50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States. Copyright ...... control assembly of the drift tube is mandatory to assess mass accuracy ...... Journal of Chromatography A, 1218(42), 7615e7626. ...... nation of the mean value (Xm), the systematic error (d), and the random error (si). ...... with security guard.
Applications in High Resolution Mass Spectrometry
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Applications in High Resolution Mass Spectrometry Food Safety and Pesticide Residue Analysis
Edited by
Roberto Romero-Gonza´lez Antonia Garrido Frenich
Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2017 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-809464-8 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals
Publisher: Nikki Levy Acquisition Editor: Patricia Osborn Editorial Project Manager: Jaclyn A. Truesdell Production Project Manager: Lisa Jones Designer: Alan Studholme Typeset by TNQ Books and Journals
Contents List of Contributors.................................................................................. xi Preface................................................................................................. xiii
CHAPTER 1 HRMS: Fundamentals and Basic Concepts................ 1 Franciso Javier Arrebola-Lie´banas, Roberto Romero-Gonza´lez, Antonia Garrido Frenich 1.1 Introduction (To High-Resolution Mass Spectrometry)............. 1 1.1.1 Basic Concepts (Units and Definitions) ......................... 1 1.1.2 Low-Resolution Mass Spectrometry Versus High-Resolution Mass Spectrometry............................. 3 1.2 Resolution and Mass Resolving Power.................................. 6 1.3 Accurate Mass Measurement: Exact Mass and Mass Defect...... 7 1.4 Mass Calibration in High-Resolution Mass Spectrometry ......... 9 1.5 General Considerations......................................................12 Acknowledgments...................................................................13 References.............................................................................13
CHAPTER 2 HRMS: Hardware and Software ...............................15 Juan F. Garcı´a-Reyes, David Moreno-Gonza´lez, Rocı´o NortesMe´ndez, Bienvenida Gilbert-Lo´pez, Antonio Molina Dı´az 2.1 Introduction.....................................................................15 2.2 Principles of High-Resolution Mass Spectrometry Analyzers........................................................................16 2.2.1 Time-of-Flight ........................................................ 17 2.2.2 Fourier Transform Ion Cyclotron Resonance ................ 19 2.2.3 Orbitrap................................................................. 21 2.3 Time-of-Flight Mass Spectrometry: Instrument Configuration and Main Features ........................................22 2.3.1 Stand-alone Electrospray Ionization Time-of-Flight and Hybrid Quadrupole Time-of-Flight Instrumentation....................................................... 22 2.3.2 Improvements of Current (Quadrupole) Time-of-Flight Instrumentation....................................................... 24 2.3.3 Ion Mobility Quadrupole Time-of-Flight ..................... 29 2.3.4 Hybrid Ion Trap Time-of-Flight ................................. 32 2.3.5 Gas ChromatographyeTime-of-Flight and Gas ChromatographyeQuadrupole Time-of-Flight .............. 32
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2.4 Orbitrap Analyzers: Instrument Configurations and Main Features..........................................................................33 2.5 Acquisition Modes in High-Resolution Mass Spectrometry......39 2.5.1 Data-Dependent Acquisition...................................... 39 2.5.2 Data-Independent Acquisition.................................... 43 2.5.3 Postacquisition Approaches....................................... 44 2.6 Databases and the Internet Resources for High-Resolution Mass Spectrometry ...........................................................44 Acknowledgments...................................................................50 References.............................................................................50
CHAPTER 3 Analytical Strategies Used in HRMS .......................59 Aan Agu¨era, Ana Bele´n Martı´nez-Piernas, Marina Celia Campos-Man˜as 3.1 Introduction.....................................................................59 3.2 Advantages of High-Resolution Mass Spectrometry in Pesticide Analysis.........................................................60 3.2.1 Selectivity in High-Resolution Mass Spectrometry: Accurate Mass and Resolution in Qualitative Analysis ................................................................ 60 3.2.2 Improving Selectivity by Tandem Mass Spectrometry Information ............................................................ 62 3.2.3 Quantitative Performance.......................................... 65 3.3 Data Analysis Workflows in High-Resolution Mass Spectrometry ...................................................................68 3.3.1 Qualitative Screening Method Validation..................... 73 3.3.2 Nontarget Analysis .................................................. 75 3.4 Conclusions.....................................................................78 Acknowledgments...................................................................78 References.............................................................................78 Further Reading......................................................................81
