Speciation work

Kolasa et al. [562] have reported changes in the degree of oxidation of chromium (from Cr6+ to Cr3+) in the course of probe mineralisation of PE for AAS analysis. HPLC-ICP-MS has been used as a selenium-specific detector [563]. Other selenium speciation work by ICP-MS has been reported [564]. Numerous other examples of speciation analysis have been described for the most appropriate techniques (Chapter 7). 

In comparison with other elements, there is only a small amount of information on the environmental chemistry of antimony. The reasons for this are the low environmental concentrations of antimony and the lack of analytical techniques that are sufficiently sensitive for antimony speciation. The toxicity and physiological behaviour of antimony depend upon its oxidation state, with Sbm being more biologically active than Sbv. For speciation work, the objectives of many analytical methods have been the determination of Sbm and total antimony, with Sbv being calculated by difference. Most of the information about chemical speciation of antimony in the environment has been obtained from analyses of natural and polluted waters. 

Another area of growing interest is the chemical speciation of elements to provide more useful information than that afforded by the total concentrations. In this context, alkaline solubilization procedures are especially suitable for speciation work, because this approach allows samples to be directly analyzed without prior preparation (destruction of the matrix), thus preserving the chemical forms of the analytes of interest. 

Since LPAS application to actinide chemistry is in its infancy, only a limited number of works are available in the published literature. Experiments hitherto performed are confined to either hydrolysis, complexation reactions with carbonate, EDTA and humate ligands and a variety of speciation works for Am(III) and to much lesser extent for U(IV), U(VI) Np(IV), Np(V), Np(VI) Pu(IV), Pu(VI). Of considerable interest is the LPAS application to the direct speciation of actinides in natural aquifer systems, where the solubility of actinides is in general very low and multi-component constituent elements as well as compounds are in much higher concentrations than actinide solubilities. The study of the chemical behaviour of actinides in such natural systems requires a selective spectroscopic method of high sensitivity. LPAS is an invaluable method for this purpose but its application to the problem is only just beginning. 

Differences in the volatility of the elements and their compounds may also be used for speciation work. Accordingly, it is possible, for example, to determine different organolead compounds such as Pb(CH3)4 and Pb(C2Hs)4 directly by furnace AAS (see e.g. Refs. [284, 285]) or after on-line coupling with chromatographic separations. This approach can be used for other species as well but must be developed carefully for each special case. 

AAS is a useful tool for speciation work as well. This applies for combinations of species-specific extraction procedures and subsequent determination of the respective species by AAS. Just such a method was worked out for the case of Crln and CrVI compounds by Subramanian et al. [320], Use is made of the fact that the Crln is slower than the CrVI to form complexes, as a result of complexation with 6 molecules of H2O. 

Gas and liquid chromatography Coupled with ICP-AES these are useful approaches for speciation work, as shown by the early work of Cox et al. [418] for Crm/Crvl speciation, where AI2O3 filled columns and elution with acids and NaOH were used. Anion exchangers could also be used. New impetus has arisen from the interfacing of thermospray and high-pressure nebulization to ICP-AES. ICP-AES remains of interest for the element-specific detection in the speciation of silicon compounds and is a good alternative to ICP-MS, where considerable spectral interferences hamper the power of detection that can be obtained. 

Helium MIPs are excellent for element-specific detection in gas chromatography, as has been commercially realized [451]. In this way, not only are the halogens and other elements relevant in pesticide residue analysis but also organolead and or-ganotin compounds determined down to low concentrations. This makes MIP-AES very useful for speciation work [321, 452]. It has been shown that the delocalized helium MIP gave low detection limits for elements with high excitation potentials... 

The isolation of analytes by leaching, as is often done in soil analysis, is often dictated by the difficulties encountered when dissolving samples completely. However, also for speciation work, where the analytes are labile such procedures are often the only way to obtain the analyte solutions (see e.g. Ref. [687] for the isolation of organomercury compounds from fish samples). 

More chemical speciation work is needed with the incorporation of various natural organics, complexes, micropollutants and precipitates into the database. Soludon speciation is of pardcular importance in NF. 

In contrast to the speciation work there have only been a modest number of reports on kinetic and mechanistic details. The formation of Zn" complexes of GSH proceeds by two pH-dependent mechanisms involving coordination at the thiol group, although no thiol involvement is apparent in Ni" complex formation. Different behaviom is found again with MeHg", which binds to GSH by an associative pathway. The Cu"-GSH system is further complicated by the observation that the disulfide hydrolysis in alkaline solution is accelerated by Cu" ions (equations 3-5). 

There are a number of other GC detectors commercially available. Photoionization detectors (PIDs) are primarily used for the selective, low-level detection of the compounds which have double or triple bonds or an aromatic moiety in their structures. Electrolytic conductivity detectors (ELCDs) are used for the selective detection of chlorine-, nitrogen-, or sulfur-containing compounds at low levels. Chemiluminescence detectors are usually employed for the detection of sulfur compounds. The atomic emission detectors (AEDs) can be set up to respond only to selected atoms, or group of atoms, and they are very useful for element-specific detection and element-speciation work. 

Chapter 8 provides the reader with the basic principles of isotope dilution mass spectrometry used for elemental analysis and also discusses more advanced features of this calibration approach, such as its use in direct solid sample analysis and in elemental speciation work, wherein not the total amount but that of various chemical species of a target element need to be determined. 

Finally, in Chapter 17, the relatively recent use of MC-ICP-MS in elemental speciation work - realized by coupling a chromatographic separation technique to an MC-ICP-MS device as an isotope-specific detector - is discussed. Also, the consequences of the transient nature of the signals thus obtained on the data collection are considered. 

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