General strategies for speciation

Interest in chemical speciation procedures is predicted to expand rapidly as a wider spectrum of the scientific community recognises that assessments of health hazards, toxicity and bioavailability must be based on levels of specific chemical forms, rather than on total element levels. Literature listings on this topic, however, are not extensive and most reviews, conference proceedings and books have appeared from the 1980s onwards (a typical few are listed in Further reading1 at the end of the chapter). 

The aim in this contribution is to introduce the reader to some of the alternative experimental strategies that have been used to study two particular categories of inorganic chemical speciation, namely  

The determination of species concentrations tends to be more challenging than determination of total element contents because 

If sediment was collected from a particular waterway, the distribution of the element of interest between different components of the sediment was found to vary with the degree of exposure to air and the temperature of any drying stages (Rapin et al., 1986 Kersten and Foerstner, 1986). The minor elements present in sediments (and soils) are not uniformly distributed. Part can be present as mineral fragments derived from the original parent rock, while other parts can be associated with distinct component phases such as carbonate compounds, hydrous oxides of Fe, Al, Mn and organic matter. Some fractions are loosely sorbed on particle surfaces or are held on ion exchange sites. 

The distribution of an element in biological samples collected from an ecosystem can be equally complex. In plants, for example, the distribution of minor elements or particular species is not uniform throughout the whole organism, i.e. leaf material values differ from those derived from ribs, stems, roots and fruit. For speciation, sub-sectioning of the semi-rigid matrix may thus have to precede isolation of the component of interest. 

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Some changes in chapter authorship have occurred since the first edition for a variety of reasons. Chapter 2, General Strategies for Speciation has been left... 

The determination of chemical species in an environmental or biological sample can be difficult and time consuming. The accuracy and precision generally achieved for this type of work is not as good as that for total elemental analysis, even for laboratories with a proven track record in speciation analysis. Only a limited number of high purity standards are available, and so calibration of the method is an important area for improvement in methodology. Standard additions and external calibration strategies have been applied and recently, the use of IDMS has been described. 

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