CHAPTER 4 Current Legislation on Pesticides ...........................83 Helen Botitsi, Despina Tsipi, Anastasios Economou 4.1 Introduction.....................................................................83 4.2 Pesticides........................................................................83 4.2.1 Identity and Physicochemical Properties...................... 83 4.2.2 Pesticides Classification............................................ 84 4.2.3 Pesticide Metabolites and Transformation Products ....... 85 4.3 Legislation ......................................................................86 4.3.1 Pesticides Authorization ........................................... 86
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4.3.2 Maximum Residue Limits......................................... 88 4.3.3 Monitoring Programs ............................................... 90 4.4 Analytical Quality ControldMethod Validation ....................92 4.4.1 Guidelines for Pesticide Residue Analysis ................... 98 4.5 Mass Spectrometry in Pesticide Residue Analysis ................ 110 4.5.1 Mass Spectrometry Identification and Confirmation......110 4.5.2 Potential of High-Resolution Mass Spectrometry in Pesticide Residue Analysis ...................................115 References........................................................................... 119
CHAPTER 5 Advanced Sample Preparation Techniques for Pesticide Residues Determination by HRMS Analysis .............................................................. 131 Renato Zanella, Osmar D. Prestes, Gabrieli Bernardi, Martha B. Adaime 5.1 Introduction................................................................... 131 5.2 Matrix Effects and the Influence of Coextracted Components .................................................................. 133 5.3 Sample Preparation Techniques for Pesticide Residue Determination by Chromatographic Techniques Coupled to High-Resolution Mass Spectrometry .............................. 135 5.3.1 Dilute-and-Shoot....................................................137 5.3.2 QuEChERS Method................................................138 5.3.3 Matrix Solid-Phase Extraction ..................................141 5.3.4 Solid-Phase Extraction ............................................142 5.3.5 Solid-Phase Microextraction and Stir Bar Sorptive Extraction .............................................................147 5.3.6 Microwave-Assisted Extraction.................................149 5.3.7 Pressurized Liquid Extraction...................................150 5.4 Perspectives and Conclusions ........................................... 151 References........................................................................... 154
CHAPTER 6 Applications of Liquid Chromatography Coupled With High-Resolution Mass Spectrometry for Pesticide Residue Analysis in Fruit and Vegetable Matrices ............................ 165 P. Sivaperumal 6.1 Introduction................................................................... 165 6.2 Applications of Pesticide Residue Analysis in Fruit and Vegetable Samples by LC-HRMS................................ 168
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6.3 Optimized Sample Preparation and Chromatographic Conditions for Mass Analyzers ......................................... 186 6.4 Analytical Method Validation ........................................... 187 6.4.1 Matrix Effect.........................................................187 6.4.2 Evaluation of the Matrix Interferences by UHPLC-HR/MS.....................................................189 6.4.3 Limit of Detection, Limit of Quantitation, Accuracy, and Precision ..........................................192 6.5 Accurate Measurement of Pesticide Residues in Fruit and Vegetable Samples.................................................... 192 6.5.1 Determination of Chlorine Isotope in Food Samples................................................................192 6.5.2 Determination of Carbon, Chlorine, and Bromine Isotope in Food Samples.............................195 6.6 Evaluation of Pesticide Residues in Fruit and Vegetable Samples........................................................................ 195 6.7 Conclusion.................................................................... 197 Acknowledgments................................................................. 198 References........................................................................... 198
