass-Spectrometer The mass-spectrometer is an instrument that measures the masses of individual molecules that have been converted into ions, i.e., molecules that have been electrically charged. A mass-spectrometer does not actually measure the molecular mass directly, but rather measures the mass-to-charge ( m/z ) ratio of the ions formed from the molecules. The mass spectrometry is a destructive technique and requires only a few nanomoles of sample to obtain its characteristic information. The first step in the mass spectrometric analysis of compounds involves the production of gas-phase ions of the compound, e.g. by electron ionization: MeM2e This molecular ion may further undergo fragmentation giving new radical cations and neutral molecules. All these ions are separated in the mass-spectrometer according to their m/z ratio and are detected in proportion to their abundance. Any typical mass spectrometer always contains the following functional units ( see Figure 2-3): Electrodes Glass tube Sample in

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13 1. A device to introduce the compound that is analyzed, e.g. a direct insertion probe or a gas chromatograph. 2. A source to produce ions from the sample. 3. An analyzer(s) to separate the various ions according to their m/z ratio. 4. A detector to count the ions emerging from the analyzer and to measure their abundance. 5. A computer to process the data, which produces the mass spectrum in a suitable form and controls the instrument through feedback. Figure 2-3. Basic components of a mass spectrometer with feedback control carried by a computer. Inlet Source Analyzer Detecto r Compute r Printe r Mass-S p ectrum Vacuum Sample Ions Ions Signal Data

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14 2.4.1 Ion Sources Ion sources exist as two types: liquid-phase ion sources and solid-state ion sources. In a liquid-phase ion source the sample enters as solution. This solution is introduced, by nebulization, as droplets into the mass spectrometer through some vacuum pumping stages. Electrospray and thermospray correspond to this type. In a solid-state ion source the analyte is an involatile deposit. This is deposit is irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focussed on to the analyzer. Field desorption, plasma desorption and matrix-assisted laser desorption sources use this technique to produce ions. ABC* + eExcitation ABC+ + 2eIonization AB+ + C + 2eA+ + BC + 2eA+ + B + C + 2eABC + eAC+ + B + 2eB + AC + 2eABCElectron Capture AB+ C A+ BC A+ BC+ + eAB+ C+ + eThe most important considerations applied while choosing a ionization technique are the internal energy transferred during ionization process and the physical and Rearrangement Dissociative Ionization Dissociative Electron Capture Ion Pair Production

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15 chemical properties of the sample that has to be ionized. Some ionization techniques are very energetic and cause extensive fragmentation while others are less energetic and only produce molecular species. The ion sources produce ions mainly by ionizing a neutral molecule through electron ejection, electron capture, etc. Some ion source reactions were described in the previous page. In electron ionization (EI) source, the sample is bombarded with a beam of energetic electrons. This ionization technique works well for many gas-phase molecules but induces extensive fragmentation so that the molecular ions are not always observed. This source consists of heated filament giving off electrons. These electrons are accelerated and made to collide with the gaseous molecules of the analyzed sample injected in to the source. Each electron is associated with an energy ( eV ) given by, / mvh, where m is its mass, v is its velocity, its wavelength and h is Planck’s constant. Usually, the energy of the electron beam can be selected from about 25 to 100 eV depending on the operation requirements, but by convention normally operates at 70 eV It is found that on average only one ion is produced for every 1000 molecules entering the source under the usual spectrometer conditions, at 70 eV Some important ion sources are briefly described in the proceeding paragraphs: In chemical ionization (CI) source, the analyte is ionized by reaction with a set of reagent ions. These reagent ions are formed from the reagent gas by a combination of electron ionization and ion-molecule reactions. The proportion of compound to reactant gas is usually of the order of 1 to 1000, so the electron ionization of the analyte does not occur. One of the most popular reagent gases is methane. Isobutane and ammonia have also been used as reagent gases. In a thermospray (TSP) source, a solution containing a

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16 salt and the sample are quickly heated in a steel capillary tube that is heated to a high temperature. The solution then passes through a vacuum chamber as a supersonic beam. As a result a fine droplet spray containing ions and solvent and sample molecules occurs. The ions in the solution are extracted and accelerated towards the mass analyzer. Electrospray (ESP) uses a strong electric field, applied to a liquid with a weak flux (normally 1-10 l/min ) passing through a capillary tube, under atmospheric pressure. An electric field of the order of 106 V/m is generated by applying a potential difference of 3-6 kV between the capillary and the counter electrode separated by 0.3-2 cm This field induces a charge accumulation at the liquid surface located at the end of the capillary that will break to form highly charged droplets. These droplets undergo a cascade of ruptures, yielding smaller and smaller droplets. When the electric field on their surface becomes large enough, desoprtion of ions from the surface occurs. In spark sources, electric discharges are used to desorb and ionize the analytes from solid samples. The source consists of a vacuum chamber in which two electrodes are mounted. A pulsed 1 MHz rf voltage of several kilovolts is applied in short pulses across a small gap between these electrodes to produce the electric discharge. One of the electrodes is the sample, if the sample is a nonmetal, it can be mixed with graphite and placed in a cup-shaped electrode. The ionization of the atomized sample atoms occurs in the plasma (spark). In plasma desorption (PD), the sample deposited on a small-aluminized nylon foil is exposed in the source to fission fragments of 252Cf having energy of several MeV The shock waves resulting from the bombardment of a few thousand fragments per second induce the desorption of neutrals and ions.

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17 2.4.2 Mass Analyzers The ions which are produced in the ion source are now transmitted in to the mass analyzer where they are separated according to their masses (rather m/z ratio), which must be determined. Scanning analyzers transmit ions of different masses successively along a time scale. Examples of this type are the magnetic sector instruments and quadrupole instruments. Although mass analyzers are generally scanning devices, some allow simultaneous transmission of all ions as in the dispersive magnetic analyzers, the time-offlight mass analyzer and the ion trap or the ion cyclotron resonance instruments. In tandem mass spectrometry (MS/MS), systems use several analyzers in sequence. The three important characteristics of an analyzer are the upper mass limit, the transmission and the resolution. The mass limit determines the highest value of the m/z ratio that can be measured. The transmission is the ratio between the number of ions reaching the detector and the number of the ions produced in the source. The resolving power is the ability to yield distinct signals for two ions with a small mass difference. The quadrupolar analyzer uses the stability of the (ion) trajectories in oscillating electric fields to separate ions according to their m/z ratio. This analyzer has four perfectly parallel pole or rods or, ideally, hyperbolic section. Mass sorting depends on ion motion resulting from simultaneously applied constant (dc) and radio frequency (rf) electric fields. Mass scanning is accomplished by systematically changing the field strengths, thereby changing the m/z value that is transmitted through the analyzer. Some main mass analyzers are briefly described in the following paragraph: The quadrupole ion trap or quistor operates on a principle similar to a quadrupole mass analyzer. It is made up of a circular electrode, with two ellipsoid caps on the top

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18 and the bottom. The overlapping of a direct potential with an alternative one gives a kind of ‘three-dimensional quadrupole’ in which the ions of all masses are trapped on a threedimensional trajectory, i.e. the space bounded by the electrodes. The mass scanning is done by varying the applied r.f. voltages to eject the trapped ions sequentially of increasing m/z ratio. In time-of-flight (TOF) analyzer, ions that are expelled from the source are accelerated by potential Vs and made to fly a distance d before reaching the detector. Mass-to-charge ( m/z ) ratios are determined by measuring the time that ions take to move through a field-free region between the source and the detector. Magnetic and electromagnetic analyzers use the action of the magnetic fields on the motion of the ions to sort the masses. The masses are filtered according to 222 s mqrBV 2.4.3 Detectors The ion beam after being sorted by the analyzer is detected and transformed into a usable signal by a detector. The detectors can be put into two categories: the Faraday cage or cup, allows a direct measurement of the charges that reach the detector, and an electron multiplier detectors and array detectors that increase the intensity of the signal. In Faraday detector the ion give up their charge on the inside of the cup. The discharge current is then amplified and measured. Electron multipliers use a plate (conversion dynode), which causes secondary emissions when positive or negative ions reach it. These secondary particles are accelerated into the continuous-dynode electron multiplier dislodging electrons as they collide with its curving inner walls. This cascade of electrons produced results in a measurable current at the end of the multiplier.

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19 2.5 The SRS Residual Gas Analyzer Stanford Research System’s (SRS) Residual Gas Analyzer (RGA) 300 was used to conduct studies. The SRS RGA is a mass spectrometer consisting of a quadrupole probe, and an electronics control unit (ECU) which mounts directly on the probe’s flange, and contains all the electronics necessary to operate the instrument ( see Figure 2-4). The probe has the quadrupole mass analyzer that is mounted directly onto a 6.985 cm (2 ”) CF port of the vacuum chamber. The total probe equipment consists of three parts: the ionizer (electron impact), the quadrupole mass analyzer and the ion detector. All these parts reside in the vacuum space formed by a stainless steel tube (CF cover nipple) covering the probe assembly with the exception of the ionizer. The ECU connects directly to the probes’ feedthru-flange and also to the computer through a standard RS232 communications port. The SRS RGA 300 has mass range from 1 to 300 amu and operating pressures are from UHV to 410Torr Figure 2-4. SRS Mass spectrometer. ECU To diffusion pump Four-way cross Quadrupole probe Heating jacket

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20 2.5.1 Ionizer The SRS RGA ionizer is of wire mesh construction with cylindrical symmetry and mounted co-axially with the mass filter assembly. The main parts of the ionizer are the repeller, the anode grid, the filament, and the focus plate ( see Figure 2-5). The filament is an oxidation-resistant thoria coated iridium wire. It operates at a negative potential relative to ground and resistively heated with an electric current. The emitted electrons are accelerated towards the anode grid, which is positively charged with reference to the filament and the ground. Because of the design of the anode grid cage, most electrons do not strike the anode immediately, but pass through the cage where they Figure 2-5. Ionizer Schematic. Reproduced from Ref. 4. create ions. Ions once formed stay within the anode grid structure, and ion distribution is more localized along the axis. The ions formed with in the anode grid volume are extracted from the ionizer by the electric field produced by the difference in voltage bias between the anode grid and the focus plate. The focus plate is kept at a negative potential relative to ground and its function is to draw the ions out of the anode cage and focus

