Speciation procedures

Christensen and Lun [57] developed a speciation procedure using a cation-exchange resin (Chelex 100) in a sequential batch/column/batch system for determining free divalent cadmium and cadmium complexes of various stabilities at the cadmium concentrations typically found in landfill leachates... 

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 guiding principles for the selection or development of speciation procedures are similar to those recommended for other forms of chemical analysis. For example, the initial step should be careful definition of the problem, including listing of the analytical specifications (e.g. type of analysis, concentration range, potential sources of error). This step can be followed by selection of a suitable measurement procedure, nomination of a selective separation procedure (if required) and organisation of the total protocol. 

The unique aspects of speciation procedures arise from the additional specification that the procedure adopted should not disturb existing equilibrium conditions. The choice of procedure is further restricted by the fact that the total concentration of element present in a sample (e.g. Cu, Pb, Cd, Zn in water samples) is often near the detection limits of many standard analytical techniques, and modified or refined techniques are required to handle the even lower levels present in isolated sub-categories. In biological matrices, the concentrations of inorganic and organo-metallic compounds present can range from 10 3 to 10 12 mol dm 3, and at the lower levels even the determination of total element content can be greatly in error, if suitable correction is not made for interference effects which can arise from the nature of the sample. 

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. 

Florence, T.M. (1977) Trace metal species in fresh waters. Water Res., 11, 681-687. Florence, T.M. (1982) Development of physicochemical speciation procedures to investigate the toxicity of copper, lead, cadmium and zinc towards aquatic biota. Anal. Chim. Acta, 141, 73-94. 

Metal speciation procedures, which have been verified under controlled laboratory conditions and evaluated by means of bioassays, will require further verification in order to determine their ecological effects. For example, how does the response of the bioassay test species to a toxic metal fraction relate to the toxicity to larger organisms such as fish in the natural environment Bioaccumulation of metals in populations has been very difficult to relate to metal speciation measurements. There is a challenge for analytical chemists to develop metal speciation procedures that are relevant to ecotoxicology (Morrison and Wei, 1991). 

Morrison, G.M.P. and Florence, T.M. (1988) Comparison of physiochemical speciation procedures with metal toxicity to Chlorella pyrenoidosa. Anal. Chim. Acta, 209, 97-109. 

Traditionally, in order to prevent oxidation of unstable Fe(II) species, the water sample has to be filtered immediately after sampling (filtration of the sample in the field needs to be carried out under completely oxygen-free conditions) and stabilized stabilization depends on the subsequent analytical method.51 Even when all sample treatment protocols are rigorously applied, Fe(II) is so easily oxidized that the initial speciation can be distorted simply by contact of the sample with air. Mn(II) oxidizes much more slowly than Fe(II) this reaction is about 107 times slower than that of Fe(II) at pH 8 and 25°C,52 reducing the risk of error during the speciation procedure. After filtration, only Mn(II) and Mn(IV) colloids remain in the sample. Filtered samples, mostly acidified, are commonly stored in precleaned Teflon bottles at 4°C. 

Se-Cys, y-glutamyl MeSe-Cys, inorganic Se y-glutamyl MeSe-Cys identity conjoined by ESI-MS and ESI-MS-MS Recovery of speciation procedure 60D85% [52]... 

Studies in this laboratory require the use of analytical and speciation procedures which can be routinely performed on a variety of samples. Most of the above mentioned speciation techniques require that a considerable amount of equipment be... 

In this section the instrumentation and the experimental procedure to determine the Cd, Cu and Pb total dissolved concentration is described in some detail. Several reviews regarding the general theoretical and experimental aspects of anodic stripping voltammetry and its application in sea water (or natural water) analysis can be found in the literature (15 17, 41, 59, 62, 67, 68). Details of the speciation procedure are reported below. 

Considering the doubts and criticisms directed at operational speciation procedures because of the potential perturbation of the equilibrium of the system... 

Comprehensive chromatographic speciation procedures for arsenic compounds often have to be performed without any pretreatment in fresh urine (e.g. Norin and Vahter, 1981). If chromatography is applied after refrigerated storage in PTFE bottles, filtration through 0.45 fim (Ghana and Smith, 1987 Sheppard et al., 1992) or even 0.2 /membrane filters (Heitkemper et al., 1989) has been reported as well. 

