Trace Quantitative Analysis by Mass Spectrometry ebook

Contents

Preface

Acknowledgements

1 Measurement, Dimensions and Units

1.1 Introduction

1.2 The International System of Units (SI)

1.3 ‘Mass-to-Charge Ratio’ in Mass Spectrometry

1.4 Achievable Precision in Measurement of SI Base Quantities

1.5 Molecular Mass Limit for Trace Quantitation by Mass Spectrometry

1.6 Summary of Key Concepts

2 Tools of the Trade I. The Classical Tools

2.1 Introduction

2.2 Analytical and Internal Standards: Reference Materials

2.3 The Analytical Balance

2.4 Measurement and Dispensing of Volume

2.5 Preparation of Solutions for Calibration

2.6 Introduction to Calibration Methods for Quantitative Analysis

2.7 Summary of Key Concepts

3 Tools of the Trade II. Theory of Chromatography

3.1 Introduction

3.2 General Principles of Chemical Separations

3.3 Summary of Important Concepts

3.4 Plate Theory of Chromatography

3.5 Nonequilibrium Effects in Chromatography: the van Deemter Equation

3.6 Gradient Elution

3.7 Capillary Electrophoresis and Capillary Electrochromatography

Appendix 3.1 Derivation of the Plate Theory Equation for Chromatographic Elution

Appendix 3.2 Transformation of the Plate Theory Elution Equation from Poisson to Gaussian Form

Appendix 3.3 A Brief Introduction to Snyder’s Theory of Gradient Elution

List of Symbols Used in Chapter 3

4 Tools of the Trade III. Separation Practicalities

4.1 Introduction

4.2 The Analyte and the Matrix

4.3 Extraction and Clean-Up: Sample Preparation Methods

4.4 Chromatographic Practicalities

4.5 Summary of Key Concepts

Appendix 4.1 Responses of Chromatographic Detectors: Concentration vs Mass–Flux Dependence

5 Tools of the Trade IV. Interfaces and Ion Sources for Chromatography–Mass Spectrometry

5.1 Introduction

5.2 Ion Sources that can Require a Discrete Interface Between Chromatograph and Source

5.3 Ion Sources not Requiring a Discrete Interface

5.4 Source–Analyzer Interfaces Based on Ion Mobility

5.5 Summary of Key Concepts

Appendix 5.1: Methods of Sample Preparation for Analysis by MALDI

6 Tools of the Trade V. Mass Analyzers for Quantitation: Separation of Ions by m/z Values

6.1 Introduction

6.2 Mass Analyzer Operation Modes and Tandem Mass Spectrometry

6.3 Motion of Ions in Electric and Magnetic Fields

6.4 Mass Analyzers

6.5 Activation and Dissociation of Ions

6.6 Vacuum Systems

6.7 Summary of Key Concepts

Appendix 6.1 Interaction of Electric and Magnetic Fields with Charged Particles

Appendix 6.2 Leak Detection Appendix

Appendix 6.3 List of Symbols Used in Chapter 6

7 Tools of the Trade VI. Ion Detection and Data Processing

7.1 Introduction

7.2 Faraday Cup Detectors

7.3 Electron Multipliers

7.4 Post-Detector Electronics

7.5 Summary of Key Concepts

8 Tools of the Trade VII: Statistics of Calibration, Measurement and Sampling

8.1 Introduction

8.2 Univariate Data: Tools and Tests for Determining Accuracy and Precision

8.3 Bivariate Data: Tools and Tests for Regression and Correlation

8.4 Limits of Detection and Quantitation

8.5 Calibration and Measurement: Systematic and Random Errors

8.6 Statistics of Sampling of Heterogeneous Matrices

8.7 Summary of Key Concepts

Appendix 8.1 A Brief Statistics Glossary

Appendix 8.2 Symbols Used in Discussion of Calibration Methods

9 Method Development and Fitness for Purpose

9.1 Introduction

9.2 Fitness for Purpose and Managing Uncertainty

9.3 Issues Between Analyst and Client: Examining What’s at Stake

9.4 Formulating a Strategy

9.5 Method Development

9.6 Matrix Effects

9.7 Contamination and Carryover

9.8 Establishing the Final Method

10 Method Validation and Sample Analysis in a Controlled Laboratory Environment

10.1 Introduction

10.2 Method Validation

10.3 Conduct of the Validaton

10.4 Examples of Methods and Validations Fit for Purpose

10.5 Validated Sample Analysis

10.6 Documentation

10.7 Traceability

11 Examples from the Literature

11.1 Introduction

11.2 Food Contaminants

11.3 Anthropogenic Pollutants in Water

11.4 GC–MS Analyses of Persistent Environmental Pollutants

11.5 Bioanalytical Applications

11.6 Quantitative Proteomics

11.7 Analysis of Endogenous Analytes

Epilog

References

Index

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Library of Congress Cataloging in Publication Data

