Principles and Practice of Analytical Techniques in Geosciences

Kliti Grice, Kliti Grice

• Publisher: Royal Society of Chemistry
• Year: 2014
• ISBN:
  9781782623052
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

The pace of revolution in analytical chemistry in the field of Geosciences has been dramatic over recent decades and includes fundamental developments that have become common place in many related and unrelated disciplines. The analytical tools (nano to macro-scale from stable to radioactive isotopes, compound specific sulfur isotopes) used have been applied to wide-ranging applications from inorganic to organic geochemistry, biodiversity and chronological tools, to build an understanding of how the Earth system evolved to its present state.

This book will provide an essential guide to exploring the earth's natural resources and changing climate by detection science. Individual chapters bring together expertise from across the globe to present a comprehensive outlook on the analytical technologies available to the geoscientist today. Experienced researchers will appreciate the broad treatment of the subject as a valuable reference, while students and those new to the field will quickly gain an appreciation of both the techniques at hand, and the importance of constructing, and analysing, the complex data sets they can generate.

CHAPTER 1

Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) as an Analytical Tool in the Geosciences

MATT R. KILBURN^*a AND DAVID WACEY^a,b

^aARC Centre of Excellence for Core to Crust Fluid Systems, and Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia; ^b Department of Earth Science and Centre of Excellence in Geobiology, University of Bergen, Allegaten 41, N-5007, Bergen, Norway

^*Email: [email protected]

1.1 Introduction

1.1.1 Overview

The in situ chemical characterisation of rocks, minerals, and soils is fundamental to our understanding of the geological and environmental processes that have shaped our planet. Furthermore, we are becoming increasingly aware that evidence of large-scale phenomena, such as crustal evolution, mantle metasomatism, changes in atmospheric composition, or the emergence of life, is often only apparent at the micro- to nanoscale. Our ability to piece together clues about the Earth’s evolution is therefore limited by the sensitivity and resolution of our analytical techniques. Over the past four decades, the development of microbeam technologies using electrons, ions, lasers, and X-rays has pushed to reduce the volume of material analysed while increasing sensitivity almost to the limits of counting statistics.

Of all the microbeam techniques available to geoscientists, secondary ion mass spectrometry (SIMS) is perhaps the most versatile. Combining in situ microbeam measurements with the high sensitivity and specificity of mass spectrometry, the technique has the ability to detect most of the elements in the periodic table with a high dynamic range (allowing both major and trace element analysis simultaneously), in a wide range of materials. SIMS is a well-established technique in the semiconductor industry, where it has been routinely used to measure the concentration of dopants and implants, and in forensic device failure analysis. The relative similarity between semiconductor materials and minerals has meant that the geological community has also employed SIMS to quantify trace elements in minerals at concentrations too low for detection by more typical methods such as electron microprobe analysis (EPMA). Furthermore, as a mass spectrometry technique, SIMS has proven itself to be highly effective in the analysis of stable and radiogenic isotopes in minerals, becoming the gold standard for uranium–lead geochronology. The lateral resolution, however, has typically been limited to tens of microns, so the demand for ever smaller length scales has necessitated the development of instruments that can produce an ion beam with orders-of-magnitude reductions in spot size while retaining ultra-high levels of sensitivity. In this chapter, we consider one such technological development, namely nanoscale secondary ion mass spectrometry (NanoSIMS), and discuss its application within the geosciences.

1.1.2 Secondary Ion Mass Spectrometry

SIMS is a surface analysis technique that uses a highly energetic ion beam to ‘sputter’ material from a sample surface, which is then analysed in a mass spectrometer. The primary ion beam impacts the sample surface with energies ranging from a few hundred electron volts to more than 10 keV, causing a cascade-collision within the top few atomic layers which releases material from the surface. The sputtered material consists of atoms, atom clusters, molecular fragments, and backscattered primary ions, and the small proportion of this material that is ionised (secondary ions) is extracted into a mass spectrometer using electrostatic fields. The amount of ionised material sputtered is dependent on a number of factors, including the chemical structure of the substrate, the ionisation efficiency of the elements within the substrate, and the nature and energy of the primary ion used.

