Proton Transfer Reaction Mass Spectrometry Principles and Applications

ANDREW M. ELLIS Department of Chemistry, University of Leicester, UK

CHRISTOPHER A. MAYHEW School of Physics & Astronomy, University of Birmingham, UK

WI LEY

Contents

Preface

xiii

SECTION 1 PRINCIPLES 1 Background 1.1 Volatile Organic Compounds in the Earth's Atmosphere 1.2 Volatile Organic Compounds in Other Environments 1.3 Techniques for VOC Measurements 1.3.1 Gas Chromatography 1.3.2 Ion Mobility Spectrometry 1.3.3 The Flowing Afterglow Technique 1.3.4 The Selected Ion Flow Tube 1.4 Emergence of Proton Transfer Reaction Mass Spectrometry 1.4.1 Historical Background 1.4.2 Compound Identification Using PTR-MS 1.4.3 An Introduction to Quantitative Aspects of PTR-MS 1.4.4 A Comparison between PTR-MS and SIFT-MS References

3 3 5 6 6 9 11 14 15 15 17 21 22 23

2 Chemical Ionization: Chemistry, Thermodynamics and Kinetics 2.1 Introduction 2.2 Proton Transfer 2.2.1 Energy Units 2.2.2 Thermodynamics of Proton Transfer 2.2.3 Kinetics of Proton Transfer 2.2.3.1 Background 2.2.3.2 Theoretical Prediction of Proton Transfer Rate Coefficients 2.2.3.3 Illustrative Calculations of Proton Transfer Rate Coefficients and Comparison with Experiment 2.2.4 Reagents and Mechanisms Reactions 2.2.4.1 Chemistry of }13 2.2.4.2 Reactions of Hydrated Hydronium Clusters 2.2.4.3 Alternative Proton Donors 2.3 Other Chemical Ionization Processes References

25 25 27 27 27 31 31 33 37 38 38 42 43 44 45

vi Contents

3 Experimental: Components and Principles 3.1 Introduction 3.2 Ion Extraction and Ion Optics 3.2.1 Ion Acceleration 3.2.2 Ion Steering 3.2.3 Ion Lenses 3.2.4 Simulation of Ion Trajectories 3.3 Ion Sources 3.3.1 Hollow Cathode Discharge Ion Source 3.3.2 Ion—Molecule Chemistry Leading to H3 0 + Production 3.3.3 Alternative Ion Sources 3.3.4 Generating Reagent Ions Other Than H30+ 3.4 Drift Tubes 3.4.1 Practical Aspects 3.4.2 Ion Mobility and Transit Times 3.4.3 Ion—Molecule Collision Energies 3.4.4 Ion Cluster Distributions 3.5 Mass Spectrometry 3.5.1 Some Important Definitions 3.5.1.1 Ion Mass and Mass-to-Charge Ratio 3.5.1.2 Mass Resolution 3.5.1.3 Transmission and Dynamic Range 3.5.2 Quadrupole Mass Spectrometry 3.5.2.1 Basic Principles of the Quadrupole Mass Spectrometer 3.5.2.2 Practical Issues 3.5.3 Quadrupole Ion Trap Mass Spectrometry 3.5.3.1 Basic Principles 3.5.3.2 Collision-Induced Dissociation 3.5.3.3 Three-Dimensional Quadrupole Ion Traps in PTR-MS 3.5.3.4 The Linear Ion Trap in PTR-MS 3.5.4 Time-of-flight Mass Spectrometry 3.5.4.1 Basic Principles of TOF-MS 3.5.4.2 Improving the Resolution: Spatial Focusing 3.5.4.3 Reflectron TOF-MS 3.5.4.4 Mass Calibration in TOF-MS 3.5.4.5 Advantages and Limitations of TOF-MS 3.5.4.6 TOF-MS Analysers in PTR MS 3.6 Ion Detectors 3.6.1 Discrete Dynode Detector 3.6.2 Channel Electron Multiplier 3.6.3 Microchannel Plate Detector 3.7 Analogue versus Digital Signal Processing References -

