Speciation of some Elements

In some instances it is only necessary to determine the metal by any appropriate analytical method. However, speciation by IC is often needed when the particular form of a metal needs to be known. The basic chemistry of several important metal elements is reviewed briefly in the following sections. 

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Table 2 The residence time and speciation of some elements in the ocean...

Although the speciation of some minor elements has been determined directly by experimental means (e.g., ion selective electrodes, polarography, electron spin resonance) most of our thinking about speciation is based on equilibrium calculations. Garrels and Thompson... 

The combustion process activates mineral ash with the result that leachates extract relatively high proportions of elements whose concentrations in potable water are limited. We are as yet some way from understanding the speciations of these elements in combustion waste as well as the geochemical evolution of waste in its disposal environment. Preliminary studies show that the design, construction, and operation of disposal sites have a major influence on releases. The underlying geochemical processes are at present only known in outline and provide a fascinating field for interdisciplinary studies. 

Another important consideration is the effect of the reactions that continue, albeit at different rates, and in some cases along new pathways, after sampling. These reactions could introduce new products, modify existing species and caixse the loss of volatile components. Consequently, the speciation profile of some elements may be changed. In order to minimize this effect, it is essential to process the samples as soon as possible after collection. 

The addition of anticoagulents and/or preservatives, a practice that may be tolerated for total element determination, should be avoided for two reasons. First, the compounds used are usually complexing agents. They therefore could bind various trace elements. Second, they could destroy some species. For example, the addition of potassium dichromate to urine in the presence of nitric acid could destroy methylmercury If the use of such compounds is unavoidable, then there should be experimental evidence to show that the speciation of the element imder study has not been adversely affected. 

EXuring storage there are changes in the sample that may have some consequence for the speciation of an element. First, the complex three-dimensional structure of most biological molecules may be destroyed. This may lead to the loss of enzymatic activity, and in some cases the associated element may be lost. Second, the natural proteolysis and/or autolysis reactions could result in the breakdown of the molecules, and as a con uence the results of the speciation experiments may indicate that the element is associated with only a fragment of the original molecule. Indeed,... 

Although lEC has been used to separate large molecules most application have been in the fractionation of low molecular mass species. Some of the applications to the study of the speciation of trace elements in biological materials and the necessary pretreatment steps are summarised in Table 2. 

While some elements, such as Fe, can be singled out for their special biogeo-chemical significance, the speciation of other elements can be highlighted due to an extreme paucity of quantitative assessments with respect to both speciation and distribution. Elements such as Nb, Ta, Ru, Os, Rh, Ir, Pt and Au are especially deserving of further distributional characterisations and speciation assessments. 

Methods involve extractions of analytes into organic solvents, as well as treatments with acidic or basic reagents. Solid-phase extraction can be used for removal and pre-concentrations of analytes in aqueous solutions. Applications of low-power focused microwave technology have been investigated as a means of dissolution, and good results have been reported for extractions of organometal-lic compounds of tin and mercury (Schmitt et al., 1996 Szpunar et al., 1996). Analyses of CRMs were used for verification. The time necessary for quantitative isolations of the analytes was greatly reduced, e.g. 24 h to 5 min. In addition, there were reductions in solvent volumes, and improvement in analyte recoveries. Some of the analytical procedures for speciation of particular elements such as mercury, described later in this chapter, include microwave-assisted sample preparation. 

The Elemental Content of Human Diets and Excreta The Elemental Constituents of Soils Mycotoxins Occurrence, Distribution, and Chemical Speciation of some Minor Dissolved Constituents in Ocean Waters. 

Trace metals in sea water are present at very low concentrations (normally lower than 10 A/) thus, the study of metal distribution and speciation poses problems that have been overcome only in the last few decades. A prerequisite for determination of trace metals is the collection and processing of uncontaminated samples for the element of interest. Improvements in analytical procedures have led to a decrease by some orders of magnitude in the reported concentration of some elements, especially so for ubiquitous and contamination-prone elements, such as Fe or Pb. These changes lead to the concept of oceanographic consistency as a criterion for accepting trace chemical measurements (83). 