Boyd, Bob, 1938-

Trace quantitative analysis by mass spectrometry /Bob Boyd, Robert Bethem, Cecilia Basic. p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-05771-1 (cloth : alk. paper)

1. Mass spectrometry. 2. Chemistry, Analytic—Quantitative. I. Bethem, Robert. II. Basic, Cecilia. III. Title.

QD272.S6B69 2008

543′.65—dc22

2007046641

The authors dedicate this book to all of our mentors and colleagues, too many to mention by name, with whom we have had the privilege of working over many years and who have taught us so much.

Acknowledgements

While all errors and obfuscations remain the respon­sibility of the authors, this book has benefited from advice and contributions generously provided by several colleagues, including Keith Gallicano, Lynn Heiman, David Heller, Bob Peterson, Eric Reiner and Vince Taguchi. We are indebted to the family of Dr A.J.P. Martin for permission to reproduce his photograph in Chapter 3 and to the Rowett Institute in Aberdeen, Scotland, for permission to reproduce the photograph of Dr R.L.M. Synge. The photographs of Dr J.J. van Deemter and Dr M.J.E. Golay were kindly provided by Drs Ted Adlard and L.S. Ettre, and the pictures of M.S. Tswett and his apparatus by Dr Klaus Beneke of the University of Kiel. The portrait of William Thomson (Lord Kelvin) was kindly provided by the Department of Physics, Strathclyde University, Scotland, and that of Joseph Black by the Department of Chemistry, University of Glasgow, Scotland. Dr Ron Majors generously sent us the original graphics from several of his articles in LCGC magazine.

Finally, the authors thank their families and friends for their unwavering patience, support and encouragement in the face of our often obsessive burning of midnight oil while writing this book.

1

Measurement, Dimensions and Units

1.1 Introduction

All quantitative measurements are really comparisons between an unknown quantity (such as the height of a person) and a measuring instrument of some kind (e.g., a measuring tape). But to be able to communicate the results of our measurements among one another we have to agree on exactly what we are comparing our measurements to. If I say that I measured my height and the reading on the tape was 72, that does not tell you much. But if I say the value was 72 inches, that does provide some meaningful information provided that you know what an inch is (tradition tells us that the inch was originally defined as the length of part of the thumb of some long-forgotten potentate but that does not help us much). But even that information is incomplete as we do not know the uncertainty in the measurement. Most people understand in a general way the concepts of accuracy (deviation of the measured value from the ‘true’ value) and precision (a measure of how close is the agreement among repeated measurements of the same quantity) as different aspects of total uncertainty, and such a general understanding will suffice for the first few chapters of this book. However, the result of a measurement without an accompanying estimate of its uncertainty is of little value, and a more complete discussion of experimental uncertainty is provided in Chapter 8 in preparation for the practical discussions of Chapters 9 and 10.

Actually, the only correct answer to the question ‘what is an inch’ is that one inch is defined as exactly 2.54 centimeters (zero uncertainty in this defined conversion factor). So now we have to ask what is a centimeter, and most of us know that a centimeter is 1/100 of a meter. So what is a meter? This is starting to sound about as arbitrary as the Roman horses’ hind quarters mentioned in the text box but in this case we can give a more useful if less entertaining answer: The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 meters per second, and that we still have not arrived at a final definition of the meter until we have defined the second (). This is the internationally accepted definition of the meter, established in 1983, and forms part of the International System of Units (Système Internationale d’Unites, known as SI for short). The SI establishes the standards of comparison used by all countries when the measured values of physical and chemical properties are reported. Such an international agreement is essential not only for science and technology, but also for trade. For example, consider the potential confusion arising from the following example: