This book was triggered by the success story of sector field mass spectrometry in elemental and isotopic analysis since the first presentation of the mass spectrum of Ne a hundred years ago. The outstanding and unique features of sector field mass spectrometry - high sensitivity, high mass resolution and simultaneous multiple ion detection – have paved the way for its widespread and successful application across different scientific disciplines. Written, compiled and edited by world renowned experts, this book is intended to provide deep insight into the topic along with fundamental knowledge about elemental and isotopic analysis. Aimed at scientists in the field of natural and life sciences, instrument manufacturers, practitioners and graduate students, it provides solid information about the methodological background and analytical capabilities of sector field mass spectrometry. A detailed description of peculiarities and an overview of the most relevant applications making use of specific techniques employing sector field mass analysers (ICP-MS, GDMS, TIMS, SIMS and IRMS) are given, including a presentation of the currently available commercial instruments. This approach guarantees that readers are thoroughly introduced to and familiarized with the fascinating inter- and transdisciplinary field of sector field mass spectrometry.
About the Author
Thomas Prohaska is professor for analytical chemistry at the BOKU (University of Natural Resources and Life Sciences Vienna, Tulln, Austria). He studied chemistry at the Vienna University of Technology and received his PhD with summa cum laude in 1995. He became scientific researcher at the BOKU in the same year to build up a laboratory for elemental trace analysis. From 1998 to 2000 he was researcher at the EC joint research center IRMM in Geel, Belgium.In 2004, he received the START award for the setup of a new research laboratory (VIRIS).
Johanna Irrgeher is currently postdoctoral researcher in the field of analytical chemistry at the VIRIS Laboratory for analytical ecogeochemistry at BOKU. She holds a master degree in biotechnology and obtained her PhD granted by the Austrian Academy of Sciences in 2013 with honours for her work on stable strontium isotope ratio analysis by (LA)-MC ICP-MS. Her current research focuses on analytical method development for elemental and isotopic analysis in the field of analytical ecogeochemistry dealing with both aquatic and terrestrial ecosystems.
Andreas Zitek is currently a postdoctoral researcher at BOKU and a member of the VIRIS Laboratory for Analytical Ecogeochemistry in Tulln. His current research activities aim at the application of elemental and isotopic analyses to basic questions in the field of aquatic ecology (‘aquatic ecogeochemistry’), with a special focus on the chemical analysis of hard parts of freshwater fish.
Norbert Jakubowski is presently head of the division Inorganic Analysis at the BAM - Federal Institute for Materials Research and Testing, Berlin, Germany. He received his PhD at the University of Stuttgart Hohenheim with summa cum laude in 1991 and in 2013 he received the "European Award for Plasma Spectrochemistry" for his contribution in elemental mass spectrometry. His research interests are related to Analytical Chemistry in general with special interest in development of instruments, methods and problem-orientated procedures based on the use of plasma sources for elemental mass spectrometry of solid and liquid samples. Recently he started a new research direction towards labelling of biomolecules with metals by use of chelating compounds for detection of biomarkers in clinical research.
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Sector Field Mass Spectrometry for Elemental and Isotopic Analysis
By Thomas Prohaska, Johanna Irrgeher, Andreas Zitek, Norbert Jakubowski
The Royal Society of ChemistryCopyright © 2015 The Royal Society of Chemistry
All rights reserved.
THOMAS PROHASKA AND ANDREAS ZITEK
University of Natural Resources and Life Sciences Vienna (BOKU), Department of Chemistry, Division of Analytical Chemistry, VIRIS Laboratory for Analytical Ecogeochemistry, Tulln, Austria
