The rapid development of inorganic instrumental analysis
after World War II, mainly driven by the development of atomic spectrometry
, enabled the analytical community to look into the role of trace elements
in different areas, such as health and environment, geochemistry and material sciences, to name only some.
The concept of “trace element analysis” grew fast during the three decades following WW II, with a major impact on social and political developments such as the ecological movement and the concept of environmental protection. During this period, it became evident that trace elements play a major role whenever biological activities, environmental chemistry, or material characteristics are discussed. With respect to environmental issues it was also increasingly realised that the distribution, mobility and biological availability of chemical elements depends not simply on their concentration as discussed by the trace element analysis but, critically, on the chemical and physical association which they undergo in natural systems. Thus, for example, arsenic is extremely toxic in its inorganic forms but relatively innocuous as arsenobetaine, a common arsenic species in fish. Organo-tin compounds, of which perhaps the best known is the antifouling agent tributyltin (TBT), are generally far more toxic than inorganic tin species. Even more extreme, hexavalent chromium [Cr(VI)] is a carcinogenic agent, while trivalent chromium [Cr(III)] is an essential element for humans.
While the concept of “element speciation” in the sense of distinguishing fractions of the total element concentration according to some peculiar properties such as being “bioavailable” grew slowly in the late 1950s, it was only during the late 80’s, that instrumental elemental analysis reached the detection power necessary to measure small fractions of trace elements in environmental and biological samples.
Today, speciation science seeks to characterise at least some of the most important forms of an element, in order to understand the transformation between forms which can occur, and to infer from such information the likely consequences for example in terms of risk assessment, toxicity or biological activity. As such, it is a discipline which is of relevance to scientists with many different backgrounds: Chemists, toxicologists, biologists, soil and sediment scientists, physicists and specialists in various aspects of nutrition and medicine; all require this type of information. In fact this area of analytical chemistry has become one of the most crucial, pertinent and challenging issues because of its impact on environmental chemistry, eco- and clinical toxicology, medical and nutrition science, food and energy industries.
Often, chemical species present in a given sample are not stable enough to be determined as such. The practice in this case has been to identify various classes of species of an element and to determine the sum of its concentration in each class. Such fractionations can be based on many different properties of the chemical species, such as size, solubility, affinity, charge, and hydrophobicity. Fractionation is often operationally defined based on the procedures used, such as physical separation (sieving, filtration) or chemical separation (dissolution in a special solvent). The additional information provided by these operationally defined fractions make this practice useful, which therefore will continue as being fit-for-purpose in some cases.
Further chapters: About Speciation