CHAPTER 7 Application of HRMS in Pesticide Residue Analysis in Food From Animal Origin ..................... 203 Roberto Romero-Gonza´lez, Antonia Garrido Frenich 7.1 Introduction................................................................... 203 7.2 Instrumental Requirements............................................... 204 7.3 Analytical Procedures: Extraction and Chromatographic Conditions..................................................................... 210 7.4 Quantitative and Qualitative Applications ........................... 215 7.5 Differences Between Low-Resolution Mass Spectrometry and High-Resolution Mass Spectrometry Analytical Methods........................................................................ 226 7.6 Overview and Future Perspectives..................................... 228 References........................................................................... 229
CHAPTER 8 Recent Advances in HRMS Analysis of Pesticide Residues Using Atmospheric Pressure Gas Chromatography and Ion Mobility ......................... 233 Lauren Mullin, Gareth Cleland, Jennifer A. Burgess 8.1 Introduction................................................................... 234 8.2 Atmospheric Pressure Gas Chromatography........................ 236 8.2.1 Introduction...........................................................236 8.2.2 Background...........................................................236
Contents
8.2.3 8.2.4 8.2.5 8.2.6
Current Design ......................................................237 Ionization Mechanisms............................................238 Ionization Trends for Pesticides ................................239 Pesticide Screening Using Atmospheric Pressure Gas Chromatography With High-Resolution Mass Spectrometry .........................................................239 8.2.7 Improved Selectivity and Sensitivity With Atmospheric Pressure Gas Chromatography ..................................245 8.2.8 Carrier Gas Flow Rate Increase ................................246 8.3 Time-of-Flight Mass Spectrometry .................................... 247 8.4 Ion Mobility Separation................................................... 249 8.4.1 Background and Theory ..........................................249 8.4.2 Application of Traveling Wave Ion Mobility Spectrometry to Pesticide Screening ..........................251 8.4.3 Spectral Selectivity Enhancement..............................253 8.4.4 Protomer Visualization ............................................255 8.5 Summary and Conclusion ................................................ 258 Acknowledgments................................................................. 261 References........................................................................... 261
CHAPTER 9 Direct Analysis of Pesticides by Stand-Alone Mass Spectrometry: Flow Injection and Ambient Ionization ............................................................ 265 E. Moyano, M.T. Galceran 9.1 Introduction................................................................... 265 9.2 Flow Injection Analysis................................................... 268 9.2.1 FIA Method Development .......................................273 9.2.2 Sample Preparation for FIA-MS-Based Methodologies .......................................................277 9.2.3 FIA-MS Methods Performance .................................279 9.2.4 FIA-MS Applications..............................................282 9.3 Ambient Mass Spectrometry ............................................ 283 9.3.1 Electrospray-Based Techniques.................................284 9.3.2 Plasma-Based Techniques ........................................290 9.3.3 Ambient Mass Spectrometry Method Development ......292 9.3.4 Sample Handling for Ambient Mass SpectrometryBased Methodologies..............................................298 9.3.5 Ambient Mass Spectrometry Method Performance.......302 9.3.6 Ambient Mass Spectrometry Applications ..................305 9.4 Final Remarks and Future Trends...................................... 307 Acknowledgments................................................................. 309 References........................................................................... 309
