Sequential extraction methodology

There have been some questions raised about the validity of results of extensive and sequential extraction methods. There is the possibility that species of an analyte may change during the extraction process. It is also possible that a species may be liberated and then reabsorbed during extraction or subsequent isolation. The same or similar question could be raised about various initial sampling and subsequent storage methodologies as described next. In spite of these questions, the BCR methodology has been widely accepted [39-41],... 

An interesting, and somewhat radical, departure from traditional extraction methodology was proposed by Cave and Wragg (1997). They demonstrated that, with an appropriate chemometric mixture resolution procedure, a simple, nonspecific extraction could provide information on metal binding in soil SRM 2710 similar to that obtained by a Tessier sequential extraction. The method used a central composite design, with extraction time, nitric acid concentration and sample extractant ratio as variables, together with PCA. 

Classical speciation of radionuclides is described in Chapter 13. Methodology for single and sequential extraction of soil to assess radionuclide availability to plants is similar to that used for heavy metals, and has recently been reviewed (Kennedy et al., 1997). Therefore, only recent applications of sequential extraction to speci-ate both natural and anthropogenic nuclides are discussed below. 

Figure 8.2.1 Schematic overview on traceability aspects of chemical sediment analysis (modified from Fdrstner Heise, 2006). X = sediment-specific property. Numbers relate to levels of methodological standardisation (section 8.3.4) 1) certified reference material, 2) standardised method, 3) well-documented method, and 4) special method. AVS/E SEM = Acid Volatile Sul-fide/Sum of Simultaneously Extractable Metals (DiToro et al. 1992) Wet Sample sub-sampling for tests under oxygen-free atmosphere (pore water, sequential extraction, etc)...

Indeed, recent results from our laboratory indicate that dendrimer-encapsulated CdS QDs can be prepared by either of two methods [192]. The first approach is analogous to the methodology described earlier for preparing dendrimer-encapsulated metal particles. First, Cd and S salts are added to an aqueous or methanolic PAMAM dendrimer solution. This yields a mixture of intradendrimer (templated) and interdendrimer particles. The smaller, dendrimer-encapsulated nanoparticles may then be separated via size-selective photo etching [193], dendrimer modification and extraction into a nonpolar phase [19], or by washing with solvent in which the dendrimer-encapsulated particles are preferentially soluble. An alternative, higher-yield method relies on sequential addition of very small aliquots of Cd + and S " to alcoholic dendrimer solutions. 

To alleviate these drawbacks, alternative methodologies relying on the continuous provision of fresh extractant volumes to the solid sample under mvestigation have been developed, characterized, and contrasted with the classical end-over-end extraction procedures. The fundamental principles of these novel, dynamic (nonequilibrium) strategies, based primarily on the use of continuous-flow analysis (Ruzicka and Hansen, 1988), flow injection analysis (Ruzicka and Hansen, 1988 Trojanowicz, 2000 Miro and Frenzel, 2004b), or sequential injection analysis (Ruzicka and Marshall, 1990 Lenehan et al., 2002), are described in detail below, and their advantageous features and limitations for fractionation explorations are discussed critically. 

The flexibility of this methodology allows for a variety of applications from determinations with no additional chemical reaction to sequential reactions involving five or six reactants from gradient techniques to stopped-flow or kinetic determinations from incorporation of liquid-iiquid extraction units to Insertion of gas samples. 

The segregated core-shell microstmctures, consisting of a hydrophobic core surrounded by a hydrophilic shell, are of great praaical interest as their mechanical properties are mainly influenced by the core polymer and the chemical properties and solubility mainly by the shell monomer units. These materials have attracted increased attention because they not only maintain the funaions of both the core and shell components but also exhibit additional excellent optical, electrical, and magnetic properties. Polymeric core-shell microstmctures can be obtained through phase separation, which is realized by inductive polymerization," solvent extraction and evaporation,"" self-assembly of amphiphilic block copolymers,"" or sequential predpitation,"" which is intrinsically a self-assembly and phase-separation process. However, these methodologies are usually complicated and tedious. 

Accelerated solvent extraction (ASE) is a relatively recent advance in sample preparation for trace environmental analysis. This techiuque uses conventional solvents at elevated pressures and temperatures to extract solid samples quickly. The process takes advantage of the increased analyte solubilities at temperatures well above the boiling points of common solvents. Under these conditions, the kinetic processes for the desorption of analytes from the matrix are accelerated. Currently a commercial unit is available in which automated extractions can be carried out on 24 samples sequentially (Richter et al., 1995, 1996). This technique offers some significant advantages over SEE and MAP. SEE uses supercritical CO2, which is a nonpolar fluid, whereas MAP requires the presence of a polar solvent that couples with microwave to promote heating. By comparison, ASE uses the same solvent as traditional Soxhlet extractions, which means a (firect transfer of methodology is feasible without any of the restrictions or limitations of the two other methods. Method development time is therefore shortened. 

Valcarcel and co-workers proposed in 2003 an automatic system for the determination of riboflavin in lyophilized food products using a solid-phase extraction process that included columns filled with cotton or silica C18 for the sequential retention of synthetic colorants and natural colorants, respectively (Gonzalez et al. 2003). In this case riboflavin was assessed as an authorized natural colorant for total estimation of this class of compounds. The analytical characteristics of this methodology are summarized in Table 18.3. 

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