4 Quantitative Analysis 4.1 Introduction 4.2 Extracting the Concentration of a Trace Gas from PTR-MS

49 49 50 51 53 54 56 57 57 59 61 63 64 64 69 71 73 76 77 77 78 79 81 81 83 85 85 87 88 90 90 90 92 93 94 95 96 97 98 100 101 103 106 111 111 111

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4.3 Normalized Counts per Second 4.4 Why Calibrate? 4.5 Calibration Techniques 4.5.1 Static Gas Calibration 4.5.2 Dynamic Methods 4.5.3 Alternative Dynamic Calibration Procedures 4.6 Effect of Humidity 4.7 Accuracy, Precision and Limit of Detection 4.8 Validation of PTR-MS References

113 113 116 116 117 119 120 122 125 126

SECTION 2 APPLICATIONS 5 PTR-MS in the Environmental Sciences 5.1 Background 5.2 Use of Reagent Ions Other Than H30+ 5.3 Biogenic VOCs 5.3.1 General Details 5.3.2 Forest Emissions Tropical Rainforests 5.3.2.1 5.3.2.2 Coniferous Forests 5.3.2.3 Deciduous Forests 5.3.2.4 Eddy Covariance Measuring Methodologies 5.3.2.5 Forest VOCs and mlz Assignments 5.3.3 Plantations 5.3.4 Various Land Emissions Woodland and Grassland Savannahs 5.3.4.1 5.3.4.2 Shrubland 5.3.4.3 Alfalfa and Grass Fields 5.3.5 Oceans and Seas Norwegian Fjord 5.3.5.1 5.3.5.2 Coastal Regions 5.3.5.3 Indian Ocean 5.3.5.4 Tropical Atlantic Ocean 5.4 Anthropogenic VOCs 5.4.1 Background 5.4.2 VOCs in Urban and Rural Sites Innsbruck 5.4.2.1 5.4.2.2 Caracas 5.4.2.3 Houston 5.4.2.4 Tokyo 5.4.2.5 Barcelona 5.4.2.6 Manchester and London 5.4.2.7 Mexico City 5.4.2.8 Toronto and Environs 5.4.2.9 Paris 5.4.2.10 Boston, New York and Los Angeles

131 131 138 141 141 142 142 144 146 147 150 150 151 151 152 152 154 154 155 156 156 157 157 157 157 158 158 159 159 160 160 162 163 163



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Contents

Diesel Engine Emissions Aircraft Emissions VOC Emissions Associated with Farming 5.4.5.1 Cattle 5.4.5.2 Pigs and Sheep 5.4.6 Other Studies of Anthropogenic VOCs 5.4.6.1 Air Quality 5.4.6.2 Firework Emissions 5.5 Biomass Burning 5.6 Applications of PTR-MS to Laboratory Studies of Atmospheric Chemistry 5.6.1 Laboratory Studies of Biomass Burning 5.6.2 Reaction Products and Reactive Species 5.6.3 Simulation Chamber and Container Measurements 5.7 Plant Studies 5.7.1 Isoprene Emissions 5.7.2 Acetaldehyde Emissions 5.7.3 Pollination 5.7.4 Roots and Soil 5.7.5 Other Plant Studies 5.7.5.1 Root-secreted VOCs 5.7.5.2 Methanol Release and Bacterial Growth: Plant-Methylobacterium Association 5.7.5.3 Comparison of VOC Emissions from Young and Mature Leaves 5.7.6 Stress-Related Emissions 5.7.7 VOC Emissions Linked to Plant Damage 5.7.7.1 Mechanical Wounding 5.7.7.2 Weather Damage 5.7.7.3 Harvesting and Mowing 5.7.7.4 Biofuel Crops 5.7.7.5 Herbivore Attack by Small Predators 5.7.7.6 Large Herbivore Attack VOC Uptake by Plants 5.7.8 Outlook for Atmospheric and Environmental Applications 5.8 of PTR-MS References

5.4.3 5.4.4 5.4.5

6 PTR-MS in the Food Sciences 6.1 Background 6.2 Combined GC—MS and PTR-MS Studies for Food Analysis 6.3 Mass Spectral Fingerprinting 6.4 Flavour Release and Perception 6.4.1 Drinks 6.4.1.1 Coffee 6.4.1.2 Tea