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CHAPTER 10 Identification of Pesticide Transformation Products in Food Applying High-Resolution Mass Spectrometry ..................................................... 315 Imma Ferrer, Jerry A. Zweigenbaum, E. Michael Thurman 10.1 Introduction................................................................. 315 10.2 Experimental ............................................................... 316 10.2.1 Chemicals and Reagents ...................................... 316 10.2.2 Greenhouse Study............................................... 316 10.2.3 Plant Extraction.................................................. 316 10.2.4 Liquid Chromatography/Quadrupole-Time-of-Flight Mass Spectrometry Analysis................................. 317 10.2.5 Mass Profiler Software ........................................ 318 10.3 Imidacloprid Metabolites in Plants .................................. 318 10.3.1 Accurate Mass Databases..................................... 319 10.3.2 Mass Profiler Professional.................................... 326 10.3.3 Metabolite Distribution and Mass Balance for Imidacloprid Metabolites ................................ 329 10.4 Imazalil Metabolites in Plants and Soil............................. 329 10.4.1 Chlorine Filter Approach ..................................... 330 10.4.2 Soil Metabolites ................................................. 332 10.5 Propiconazole Metabolites in Plants and Soil .................... 332 10.5.1 Chlorine Filter Approach ..................................... 332 10.6 Conclusions................................................................. 334 References........................................................................... 334 Index...................................................................................................337
List of Contributors Martha B. Adaime Federal University of Santa Maria, Santa Maria, Brazil Ana Agu¨era CIESOL, Joint Centre of the University of Almerı´a e CIEMAT, Almerı´a, Spain Franciso Javier Arrebola-Lie´banas University of Almerı´a, Almerı´a, Spain Gabrieli Bernardi Federal University of Santa Maria, Santa Maria, Brazil Helen Botitsi General Chemical State Laboratory, Athens, Greece Jennifer A. Burgess Waters Corporation, Milford, MA, United States Marina Celia Campos-Man˜as CIESOL, Joint Centre of the University of Almerı´a e CIEMAT, Almerı´a, Spain Gareth Cleland Waters Corporation, Milford, MA, United States Anastasios Economou National and Kapodistrian University of Athens, Athens, Greece Imma Ferrer University of Colorado, Boulder, CO, United States Antonia Garrido Frenich University of Almerı´a, Almerı´a, Spain M.T. Galceran University of Barcelona, Barcelona, Spain Juan F. Garcı´a-Reyes University of Jae´n, Jae´n, Spain Bienvenida Gilbert-Lo´pez University of Jae´n, Jae´n, Spain Ana Bele´n Martı´nez-Piernas CIESOL, Joint Centre of the University of Almerı´a e CIEMAT, Almerı´a, Spain Antonio Molina Dı´az University of Jae´n, Jae´n, Spain
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David Moreno-Gonza´lez University of Jae´n, Jae´n, Spain E. Moyano University of Barcelona, Barcelona, Spain Lauren Mullin Waters Corporation, Milford, MA, United States Rocı´o Nortes-Me´ndez University of Jae´n, Jae´n, Spain P. Sivaperumal National Institute of Occupational Health, Ahmedabad, Gujarat, India Osmar D. Prestes Federal University of Santa Maria, Santa Maria, Brazil Roberto Romero-Gonza´lez University of Almerı´a, Almerı´a, Spain E. Michael Thurman University of Colorado, Boulder, CO, United States Despina Tsipi General Chemical State Laboratory, Athens, Greece Renato Zanella Federal University of Santa Maria, Santa Maria, Brazil Jerry A. Zweigenbaum Agilent Technologies Inc., Wilmington, DE, United States
Preface Each problem that I solved became a rule, which served afterward to solve other problems. Rene Descartes
Currently, mass spectrometry is an essential tool in food safety and its output has reached an unprecedented level, being the most adequate detection system for the analysis of pesticide residues in food matrices. In the last few years, new analyzers, ionization methods, and analytical strategies have been developed to increase the scope of analytical methods as well as the reliability of the results. Up to now, high-resolution mass spectrometry (HRMS) has been considered as a complementary tool of conventional triple quadrupole (QqQ) analyzers. However, time of flight and/or Orbitrap are replacing low-resolution mass spectrometry analyzers because they can increase the number of compounds simultaneously monitored, retrospective analysis can be carried out, and unknown compounds can be identified. All these tasks can be performed using one benchtop platform at suitable sensitivity. In addition, if HRMS analyzers are coupled with ultrahigh-performance liquid chromatography (UHPLC) the number of compounds analyzed in one single run can increase considerably. Furthermore, when ambient ionization techniques are used with HRMS analyzers, sample treatment could be minimized, increasing sample throughput. The use of these HRMS analyzers opens a new scenario in pesticide residue analysis, and potential users should