SIMS works in both ‘static’ and ‘dynamic’ regimes depending on the current density of the primary ion beam. In static SIMS, the primary ion dose is typically less than 10¹² ions cm⁻², which is low enough that each impinging primary ion interacts with an essentially pristine area of the sample surface. This results in the sputtering of low-energy secondary ions from the topmost monolayers only. The surface is not actively eroded by the primary ion beam, hence the term ‘static’. The secondary ions have velocities dependent on their mass, and by measuring the time of flight to reach the detector it is possible to differentiate between different elements, molecules and molecular fragments.¹ The pulsed primary ion beam, typically consisting of cluster projectiles such as C₆₀ or Bi₃₀₀, can be rastered over the sample surface, producing images with lateral resolution up to 100 nm, which record the entire mass spectrum (up to tens of thousands of amu) on each pixel. The technique, however, must trade off lateral resolution for mass resolution and sensitivity, and even under optimum conditions cannot achieve the mass separation necessary to provide high-precision isotopic analyses.

Dynamic SIMS uses a high-energy beam that erodes the sample surface, producing a higher secondary ion flux, but destroying the molecular bonds between the atoms in the sample. This results in the detection of elemental ions only (although some non-stoichiometric ion clusters are formed in the sputtering process, as discussed below). Dynamic SIMS typically employs magnetic sector mass analysers, in which secondary ions are deflected in a magnetic field depending on their mass to charge ratio, m/z. Double-focusing mass spectrometers also use an electrostatic sector to filter the secondary ions by kinetic energy either before or after the magnetic sector. So-called sector-field mass analysers allow high mass resolution to be achieved with high transmission, providing optimal conditions for high-precision elemental and isotopic analyses. The development of dynamic SIMS over recent years has pushed towards higher sensitivity, for the measurement of high-precision isotope ratios, and high lateral resolution, for imaging.

1.1.3 Historical Use of SIMS in Geoscience

The use of SIMS in geoscience really began in earnest with the development of the sensitive high-resolution ion microprobe (SHRIMP) in the 1970s. With larger sector fields than had previously been employed in SIMS, its larger radius allowed high mass resolution to be routinely achieved. The ability to make spatially resolved measurements of uranium–lead isotopes within individual zircon grains revolutionised the field of geochronology, and the SHRIMP II is still the gold standard for uranium–lead dating in zircons and other minerals, today.² At the same time, smaller-radius instruments (namely, the Cameca IMS f-series) were beginning to gain popularity for the measurement of trace elements and stable isotopes. This was especially true among the cosmochemistry community, as SIMS had the ability to locate and measure tiny grains of isotopically unusual material (i.e. with pre-solar composition) against a background of ‘normal’ material.³ This approach is also used by the nuclear safeguards community for identifying enriched uranium particles in environmental dust samples.⁴ Cameca also developed a large-geometry ion probe in the 1990s, the IMS1270 (and later the IMS1280), based on the ion microscope concept of the f-series, but tailored towards the high-precision measurement of stable isotopes within minerals. The SHRIMP II and IMS1280 still dominate geoscience applications, with increasing degrees of overlap in their respective capabilities. The lateral resolution of these instruments, however, is still limited to microns or tens of microns. With the increasing need to reduce spot size, a new platform was sought that would allow the submicron elemental and isotopic characterisation of materials. An excellent history of the application of SIMS in geosciences is given in Stern.⁵

