Sequential extraction procedures specificity

Various workers have questioned the ability of sequential extraction to provide accurate information on the mineralogical phases with which trace elements are associated in soil or sediments (e.g. Nirel and Morel, 1990). Problems, including non-selectivity of reagents and readsorption of analytes following release, are frequently reported. Hence, nowadays, most environmental analytical chemists accept that sequential extraction should be considered an operational form of speciation, in which the fractions isolated are defined purely by the sequence of reagents used, and not as a means to determine information on the specific mineralogical phases to which trace elements are bound. Modern sequential extraction procedures label the fractions obtained in terms of the type of chemical reaction used to isolate them, in order to emphasise this, e.g. reducible or oxidisable species. Unfortunately, this distinction is not always made clear in the wider environmental literature. 

In common with other sequential extraction procedures, the BCR scheme suffers from a degree of non-specificity (Whalley and Grant, 1994 Coetzee et d., 1995) and redistribution of analytes during extraction (Raksasataya et d., 1996). Some success in limiting lead redistribution by addition of cryptand 2.2.2 or nitriloacetic acid to the acetic acid in Step 1 has been reported, but the effectiveness of the complexing agent was found to be strongly dependent on the bulk composition of the model soil system studied (Raksasataya et d., 1997). 

Chemometrics can also be used to overcome some of the intrinsic deficiencies of sequential extraction, such as non-specificity. Barona and Romero (1996a) used principal components analysis (PCA) to establish relationships between the amounts of metals released at each stage of a sequential extraction procedure and bulk soil properties, and demonstrated that carbonates played a dominant role in governing metal partitioning in the soil studied. The same workers employed multiple regression analysis to study soil remediation (see Section 10.11.1.1). Zufiaurre et al. (1998) also used PCA to confirm their interpretation of phase association and hence potential bioavailability of heavy metals in sewage sludge. 

More widely applied to determine the potential, plant and human bioavailability are the methods of PTMs speciation which involve selective chemical extraction techniques. Estimation of the plant- or human-available element content of soil using single chemical extractants is an example of functionally defined speciation, in which the function is plant or human availability. In operationally defined speciation, single extractants are classified according to their ability to release elements from specific soil phases. Selective sequential extraction procedures are examples of operational speciation (Ure and Davidson, 2002). 

The undisputed advantage of sequential extraction procedures lies in the fact, that rearrangement of specific solid phases can be evaluated prior to the actual remobUization of certain proportions of an element into the dissolved... 

Specific chapters then focus on different projects on speciation analysis. Chapter 4 deals with interlaboratory studies on methylmercury in fish and sediment Chapter 5 describes the collaborative projects to certify organotins in sediment RMs and mussel tissues Chapter 6 gives an overview of the certification project on trimethyllead in simulated rainwater and urban dust Chapter 7 describes the certification project on arsenic species in fish tissues Chapter 8 focuses on the intercomparison and tentative certification of Se(IV) and Se(VI) in simulated freshwater Chapter 9 deals with a feasibility study to stabilize Cr species in solution followed by the certification of Cr(III) and Cr(VI) in lyophilized solutions and welding dust Chapter 10 gives a review of methods used for A1 speciation Chapter 11 develops the overall collaborative project to standardize single and sequential extraction procedures for soil and sediment analysis, followed by interlaboratory studies and certification of soil and sediment reference materials. 

The isolation of polysaccharides from soy meal was successful, WUS contained only 2.1% of protein and 92% of the polysaccharides present in soy meal were recovered in WUS. The sugar composition of both the soy meal and WUS are similar and allow the conclusion that during the isolation procedure no sugar residues were specifically removed. The pectin-rich extracts (ChSS and DASS) obtained after sequential extraction of the WUS were the most abundant. 

There are a number of selective sequential dissolution procedures which have been developed for specific elements, matrices, regional soils, and specific purposes (Table 4.1). Additionally, various extractants are used in different sequential procedures for the similar targeted solid-phase component. The review on the extractants of individual fractions of metals in soil was made by Shuman (1991). However, in most protocols the trace elements/heavy metals in their native and waste-amended soils are divided into the following physicochemical forms ... 

Various sequential dissolution protocols have been developed by different research groups in order to accommodate their types of soils, experimental conditions, and objectives. This makes it difficult to compare the results with different procedures. We compared two SSD procedures for humid zone and arid zone soils, developed by German and Israeli soil scientists, respectively, based on aggressiveness of extractants, their specificity and selectivity, completeness of phase-extraction by each extractant from defined phases and their effects on subsequent fractions. We also appraised the applicability as well as the limitations of each procedure under different conditions. 

By applying methods of sequential leaching, direct chemical information can be obtained and a rough assessment of the remobilization potential of the sedimented heavy metals is possible. Because of the lack of specifity of the extraction procedures particular species cannot be identified (see also critical hints in [KHEBOIAN and BAUER,... 

Just choosing the most widely applied procedure (namely that of Tessier et al., 1979) could yield data of doubtful reliability for a particular matrix or objective, but may nevertheless allow comparison with results of many other studies. In practice, there is always an optimisation necessary between compatibility and reliability. The limitations reported here and elsewhere lead to the conclusion that results given by sequential sediment extraction experiments can be used for an assessment of specific release scenarios particularly related to changing pH, complexing ligand availability and redox environments rather than for true metal speciation in sediments. The latter can be achieved only by using intrumental speciation techniques, either alone or in combination with sequential extraction. It is in this area of research that new developments have appeared since the first edition of this volume. Particularly... 