know the strategies that can be applied to get all the information that these analyzers could provide, as well as the pros and cons in relation to QqQ. Because of the fast application and implementation of these new procedures, we consider that setting the main principles and strategies based on these approaches is necessary to provide a comprehensive view of the cited tools, being a cornerstone in the food safety field. Thus, this book would provide a complete overview of the possibilities that HRMS could offer in pesticide residue analysis in food, as well as the suitable workflow needed to achieve the goals proposed by scientists. Chapters 1e4 provide an overview of HRMS, basic concepts, hardware, general approaches (target and nontarget analysis), and current legislation related to this topic. Chapters 5e7 are “applied chapters” where the main extraction procedures used, as well as the application of HRMS in pesticide residue analysis in several types of matrices, are described. Finally, Chapters 8e10 describe new advances on the use of HRMS such as gas chromatography (GC)eHRMS, ambient ionization techniques, and identification of transformation products. The content of each chapter is described in more detail as follows.
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To facilitate the introduction to the topics presented in this book, Chapter 1 describes the principles of HRMS, explaining several concepts such as monoisotopic mass, resolution, mass accuracy, isotopic pattern, etc. Moreover, the differences between low- and high-resolution mass spectrometry are also discussed. In Chapter 2, the several analyzers that could be used in HRMS are described, indicating the differences between the HRMS systems from different vendors, as well as the software that, nowadays, is available to process all the data provided by these analyzers. Finally, online resources such as ChemSpider and MassBank are described. Target, nontarget, and unknown analysis strategies are explained in Chapter 3, describing how a database is prepared and the workflow commonly used for nontarget and unknown analysis. Chapter 4 provides a complete overview of the general requirements indicated by international guidelines (i.e., SANTE) regarding the use of HRMS in pesticide residue analysis, as well as the parameters that should be validated. A section describing pesticides legislation (indicating MRLs) is also included. Bearing in mind that theoretically, unlimited number of compounds could be determined by HRMS, Chapter 5 describes the development of generic extraction methods that allow the simultaneous extraction of a huge number of pesticides that are needed. Chapters 6 and 7 cover the main applications describing the use of HRMS coupled with UHPLC during the analysis of pesticide residue analysis in fruits and vegetables (Chapter 6) and in food from animal origin (Chapter 7). Although most of the current applications focused on pesticide residue applying HRMS use liquid chromatography as the separation technique, in the last few years, GC has also been utilized, because of the development of new ionization sources, such as atmospheric pressure gas chromatography or new couplings such as GCOrbitrap. Therefore, Chapter 8 is devoted to this topic to highlight the potentiality of GCeHRMS in pesticide residue analysis as well as new approaches such as ion mobility. Because of the potentiality of HRMS, chromatographic step could be removed from the conventional analytical method, and rapid detection of the target compounds could be performed. This approach is interesting for pesticides that cannot be commonly analyzed by multiresidue methods (i.e., very polar pesticides), and the use of HRMS could simplify the analytical strategy. Chapter 9 is focused on this approach as well as in the use of ambient mass spectrometry, which allows for the direct analysis of samples without sample extraction and chromatographic separation, especially when HRMS analyzers are used. Finally, Chapter 10 is dedicated to the advantages that HRMS provides for the identification of pesticide transformation products. The book is intended for researchers and professionals working with LCeMS, such as food chemists, analytical chemists, toxicologists, food scientists, and everyone who uses/needs this technique to evaluate food safety. Moreover, undergraduate students would also be interested. Finally, it has been a great pleasure to thank all the authors of this book for their work. All of them are specialists and we really appreciate their effort and time. We also give special thanks to the editorial and production teams of the publisher,
Preface
Elsevier, especially Karen R. Miller, who started this adventure, and Jackie Truesdell, for her patience and help, allowing this work to come to fruition. Roberto Romero-Gonza´lez Antonia Garrido Frenich Almerı´a, Spain November, 2016.