Because the toxicity of some elements (especially organometallics) can depend on the species in which they are present rather than on their total concentration, the individual species may require chromatographic separation followed by ICP-MS detection. Hence, LC-ICP-MS has evolved as an important analytical technique for speciation analysis. Sometimes a UV detector is included in the system so that the spectra from both detectors can be overlaid for extra analytical information. 

Presently, RPC is used as part of the fractionation protocol for the study of organome-tallic species. In a number of applications, the species are fractionated after an appropriate derivatization step. Some examples of the application of RPC for the study of the speciation of various elements are As (Francesconi et al.. 1985), Cr (Krull et al.. 1983), Pb (Robinson and Boothe. 1984), Sn (Krull and Panaro, 1985). 

In contrast to liquid chromatography which is the technique of choice when considering high molecular mass constituents, gas chromatography (GC) is well-suited for the fractionation of low molecular mass species which are volatile, thermally stable and preferably neutral. Some species that do not already possess the above properties can be converted into forms that are amenable to GC. Various derivatization procedures that can be used to perform such conversions have been described by Poole and Schuette (1984). However, it Is worth mentioning that hydride formation and alkylation are the two most commonly used derivatization methods that have found application in the study of the speciation of various elements like As, Bi, Ge, Hg, Pb, Sb, Se, Sn, Te and Tl. Two points have to be considered when derivatization is performed first, the specificity of the chemical conversion second, the percentage yield of the reaction. A specific reaction is desirable in order to avoid the introduction of artefacts. Moreover, if a quantitative estimate of the amount of the original species is required, then it is essential that the extent of the conversion is known. 

Almost all publications on substoichiometry which have appeared to date have dealt with the determination of the total amount of the element. Recently, some attention has been focused on the determination of the element of interest in different chemical states. This speciation of trace elements of interest is almost impossible by instrumental analytical methods alone. For example, suppose a sample contains the element of interest in different oxidation states, abbreviated as M(III) and M(V). If one state, e.g., M(III) can be determined, the total amount of M can be ascertained after appropriate reduction of M(V) to M(III) in the sample. Then the amount of M(V) can be estimated as the difference in the total amount of M and that of M(III) in sample. 

Nuclear activation methods can provide high sensitivities for many but not all elements. These methods are capable of simultaneous multielement analysis and in some cases they may be essentially nondestructive analytical methods. One of the main disadvantages is the lack of information on chemical form (speciation) of the elements in the analyte. Neutron activation analysis is the most common form of activation analysis, but charged particle and photon activation analysis methods are also applied. 

If the speciation of trace elements is to be studied, measurements must be performed immediately after sampling. If for some elements minor changes in original speciation are acceptable, then the samples should be stored unacidified but frozen (see also the storage of As, Sb and Ge outlined in Chapter 12) to preserve as much as possible the original distribution of species. A few studies indicate, however, that some metal associations with humic substances might be stable even down to pH 2.3 (e.g., Helmers, 1994). 

The information on the chemical speciation of trace elements in biological systems is much needed to evaluate their biological significance. Although a number of analytical techniques based on atomic behavior are available for the analysis of chemical speciation of trace elements, neutron activation analysis, as a nuclear analytical technique, can be successfully used in chemical speciation studies, after appropriate fractionation steps. Table 2.5 lists some typical applications of NAA in chemical speciation analysis of metalloproteins. The main advantages of NAA are of its high sensitivity and the absence of matrix effects inherited from the conventional neutron activation analysis. It can, therefore, be used to analyze the chemical species of trace elements in very small samples or complicated matrices, which is often impossible for non-nuclear techniques. 

The speciation of certain elements in water is important, as it throws light on the transport of the elements between other media, e.g. the soil-plant interface. In this context, atomic spectrometry has the role of providing a sensitive and virtually specific detector for the individual elements, after some... 

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