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21 them into the analyzer section. The repeller is biased negative relative to the filament and prevents the loss of electrons from the ion source. 2.5.2 Quadrupole Mass Filter: The analyzer is built of four stainless steel (type 304) cylindrical rods (11.43 cm long, 0.635 cm diameter) accurately held in place by a set of two high-purity alumina insulators. The electrodynamic quadrupole field is produced by a combination of dc and rf voltages. During operation, a 2-d ( x-y ) quadrupole field is established between the four cylindrical electrodes with the two opposite rods connected together electrically. Ions enter the filter along the zaxis and start oscillating in the xand ydirections. Figure 2-6. Quadrupole rod connections. Reproduced from Ref. 4. The general principle of operation of the analyzer can be visualized qualitatively as: one rod pair ( xz plane) is connected to a positive dc voltage with a superimposed sinusoidal rf voltage. The other rod pair ( yz plane) is connected to a negative dc voltage

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22 upon which a sinusoidal rf voltage is superimposed, 1800 out of phase with the rf voltage of the first set of rods ( see Figure 2-6). The potentials are given by the expression: 0 0(cos) (cos) UVt UVt In this equation, U is the magnitude of the dc voltage applied to either pair of rods, V is the amplitude of the rf voltage applied to either set of rods, and is the angular frequency (=2 f) of the rf The ions accelerated along the zaxis are submitted to the following forces: 2 2 2 2 x ydx Fmze dtx dy Fmze dty where is a function of 0 : 222222 (,)000()()(cos)xy x yrxyUVtr where0ris the radius of the circle inscribed by the quadrupole rods. Derivatizing and rearranging the terms leads to following equations of motion: 2 22 0 2 22 02 (cos)0 2 (cos)0 dxze UVt dtmr dyze UVt dtmr

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23 For 0, x yr the trajectory of the ion will be stable. That means the ion never hits the rods. The above equations are in the form of the Mathieu equation: 2 2(2cos2)0uudu aqu d comparing the preceding equations with this one, we have: 22 0 22 08 2 4 and, uxy uxytzeU aaa mr zeV qqq mr from the above relations we can deduce: 2222 00 and 84uumrmr UaVq z eze The last two terms of both the U and V equations are constant for a given quadrupole instrument as they operate at constant (2) f 2.5.3 Ion Detector Positive ions that successfully pass through the quadrupole are then focussed towards the detector by an exit aperture held at ground potential. The detector components are shown in the Figure 2-7. The Faraday Cup (FC) detector, is a 304 stainless steel bucket, measures the incident ion current directly. It is shielded from the intense RF and DC fields of the quadrupole by the grounded exit plate. A cylindrical tube (FC shield) encloses the FC, protecting it from the strong electrodynamic potentials of the adjacent rods and from

PAGE 34

24 collecting ions originated other than the ionizer. Positive ions enter the grounded detector and give up their charge on the wall. The electrons given up in this process establish an electrical current that has the same intensity as the incoming ion current. The nominal sensitivity of the RGA is in the order of 10-4 amps/torr. Minimum-detectable partial pressures as low as 5.10-11 Torr are possible with FC. Figure 2-7. Ion Detector Components. Reproduced from Ref. 4. 2.5.4 Modes of Operation of SRS RGA There are four basic modes of operation of the mass spectrometer: analog scanning, histogram scanning, single mass measurement and total pressure measurement. In analog scanning the mass spectrometer is stepped at fixed mass increments through a prespecified mass range. The ion current is measured after each mass increment step and transferred to the host computer over RS232. In leak detection measurement mode (single mass measurement), the RGA can measure individual peak heights at any integer mass with in its mass range. This mode of operation is used to generate data for leak testing measurements, and to track changes in the concentrations of several different components of a mixture as a function of time.

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25 2.6 Vacuum System The SRS mass spectrometer operates at pressure ranging from 10-4 Torr to UHV. The mass spectrometer was mounted onto a standard 6.985 cm (2”) CF port of a 4-way cross connector. The diffusion pump is connected to one side of the 4-way cross. The vacuum chamber is comprised of the probe volume and the 4-way cross volume. The required vacuum environment in this chamber was achieved using a diffusion pump and a rotary pump connected in series. A second rotary pump was connected to the sampling port to suck in the sample gas and also pumps away negative ions and gases from the vacuum chamber (see Figure 2-8). Figure 2-8. Vacuum pumping system. Chiller Rotary pumps Diffusion pump Vacuum chamber pressure read-out To second rotary pump first second

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26 CHAPTER 3 RESULTS AND SUMMARY 3.1 Introduction In this study, preliminary work has been done to evaluate the application of quadrupole mass spectrometry (SRS RGA 300) to two distinct situations: gas leak detection and ionization of fuels. For the leak detection studies the focus was to determine the trend of concentration gradients for a gas leak until the time it takes to reach a steady state. In the ionization studies effort was made to determine the intensity of ion production as a function of ionization potential, i.e. for various degrees of ionization. 3.2 Data Acquisition and Calibration The first step in data acquisition (scanning) requires establishing a connection between the RGA program and the head. The data is carried out over the RS232 cable to the computer. Then a desired scanning mode is selected and the filament is turned on. Next the desired scan parameters and trigger rate are selected to start the scan. The SRS mass spectrometer was calibrated at the factory and it was checked for the accuracy of mass scale calibration prior to testing. Towards this effort, few analog scans for a mixture of argon and helium and air were taken and checked for the peak positions of Ar, He, N2, O2, CO2, and H2O. The peaks were seen at the desired positions on the mass scale (see Figure 3-1).

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27 0 10 20 30 40 50 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 x 10-7 Atomic Mass UnitsPartial Pressure (torr)Argon-Helium Mixture 0 10 20 30 40 50 0 0.5 1 1.5 2 2.5 x 10-8 Atomic Mass UnitsPartial Pressure (torr)Air Figure 3-1. Analog scansmass scale calibration. Ar+ Ar++ He+ H2O+ N2 + He++ H2O+ H2 + N2 + O2 +

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28 3.2 Leak Detection Tests The leak detection tests were carried out using the Leak Test mode available in the RGA Windows program. This mode provides the most effective way to monitor a single gas species. During the initial runs, a helium (amu=4) leak at a pressure of PHe=241 kPa was initiated through a 1.6 mm tygon tube at a predetermined location inside the aft compartment model. The air purge rate into the model was 0.012 SCMS. The sampling to the mass spectrometer was done with a 1.6 mm tygon tube taken from a predetermined sampling point. The tests were conducted at Scan Speed (SS) of 3, i.e. Noise Floor (1)SS of 2 and the mass spectrometer was triggered every 5 sec. The mass spectrometer did not detect any trace of helium even after 1 hr and the run was terminated. Various runs were repeated at similar conditions and for different sample and leak ports without He detection. Therefore calibration test have been initiated into a smaller volume. A helium leak at PHe=14 kPa was initiated into a small box with a volume of 1500 ml. The runs were conducted at SS=3 and triggering of 4 sec and for two different lengths of separation of L=1.3 cm and L=2.5 cm between the leak and the sample point. Helium detection was observed for this set of runs. Figures. 3-2 and 3-3 shows the curves of He concentration for the four leak test runs as Mean Normalized Pressure vs. Time, each taken at L=1.3 and L=2.5 cm respectively. The steady state detection is achieved within 5.25 sec for L=1.3 cm and 5.5 sec for L=2.5 cm.

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29 Figure 3-2. Helium leak detection concentration curve for L=1.3 cm Figure 3-3. Helium leak detection concentration curve for L=2.5 cm.

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30 3.3 Ionization Tests For the ionization tests ‘Analog mode’ was used to take the scans. In Analog mode the RGA Head scans from the start to the stop mass using the points per amu variable specified in the Mass Spec Parameters dialog box. The x-axis represents the mass range chosen and the y-axis represents the partial pressures of individual species in the gaseous mixture. The mass spectrometer head and scan parameters used for the scans were the default values. In the SRS mass spectrometer a scan cannot be taken without switching the filament on. This is an inherent limitation with regards to the operation of this mass spectrometer. The ionization tube is an external ion-generating source. The degree of ionization obtained with in the ionization tube can be truly quantified when the internal ionization due to the mass spectrometer’s ionizer is completely eliminated. Therefore to accomplish this the filament must be removed from the ionizer assembly. Tests were conducted to study the intensity of ionization/fragmentation of butane (C4H10) gas. Butane was introduced into the mass spectrometer bypassing the ionization tube. The effects of internal ionization on butane were studied at 25, 40 and 70 eV ionization potential. The scans shown in Figure 3-4 indicate increased intensity and fragmentation of butane into smaller species with increasing ionization potential. The major fragmentation species observed were CH3 +, C2H3 +, C2H4 +, C2H5 +, C3H5 +, C3H6 +, and C3H7 +. The Figure 3-5 shows a plot of fragmentation species pressure normalized by parent ion pressure (C4H10 +) versus atomic mass units. From this plot it can be seen that the methyl radical, CH3 +, is not formed at 25 eV ionization potential. Further tests were conducted on argon gas to compare the intensity of external ionization obtained in the ionization tube to that obtained within the mass spectrometer ionizer. The results show

PAGE 41

31 that the ionization obtained within the ionization was a miniscule when compared to that produced within the mass spectrometer. 0 20 40 60 80 100 0 0.5 1 1.5 2 2.5 3 x 10-7 Atomic Mass UnitsPartial Pressure (torr)Butane Ionization at 25 eV Figure 3-4. Butane (C4H10) ionization. C2H4 + C2H5 + C3H5 + C3H7 + C4H10 + C3H6 +

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32 0 20 40 60 80 100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x 10-6 Atomic Mass UnitsPartial Pressure (torr)Butane Ionization at 40 eV 0 20 40 60 80 100 0 0.2 0.4 0.6 0.8 1 1.2 x 10-6 Atomic Mass UnitsPartial Pressure (torr)Butane Ionization at 70 eV Figure 3-4 (contd.). Butane (C4H10) ionization. CH3 + C2H3 + C2H4 + C2H5 + C3H5 + C3H7 + C4H10 + C3H6 + CH3 + C2H3 + C2H4 + C2H5 + C3H5 + C3H6 + C3H7 + C4H10 +

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33 Figure 3-5. Normalized fragmentation species pressure. 3.5 Summary The present study evaluated the mass spectrometry system developed for two distinct applications of leak detection and ionization of hydrocarbon gases. The results are summarized in the proceeding paragraphs: The mass spectrometry system was not able to detect any trace of He leak in the aft compartment model for both purge on and off modes. Some of the possible reasons for this problem are: (1) The amount of leak initiated into the aft compartment model was low at the conditions tested and the detection results were, therefore, inconsistent. (2) In the case of ionization tube, the true amount of ionization produced in the tube might be higher than that produced internally in the ionizer. However the study has shown that the internal ionization is more than that of the external ionization. One of the reason for this defect may be the externally produced ions might not be reaching the ion detector due to kinetic energy losses and

PAGE 44

34 divergence. This problem may be overcome by accelerating the ions into the mass spectrometer using a set of electrostatic lenses with increasing negative potential applied to the successive lens.

