Direct methods of metal speciation

The speciation or determination of the chemical nature of an element involves two major processes separation of mixtures and identification of their components. In some cases, adequate identification of the nature of chemical species may be obtained from separation data alone, whereas complete quantitative determination of molecular species requires an initial separation followed by purification of individual compounds. 

Separation techniques are not considered in this chapter and attention will be devoted almost entirely to chemical identification techniques. The methods which are discussed here are summarised in Table 3.1, which also lists some of the main separation techniques that are available. Emphasis will be placed on the various types of spectroscopy, since this chapter is concerned with non-destructive methods for the determination of speciation. 

Analytes may be small neutral molecules, simple anions or cations, complex ions or molecules, non-stoichiometric compounds, polymers, both soluble and insoluble, or clathrates. The chemical forms of the alkali and alkaline earth metals are either simple cations or hydrated cations and do not exhibit great variability so, of the metals of biological interest, this chapter will restrict itself to the 

Gel permeation chromatography Ion chromatography Ion-exchange chromatography Supercritical fluid chromatography Planar chromatography 

Vibrational spectroscopy Infrared spectroscopy Raman spectroscopy 

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The ideal speciation procedure is one which allows positive identification and quantitative evaluation of one particular species. Some of the better known approaches are summarised in Table 2.1. Techniques (usually spectroscopic) which have been used to identify and determine directly a particular chemical species in biological samples, are summarised in Table 2.2 and the topic of Direct Methods of Metal Speciation is dealt with in Chapter 3. 

A. M. Mota, L. Simaes Gonsalves, Direct method of speciation of heavy metals in natural waters, in S. Caroli (ed.), Element Speciation in Bioinorganic Chemistry, Wiley, New York, 1996, pp. 21-96. 

Ruzic [278 ] considered the theoretical aspects of the direct titration of copper in seawaters and the information this technique provides regarding copper speciation. The method is based on a graph of the ratio between the free and bound metal concentration versus the free metal concentration. The application of this method, which is based on a 1 1 complex formation model, is discussed with respect to trace metal speciation in natural waters. Procedures for interpretation of experimental results are proposed for those cases in which two types of complexes with different conditional stability constants are formed, or om which the metal is adsorbed on colloidal particles. The advantages of the method in comparison with earlier methods are presented theoretically and illustrated with some experiments on copper (II) in seawater. The limitations of the method are also discussed. 

A combination of direct observation and extraction may be carried out. The whole soil may be analyzed by various methods and then specific components sequentially extracted and measured. This approach has been used in the investigation of the speciation of metals in soil under various conditions. Using X-ray spectroscopy, metals and their various ionic forms in soil can be directly identified [1],... 

There are few methods which can measure well-defined metal fractions with sufficient sensitivity for direct use with environmental samples (approach B in Fig. 8.2). Nevertheless, this approach is necessary in the experimental determination of the distribution of compounds that are labile with respect to the time scales of the analytical method. Recent literature indicates that high-performance liquid (HPLC) and gas chromatographic (GC) based techniques may have such capabilities (Batley and Low, 1989 Chau and Wong, 1989 van Loon and Barefoot, 1992 Kitazume et al, 1993 Rottmann and Heumann, 1994 Baxter and Freeh, 1995 Szpunar-Lobinska et al, 1995 Ellis and Roberts, 1997 Vogl and Heumann, 1998). The ability to vary both the stationary and mobile phases, in conjunction with suitable detector selection (e.g. ICP-MS), provides considerable discriminatory power. HPLC is the superior method GC has the disadvantage that species normally need to be derivatised to volatile forms prior to analysis. Capillary electrophoresis also shows promise as a metal speciation tool its main advantage is the absence of potential equilibria perturbation, interactions... 

Fluorescence spectroscopy has, however, several advantages over most other methods for studying metal-HS complexation in aqueous media. The method is relatively rapid since no separation is required between bound and free metal ion thus, errors associated with the separation step in most speciation methods are avoided. Unlike most other methods, it allows direct measurement of the CC of the ligand through a determination of the concentration of free ligands, thus... 

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