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CHAPTER
HRMS: Fundamentals and Basic Concepts
1
Franciso Javier Arrebola-Lie´banas, Roberto Romero-Gonza´lez, Antonia Garrido Frenich University of Almerı´a, Almerı´a, Spain
1.1 INTRODUCTION (TO HIGH-RESOLUTION MASS SPECTROMETRY) 1.1.1 BASIC CONCEPTS (UNITS AND DEFINITIONS) Mass spectrometry (MS) is an analytical technique commonly used for qualitative and quantitative chemical analysis. MS measures the massecharge ratio (m/z) of any analyte, of both organic and inorganic nature, which has previously been ionized. Only the ions are registered in MS, but the particles with zero net electric charge (molecules or radicals) are not detected. Therefore, MS does not directly measure mass, but it determines the m/z, being m the relative mass of an ion on the unified atomic scale divided by the charge number, z, of the ion (regardless of sign). The m/z value is a dimensionless number. Because the mass of atoms and molecules is very small, the kilogram as standard international (SI) base unit cannot be used for its measurement. For that, a non-SI unit of mass, unified atomic mass unit (u) is used. At this point, in this introductory section, it is worth clarifying some basic terms (units and definitions) in MS according to the International Union of Pure and Applied Chemistry (IUPAC) recommendations (IUPAC, 1997; Murray et al., 2013). The u also called Dalton (Da), is defined as 1/12th of the mass of one atom of 12C at rest in its ground state, being 1 u ¼ 1 Da ¼ 1.660538921 (73) 1027 kg (number in parentheses indicates the estimated uncertainty). In this way, the mass of other atoms or molecules is expressed relative to the mass of the most abundant stable isotope of carbon, 12C, and this value is dimensionless. The z is defined as absolute value of charge of an ion divided by the value of the elementary charge of the electron (e) rounded to the nearest integer, being e ¼ 1.602177 1019 C. The m/z unit is the thomson (Th), although it is now a deprecated term, being 1 Th ¼ 1 u/e ¼ 1.036426 108 kg/C. For that, use of the dimensionless term m/z is accepted in the literature, and this criterion will be followed throughout this book. Other basic concepts that are commonly used in MS will be shortly described to clarify the meaning of these throughout the following chapters. Applications in High Resolution Mass Spectrometry. http://dx.doi.org/10.1016/B978-0-12-809464-8.00001-4 Copyright © 2017 Elsevier Inc. All rights reserved.