1.1.4 Development of NanoSIMS

The NanoSIMS 50, conceived by Georges Slodzian⁶ and developed by CAMECA⁷ (Gennevilleirs, France), was designed for imaging with submicron lateral resolution. This is achieved by positioning the primary probe-forming lens parallel and very close to the sample, allowing the beam to be focused to a very small diameter. The primary ion beam impacts the sample surface at 90°, with the secondary ions extracted back through the same lens assembly. As the coaxial lens uses common focusing optics for both the primary and secondary ion beams, the polarity of the secondary ions must be opposite to that of the primary ions. Two primary ion sources are available for the NanoSIMS: a Cs⁺ source for the generation of negative secondary ions, which can be focused to a sub-50 nm probe diameter; and a duoplasmatron source which can produce O⁻ and O₂⁺ primary ions, with a smallest beam diameter of about 150 nm. Using the Cs⁺ primary beam also allows secondary electrons, generated during the sputtering process, to be extracted to a photomultiplier, providing a simultaneous secondary electron image of the sample.

High mass resolution is achieved through the use of a double-focusing sector-field spectrometer, consisting of an electrostatic filter (to filter secondary ions according to their kinetic energy) and a magnetic sector (to separate ions by their mass). The geometry of the mass spectrometer has been optimised to give high transmission and high mass resolution at high lateral resolution, such that a M/ΔM mass resolution in excess of 9000 with a 100 nm beam diameter is achievable with only approximately 60% loss in transmission.

The secondary ions are focused along the plane of the magnetic sector where up to seven detectors can be positioned, allowing a large number of mass combinations to be measured. This multi-collection capability allows the same microvolume to be sampled on up to seven detectors simultaneously, minimising the loss of material and increasing throughput. Magnetic peak switching is also possible using a single detector. The latest generation of NanoSIMS instruments come equipped with both electron multipliers (EM) and Faraday cup (FC) detectors, and electronics that are housed under vacuum and regulated thermally. Imaging is achieved by scanning the focused primary ion beam across the sample, recording the number of incoming ions at the EM for each pixel of the scan (raster).

1.2 Technical

1.2.1 Instrument Specifications

1.2.1.1 Primary Ion Beam

The NanoSIMS is equipped with a Cs⁺ microbeam source and an O⁻ duoplasmatron source. Due to the coaxial lens configuration, the polarity of the primary ion beam must be opposite to that of the secondary ions. Thus, the choice of a Cs⁺ or O⁻ primary beam is determined by the polarity of the secondary ions to be analysed. The Cs⁺ primary beam is generated by the thermal decomposition of CsCO₃, and the extraction of the resulting ionised Cs. The beam can be focused to a very fine probe diameter, depending on the beam current, which can be controlled through the use of an aperture. At each successively smaller aperture the beam current is decreased, resulting in a decrease in the secondary ion signal but producing a sharper image due to the smaller probe diameter.

As mentioned above, the impinging primary beam causes a cascade-collision in the top few atomic layers...

Table of contents

• Cover image
• Title page
• Copyright
• Preface
• Abbreviations
• Contents
• Chapter 1 Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) as an Analytical Tool in the Geosciences
• Chapter 2 Clumped Isotope Geochemistry
• Chapter 3 Application of Radiogenic Isotopes in Geosciences: Overview and Perspectives
• Chapter 4 Advances in Fluorescence Spectroscopy for Petroleum Geosciences
• Chapter 5 Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS): Principles and Practice in the Biogeosciences
• Chapter 6 Development and Use of Catalytic Hydropyrolysis (HyPy) as an Analytical Tool for Organic Geochemical Applications
• Chapter 7 Microscale Sealed Vessel Pyrolysis
• Chapter 8 High-Precision MC-ICP-MS Measurements of δ11B: Matrix Effects in Direct Injection and Spray Chamber Sample Introduction Systems
• Chapter 9 Radioactive Carbon in Environmental Science
• Chapter 10 Development and Initial Biogeochemical Applications of Compound-Specific Sulfur Isotope Analysis
• Chapter 11 Applications of Liquid Chromatography–Isotope Ratio Mass Spectrometry in Geochemistry and Archaeological Science
• Chapter 12 Advances in Comprehensive Two-Dimensional Gas Chromatography (GC×GC)
• Subject Index