It should be noted that the results obtained by the use of different SEPs are often diffieult to eonipare. For example, fraction-specific concentrations, percentages, and relative recoveries for Cu, Pb, and Zn in SRM 2710 and 2711 were used to explore differences between the optimized (modified) SM T procedure and three other sequential extraction schemes (i.e., the Tessier scheme, tlie Geological Survey of Canada scheme, and the original SM T scheme) (Sutherland and Tack, 2003). The experimental results revealed significant differences among the four schemes, even for those closely associated. Thus, the TEs released by the various SEPs in the different fractions must be assessed carefully because different trends may be obtained for the same metal species, soil-phase association, and environmental solid. 

Phosphorus is not a TE but a major nutrient element. Nevertheless, fractionation of this element is essential for environmental studies, and hence it seems reasonable to highlight here some relevant SEPs. Four different procedures for the fractionation of P in lake sediment samples have been tested in an interlaboratory study in the framework of the SM T program (Ruban et al., 1999). As a result, a novel scheme based on the Williams protocol (Williams et al., 1976) has been developed aimed at the restoration of lake sediments. The scheme comprises three separate assays (1) sequential extraction of NaOH-extractable (Fe- and Al-bound) and HCl-extractable (Ca-bound) fractions, (2) sequential extraction of inorganic and organic phosphorus and (3) single extraction, after calcination, of concentrated HCl-extractable (total P) fraction (see Table 12.3 for further details). Further discrimination of specific compounds is made feasible by the use of chromatographic and capillary separation techniques as reviewed by Spivakov et al. (1999). 

Metal species resulted from the partitioning of the total metal content that is associated with various solid soil fractions, and usually are estimated using specific operational extraction procedures. Comprehensive reviews of methods applied for the sequential extraction have been provided in several publications (Bourg 1995, Briimmer 1986, Fbrstner 1986, Kersten and Fbrstner 1991, Salomons and Fbrstner 1984, Sauve 2001, Sheppard and Stephenson 1997, Mortved etal. 1991, Ure and Davidson... 

Sequential chemical extraction involves treatment of a sample of soil or sediment with a series of reagents in order to partition the trace element content. The principal advantage claimed for sequential extraction over the use of single extractants is that the phase specificity is improved. This technique has been used to determine the chemical forms of trace elements in soils, sediments, and suspended solids in natural waters, and is based theoretically and experimentally on more than 100 years of research (Jackson, 1985). A basic requirement of any extraction procedure should be the ability of the extractant to dissolve a specific component of a soil or sediment (Chao, 1984). Many different methods have been employed to fractionate trace elements, and these have proved useful for metal speciafion (Jones and Hao, 1993). Reviews of the fractionation methods used to determine the chemical forms of trace elements in soils and sediments (e.g. Pickering, 1981, 1986 Ross, 1994 Sheppard and Stevenson, 1997), in geochemical exploration (Chao, 1984), and in natural waters (Florence and Batley, 1977) are available. A few commonly used fractionation schemes for delineating different forms of trace elements are given in Table 6. 

In general, three conventional methods were used for the extraction of bioactive compounds such as solvents, steam, and supercritical fluids. On a global level, water extraction is practised while making cofiee or tea. Basically, pretreated plant material is extracted with hot water which takes up the flavor, taste, and color of the components. After filtration, the extract is ready for consumption. In case of the isolation of certain bioactive compounds from plant material by means of liquid extraction, some technological problems needs to be resolved [3]. First the plant material has to be pretreated in order to obtain reasonable extraction yields. Another problem is the need for special solvents to be used in the extraction procedure [4]. More recently, attention has been focussed towards the isolation of specific compounds that can be used in the food industry. Of particular interest is the isolation of bioactive compounds, aromas, and fiiagrances from plants and fruits [5,6]. The sequential extractions of bioactives using nonpolar to polar solvents are depicted in Figure 7.1. Various polarity solvents are reported as follows (1) nonpolar solvents (hexane, heptanes, petroleum ether,... 

The basic assumption behind the sequential dissolution methods is that a particular extractant is phase or retention mode specific in its chemical attachment on a mixture of forms (D Amore et al., 2005). The procedure... 

Specific extractants and sequential fractionation are also widely used procedures to estimate metal or nutrient availability to plants (see, e.g., Houba et al. 1996). Several extractants can be considered, like diethylene triamine penta acetate (DTPA), ethylene diamine tetra acetic acid (EDTA), acetic acid, HN03, HC1, or other... 

Immunoassays of various designs for estimating free thyroid hormones using antibody extraction techniques have been developed. These assays are subdivided as either sequential two-step assays or simultaneous one-step Canalogue ) assays. Each procedure involves the direct incubation of serum with a specific anti-T4 or anti-T3 antibody, during which thyroid hormones reach a new equilibrium with all of the binders present. A slight decrease in free hormone concentration occurs, but is insignificant if the antibody sequesters less than 5% of the total amount of hormone present in the specimen. Thus the amount of immunoex-tracted T4 or T3 closely approximates the undisturbed free hormone concentration that preexists in serum at equilibrium. 

In contrast to the enhanced release of organic compounds by a sequential application of hydrolysis and BBr3 the sequential procedure led in the case of Ru04 to a minor portion of released organic compounds. Hence specific contaminants (e.g. halogenated arenes, nitro compounds) were not observed within the extracts of Ru04 oxidation products applied to the saponified residues. 

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