Über die Autorin bzw. den Autor

Rita Cornelis and Joseph A. Caruso are the authors of Handbook of Elemental Speciation, 2 Volume Set, published by Wiley.

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Handbook of Elemental Speciation, 2 Volume Set

John Wiley & Sons

Chapter One

R. C. Cornelis Laboratory for Analytical Chemistry, Ghent University, Belgium

H. M. Crews Central Science Laboratory, Sand Hutton, York, UK

J. A. Caruso University of Cincinnati, Ohio, USA

K. G. Heumann Institut fur Anorganische and Analytische Chemie, Mainz, Germany

1 Definition of Elemental Speciation and of Fractionation 2

2 Problems to be Solved 2

3 Speciation Strategies 3

4 References 5

'Speciation', a word borrowed from the biological sciences, has become a concept in analytical chemistry, expressing the idea that the specific chemical forms of an element should be considered individually. The underlying reason for this is that the characteristics of just one species of an element may have such a radical impact on living systems (even at extremely low levels) that the total element concentration becomes of little value in determining the impact of the trace element. Dramatic examples are the species of tin and mercury, to name just these two. The inorganic forms of these elements are much less toxic or even do not show toxic properties but the alkylated forms are highly toxic. No wonder analytical chemists had to study elemental speciation and devise analytical techniques that produce qualitative and quantitative information on chemical compounds that affect the quality of life.

Before embarking on the definitions of elemental speciation and species, it may be interesting to give a short historical setting of this emerging branch of analytical chemistry. Analytical chemistry began as a science in the early 19th century. A major milestone was the book by Wilhelm Ostwald 'Die Wissenschaftlichen Grundlagen der analytischen Chemie' (Scientific Fundamentals of Analytical Chemistry) in 1894 [1]. A personality who contributed substantially to the development of analytical chemistry and chemical analysis was Carl Remigus Fresenius. In 1841 he published a very interesting book on qualitative chemical analysis [2]. It was followed over the next 100 years by a series of standard works on qualitative and quantitative analysis by several generations of the Fresenius family and by the publications by Treadwell [3, 4], Feigl [5] and Kolthoff [6, 7], to name just these few. The interest remained largely focused on inorganic analytical chemistry. The term 'trace elements' dates back to the early 20th century, in recognition of the fact that many elements occurred at such low concentrations that their presence could only just be detected. During the following 60 years all efforts were focused on total trace element concentrations. Scientists developed methods with increasing sensitivity. It was only in the early 1960s that questions were raised concerning the chemical form of the trace elements and that the need for an analytical methodology developed subsequently. This development has been growing exponentially to the point that research on trace element analysis today appears almost exclusively focused on trace element species.

Extensive literature is available on the speciation and fractionation of elements. Newcomers have to absorb a wealth of highly specialised publications and they miss the broader overview to guide them. This handbook aims to provide all the necessary background and analytical information for the study of the speciation of elements.

The objective of this handbook is to present a concise, critical, comprehensive and systematic (but not exhaustive), treatment of all aspects of analytical elemental speciation analysis. The general level of the handbook makes it most useful to the newcomers in the field, while it may be profitably read by the analytical chemist already experienced in speciation analysis.

1 DEFINITION OF ELEMENTAL SPECIATION AND OF FRACTIONATION

The International Union for Pure and Applied Chemistry (IUPAC) has defined elemental speciation in chemistry as follows:

(i) Chemical species. Chemical element: specific form of an element defined as to isotopic composition, electronic or oxidation state, and/or complex or molecular structure.

(ii) Speciation analysis. Analytical chemistry: analytical activities of identifying and/or measuring the quantities of one or more individual chemical species in a sample.

(iii) Speciation of an element; speciation. Distribution of an element amongst defined chemical species in a system.

When elemental speciation is not feasible, the term fractionation is in use, being defined as follows:

(iv) Fractionation. Process of classification of an analyte or a group of analytes from a certain sample according to physical (e.g., size, solubility) or chemical (e.g., bonding, reactivity) properties.

As explained in the IUPAC paper [8], it is often not possible to determine the concentrations of the different chemical species that sum up to the total concentration of an element in a given matrix. Often, chemical species present in a given sample are not stable enough to be determined as such. During the procedure, the partitioning of the element among its species may be changed. For example, this can be caused by a change in pH necessitated by the analytical procedure, or by intrinsic properties of measurement methods that affect the equilibrium between species. Also in many cases the large number of individual species (e.g., in metal-humic acid complexes or metal complexes in biological fluids) will make it impossible to determine the exact speciation. The practice is then to identify various classes of the elemental species.

2 PROBLEMS TO BE SOLVED

While the incentive to embark on speciation and fractionation of elements is expanding, it becomes more and more evident that the matter has to be handled with great circumspection. Major questions include: What are the species we want to measure? How should we sample the material and isolate the species without changing its composition? Can we detect very low amounts of the isolated species, which may represent only a minute fraction of the total, already ultra-trace element concentration? How do we calibrate the species, many of these not being available as commercial compounds? How do we validate methods of elemental analysis? All of these questions will be carefully dealt with in the first volume of the handbook. The second and third volumes will extensively address elemental species of specific elements and the analysis of various classes of species.

Advances in instrumentation have been crucial to the development of elemental speciation. There has been a very good trend towards lower and lower detection limits in optical atomic spectrometry and mass spectrometry. This has allowed the barrier between total element and element species to be crossed. While the limit of detection for many polluting species is sufficient for their measurement in a major share of environmental samples, this is not yet the case for human, animal and perhaps plant samples at 'background levels'. The background concentration of elemental species of anthropogenic origin was originally zero. Today they are present, because they have been and continue to be distributed in a manner that affects the life cycle. However, because we cannot measure them...