PAGE 45

35 APPENDIX A AFT COMPARTMENT MODEL This appendix gives the drawings used to construct the hull of the aft compartment model. The compartment model is scaled to 1:4, and of plexiglass outer construction. All the dimensions in the drawings are in meter. Figure A-1. Hull.

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36 Figure A-2. Hull-front piece. Figure A-3. Hullrear piece. 1.34 0.95 0.60 0.48 0.43 0.94 1.42 1.22 0.061.04 1.63 0.76 0.62 0.50 1.03 1.63 0.03

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37 Figure A-4. Hullside piece. Figure A-5. Hullshoulder piece. 1.40 0.95 1.26 0.61 0.410.0.15 0.06 0.01 1.26 1.16 0.61 0.11 0.10 0.05 0.41 0.61 0.1

PAGE 48

38 Figure A-6. Hulltop piece. Figure A-7. Hullbottom piece. 1.14 0.62 0.48 0.07 1.37 1.63 1.33 0.14

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39 APPENDIX B STANFORD RESEARCH SYSTEMS RESIDUAL GAS ANALYZER 300 This appendix gives some of the specifications and figures (internal components) of the SRS RGA 300 mass spectrometer and a typical electrostatic ion lens assembly used to accelerate ions in to the mass spectrometer. B.1 SRS Mass Spectrometer B.1.1 Specifications Mass Range: 1 to 300 amu Mass Filter Type: Quadrupole Detector Type: Faraday Cup (FC) Resolution: Better than 0.5 amu @ 10% peak height Sensitivity (A/Torr): 2.10-4 (FC) Minimum Detectable Partial Pressure: 5.10-11 Torr Operating Pressure Range: 10-4 Torr to UHV Design: Open source, cylindrical symmetry Operation: Electron ionization Material: Stainless steel, type 304 Filament: Thoriated iridium Electron Energy: 25 to 105 V Ion Energy: 8 or 12 V Focus Voltage: 0 to 150 V Electron Emission Current: 0 to 3.5 mA Computer Interface: RS-232C, 28800 Baud Software: Windows OS based application (RGA Windows)

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40 B.1.2 Internal Components Figure B-1. Ionizer Components. Figure B-2. Quadrupole Mass Filter Components.

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41 B.2 Electrostatic Ion Lens System The high-speed hydrocarbon ions produced in the ionization tube tend to have decreasing kinetic energy and increasing divergence as they move along the length of the vacuum system until they reach the quadrupole mass analyzer. These problems can be overcome with the help of an ion lens system. An electrostatic ion lens system is placed between the ion source (ionization tube) and the quadrupole mass analyzer and is enclosed in the vacuum chamber. The function of an electrostatic ion lens system is to collect positive ions from the supersonic jet of sampled gas while neutral particles are pumped away. The ions are then focussed and transmitted to the mass analyzer. A typical ion lens system is shown in Figure B.3, which consists of a set of coaxial, sequential cylinders, each biased at a particular dc voltage. The ion system is tuned to a particular set of dc voltages such that maximum ion signals are obtained. Figure B-3. Electrostatic ion lens system. Lenses (6)

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42 A schematic diagram of ICP, ion sampling interface and vacuum system is shown in Figure B.4. The components shown in the schematic are: (1) analyte introduction, (2) ICP torch and load coil, (3) shielding box, (4) skimmer with plasma plume shown streaming through the central hole, (5) sampler cone with extraction orifice, (6) electrostatic lens assembly, (7) quadrupole mass analyzer, (8) electron multiplier, (9) pumping port to fist pumping stage, and (10) pumping port to second pumping stage. Figure B-4. ICP, ion sampling interface, and vacuum system. Reproduced from Ref. 21. The system shown above can be adapted for the SRS mass spectrometer system as shown in the Figure B-5. The vacuum chamber houses an electrostatic ion lens system similar to that shown in Figure B-3. The chamber has 3 ports namely, a pinhole, a pumping port and a port to connect to the 2 ” CF flange of the SRS quadrupole probe. The entire ionizer assembly of the SRS mass spectrometer is physically removed for this arrangement except the focus plate.

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43 Figure B-5. Schematic of SRS mass spectrometer with the electrostatic ion lens assembly for ionization studies. ECU 2 CF flange To pumping system Quadrupole p robe Electrostatic ion lens assembly Pinhole Ions from ionization tube Vacuum chamber

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44 LIST OF REFERENCES 1. T.P. Griffin, G.R. Naylor, W.D. Haskell, C. Curley, R.J. Hritz, G.S. Breznik, C. Mizell, “Hazardous Gas Detection System 2000,” NASA Technical Briefs, 2001. 2. NASA KSC WebPages, “Portable Aft Mass Spectrometer (PAMS),” URL: http://technology.ksc.nasa.gov/WWWaccess/techreports/96report/instf/inst17.html 3. T.P. Griffin, G.S. Breznik, C.A. Mizell, W.R. Helms, G.R. Naylor, W.D. Haskell, “A Fully-redundant, On-line, Mass-spectrometer System Used to Monitor Cryogenic Fuel Leaks on the Space Shuttle,” Trends in Analytical Chemistry, vol. 21, no. 8,pp 488-497, 2002. 4. Stanford Research Systems, Operating Manual and Programming Reference, Models RGA 100, RGA 200, RGA 300 Residual Gas Analyzer. 5. M. Mizukami, G.P. Corpening, R.J. Ray, N. Hass, K.A. Ennix, S.M. Lazaroff, “Linear Aerospike SR-71 Experiment (LASRE): Aerospace Propulsion Hazard Mitigation Systems,” NASA/TM, 206561, 1998. 6. N. Hass, M. Mizukami, B.A. Neal, C.St. John, R.J. Beil, T.P. Griffin. “Propellant feed system leak detection-Lessons learned from the Linear Aerospike SR-71 Experiment (LASRE),” NASA/TM, 206590, 1999. 7. H.K. Rivers, J.G. Sikora, S.N. Sankaran, “Detection of Hydrogen Leakage in a Composite Sandwich Structure at Cryogenic Temperature,” J. of Spacecraft and Rockets, 39, pp452-459, 2002. 8. A. Chutjian, M.R. Darrach, V. Garkanian, S.P. Jackson, T.D. Molsberry, O.J. Orient, D. Karmon, P.M. Holland, D. Aalami, “A Miniature Quadrupole Mass Spectrometer Array and GC for Space Flight: Astronaut EVA and Cabin-Air Monitoring,” Society of Automotive Engineers, Inc., 2000-01-2300. 9. G.W. Hunter, Liang-Yu Chen, P.G. Neudeck, D. Knight, Chung-Chiun Liu, QuingHai Wu, Huan-jun Zhou, “Chemical Gas Sensors for Aeronautic and Space Applications,” NASA/TM, 107444, 1999. 10. A. Vertes, R. Gijbels, F. Adams, “Diagnostics and Modeling of Plasma Processes in Ion Sources,” Mass Spectrometry Reviews, 9, pp71-113, 1990. 11. M.A. Duncan, “Frontiers in the Spectroscopy of Mass-selected Molecular Ions,” Int. J. of Mass Spectrometry, 200, pp545-569, 2000.

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45 12. Yu.Ya. Buriko, V.A. Vinogradov, V.F. Goltsev, “Estimation of the Possibility of a Rise in Chemical Activity of Hydrocarbon Fuel Through Injection Free Radicals,” Central Institute of Aviation Motors, Moscow, Russia. 13. S. Williams, A.J. Midey, S.T. Arnold, P.M. Bench, A.A. Viggiano, R.A. Morris, L.Q. Maurice, C.D. Carter, “Progress on the Investigation of the Effects of Ionization on Hydrocarbon/Air Combustion Chemistry,” AIAA 99-4907, 1999. 14. S.T. Arnold, A.A. Viggiano, R.A. Morris, “Rate Constants and Branching Ratios for the Reactions of Selected Atmospheric Primary Cations with n-Octane and Isooctane (2,2,4-trimethylpentane),” J. Phys. Chem. A, 101, pp9351-9358, 1997. 15. S.T. Arnold, A.A. Viggiano, R.A. Morris, “Rate Constants and Product Branching Fractions for the Reactions of H3O+ and NO+ with C2-C12 Alkanes,” J. Phys. Chem. A, 102, pp8881-8887, 1998. 16. S.T. Arnold, R.A. Morris, A.A. Viggiano, “Reactions of Owith Various Alkanes: Competition between Hydrogen Abstraction and Reactive Detachment,” J. Phys. Chem. A, 102, pp1345-1348, 1998. 17. S.T. Arnold, S. Williams, Itzhak Doton, A.J. Midey, R.A. Morris, A.A. Viggiano, “Flow Tube Studies of Benzene Charge Transfer Reactions from 250 to 1400 K,” J. Phys. Chem. A, 103, pp8421-8432, 1999. 18. S.T. Arnold, J.V. Seeley, J.S. Williamson, P.L. Mundis, A.A. Viggiano, “New Apparatus for the Study of Ion-Molecule Reactions at Very High Pressure (25-700 Torr): A Turbulent Ion Flow Tube (TIFT) study of Reactions of SF6 + SO2,” J. Phys. Chem. A, 104, pp5511-5516, 2000. 19. M.S.B. Munson and F.H. Field, “Chemical Ionization Mass Spectrometry,” J. Am. Chem. Soc., 88, pp2621-2630, 1966. 20. P.T. Palmer, T.F. Limero, “Mass Spectrometry in the U.S. Space Program: Past, Present, and Future,” Am. Soc. For Mass Spectrometry, 12, pp656-675, 2001. 21. R.S. Houk, V.A. Fassel, G.D. Flesch, H.J. Svec, A.L. Gray, C.E. Taylor, “Inductively Coupled Argon Plasma as an Ion Source for Mass Spectrometric Determination of Trace Elements,” Analytical Chemistry, 52, pp2283-2289, 1980.

