Modern Analytical Geochemistry

An Introduction to Quantitative Chemical Analysis Techniques for Earth, Environmental and Materials Scientists

Robin Gill

• Publisher: Routledge
• Year: 2014
• ISBN:
  9781317894704
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

A comprehensive handbook of analytical techniques in geochemistry which provides the student and the professional with an understanding of the wide spectrum of different analytical methods that can be applied to Earth and environmental materials, together with a critical appreciation of their relative merits and limitations.

Chapter 1

What a geochemical analysis means

Robin Gill and Michael H. Ramsey

Why analyse?

The term ‘analysis’ as used in this book includes any measurement that provides information about the chemical composition of a sample. Depending on the application, this information could take various forms. For example:

• the concentrations of certain elements or compounds in the sample;

• the relative amounts (abundance ratio) of two or more isotopes of a particular element;

• what proportion of an element occurs in a specific chemical form (e.g. how much of the sulphur present is in the form of sulphate and how much is sulphide).

Geochemical analyses are those carried out on natural earth or environmental materials such as air, volcanic gas, water, dust, soil or rock, or on processed materials that are relevant to the quality of our environment, such as sewage sludge or industrial effluent.

What purposes do geochemical analyses serve? The following examples illustrate some of the many geological, environmental or industrial applications for accurate chemical analyses:

• Identifying or characterising a completely unknown natural material.

• Verifying the quality of a processed product, or testing the contamination of a natural material, against statutory or recommended limits.

• Measuring a particular detail of a sample’s composition to determine aspects of its history (e.g. isotopic dating of rocks, geothermobarometry (Chapter 14), forensic applications).

• Investigating a geochemical process (natural or anthropogenic) by following the movement of tracer elements (e.g. Ni) or isotopes (e.g. ¹⁸O/¹⁶O), or their distribution in a representative range of samples.

• Determining how composition varies with time at a single place (e.g. to determine the residence time of radioactive Cs in Cumbrian soil following the Chernobyl incident).

• Mapping the spatial distribution of an element or compound at the present time, perhaps to locate its source (e.g. source of river pollution, or the location of an exposed ore deposit from stream sediment analyses).

• Monitoring the efficiency of an experimental or industrial process as physical or chemical parameters are varied to determine optimum conditions.

Thus in most cases the motivation for carrying out analyses is not simply to acquire data, but to solve a problem, to locate a source, to test a theory, or to see if a sample satisfies some pre-determined quality standard. Most analyses are carried out to determine the concentration of either a particular element of interest (referred to as the analyte), or a suite of elements (analytes) sharing a particular concentration range or behaviour. Geochemical analysis involves determining not only concentrations themselves but also the uncertainties (‘errors’) that are associated with them – as indeed with any scientific measurement – because these restrict the conclusions that can objectively be drawn from the data.

The professional analyst bears the primary responsibility for selecting the least costly technique whose results are sufnciendy precise and accurate for the intended application (see Chapter 13). For this reason, he or she needs to be acquainted with a range of alternative techniques that could be utilised according to the circumstances and the demands of the application. But clearly the non-specialist user of geochemical analyses, and the beginner with a specific problem to solve, also need an up-to-date overview of the relative merits of alternative geochemical methods in order to be able to plan and budget their research effectively, and it is for them, as well as the student of geochemistry or environmental science, that this introductory book has been written.

Types of analysis – terminology

Qualitative versus quantitative analysis

A qualitative analysis simply reports a list of elements or compounds that are present at detectable levels in a sample, information which has relatively little use except for identification. This book is concerned primarily with quantitative analysis, which measures the concentrations of the analyte(s) in the sample. The analytes may be elements, compounds, isotopes or chemical species (e.g. Fe²⁺ as distinct from Fe³⁺ – see Chapter 5).

Major, minor and trace constituents

In dealing with rocks, sediments and minerals, it is useful to distinguish between major elements (those present at concentrations exceeding 1% by mass, making up the main minerals of the rock), minor elements (concentrations between 0.1 and 1.0%) and trace elements (concentrations less than 0.1%). In drawing these distinctions, one should recognise that the same element may be a major element in one type of sample (e.g. sulphur in an ore concentrate) but a trace element in another (sulphur in a fresh basalt).

Bulk analysis versus spatially resolved analysis

Most analyses are designed to measure the overall composition of a homogeneous sample such as a rock powder; they are referred to as bulk or (in a geological context) ‘whole-rock’ analyses. Techniques for carrying out such analyses are described in Chapters 4–12. Certain applications may, on the other hand, require analysis of material that is available only in minute amounts, or which is only one of several materials present in a mixed sample (e.g. analysing individual crystals of a specific mineral in situ in a rock section), or the task may be to map the distribution of one constituent in a heterogeneous material. In such cases one must use a technique with spatially resolved analysis capability, such as electron probe microanalysis (Chapter 14) or a laser-based method (see Chapters 9, 10, 11).

Chemical form – ‘speciation’

The chemical form (e.g. the oxidation state) that an element adopts in a sample, or the relative amounts of the element that exist in alternative forms, may sometimes be of more interest than the element’s gross concentration. Sulphur, for example, may exist in a sedimentary rock as sulphide or as sulphate (or even as elemental sulphur), and separate analyses would be required to determine how much sulphur is present in each form. The same is true for other elements that exhibit multiple oxidation states in nature, such as iron, carbon and nitrogen. Another form of speciation is establishing the proportions of carbon present in water in the forms H₂CO₃, HCC₃⁻ and CO₃²⁻. These questions are discussed in Chapters 5, 9 and 16.

Isotopic composition of an element

The goal of geological isotope analysis is usually to determine accurately the atomic abundance ratio of two or more isotopes of the same element (e.g. ¹⁴³Nd/¹⁴⁴Nd, ¹⁸O/¹⁶O), either in a whole-rock sample or in individual minerals. Such ratios, which can be measured with greater precision than individual isotope concentrations, carry information about the age or derivation of a geological sample. ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Contributors
• Acknowledgements
• Chapter 1 What a geochemical analysis means
• Chapter 2 Sampling and sample preparation
• Chapter 3 Dissolution procedures for geological and environmental sampless
• Chapter 4 Inductively coupled plasma-atomic emission spectrometry (ICP-AES)
• Chapter 5 Atomic absorption spectrometry and other solution methods
• Chapter 6 X-ray fluorescence spectrometry
• Chapter 7 Neutron activation analysis
• Chapter 8 Thermal ionisation mass spectrometry (TIMS)
• Chapter 9 Gas source mass spectrometry: isotopic composition of lighter elements
• Chapter 10 Inductively coupled plasma-mass spectrometry
• Chapter 11 Elemental analysis by spark source mass spectrometry
• Chapter 12 Accelerator mass spectrometry
• Chapter 13 Which method should I use?
• Chapter 14 Electron beam methods
• Chapter 15 Principles of SIMS and modern ion microprobes
• Chapter 16 Analytical techniques in organic chemistry
• Appendix A: Vacuum technology
• Appendix B: Glossary
• Appendix C: Symbols and constants used in this book
• Appendix D: SI units
• Bibliography
• Index