1.1 Why Another Book on Mass Spectrometry?
Mass spectrometry has been used since its advent as a tool to investigate challenging scientific questions that could not be tackled with conventional analytical methods before in many different scientific disciplines. As a consequence of the steadily growing interest in mass spectrometry, the increasing number of installed instruments and the massive expansion in augmenting numbers of fields of applications, mass spectrometry has developed steadily into a black box for the common user often having little knowledge about what happens between the sample introduction and the final data provided in electronic form (Figure 1.1). However, a basic understanding of the technical and analytical factors that might significantly influence the measurement and the final result obtained is crucial to avoid misleading and even wrong interpretations. It is especially the increasingly transdisciplinary application of mass spectrometry that creates the basic need for a transfer of the related analytical knowledge between scientific disciplines, e.g. about the general methodological background and uncertainties related to the final result including a clear picture on the potential and limitations of each method. Therefore, a complex, extensive and far-ranging topic such as mass spectrometry requires to be covered by a variety of educating tools such as textbooks organised along specific main subjects and providing different points of view on necessarily redundant topics.
1.1.1 The Success of Magnetic Sector Field Mass Spectrometry
Mass spectrometry has developed from a scientific instrument providing profound knowledge of atoms and isotopes into an affordable and routine analytical tool in many laboratories. Magnetic sector fields as mass separator laid the basis for modern mass spectrometry. From the early days at the beginning of the 20th century until around 1940, mass spectroscopy was mainly applied for the determination of masses and abundances of isotopes. Since then, the applications have spread rapidly from fundamental investigations into elemental and isotopic applications in inorganic mass spectrometry and into the gathering of compositional and structural information of organic molecules in organic mass spectrometry. Historically, the first mass spectrometers at the beginning of the 20th century were based on a sector field device, which were exclusively used for a long period of time. Their importance increased in the second half of the 20th century mainly due to their application for determining organic molecules, where high mass resolution capabilities were required. New designs of mass separators were developed during the second half of 20th century. The major advantages of sector field mass spectrometers have underpinned their importance ever since: the high sensitivity for ultratrace levels of elements, the simultaneous measurement capabilities of multiple isotopes for precise isotope ratio measurements and the high-resolution capabilities to resolve spectral interferences.
1.1.2 Magnetic Sector Field Devices for Elemental and Isotopic Analysis
Throughout the last 110 years, the basic principle of all mass spectrometers has remained the same: A sample is ionised in an ion source, the ions are separated via a mass separator according to their mass/charge (m/z) ratio and finally detected at a detection unit. The devices, which are described in this book, are based on a magnetic sector field as mass separator and applied for quantitative elemental analysis and isotope ratio measurements (of isotopes of one element or of different elements).
Modern mass spectrometers use sector field devices for quantitative low- level elemental analysis due to their high sensitivity and for precise isotope ratio measurements due to their outstanding possibility of separating and letting pass ions of different m/z ratio simultaneously. Another important feature of magnetic sector field devices is the capability to be operated at higher mass resolution (i.e. greater than unity) in order to resolve interfering ions from the ions of interest. Most often, the combination of a magnetic sector and an electric sector can be found in a variety of geometries (see Chapter 4) and results in a so-called double-focusing mass spectrometer. Whereas the magnet is used as mass separator and for directional focusing, the electrostatic sector is used for focusing ions of different energies that the energy spread of the ions does not compromise the achievable mass resolution. The electrostatic sector field is not necessarily required if the ionisation source provides ions with little energy spread.
The major difference of the described instruments within this book is based on different ionisation strategies. The goal of this book is to discuss the main features, principles of operation and the resulting applications of these different sector field mass spectrometers. The operation principles are explained in detail based on currently commercially available state-of-the-art instruments.
1.1.3 Metrology in Chemistry – The Science of Measurement
We want to make use of this chapter to underpin the necessity of the full understanding of the analytical process, what is usually known under the topic "Science of Measurements" or "Metrology", as defined by the International Bureau of Weights and Measures (BIPM). It is a declared aim of this book to enhance knowledge about the measurement and thus to contribute to a better understanding of the applied methods. Even though instrumental developments have resulted in robust and field-proven devices, all measurement strategies require in any case a sound metrological background – and thus a full understanding of the analytical process from sampling to the final result.