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46 BIOGRAPHICAL SKETCH Jayanth Pothireddy was born in India on March 14, 1977. He grew up in the town of Porumamilla and completed his schooling in 1992 from Vasista School, Madanapalle. He finished his high school studies from Vani Junior College, in 1994. Jayanth graduated from RV College of Engineering, Bangalore University, in 1999 where he obtained a Bachelor of Engineering in mechanical engineering. He pursued graduate studies at the University of Florida from 2001 to 2003 and obtained a Master of Science degree from the Department of Mechanical and Aerospace Engineering.


Permanent Link: http://ufdc.ufl.edu/UFE0000880/00001

Material Information

Title: Mass Spectroscopy Applied to Gas Leak Detection and Fuel Ionization
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0000880:00001

Permanent Link: http://ufdc.ufl.edu/UFE0000880/00001

Material Information

Title: Mass Spectroscopy Applied to Gas Leak Detection and Fuel Ionization
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0000880:00001


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MASS SPECTROSCOPY APPLIED TO
GAS LEAK DETECTION AND FUEL IONIZATION















By

JAYANTH POTHIREDDY


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2003

































Copyright 2003

by

Jayanth Pothireddy




























To my friends















ACKNOWLEDGMENTS

I would like to sincerely thank Dr. Corin Segal for giving me this wonderful

opportunity and for his unlimited support, understanding and incredible patience without

which I would not have successfully completed my thesis. I would also like to thank Dr.

David Mikolaitis and Dr. Martin Vala for their support and Dr. Norman Fitz-Coy for his

valuable suggestions.

Many thanks go to Mr. Jan Szczepanski for giving me "first lessons" on mass

spectrometry, invaluable suggestions, tremendous support and help in developing the

mass spectrometry system.

My heartfelt thanks go to my friends Venkat, Sujith, Gopi, Anand, "Iranian bros"

Amir and Omid, Weizhong, Sampath, Harsha, Adil, Anant, Sasidhar, Charan, Danny,

Jonas, Nelson, Zhu Wei, Ron, "Dr" Amit, Errol, Anurag, Priya, Amit "Saale" and Balaji

for their support and understanding.

I thank Ron Brown for constructing a nice model of the Space Shuttle aft

compartment and for his help in moving the mass spectrometer cart to and fro from

"chemistry" to Aeroo."
















TABLE OF CONTENTS
Page

A C K N O W L E D G M E N T S ......... .................................................................................... iv

LIST OF FIGURES .............. .......... ... ...... ............ vii

A B S T R A C T ............................................................................ ............... ix

CHAPTERS

1 INTRODUCTION..................... .................. 1

1.1 Introduction ............... ......... ..... ....... ..... ......... ................. 1
1.2 Cryogenic Fuel Leaks ... ......... ................ ............. ........ 1
1.3 Ionization of Fuels ........ ........... .................. ........ ...... ............ 4

2 EX PER IM EN TA L SE TU P ................................................................................... 9

2 .1 Introdu action ...................... ................ ............... .............................. . 9
2.2 A ft-C om partm ent M odel......................................... .......................................... 9
2 .3 Ion ization T u b e ....................................................................... 1 1
2.4 M ass-Spectrom eter ............................................................... ........ ............ .. 12
2.4.1 Ion Sources .. ...................................................... ....... .............. 14
2.4.2 M ass A nalyzers .................. ............................ .... .. .. ............... 17
2 .4 .3 D detectors ............................................................... ................ .... 18
2.5 The SRS Residual Gas Analyzer .................. ...................................... 19
2.5.1 Ionizer ............. ........... ....................... .................. 20
2.5.2 Quadrupole M ass Filter: ........................................................ 21
2.5.3 Ion Detector ................................................... 23
2.5.4 Modes of Operation of SRS RGA ....................................................... 24
2 .6 V acuum Sy stem ........ ........ ................................... ..................... 2 5

3 RESULTS AND SUM M ARY .......................................................... .............. 26

3.1 Introduction .................. ...... ................. 26
3.2 D ata A acquisition and C alibration................................................. ... .. .............. 26
3.2 Leak D election Tests.................... ......................................... .......................... 28
3.3 Ionization Tests .............................................. ................. 30
3 .5 S u m m a ry ............. ......... .. .............. .. .................................................. 3 3











APPENDIX

A AFT COM PAR TM EN T M OD EL ................................................................................ 35

B STANFORD RESEARCH SYSTEMS RESIDUAL GAS ANALYZER 300........... 39

B 1 SR S M ass Spectrom eter ............................................................ .................... 39
B .1.1 Specifications ................................................ .. .. .... .. .......... .. 39
B 1.2 Internal Com ponents ....................................................... ....... .... 40
B .2 Electrostatic Ion Lens System ........................................ .................. ...... 41

LIST O F R EFER EN CE S ..................................................... ................................. 44

BIOGRAPHICAL SKETCH ........ ................................................... .............. 46
















LIST OF FIGURES

Figure p

2-1. Aft compartment model- left side view. ...................................................... 11

2-2. Ionization tube ........... ........................ ................................................. .. 12

2-3. Basic components of a mass spectrometer with feedback control carried by a
computer. ............................................. 13

2-4. SRS M ass spectrum eter. ................................................... ............................... 19

2-5. Ionizer Schematic. Reproduced from Ref. 4..................................................... 20

2-6. Quadrupole rod connections. Reproduced from Ref 4. ........................................ 21

2-7. Ion Detector Components. Reproduced from Ref 4. ........................................... 24

2-8. V acuum pum ping system .................................................................................... ....... 25

3-1. Analog scans- m ass scale calibration...................................................................... 27

3-2. Helium leak detection concentration curve for L=1.3 cm......................................... 29

3-3. Helium leak detection concentration curve for L=2.5 cm......................................... 29

3-4. B utane (C 4H 10) ionization. ................................................ ............... .............. 31

3-4 (contd.). Butane (C4H10) ionization. ............................................... 32

3-5. Normalized fragmentation species pressure. ...................................................... 33

A-1. Hull.......................................................... 35

A-2. Hull-front piece..................... ... .............. 36

A-3. Hull- rear piece ............................... ................ .......... .............. ... 36

A-4. Hull- side piece. .......................................... 37

A-5. Hull- shoulder piece ....................... .. .................... 37









A-6. Hull- top piece............... ......................................................... 38

A -7. H ull- bottom piece. ............. .................. .................. ............. .. 38

B-1. Ionizer Components. ............. ........ ........ .................. ... .......... .. 40

B -2. Quadrupole M ass Filter Com ponents............................................. ... ................. 40

B-3. Electrostatic ion lens system. ................... ......... ....................... 41

B-4. ICP, ion sampling interface, and vacuum system. Reproduced from Ref 21. ......... 42

B-5. Schematic of SRS mass spectrometer with the electrostatic ion lens assembly for
ionization studies. .......................................... 43








































viii















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

MASS SPECTROSCOPY APPLIED TO
GAS LEAK DETECTION AND FUEL IONIZATION

By

Jayanth Pothireddy

August 2003

Chair: Corin Segal
Major Department: Mechanical and Aerospace Engineering

A quadrupole mass spectrometry system was evaluated for two distinct

applications: gas leak detection in a scaled model of the space shuttle aft compartment

and to characterize the effect of fuel ionization. Stanford Research Systems Residual Gas

Analyzer 300 was evaluated in this study.

The leak detection experiments were conducted by simulating He gas leaks for

cryogenic fuel leaks, inside a model of the space shuttle aft compartment. For this study,

He gas leak at a pressure of 241 kPa was initiated through a 1.6 mm tygon tube at six

predetermined locations inside the aft compartment model. The air purge rate into the

model was 0.012 SCMS. The mass spectrometer did not detect the presence of He in the

model even after an hour of leak. The non-detection of leak by the SRS mass

spectrometer might be that the amount of He leak initiated into the model was low at the

conditions tested.









In the ionization studies, butane (C4H10) gas was introduced into the SRS mass

spectrometer bypassing the ionization tube. The effects of internal ionization on butane

were studied at 25, 40 and 70 eV ionization potential (IP). The results showed increasing

intensity and fragmentation of butane into smaller species with increasing IP. The study

indicated the absence of methyl radical (CH3+) for butane ionization at 25 eV. Also the

study showed the inherent limitation of SRS mass spectrometer, that it cannot be operated

without switching the filament ON. Hence, for the ionization studies using the SRS mass

spectrometer system, the filament has to be physically isolated from the system to

characterize the effect of external ionization in the ionization tube.














CHAPTER 1
INTRODUCTION

1.1 Introduction

A gas analysis mass spectrometry system has been evaluated for two distinct

applications: (1) a model of the space shuttle engine compartment to detect the presence

of potentially explosive mixtures, and (2) high-speed gaseous hydrocarbons ionized to

provide accelerated chemical reactions in a model scramjet. Gas composition detection

with application to these issues are described, generally, in this section.

1.2 Cryogenic Fuel Leaks

The danger of a fire on the space shuttle can be generated by leak of fuel or

oxidizer. The space shuttle main engines use liquid hydrogen (LH,) and liquid oxygen

(LO,) as propellants. The cryogenic fuel, LHand the oxidizer, LOare stored in the

external tank and fed to the main engines located in the aft compartment of the space

shuttle during launch. A scaled model of the space shuttle aft compartment has been build

and fuel leaks have been modeled at several locations. The current mass-spectrometer

system is used to detect the spreading of this fuel throughout the compartment.

Mass-spectrometers have been used in the past to detect gas leaks in several

applications. Griffin et al.1 have noted that mass-spectrometry is the only technology

proven to monitor all necessary gas constituents at the required limits of detection, as low

as 25 parts-per-million by volume (ppmv). Traditionally, NASA used four mass-

spectrometer based leak detection systems for each launch support: Prime Hazardous Gas









Detection System (HGDS), Backup HGDS, Portable Aft Mass Spectrometer2 (PAMS),

and Hydrogen Umbilical Mass Spectrometer (HUMS). NASA has introduced a new

replacement system called the Hazardous Gas Detection System (HGDS) 2000 with new

features including Multiple Detector Sample Delivery system, control of inlet pressure,

use of an inexpensive quadrupole mass spectrometer, and new pumping technology. This

system is capable of monitoring gas in helium, nitrogen, or air background and also

enables near simultaneous monitoring of all gas constituents of interest. The system can

detect the leaks of hydrogen and oxygen as low as 25 ppmv and argon concentrations as

low as 10 ppmv, with a response time of less than 10 seconds.