Because of the stability of most of the arsenic compounds of interest for speciation studies, nearly all work commences with homogenized, predominantly lyophylized, materials. Extraction of arsenic species from dry and finely ground samples is often performed with methanol/water/chloroform mixtures or just methanol (Luten et al., 1982 Francesconi et al., 1985 Momplaisir et al., 1991 Ballin et al., 1992 ). Methanol and phosphate buffer (1 1) extraction was applied with especially good results for methylated forms of inorganic arsenic (Arenas, 1991). Rather simple approaches are the solubilization of various types of organic materials by treatment with e.g. tetramethyl ammonium hydroxide (TMAH) at ambient temperature for the subsequent determination of inorganic arsenic and its metabolites (Stoeppler and Apel, 1984, Burow and Stoeppler, 1987) and the solubilization of As(lll) and As(V) by treatment with perchloric acid and Fe2(S04)3 at elevated temperatures with subseqent separation and determination of As(lll) and As(V) (Holak and Specchio, 1991). Some speciation procedures based on NaOH treatment will be described under Speciation procedures. 

Total arsenic determination is necessary in many cases, particularly in intoxication, for balance studies, and for food control if only total arsenic contents are needed. However, it is only a complement to the various speciation procedures and the methods described in this section are very often used to determine the arsenic amount that occurs in a distinct chemical form or fraction as part of a so-called hyphenated procedure. As far as these methods are described in detail under Speciation procedures the final determination step will also be outlined. 

Several speciation procedures for Cr(lll) and Cr(VI) can be found in the literature. They are based on the distinction between cationic Cr(lll) and anionic Cr(VI) forms. 

A number of fractionation procedures have been developed over the last 15 years in order to distinguish between the various aqueous forms of aluminium. These methods include dialysis, ion exchange (both batch and column), HPLC, F ion-selective electrode, NMR, species specific extractions, filtration and computational techniques. All of these procedures usually measure operationally defined aluminium fractions i.e. groups of aluminium species are measured, rather than a single species, and the values obtained depend upon the precise procedure used for the analysis). Therefore, present speciation procedures may measure slightly different forms of aluminium depending on the conditions, and thus they should accordingly provide different results. 

From the data available, there is no consensus as to the precise experimental conditions to be used. This is especially true where the analytical separation is concerned. The resin in the column has to be conditioned with a solution of NaCl having the same ionic strength and pH as the water samples to be analysed and this is one of the main drawbacks of the method because the solution of NaCl has to be passed through the column until the pH of the effluent does not change by more than 0.2 pH units. A full Driscoll/PCV speciation procedure including all the modifications has been published elsewhere [168]. 

One interlaboratory method comparison exercise for aluminium speciation has been organized within the framework of the BCR project on A1 speciation [184]. Results obtained using a standard Driscoll/PCV method for the determination of the labile monomeric aluminium fraction were tested and compared to three other aluminium speciation procedures. Each of the three participant laboratories carried out both the standard Driscoll method, used as... 

The direct species-specific techniques would be the ideal speciation procedure, providing identification and quantification of a given individual species in situ without any sample treatment. The techniques of this type, more used for individual metal-containing species determination, are electroanalytical in nature (e.g., ion-selective electrodes have been used for determining major free alkali ion determinations in natural waters and serum). More sensitive voT tamperometric redissolution techniques can be useful for speciation of free and labile forms of metals (or their different oxidation states) in waters. 

Developments of quality assurance of the analytical speciation procedures and results are of prime importance at this point. Validation of new methodologies should be carried out if possible with certified reference materials (CRMs) (with a matrix as similar as possible to the real sample certified for the sought species). Unfortunately, very few CRMs are commercially available today for chemical speciation. Thus, alternative approaches for validation using complementary methods based on different mechanisms of separation and complementary detectors are now in order. 

To use geochemical speciation procedures in combination with bioassays and epidemiological data... 

Numerous speciation procedures have been developed in order to describe heavy metal forms in soils (Himer 1992 Rauret 1993 Zeien and Bruemmer 1989). In this study, the sequential extraction method according to Zeien and Bruemmer (1989,1991) was applied to soil samples collected from sanitary protection zones. Both surface (0-20 cm) and subsurface (30-40 cm) soil samples were examined. The method involved the sequential extraction of 7 operationally defined fractions ... 

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