Metrology in chemistry has become of crucial importance in order to avoid the situation that scientific conclusions are drawn from analytical artefacts sometimes provided by highly specialised equipment if not looked at thoroughly. This is even more important as new phenomena can be observed by steadily improving analytical techniques. The recent developments in mass spectrometers used for elemental and isotopic analysis have improved in the analytical performance especially considering sensitivity and measurement precision. Today, both parameters enable the measurement of effects of processes on the elemental and isotopic composition, which have been hidden by the measurement performance for a long time. For example, natural isotopic variations of a number of elements that have – for a long time – been considered as constant, are increasingly exploited. This has a major impact on science such as, e.g. transport phenomena in ecosystems or metabolic processes that can be described by isotopic fractionation. Furthermore, this has a consequence on the atomic weight, which is (for some elements) no longer reported as single value but as interval.
This development makes it evident that new scientific questions can be formulated requiring to collaborate across disciplinary boundaries. Therefore, analytical chemistry has been a genuine trigger of transdisciplinary science ever since: New research questions trigger the instrumental development and new developments in the analytical techniques provide access to a large variety of fields of research.
1.1.4 A Wide Field for Modern Mass Spectrometers with Unforeseen Development
As most of the magnetic sector field mass spectrometers were applied in fundamental research in the early stages, they soon gained their importance in elemental and isotopic research as well as in organic chemistry and related fields. In elemental and isotopic research, the first applications were aimed towards environmental sciences, geology and radionuclide research, where magnetic sector field mass spectrometers had an important impact from the early beginning (see Chapter 2). Nowadays, the fields of applications are manifold: geology and environmental sciences are still the most prominent fields, followed by nuclear research and technological applications in, e.g. the metal and semiconductor industries. More and more, the devices have made an important impact on the life sciences, namely food science and technology, biochemical and biomedical research or medical research. In addition, forensic science and archaeometry have become research fields, where, e.g. the measurements of isotope ratios for determining the provenance of forensic evidence or cultural goods have become indispensable tools.
1.2 A New Book on Sector Field Mass Spectrometry for Elemental And Isotopic Analysis!
As a consequence of all these considerations, this book was inspired by the idea to provide both a basic and deep insight into currently applied mass spectrometric techniques for elemental and isotopic mass spectrometry using a magnetic mass separator as common principle linking analytical chemistry and the different fields of science. This requires giving a full picture of the technique along with the provision of the spectrum of modern instruments and their potential and actual applications. Some information within the book might be redundant to existing literature but were found to be necessary to guide the reader from theoretical basics via an overview of up-to-date instruments to a summary of modern applications within one work. Even though sector field devices have been used in other devices applied for elemental and isotopic analysis (such as AMS, SNMS, SSMS, RIMS or LIMS), this book focuses on the major techniques generally applying sector field devices namely ICP-MS, GDMS, TIMS, SIMS and IRMS (including dynamic and static IRMS).
(Note: for the abbreviations see the List of Abbreviations. Instruments, which are further described in Chapters 12–17, are written in italic without the details on the manufacturer as all these instruments will be explained in detail. All other equipment, which is not listed in these sections, is given with details on the manufacturer.)
The book should encourage reading, encourage the blitheness to gain knowledge and encourage the urge of understanding – especially between the field of mass spectrometry and other scientific disciplines – to force the transdisciplinary application of mass spectrometry. The book shall be comprehensive without being overburdened with details with regard to the technical and analytical background needed to understand the potential and limitations of each method. Many aspects of mass spectrometers are covered in other works, as well, which the interested reader shall be referred to for further reading (Table 1.1).CHAPTER 2
University of Natural Resources and Life Sciences Vienna (BOKU), Department of Chemistry, Division of Analytical Chemistry, VIRIS Laboratory for Analytical Ecogeochemistry, Tulln, Austria
2.1 Where it All Started
"In reality, there is only atoms and emptiness" (Figure 2.1) (Democrit)
The general principle of the idea of separating our world into the smallest increments possible, atoms, started with Democrit (460–370 BC), the Greek philosopher, a pupil of Leucippus, who asked the origin of all questions: "What is that in truth being?" His answer was simple, but with con- sequences. Democrit can be named as a major influence on the formulation of an atomic theory of the universe. He is often cited as the "father of modern science". According to the theory of Democrit, matter is composed of impartible (greek = atomos) basic modules, which implement already the properties of the matter, which they compose. According to Democrit, these atoms act comparable to more or less hard balls. Atoms and atomic theories went along with the discovery of the elements composing the matter. The definition of elements as major components of our environment had major consequences in science. John Dalton (1766–1844), e.g. introduced the fundamental law of multiple proportions as one of the basic laws of stoichiometry. The law is based on his atomic hypothesis following the work of Jeremias Benjamin Richter (1762–1807), a German chemist.