Griffin et al.3 reported the results of performance tests done to evaluate the Stanford

Research Systems (SRS) RGA 100 quadrupole mass spectrometer for using as a detector

subsystem on the HGDS 2000. It is mentioned that while the SRS mass spectrometer

meets the needs for this system, no commercial mass spectrometer system was found to

give the performance required. Accuracy, limits-of-detection, drift, response time (the

time for the concentration reading to reach 95% of the actual value was taken as the

response time), and recovery time (the time required for the concentration reading to

measure within 5% of actual) of the system have been evaluated. For all the tests

conducted, the SRS Residual Gas Analyzer4 (RGA) was set to Noise Floor (NF) = 2 and

mass-to-charges (m/z) monitored were 2, 4, 32, and 40; and detector was Faraday Cup.

Their tests showed that the RGA maintained good linearity and all the values fell within

10% of the readings. The response time was measured as 8 sec for each component and

the recovery time was measured to be less than 20 sec except for H2, which was 2 min. It









was concluded that the mass spectrometer was the best method of leak detection for

HGDS 2000.

The flammability limits of an oxygen and hydrogen mixture are an oxygen volume

fraction < 4% or a hydrogen volume fraction < 4%. Altitude has little effect on the

flammability limits of hydrogen and oxygen mixtures until altitudes above 80,000 feet.5' 6

Leak detection has a particular role in the evaluation of new composite materials

based fuel tanks. New hydrogen tanks made of polymer-matrix composite (PMC)

material are under consideration as an enabling technology for reducing the weight of

reusable launch vehicles and increasing their payload. A key developmental issue for

these lightweight structures has been the LH, leakage through the composite material.

Rivers et al.7 have developed a method to calibrate and measure the leakage through the

X-33 liquid hydrogen tank at cryogenic temperatures and under mechanical loads. The

Flexible Microleak Detection System (FMLDS) developed for the test uses a mass

spectrometer to determine the leaks and rate of permeation in the event of low leak rates.

The results reported indicate that the errors are less than 10% for leak rates ranging from

0.3 to 200 cm3/min.

A miniature quadrupole mass spectrometer array and gas chromatograph have been

designed by Chutjian et al.8 and built for NASA flight missions. The system is used for

detection, by astronauts in EVA, of N,, 0,, the hydrazines, and NH3 leaks in the hull of

the ISS, and of absorbed hydrazines on the astronauts' suits. Also the system will find use

in long-duration human flight to monitor in the spacecraft the atmosphere, the quality of

drinking water, and the microbial content of the air and surfaces.









Hunter et al.9 discuss the needs of aeronautical and space applications and the

point-contact sensor technology to address these needs. The method of leak detection

currently used for the space shuttle is a mass-spectrometer connected to an array of

sampling tubes placed throughout the region of interest. The device was able to detect

hydrogen in a variety of ambient environments, but it had its drawbacks, delay time with

leak detection and pinpointing the exact location of the leak.

1.3 Ionization of Fuels

In many practical devices such as ramjets experimental investigations have shown

that there are serious difficulties concerning ignition of hydrocarbons due to relatively

long times to initiate chemical reactions relative to the residence times. These problems

are more predominant in time-limited combustion systems, in particular, supersonic

combustion-based propulsion devices, where the fluid residence times may approach the

chemical reaction rates. Therefore, methods are needed to accelerate the reactions to

achieve stable and efficient energy deposition in the flow.

A possibility to increase the production of free radicals from hydrocarbon fuel is to

ionize it. The ions are expected to contribute to the acceleration of chemical reactions.

The degree of ionization and the ions production are evaluated via mass spectrometry.

The main difference among the methods for plasma ionization lies in the form in which

energy is delivered to the sample: electric current (glow discharge, spark), microwave-

induced plasma (MIP), pulsed laser.10 Duncan indicated the recent developments in ionic

molecular species production and to study their spectroscopy. Also new ion production

schemes and/or modified ion sources are discussed, as are various mass spectrometry

configurations used for mass selection.11









Combustion systems benefit from the acceleration of chemical reactions to reduce

the ignition delay time and to enhance the recombination reaction rates. Buriko et al.12 in

their studies have found that air-fuel reactivity can be increased by injecting additives

that contain radicals in large concentration, or by generation of free radicals prior to

injection through ionization. Also Buriko et al. indicated that the ignition delay time can

be decreased by as much as 20% due to free radicals resulting from ionization of

hydrocarbons: CO, H-,CH3,HCO-,CH2O-, CH7 etc., and over 50% when radicals

containing atomic oxygen anion (0-) are present. These results coincide with other recent

studies conducted by Williams et al.13 which indicate that reactions can be accelerated

when ions found in air plasmas react with aromatic fuel compounds typically found in

aviation (JP) fuels.

Using experimental techniques and theoretical analyses, Arnold et al.14 have

conducted studies on gas-phase ion molecule reactions of the primary atmospheric

cations (NO+, O0, O, N', and N2) with two isomers of octane, n-CH, and iso-CH,,

(2,2,4-trimethylpentane). Reaction rate constants and branching ratios for the reactions

were measured from 300 to 500 K. It is reported that the ion-molecule reaction rates are

significantly faster and initiate radical formation at much lower temperatures.

Arnold et al.15 have done extensive studies of the reactions of air plasma ions with

a variety of alkanes. Especially the larger alkanes commonly found in fuels such as

gasoline or kerosene, to explore the possibility that air plasma ions could be used as

additives to enhance the rate of combustion of hydrocarbon fuels or to reduce the ignition

delay time. They have shown that the positive ions H3O' and NO', which appear

through the ionization of air (air plasma ions), have a significant effect on accelerating









the decomposition reactions of large alkane compounds, beginning with C6 groups. This

result is of particular interest for practical devices, such as aerospace vehicle engines, that

employ mixtures of hydrocarbons of large C groups. Also the effects of these ions are

influenced in different ways depending on the temperature at which these reactions take

place. For example, the lower effectiveness of H3,O, which decomposes thermally

between 300-400 K.

Arnold et al.16 has studied reactions of atomic oxygen anion(O-)with a large

number of alkanes. The efficiencies for the reactions of O- with ethane, propane, and

butane increase with increasing size, becoming collisional for butane at room

temperature. For pentane and larger alkanes, the reactions are collisional at all

temperatures. Extending the studies, Arnold et a1.17 reported temperature dependence of

reaction rates of the atmospheric plasma cations obtained from air ionization with

benzene, a typical aromatic compound over a broad temperature range of 250 to 1400 K

and the reactions were found to proceed at collisional rates.

In the past, gas-phase ion-molecule reactions up to 5 Torr were studied by flow

tube experiments that were based on laminar flow dynamics. Arnold et al.18 have

developed a new instrument, the turbulent ion flow tube (TIFT), for studying ion-

molecule reactions in the very high pressure range. It is designed to operate at room

temperature and over a pressure range from approximately 20-750 Torr.

Williams et al.13 reported the effect of air ions on other aromatic compounds, such

as JP-10 (a synthetic high density compound), ethyl benzene and isooctane. The studies

indicated a significant reduction in the ignition delay time, with stronger effects noted at

low temperatures and large ionization levels. Possible explanations for the observed









effects are: (i) the ion-molecule reactions discussed are associated with high

exothermicities, thus, the reduction of ignition delay time could be due to additional heat

added to the system leading to an effective rise in temperature and thereby reducing the

ignition delay time; (ii) the production of radicals initiated by the air-plasma chemistry

followed by dissociative recombination of the resulting molecular ions could be

responsible; (iii) reactions of the air-plasma ions with the fuel molecules result in the

rapid decomposition of the initial constituents to smaller fuel fragments with lower

ignition temperature.

Munson and Field presented a new technique in mass spectrometry called chemical

ionization mass spectrometry.19 This was based on the formation of the ions of an

unknown material by chemical reactions in the gas phase. A reaction gas was introduced

into the ionization chamber of a mass spectrometer that can produce a set of ions, which

are either non-reactive or only very slightly reactive with the reaction gas itself at

pressures of 1 Torr. When a small amount of another material is present in the mixture at

these high pressures, the stable ions of the reaction gas will react with the second material

to produce a spectrum of ions characteristic of the second material. It is reported that the

spectra produced by this method are often more useful for determining the structure of

compounds and identifying compounds and mixtures than electron impact spectra. Also

the fragmentation patterns of chemical ionization mass spectrometry correspond closely

to the structures of the molecules and appear to result from localized attack at reactive

centers in the molecule.

Palmer et al.20 gives a good description on the development and application of

mass spectrometry instrumentation to support the goals of U.S. space program. The main






8


focus of the study is the application of MS in studying the composition of planetary

atmospheres and monitoring air quality on manned space missions.














CHAPTER 2
EXPERIMENTAL SETUP

2.1 Introduction

This chapter describes the experimental model for the leak detection, the ionization

tube for ionization studies, the mass spectrometry theory, the description and working of

SRS mass spectrometer and finally the vacuum pumping system.

2.2 Aft-Compartment Model

The Space Shuttle aft-compartment houses the main propulsion system: main

engines, L02 and LH2 feed lines, avionics bays, tanks, etc. A scaled, 1:4, Plexiglass

(outer construction) geometric model of the space shuttle aft-compartment is constructed

to include the following components inside the hull (see Figure 2-1):

1. The main LH2 supply line connecting the interface with the external tank to hydrogen
manifold. This is a 10.2 cm diameter line.

2. The 3 LH2 lines connecting hydrogen manifold to the 3 main engines. These are 7.6
cm diameter lines.

3. The main L02 supply line connecting the interface with the external tank to the
oxygen manifold which is a 10.2 cm diameter line.