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•
Atomic mass: The number that represents the element’s mass based on the weighted average of the masses of its naturally occurring stable isotopes. For example, the integer atomic mass of bromine is 80 Da. This is because there are only two naturally occurring stable isotopes of bromine, 79Br and 81Br, which exist in nature in about equal amounts. When the relative mass (Mr) of an ion, molecule, or radical is reported, it is based on the atomic masses of its elements. • Nominal mass: Mass of a molecular ion or molecule calculated using the isotope mass of the most abundant constituent element isotope of each element (Table 1.1) rounded to the nearest integer value and multiplied by the number of atoms of each element. Example: nominal mass of H2O ¼ (2 1 þ 1 16) u ¼ 18 u. • Monoisotopic mass: Exact mass of an ion or molecule calculated using the mass of the most abundant isotope of each element. Example: monoisotopic mass of H2O ¼ (2 1.007825 þ 1 15.994915) u ¼ 18.010565 u. The exact mass of the common elements and their isotopes are provided in Table 1.1. • Exact mass: Calculated mass of an ion or molecule with specified isotopic composition. • Mass defect: Difference between the nominal mass and the monoisotopic mass of an atom, molecule, or ion. It can be a positive or negative value. • Relative isotopic mass defect (RDm): It is the mass defect between the monoisotopic mass of an element and the mass of its Aþ1 or its Aþ2 isotopic cluster (Thurman & Ferrer, 2010). For instance, RDm for the pair 35Cl:37Cl is 0.0030 Da. • Average mass: Mass of an ion or molecule weighted for its isotopic composition, i.e., the average of the isotopic masses of each element, weighted for isotopic abundance (Table 1.1). Example: average mass of H2O ¼ (2 1.00794 þ 1 15.9994) u ¼ 18.01528 u. • Accurate mass: Experimentally determined mass of an ion of known charge. • Mass accuracy: Difference between the mass measured by the mass analyzer and theoretical value. • Resolution or mass resolving power: Measure of the ability of a mass analyzer to distinguish two signals of slightly different m/z ratios. • Mass calibration: Means of determining m/z values of ions from experimentally detected signals using a theoretical or empirical relational equation. In general, this is accomplished using a computer-based data system and a calibration file obtained from a mass spectrum of a compound that produces ions of known m/z values. • Mass limit: Value of m/z above or below which ions cannot be detected in a mass spectrometer. • Mass number: The sum of the protons and neutrons in an atom, molecule, or ion. If the mass is expressed in u, mass number is similar to nominal mass. • Most abundant ion mass: The mass that corresponds to the most abundant peak in the isotopic cluster of the ion of a given empirical formula.
1.1 Introduction (To High-Resolution Mass Spectrometry)
Table 1.1 Nominal, Isotopic, and Average Masses of Some Common Stable Isotopes Element
Isotope
H
1
H H 12 C 13 C 14 N 15 N 16 O 17 O 18 O 19 F 23 Na 28 Si 29 Si 30 Si 31 P 32 S 33 S 34 S 35 Cl 37 Cl 79 Br 81 Br 127 I 2
C N O
F Na Si
P S
Cl Br I
Abundance 99.9885 0.0115 98.93 1.08 99.632 0.368 99.757 0.038 0.205 100 100 92.2297 4.6832 3.0872 100 94.93 0.76 4.29 75.78 24.22 50.69 49.32 100
Nominal Mass 1 2 12 13 14 15 16 17 18 19 23 28 29 30 31 32 33 34 35 37 79 81 127
Isotopic Mass 1.007825 2.014102 12.000000 13.003355 14.003074 15.000109 15.994915 16.999131 17.999160 18.998403 22.989770 27.976927 28.976495 29.973770 30.973762 31.972072 32.971459 33.967868 34.968853 36.965903 78.918336 80.916289 126.904476
Average Mass 1.00794 12.0110 14.00674 15.9994
18.9984 22.9898 28.0855
30.9738 32.0660
35.4527 79.9094 126.9045