Since then, the efforts of scientists have had the goal to visualise these atoms inventing tools to monitor the material world around us into its smallest possible increments. As a result, scientists involved in spectroscopy have invented and established tools to identify the elemental composition of our material world and to measure the weight of atoms by a very special balance: the mass spectrometer.
2.2 Cathode Rays and Kanalstrahlen
(Hittdorf, Goldstein, Wien) (Figure 2.2)
The roots of sector field instruments date back to the late 19th century. In 1886, the German physicist Eugen Goldstein (1850–1930), a scholar of Hermann von Helmholtz (1821–1894), coined the term "cathode rays" for the negatively charged electrons, which are extended from a negative electrode (in a glow discharge). The existence of these rays had been discovered by Julius Plücker (1801–1868) and his scholar Johann Wilhelm Hittdorf (1824–1914), who found that these rays could be deflected by a magnetic field. Franz Arthur Friedrich Schuster (1851–1934) determined the mass to charge ratio of the particles of cathode rays by measuring the degree of deflection in a magnetic field in 1890. Goldstein also discovered "Kanalstrahlen" (canal rays), positively charged particles formed when electrons are removed from gas particles in a glass tube filled with gas at reduced pressure and equipped with a perforated cathode. In 1898, Wilhelm Wien (1864 – 1928) demonstrated that these canal rays had a positive charge and could be deflected by a strong superimposed electric and magnetic sector field. The resulting velocity filter for charged particles (Wien filter) consisted of perpendicular electric and magnetic fields and could be applied as an energy analyser or mass separator of charged particles (Figure 2.3).
2.3 The First Mass Spectrometer and the Determination of Isotopes
(Thomson, Soddy, Aston) (Figure 2.4)
J. J. Thomson (1846–1940), a British physicist, showed that cathode rays consisted of an unknown negatively charged particle – the electron – and determined the m/z ratio of this particle. He received the Nobel prize in 1906 "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases".
"At first there were very few who believed in the existence of these bodies smaller than atoms. I was even told long afterwards by a distinguished physicist who had been present at my  lecture at the Royal Institution that he thought I had been "pulling their legs"."
Thomson had started his studies of the "Kanalstrahlen" in 1905 and he improved the Wien filter by reducing the pressure in his apparatus. Thomson was soon assisted by his student Francis William Aston (1877–1945), who started to work at the Cavendish Laboratories in 1909. In 1910, Frederick Soddy (1877–1956) discovered that the element lead differed in mass depending whether it had been formed via the decay of thorium or the decay of uranium. This was originally considered as a peculiarity of radioactive materials, which he called "isotopes", from the Greek, words "iso" (same) and "topos" (place) ([TEXT NOT REPRODUCIBLE IN ASCII.] = at the same place) – as they can be found at the same position within the periodic table of the elements. Even though the term nuclide is the more general expression for an atomic species characterised by the constitution of the nucleus (i.e. number of protons and neutrons), the term isotope is still commonly used.
In 1912, Thomson and Aston guided a collimated beam of ions - generated in an electrical discharge in a gas at low pressure (Figure 2.5) – through a magnetic and an electric field based on the positive ray apparatus first employed by Walter Kaufman (1871–1947) in 1901 (Figure 2.6).
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Table of Contents
Introduction; History; Fundamentals; Analytical Instrumentation; Comparative Summary; Future development/Outlook; Subject Index