4. The 3 L02 lines connecting the oxygen manifold to the 3 main engines, which are 7.6
cm diameter lines.

5. The hydrogen drain and fill lines. These are 5.1 cm diameter lines.

6. The oxygen drain and fill lines which are 5.1 cm diameter lines.

7. Three spherical hydrazine tanks which have a diameter of 17.8 cm.

8. Four spherical helium tanks which have a diameter of 15.3 cm.









9. Three boxes representing the avionics bay, two with 35.6x30.5x26.6 and one with
35.5x28.0 x20.3 dimensions (cm).

10. Three cylinders with a length of 38.0 cm and diameter of 50.8 cm represent the main
engines.

Table 2-1. Check Valve placement on the hull-front plate.
Check Valves Co-ordinates [cm]
Check Valves
Y Z
1 -23.8 134.2
2 23.8 134.2
3 -38.5 119.0
4 38.5 119.0
5 -10.9 99.5
6 18.3 76.4
7 -33.0 47.8
8 33.0 47.8
9 -47.3 8.4
10 -31.3 3.6
11 31.3 3.6
12 47.3 8.4
13 -26.7 17.9
14 26.7 17.9


The hull consists of flat-sided smooth outline planar figures and modeled without

any structural frame members such as beams, trusses, etc. The purging of aft-

compartment is done with air through the check valves placed on the hull front piece. The

total purge flow rate was 0.012 SCMS. Table 2-1 gives the co-ordinates for the check-

valve placement in the model co-ordinates. The hull ventilation is provided through four

airtight hatches carved out on the hull- side and shoulder pieces.

The leak was initiated from a 16000 kPa industrial grade He gas cylinder at six

predetermined locations inside the hull and sampling is done through six predetermined

sample locations. He is used to simulate hydrogen gas leaks. The location of leak and

sampling points are at manifold left (LI, Si), manifold right (L4, S4), bottom left (L2,

S2), bottom right (L5, S5), side left (L3, S3) and side right (L6, S6). The leak and







sampling is done through a 1.3 mm tygon tube.
in the Appendix A.

m aHBL~


The detailed model drawings are included


fN


.roeln fe


Figure 2-1. Aft compartment model- left side view.
2.3 Ionization Tube
The selected hydrocarbon gases have been ionized in the device shown in Figure 2-
2. It has a glass tube with two circular electrodes at each end. The potential will be varied
to obtain various degrees of ionization. The gases are ionized by the high dc potential of
about 900 V applied between the two electrodes. The ionized gases are then directed into
the mass spectrometer for measurements of ion production as a function of the ionization
potential applied to the electrodes.


I Lea port


Manifold
























Figure 2-2. Ionization tube.

2.4 Mass-Spectrometer

The mass-spectrometer is an instrument that measures the masses of individual

molecules that have been converted into ions, i.e., molecules that have been electrically

charged. A mass-spectrometer does not actually measure the molecular mass directly, but

rather measures the mass-to-charge (m/z) ratio of the ions formed from the molecules.

The mass spectrometry is a destructive technique and requires only a few nanomoles of

sample to obtain its characteristic information.

The first step in the mass spectrometric analysis of compounds involves the

production of gas-phase ions of the compound, e.g. by electron ionization:

M+e ->M++2e

This molecular ion may further undergo fragmentation giving new radical cations

and neutral molecules. All these ions are separated in the mass-spectrometer according to

their m/z ratio and are detected in proportion to their abundance.

Any typical mass spectrometer always contains the following functional units (see

Figure 2-3):









1. A device to introduce the compound that is analyzed, e.g. a direct insertion probe or a
gas chromatograph.

2. A source to produce ions from the sample.

3. An analyzer(s) to separate the various ions according to their m/z ratio.

4. A detector to count the ions emerging from the analyzer and to measure their
abundance.

5. A computer to process the data, which produces the mass spectrum in a suitable form
and controls the instrument through feedback.


Figure 2-3. Basic components of a mass spectrometer with feedback control carried by a
computer.









2.4.1 Ion Sources

Ion sources exist as two types: liquid-phase ion sources and solid-state ion sources.

In a liquid-phase ion source the sample enters as solution. This solution is introduced, by

nebulization, as droplets into the mass spectrometer through some vacuum pumping

stages. Electrospray and thermospray correspond to this type. In a solid-state ion source

the analyte is an involatile deposit. This is deposit is irradiated by energetic particles or

photons that desorb ions near the surface of the deposit. These ions can be extracted by

an electric field and focused on to the analyzer. Field desorption, plasma desorption and

matrix-assisted laser desorption sources use this technique to produce ions.

> ABC + e- Excitation

SABC+ + 2e- Ionization

SAB+ + C + 2e

> A + BC + 2e Dissociative
Ionization
> A++B+C+2e-

ABC+e > AC + B + 2e-
SRearrangement
> B + AC + 2e-

> ABC- Electron Capture

AB- + C
> AB- + C Dissociative
SA- + BCJ Electron Capture

> A- + BC+ + e
Ion Pair
SAB + C+ + e- Production



The most important considerations applied while choosing a ionization technique

are the internal energy transferred during ionization process and the physical and









chemical properties of the sample that has to be ionized. Some ionization techniques are

very energetic and cause extensive fragmentation while others are less energetic and only

produce molecular species. The ion sources produce ions mainly by ionizing a neutral

molecule through electron ejection, electron capture, etc. Some ion source reactions were

described in the previous page.

In electron ionization (EI) source, the sample is bombarded with a beam of

energetic electrons. This ionization technique works well for many gas-phase molecules

but induces extensive fragmentation so that the molecular ions are not always observed.

This source consists of heated filament giving off electrons. These electrons are

accelerated and made to collide with the gaseous molecules of the analyzed sample

injected in to the source. Each electron is associated with an energy (eV) given by,

my =h/A, where m is its mass, v is its velocity, A its wavelength and h is Planck's

constant. Usually, the energy of the electron beam can be selected from about 25 to 100

eV depending on the operation requirements, but by convention normally operates at 70

eV. It is found that on average only one ion is produced for every 1000 molecules

entering the source under the usual spectrometer conditions, at 70 eV.

Some important ion sources are briefly described in the proceeding paragraphs:

In chemical ionization (CI) source, the analyte is ionized by reaction with a set of

reagent ions. These reagent ions are formed from the reagent gas by a combination of

electron ionization and ion-molecule reactions. The proportion of compound to reactant

gas is usually of the order of 1 to 1000, so the electron ionization of the analyte does not

occur. One of the most popular reagent gases is methane. Isobutane and ammonia have

also been used as reagent gases. In a thermospray (TSP) source, a solution containing a









salt and the sample are quickly heated in a steel capillary tube that is heated to a high

temperature. The solution then passes through a vacuum chamber as a supersonic beam.

As a result a fine droplet spray containing ions and solvent and sample molecules occurs.

The ions in the solution are extracted and accelerated towards the mass analyzer.

Electrospray (ESP) uses a strong electric field, applied to a liquid with a weak flux

(normally 1-10 l l/min) passing through a capillary tube, under atmospheric pressure. An

electric field of the order of 106 V/m is generated by applying a potential difference of 3-6

kV between the capillary and the counter electrode separated by 0.3-2 cm. This field

induces a charge accumulation at the liquid surface located at the end of the capillary that

will break to form highly charged droplets. These droplets undergo a cascade of ruptures,

yielding smaller and smaller droplets. When the electric field on their surface becomes

large enough, desoprtion of ions from the surface occurs.

In spark sources, electric discharges are used to desorb and ionize the analytes from

solid samples. The source consists of a vacuum chamber in which two electrodes are

mounted. A pulsed 1 MHz rf voltage of several kilovolts is applied in short pulses across

a small gap between these electrodes to produce the electric discharge. One of the

electrodes is the sample, if the sample is a nonmetal, it can be mixed with graphite and

placed in a cup-shaped electrode. The ionization of the atomized sample atoms occurs in

the plasma (spark).

In plasma desorption (PD), the sample deposited on a small-aluminized nylon foil

is exposed in the source to fission fragments of 252Cf having energy of several MeV. The

shock waves resulting from the bombardment of a few thousand fragments per second

induce the desorption of neutrals and ions.









2.4.2 Mass Analyzers

The ions which are produced in the ion source are now transmitted in to the mass

analyzer where they are separated according to their masses (rather m/z ratio), which

must be determined.

Scanning analyzers transmit ions of different masses successively along a time

scale. Examples of this type are the magnetic sector instruments and quadrupole

instruments. Although mass analyzers are generally scanning devices, some allow

simultaneous transmission of all ions as in the dispersive magnetic analyzers, the time-of-

flight mass analyzer and the ion trap or the ion cyclotron resonance instruments. In

tandem mass spectrometry (MS/MS), systems use several analyzers in sequence.

The three important characteristics of an analyzer are the upper mass limit, the

transmission and the resolution. The mass limit determines the highest value of the m/z

ratio that can be measured. The transmission is the ratio between the number of ions

reaching the detector and the number of the ions produced in the source. The resolving

power is the ability to yield distinct signals for two ions with a small mass difference.

The quadrupolar analyzer uses the stability of the (ion) trajectories in oscillating

electric fields to separate ions according to their m/z ratio. This analyzer has four

perfectly parallel pole or rods or, ideally, hyperbolic section. Mass sorting depends on ion

motion resulting from simultaneously applied constant (dc) and radio frequency (rf)

electric fields. Mass scanning is accomplished by systematically changing the field

strengths, thereby changing the m/z value that is transmitted through the analyzer.

Some main mass analyzers are briefly described in the following paragraph:

The quadrupole ion trap or quistor operates on a principle similar to a quadrupole

mass analyzer. It is made up of a circular electrode, with two ellipsoid caps on the top









and the bottom. The overlapping of a direct potential with an alternative one gives a kind

of 'three-dimensional quadrupole' in which the ions of all masses are trapped on a three-

dimensional trajectory, i.e. the space bounded by the electrodes. The mass scanning is

done by varying the applied r.f voltages to eject the trapped ions sequentially of

increasing m/z ratio. In time-of-flight (TOF) analyzer, ions that are expelled from the

source are accelerated by potential Vs and made to fly a distance d before reaching the

detector. Mass-to-charge (m/z) ratios are determined by measuring the time that ions take

to move through a field-free region between the source and the detector. Magnetic and

electromagnetic analyzers use the action of the magnetic fields on the motion of the ions

to sort the masses. The masses are filtered according to m/q = r2B22V, .

2.4.3 Detectors

The ion beam after being sorted by the analyzer is detected and transformed into a

usable signal by a detector. The detectors can be put into two categories: the Faraday

cage or cup, allows a direct measurement of the charges that reach the detector, and an

electron multiplier detectors and array detectors that increase the intensity of the signal.