164 164 164 165 165 166 166 166 166 169 171 173 176 181 182 185 187 188 189 189 190 190 190 193 193 194 195 196 197 202 202 203 203

221 221 223 226 227 228 228 231

Contents ix

6.4.1.3 Carbonated Drinks 6.4.1.4 Fruit Juices 6.4.1.5 Wine 6.4.1.6 Vodka 6.4.1.7 Infant Formula 6.4.2 Food 6.4.2.1 Cheese 6.4.2.2 Bread 6.4.2.3 Onions 6.4.2.4 Wheys 6.4.2.5 Fruit 6.4.3 Flavour Release: Food Texture, Composition and Physiological Effects 6.5 Food Classification, Food Quality and Food Control 6.5.1 Geographical Location 6.5.1.1 White Truffles 6.5.1.2 Butter 6.5.1.3 Olive Oil 6.5.1.4 Roe 6.5.1.5 Dry-Cured Ham 6.5.1.6 Cumin Cheese 6.5.2 Food Classification and Quality 6.5.3 Food Freshness and Ripening 6.5.3.1 Meat Degradation 6.5.3.2 Fruit and Vegetables: Ripening, Storage and Monitoring 6.5.3.3 Ripening of Cheese 6.5.4 Process Monitoring and Biochemical Processing 6.6 Outlook for Food Science and Technology Applications References 7 PTR - MS in the Medical Sciences

7.1 Background 7.2 Breath Analysis 7.2.1 Smoking and Breath Volatiles 7.2.2 Isoprene in Breath 7.2.3 Acetone in Breath 7.2.4 Lung Studies: Cancer and Emphysema 7.2.5 Other PTR-MS Breath Studies 7.2.5.1 Crohn's Disease and Ulcerative Colitis 7.2.5.2 Carbohydrate Malabsorption 7.2.5.3 High Mass-Resolution PTR-TOF-MS Breath Studies 7.2.5.4 Kidney Function and PTR-MS 7.2.5.5 Liver Disease 7.2.6 Drug Monitoring and Pharmacokinetics Using Breath Analysis and PTR-MS

232 233 233 234 235 235 235 237 237 237 238 240 245 245 245 246 247 247 247 248 249 250 250 251 253 253 256 257 267

267 268 271 272 275 276 278 278 278 278 280 280 281

x Contents

7.2.7 Breath VOC Levels Measured Using PTR-MS versus Blood Concentrations 7.2.8 Breath Sampling and PTR-MS 7.2.8.1 Offline Breath Sampling 7.2.8.2 Online Breath Sampling 7.2.9 PTR-MS and Breath Analysis: Requirements and Future Directions 7.3 Online PTR-MS Measurements of Volatile Emissions from Microbial Cultures 7.3.1 Bacteria 7.3.2 VOC Emissions from Fungi 7.3.3 Concluding Remarks an Microbial Emissions 7.4 Other Medical Applications 7.4.1 Urine Headspace Analysis 7.4.2 Skin Emissions 7.4.3 VOC Emissions from Human Cells 7.4.4 VOCs in Clinical Environments References

8 Applications of PTR-MS to Homeland Security: The Detection of Threat Agents 8.1 Background 8.2 Explosives 8.2.1 Forensic Issues 8.2.1.1 The Unambiguous Detection of TNT 8.2.1.2 High Mass Resolution PTR-TOF-MS Measurements of TNT 8.2.1.3 Reagent Ion Switching and Explosives Detection 8.2.1.4 PTR-MS and the Detection of Traces of Explosives 8.2.2 Environmental Aspects and Explosives 8.3 Chemical Warfare Agents and Toxic Industrial Chemicals 8.4 Narcotics 8.5 Date Rape Drugs 8.6 Ion Mobility Mass Spectrometry and PTR-MS: A Brief Comparison for Homeland Security Applications 8.7 Future Directions References

284 285 286 287 287 290 290 296 297 297 301 301 301 302 302

311 311 312 312 315 318 319 320 320 321 322 325 326 327 328

9 Liquid Analysis Using PTR MS 9.1 Determination of Henry's Law Constants Using PTR-MS 9.2 Analysis of Liquids References

331 331 333 336

Index

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