1.1.2 LOW-RESOLUTION MASS SPECTROMETRY VERSUS HIGHRESOLUTION MASS SPECTROMETRY It should be noted that mass measurements in MS can be carried out at either low resolution (LRMS) or high resolution (HRMS). An LRMS measurement provides information about the nominal mass of the analyte (Dass, 2007), i.e., the m/z for each ion is measured to single-digit mass units (integer mass). However, exact mass is measured by HRMS, i.e., the m/z for each ion is measured to four to six decimal points (Ekman, Silberring, Westman-Brinkmalm, & Kraj, 2009). This is very useful to structure elucidation of unknown compounds for analytes having the same nominal mass, but with very small differences in their exact masses. As a result, by LRMS measurements it is not possible to differentiate between imazalil, C14H14Cl2N2O (14 12 þ 14 1 þ 2 35 þ 2 14 þ 1 16 ¼ 296 u), and flunixin, C14H11F3N2O2 (14 12 þ 11 1 þ 3 19 þ 2 14 þ 2 16 ¼ 296 u),
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pesticides. However, this would be possible by using exact mass measurements, imazalil C14H14Cl2N2O (14 12 þ 14 1.007825 þ 2 34.968852 þ 2 14.003074 þ 1 15.994915 ¼ 296.048317 u) and flunixin C14H11F3N2O2 (14 12 þ 11 1.007825 þ 3 18.998403 þ 2 14.003074 þ 2 15.994915 ¼ 296.077262 u). High-resolution mass spectrometers have evolved from the 1960s with the introduction of double-focusing magnetic-sector mass instruments (Pico´, 2015). Next, Fourier transform ion cyclotron resonance (FT-ICR), time-of-flight (TOF), and Orbitrap mass analyzers were also introduced in the market. Also, hybrid HRMS instruments, such as quadrupole TOF (Q-TOF), ion trap (IT)-TOF, linear trap quadrupole (LTQ)-Orbitrap, or QeOrbitrap, have been developed. These last analyzers provide tandem (MS/MS) or MSn spectra of high resolution, in addition to accurate monoisotopic mass measurements, of great applicability both for the confirmation of target compounds and the identification of unknown compounds (Lin et al., 2015). The TOF and Orbitrap analyzers, single or hybrid instruments, are the most widely used in the analysis of organic contaminants, such as pesticide residues (Lin et al., 2015; Pico´, 2015). Among the main characteristics that define the performance of a mass analyzer are (Dass, 2007; de Hoffmann & Stroobant, 2007; Mcluckey & Wells, 2001) mass range, speed, efficiency, linear dynamic range, sensitivity, resolution (or its mass resolving power), and mass accuracy. The mass range is that over which a mass spectrometer can detect ions or is operated to record a mass spectrum. When a range of m/z is indicated instead of a mass range, this should be specified explicitly. The speed or scan speed is the rate at which the analyzer measures over a particular mass range. Efficiency is defined as the product of the transmission of the analyzer by its duty cycle, where the transmission is the ratio of the number of ions reaching the detector and the number of ions entering the mass analyzer, and the duty cycle can be described as the fraction of the ions of interest formed in the ionization step that are subjected to mass analysis. Linear dynamic range is considered as the range over which ion signal is linear with analyte concentration. Sensitivity can be expressed as detection sensitivity or abundance sensitivity; the first is the smallest amount of an analyte that can be detected at a certain defined confidence level, while the second is the inverse of the ratio obtained by dividing the signal level corresponding to a large peak by the signal level of the background at one mass-to-charge unit lower or higher. A summary of these characteristics of high-resolution mass analyzers is shown in Table 1.2. As it can be observed, in terms of resolving power and accuracy, the FT-ICR analyzer presents the best values, followed by the recently introduced tribrid Orbitrap analyzer. TOF and Q-TOF analyzers have worse values, although the FTICR analyzer comprises the worst sensitivity. Last but not least, two key characteristics of high-resolution mass analyzers are resolution (or its mass resolving power) and mass accuracy, which will be treated in more detail in the following two sections.
Analyzer Magnetic sector FT-ICR TOF Q-TOF IT-TOF Exactive Orbitrap LTQ-Orbitrap Q-Orbitrap Tribrid Orbitrap
Mass Range 10,000 10,000 >300,000 10,000 4000 4000 8000 6000
Speed
Linear Dynamic Range
Sensitivity
Resolving Power (FWHM)
Accuracy (ppm)
w1 s w1 s Milliseconds wMilliseconds w0.1 s 0.1 s 0.1 s 0.05 s 0.05 s
109 103e104 106 103e104 103e104 >5000 >5000 105 105
106e109 103e104 106 106 105 106 106 106 106e107
100,000 1,000,000 30,000 30,000 100,000 100,000 100,000/240,000 240,000 500,000