In Faraday detector the ion give up their charge on the inside of the cup. The discharge

current is then amplified and measured. Electron multipliers use a plate (conversion

dynode), which causes secondary emissions when positive or negative ions reach it.

These secondary particles are accelerated into the continuous-dynode electron multiplier

dislodging electrons as they collide with its curving inner walls. This cascade of electrons

produced results in a measurable current at the end of the multiplier.









2.5 The SRS Residual Gas Analyzer

Stanford Research System's (SRS) Residual Gas Analyzer (RGA) 300 was used to

conduct studies. The SRS RGA is a mass spectrometer consisting of a quadrupole probe,

and an electronics control unit (ECU) which mounts directly on the probe's flange, and

contains all the electronics necessary to operate the instrument (see Figure 2-4). The

probe has the quadrupole mass analyzer that is mounted directly onto a 6.985 cm (2 34")

CF port of the vacuum chamber. The total probe equipment consists of three parts: the

ionizer (electron impact), the quadrupole mass analyzer and the ion detector. All these

parts reside in the vacuum space formed by a stainless steel tube (CF cover nipple)

covering the probe assembly with the exception of the ionizer. The ECU connects

directly to the probes' feedthru-flange and also to the computer through a standard RS232

communications port. The SRS RGA 300 has mass range from 1 to 300 amu and

operating pressures are from UHV to 10-4 Torr.


Figure 2-4. SRS Mass spectrometer.









2.5.1 Ionizer

The SRS RGA ionizer is of wire mesh construction with cylindrical symmetry and

mounted co-axially with the mass filter assembly. The main parts of the ionizer are the

repeller, the anode grid, the filament, and the focus plate (see Figure 2-5).

The filament is an oxidation-resistant thoria coated iridium wire. It operates at a

negative potential relative to ground and resistively heated with an electric current. The

emitted electrons are accelerated towards the anode grid, which is positively charged with

reference to the filament and the ground. Because of the design of the anode grid cage,

most electrons do not strike the anode immediately, but pass through the cage where they


Filament Repeller

------------- -------





Focus plate -- .
r-- -------

Anode grid


Figure 2-5. Ionizer Schematic. Reproduced from Ref 4.

create ions. Ions once formed stay within the anode grid structure, and ion distribution is

more localized along the axis. The ions formed with in the anode grid volume are

extracted from the ionizer by the electric field produced by the difference in voltage bias

between the anode grid and the focus plate. The focus plate is kept at a negative potential

relative to ground and its function is to draw the ions out of the anode cage and focus
Focus plate ----~- --- -~-'















relative to ground and its function is to draw the ions out of the anode cage and focus










them into the analyzer section. The repeller is biased negative relative to the filament and

prevents the loss of electrons from the ion source.

2.5.2 Quadrupole Mass Filter:

The analyzer is built of four stainless steel (type 304) cylindrical rods (11.43 cm

long, 0.635 cm diameter) accurately held in place by a set of two high-purity alumina

insulators. The electrodynamic quadrupole field is produced by a combination of dc and

rfvoltages.

During operation, a 2-d (x-y) quadrupole field is established between the four

cylindrical electrodes with the two opposite rods connected together electrically. Ions

enter the filter along the z-axis and start oscillating in the x- andy- directions.

Quadrupole
axis .-















-(U+Vo cost)


Figure 2-6. Quadrupole rod connections. Reproduced from Ref. 4.

The general principle of operation of the analyzer can be visualized qualitatively as:

one rod pair (xz plane) is connected to a positive dc voltage with a superimposed

sinusoidal rfvoltage. The other rod pair (yz plane) is connected to a negative dc voltage









upon which a sinusoidal rfvoltage is superimposed, 1800 out of phase with the rfvoltage

of the first set of rods (see Figure 2-6). The potentials are given by the expression:



0 = (U+Vcos ct)
0 = -(U+Vcos ot)


In this equation, U is the magnitude of the dc voltage applied to either pair of rods,

V is the amplitude of the rfvoltage applied to either set of rods, and co is the angular

frequency (=2;rf) of the rf The ions accelerated along the z-axis are submitted to the

following forces:


d2x a0


F =m d = -ze ao
S dt2 y


where 0 is a function of 0 :


(,,)) = O(X2 y2)/r = (x2 y)(U + V cos ct)/


where, is the radius of the circle inscribed by the quadrupole rods. Derivatizing

and rearranging the terms leads to following equations of motion:


d2x 2ze
+ (U +Vcosot)= 0
dt mr,
d2y 2ze
dt2 mro2 (U+ V cosCt) = 0
dt mr0









For x,y # r,, the trajectory of the ion will be stable. That means the ion never hits

the rods. The above equations are in the form of the Mathieu equation:


d2u
+ (au 2q, cos 2)u = 0
dJ2


comparing the preceding equations with this one, we have:


(rt 8zeU
= a, = ax = -ay =---2
2 mai rQ
4zeV
and, qu = qx = -qy = 2



from the above relations we can deduce:



U = a, and V = qu
z 8e z 4e


The last two terms of both the U and V equations are constant for a given

quadrupole instrument as they operate at constant ao(= 27 f).

2.5.3 Ion Detector

Positive ions that successfully pass through the quadrupole are then focused

towards the detector by an exit aperture held at ground potential. The detector

components are shown in the Figure 2-7.

The Faraday Cup (FC) detector, is a 304 stainless steel bucket, measures the

incident ion current directly. It is shielded from the intense RF and DC fields of the

quadrupole by the grounded exit plate. A cylindrical tube (FC shield) encloses the FC,

protecting it from the strong electrodynamic potentials of the adjacent rods and from









collecting ions originated other than the ionizer. Positive ions enter the grounded detector

and give up their charge on the wall. The electrons given up in this process establish an

electrical current that has the same intensity as the incoming ion current. The nominal

sensitivity of the RGA is in the order of 10-4 amps/torr. Minimum-detectable partial

pressures as low as 5.10-11 Torr are possible with FC.



CDEM anode
CDEM
CDEM cone

Exit Plate \ /
-x.. ..\M-- CDEM signal
ions t.- -- FC signal


Faraday cup
/ a y cp Probe flange
Faraday cup shield


Figure 2-7. Ion Detector Components. Reproduced from Ref 4.

2.5.4 Modes of Operation of SRS RGA

There are four basic modes of operation of the mass spectrometer: analog scanning,

histogram scanning, single mass measurement and total pressure measurement. In analog

scanning the mass spectrometer is stepped at fixed mass increments through a pre-

specified mass range. The ion current is measured after each mass increment step and

transferred to the host computer over RS232. In leak detection measurement mode (single

mass measurement), the RGA can measure individual peak heights at any integer mass

with in its mass range. This mode of operation is used to generate data for leak testing

measurements, and to track changes in the concentrations of several different components

of a mixture as a function of time.









2.6 Vacuum System

The SRS mass spectrometer operates at pressure ranging from 10-4 Torr to UHV.

The mass spectrometer was mounted onto a standard 6.985 cm (23/4") CF port of a 4-way

cross connector. The diffusion pump is connected to one side of the 4-way cross. The

vacuum chamber is comprised of the probe volume and the 4-way cross volume. The

required vacuum environment in this chamber was achieved using a diffusion pump and a

rotary pump connected in series. A second rotary pump was connected to the sampling

port to suck in the sample gas and also pumps away negative ions and gases from the

vacuum chamber (see Figure 2-8).


I-Rotary .u


Figure 2-8. Vacuum pumping system.


- I --- i














CHAPTER 3
RESULTS AND SUMMARY

3.1 Introduction

In this study, preliminary work has been done to evaluate the application of

quadrupole mass spectrometry (SRS RGA 300) to two distinct situations: gas leak

detection and ionization of fuels. For the leak detection studies the focus was to

determine the trend of concentration gradients for a gas leak until the time it takes to

reach a steady state. In the ionization studies effort was made to determine the intensity

of ion production as a function of ionization potential, i.e. for various degrees of

ionization.

3.2 Data Acquisition and Calibration

The first step in data acquisition (scanning) requires establishing a connection

between the RGA program and the head. The data is carried out over the RS232 cable to

the computer. Then a desired scanning mode is selected and the filament is turned on.

Next the desired scan parameters and trigger rate are selected to start the scan.

The SRS mass spectrometer was calibrated at the factory and it was checked for the

accuracy of mass scale calibration prior to testing. Towards this effort, few analog scans

for a mixture of argon and helium and air were taken and checked for the peak positions

of Ar, He, N2, 02, C02, and H20. The peaks were seen at the desired positions on the

mass scale (see Figure 3-1).












x107


0
3-
a,
a 2.5

S2

_ 1.5 c

1

0.5

n


x 108
2.5




2


0
,
" 1.5

a,


0-



0.5


Argon-Helium Mixture















+0








20 30
Atomic Mass Units


Air


20 30
Atomic Mass Units


Figure 3-1. Analog scans- mass scale calibration.









3.2 Leak Detection Tests

The leak detection tests were carried out using the Leak Test mode available in the

RGA Windows program. This mode provides the most effective way to monitor a single

gas species.

During the initial runs, a helium (amu=4) leak at a pressure of PHe=241 kPa was

initiated through a 1.6 mm tygon tube at a predetermined location inside the aft

compartment model. The air purge rate into the model was 0.012 SCMS. The sampling to

the mass spectrometer was done with a 1.6 mm tygon tube taken from a predetermined

sampling point. The tests were conducted at Scan Speed (SS) of 3, i.e. Noise Floor

(SS-1) of 2 and the mass spectrometer was triggered every 5 sec. The mass

spectrometer did not detect any trace of helium even after 1 hr and the run was

terminated. Various runs were repeated at similar conditions and for different sample and

leak ports without He detection.

Therefore calibration test have been initiated into a smaller volume. A helium leak

at PHe=14 kPa was initiated into a small box with a volume of 1500 ml. The runs were

conducted at SS=3 and triggering of 4 sec and for two different lengths of separation of

L=1.3 cm and L=2.5 cm between the leak and the sample point. Helium detection was

observed for this set of runs. Figures. 3-2 and 3-3 shows the curves of He concentration

for the four leak test runs as Mean Normalized Pressure vs. Time, each taken at L=1.3

and L=2.5 cm respectively. The steady state detection is achieved within 5.25 sec for

L= 1.3 cm and 5.5 sec for L=2.5 cm.


















MiL FrimlAd HIe FIPsiua c. T i
IH~fl~n; ft#* (..box r~ itAn~b


Figure 3-2. Helium leak detection concentration curve for L=1.3 cm






hwI NqmarWHed M p lrpFljrv w% Tinm
Ijmrbm: p~.m pl tkas irMj









I.,.-"
r;1^*^ ^**fcr-^ in.^^.^^


Thi"yl I


Figure 3-3. Helium leak detection concentration curve for L=2.5 cm.


m I


h~- -~---L ,









3.3 Ionization Tests

For the ionization tests 'Analog mode' was used to take the scans. In Analog mode

the RGA Head scans from the start to the stop mass using the points per amu variable

specified in the Mass Spec Parameters dialog box. The x-axis represents the mass range

chosen and the y-axis represents the partial pressures of individual species in the gaseous

mixture. The mass spectrometer head and scan parameters used for the scans were the

default values. In the SRS mass spectrometer a scan cannot be taken without switching

the filament on. This is an inherent limitation with regards to the operation of this mass

spectrometer. The ionization tube is an external ion-generating source. The degree of

ionization obtained with in the ionization tube can be truly quantified when the internal

ionization due to the mass spectrometer's ionizer is completely eliminated. Therefore to

accomplish this the filament must be removed from the ionizer assembly.

Tests were conducted to study the intensity of ionization/fragmentation of butane

(C4H10) gas. Butane was introduced into the mass spectrometer bypassing the ionization

tube. The effects of internal ionization on butane were studied at 25, 40 and 70 eV

ionization potential. The scans shown in Figure 3-4 indicate increased intensity and

fragmentation of butane into smaller species with increasing ionization potential. The

major fragmentation species observed were CH3+, C2H3+, C2H4+, C2H5+, C3H5+, C3H6+,

and C3H7+. The Figure 3-5 shows a plot of fragmentation species pressure normalized by

parent ion pressure (C4Hio+) versus atomic mass units. From this plot it can be seen that

the methyl radical, CH3+, is not formed at 25 eV ionization potential. Further tests were

conducted on argon gas to compare the intensity of external ionization obtained in the

ionization tube to that obtained within the mass spectrometer ionizer. The results show










that the ionization obtained within the ionization was a miniscule when compared to that

produced within the mass spectrometer.













x 107 Butane Ionization at 25 eV
3 ,


2.5



2
o
U,

S1.5
C,


i 1
0-
+ + +

0.5- 0

0
0 ^ .. ,l I` k I _
0 20 40 60 80 100
Atomic Mass Units


Figure 3-4. Butane (C4H10) ionization.











Butane ionization at 40 eV


20 40 60 80
Atomic Mass Units


Butane ionization at 70 eV

+
C--


40 60
Atomic Mass Units


Figure 3-4 (contd.). Butane (C4H10) ionization.


x 10-6
1


0.9

0.8

-0.7

0.6
",
0.5

0.4

- 0.3

0.2

0.1

0


1.2



1



0.8
o


0.6



c 0.4



0.2


0
0


















hr~~~lM---------------------------H ~------------ I


It,-I I L I..










Figure 3-5. Normalized fragmentation species pressure.



3.5 Summary

The present study evaluated the mass spectrometry system developed for two

distinct applications of leak detection and ionization of hydrocarbon gases. The results

are summarized in the proceeding paragraphs:

The mass spectrometry system was not able to detect any trace of He leak in the aft

compartment model for both purge on and off modes. Some of the possible reasons for

this problem are:

(1) The amount of leak initiated into the aft compartment model was low at the
conditions tested and the detection results were, therefore, inconsistent.

(2) In the case of ionization tube, the true amount of ionization produced in the
tube might be higher than that produced internally in the ionizer. However the
study has shown that the internal ionization is more than that of the external
ionization. One of the reason for this defect may be the externally produced
ions might not be reaching the ion detector due to kinetic energy losses and


r Orto






34


divergence. This problem may be overcome by accelerating the ions into the
mass spectrometer using a set of electrostatic lenses with increasing negative
potential applied to the successive lens.















APPENDIX A
AFT COMPARTMENT MODEL

This appendix gives the drawings used to construct the hull of the aft compartment

model. The compartment model is scaled to 1:4, and of plexiglass outer construction. All

the dimensions in the drawings are in meter.


S Ih,. id r


...: '"1: ; %" A'; "" li ,. '. *. Y'.ilC
.':-.^.g :: h. nl.e. : .



^ ^- "*^ ;:.c***;*
*, ,.. *.. *,* *- .-.: \ k






, F r.n f' :' : : !

Side1'" *.






t
l-ioi I:
'- .!: I~ dd i < c.,

.. .} ~ ~ : ; : 5:.'
:...



....~;. ....

l_" ) Ic,


Figure A-1. Hull.










S0.48 0.43




< :" ^ ', -... *-. (
S;60
''""., '' i '-- ", "': -- : '--



..- .1.42










CL
....3 4. l-- -"'. ---"----


1.34

Figure A-2. Hull-front piece.
0.95 ~:;."j i :i '~ :0.50















1 i '
.62 L 05 slight bend
_7.
a '


Figure A-3. Hull- rear piece.














0.06


0.11.
0.01


1.40


Figure A-4. Hull- side piece.


I .



0.05-


- 1.26


Figure A-5. Hull- shoulder piece.




















0.07


11.62


0.-OA


cr. erlap


Figure A-6. Hull- top piece.


0.14





1.33


1.37


A


(I


( I


Figure A-7. Hull- bottom piece.














APPENDIX B
STANFORD RESEARCH SYSTEMS RESIDUAL GAS ANALYZER 300

This appendix gives some of the specifications and figures (internal components) of

the SRS RGA 300 mass spectrometer and a typical electrostatic ion lens assembly used to

accelerate ions in to the mass spectrometer.

B.1 SRS Mass Spectrometer

B.1.1 Specifications

Mass Range: 1 to 300 amu
Mass Filter Type: Quadrupole
Detector Type: Faraday Cup (FC)
Resolution: Better than 0.5 amu @ 10% peak height
Sensitivity (A/Torr): 2.10-4 (FC)
Minimum Detectable Partial Pressure: 5.10-11 Torr
Operating Pressure Range: 10-4 Torr to UHV
Design: Open source, cylindrical symmetry
Operation: Electron ionization
Material: Stainless steel, type 304
Filament: Thoriated iridium
Electron Energy: 25 to 105 V
Ion Energy: 8 or 12 V
Focus Voltage: 0 to 150 V
Electron Emission Current: 0 to 3.5 mA
Computer Interface: RS-232C, 28800 Baud
Software: Windows OS based application (RGA Windows)









B.1.2 Internal Components


Focus Plate


O Anode Grid















Figure B-1. Ionizer Components.













Spring loaded /
perforated screws


Filament


Repeller


RIDown CLr..;Ie.r


SOuadrupcle r&o


AlurrIr3 sp3:cri


Figure B-2. Quadrupole Mass Filter Components.









B.2 Electrostatic Ion Lens System

The high-speed hydrocarbon ions produced in the ionization tube tend to have

decreasing kinetic energy and increasing divergence as they move along the length of the

vacuum system until they reach the quadrupole mass analyzer. These problems can be

overcome with the help of an ion lens system.

An electrostatic ion lens system is placed between the ion source (ionization tube)

and the quadrupole mass analyzer and is enclosed in the vacuum chamber. The function

of an electrostatic ion lens system is to collect positive ions from the supersonic jet of

sampled gas while neutral particles are pumped away. The ions are then focused and

transmitted to the mass analyzer. A typical ion lens system is shown in Figure B.3, which

consists of a set of coaxial, sequential cylinders, each biased at a particular dc voltage.

The ion system is tuned to a particular set of dc voltages such that maximum ion signals

are obtained.














Lenses (6)
i/a


Figure B-3. Electrostatic ion lens system.









A schematic diagram of ICP, ion sampling interface and vacuum system is shown

in Figure B.4. The components shown in the schematic are: (1) analyte introduction, (2)

ICP torch and load coil, (3) shielding box, (4) skimmer with plasma plume shown

streaming through the central hole, (5) sampler cone with extraction orifice, (6)

electrostatic lens assembly, (7) quadrupole mass analyzer, (8) electron multiplier, (9)

pumping port to fist pumping stage, and (10) pumping port to second pumping stage.








P /,



@ 1I !, lll "i. i'O--O-,----l
I i l tJ 1 F I i
r-;i' I-I V2 VROCu
B 2 Ia*TORR\ lxlOTORR




0 5 10 15cm

Figure B-4. ICP, ion sampling interface, and vacuum system. Reproduced from Ref 21.

The system shown above can be adapted for the SRS mass spectrometer system as

shown in the Figure B-5. The vacuum chamber houses an electrostatic ion lens system

similar to that shown in Figure B-3. The chamber has 3 ports namely, a pinhole, a

pumping port and a port to connect to the 2 34" CF flange of the SRS quadrupole probe.

The entire ionizer assembly of the SRS mass spectrometer is physically removed for this

arrangement except the focus plate.


















Ions from
ionization
tube






Pinhole


Figure B-5. Schematic of SRS mass spectrometer with the electrostatic ion lens assembly
for ionization studies.















LIST OF REFERENCES


1. T.P. Griffin, G.R. Naylor, W.D. Haskell, C. Curley, R.J. Hritz, G.S. Breznik, C.
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3. T.P. Griffin, G.S. Breznik, C.A. Mizell, W.R. Helms, G.R. Naylor, W.D. Haskell,
"A Fully-redundant, On-line, Mass-spectrometer System Used to Monitor
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for the Reactions of Selected Atmospheric Primary Cations with n-Octane and
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BIOGRAPHICAL SKETCH

Jayanth Pothireddy was born in India on March 14, 1977. He grew up in the town

of Porumamilla and completed his schooling in 1992 from Vasista School, Madanapalle.

He finished his high school studies from Vani Junior College, in 1994. Jayanth graduated

from RV College of Engineering, Bangalore University, in 1999 where he obtained a

Bachelor of Engineering in mechanical engineering. He pursued graduate studies at the

University of Florida from 2001 to 2003 and obtained a Master of Science degree from

the Department of Mechanical and Aerospace Engineering.