Sequential extraction procedures examples

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

Sequential extraction procedures have been applied for the purpose of isolating species of elements from particulate materials, soils and sediments (see Chapters 10 and 11). In sequential extraction procedures, samples are treated with a series of chemicals under rigorously controlled conditions of temperature, time and ratio of reactant to sample. The work of Tessier et al. (1979) resulted in a carefully designed procedure for the determination of species of elements in sediments. It has been used and modified by other investigators. Trace elements in the extracts are usually determined by means of AAS, ICPAES and ICP-MS. An example is the study of sequential extractions for the determination of 20 trace elements in ten certified geological reference materials (Hall et al., 1996). 

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). 

In Chapter 3.4 a method has been presented for long-term prognosis of metal pollutant mobility, which combines column circulation leaching experiments at variable pH/E -conditions with sequential extraction procedures on the solid waste material before and after these experiments (Schoer FSrstner, 1987). Temporal release patterns are different for the individual elements (Figure 6-5) While at pH 5/400 mV release of cadmium seems to be completed within the experimental period mobilization of copper is still going on and the end point cannot be estimated from the data of the "kinetic" experiments. The same effect has been found for the examples of thallium and vanadium. For the other elements, the endpoint of release can be determined as approximately 10 mg cobalt, 0.6 mg cadmium, 600 mg zinc and 0.3 mg chromium, 2 mg barium and 20 mg lead (per 100 g of solid substrate treated with 140 L solu-... 

Zasavica/Sava confluence, and downstream sites of the river Zasavica in order Modran , Valjevac , Sumareva , Ravnjanska , Rajevica , Gajica . A sequential extraction procedure was applied for partitioning of metals into five fractions. The examined river sediment samples are very interesting because of their great surface area and content of highly adsorbing minerals that are adsorbing toxic heavy metals, for example lead and cadmium. 

The procedure of simultaneous extracting-spectrophotometric determination of nitrophenols in wastewater is proposed on the example of the analysis of mixtures of mono-, di-, and trinitrophenols. The procedure consists of extraction concentrating in an acid medium, and sequential back-extractions under various pH. Such procedures give possibility for isolation o-, m-, p-nitrophenols, a-, P-, y-dinitrophenols and trinitrophenol in separate groups. Simultaneous determination is carried out by summary light-absorption of nitrophenol-ions. The error of determination concentrations on maximum contaminant level in natural waters doesn t exceed 10%. The peculiarities of application of the sequential extractions under fixed pH were studied on the example of mixture of simplest phenols (phenol, o-, m-, />-cresols). The procedure of their determination is based on the extraction to carbon tetrachloride, subsequent back-extraction and spectrophotometric measurement of interaction products with diazo-p-nitroaniline. 

Bound residnes are those chemicals retained in the subsurface matrix in the form of the parent organic contaminant or its metabolites these residnes remain after subsequent extractions, dnring which the nature of the compound or of the matrix is not altered by the extraction procedure. An example of sequential extraction of... 

Speciation science seeks to characterise the various forms in which PTMs occur or, at least, the main metal pools present in soil. This chapter provides a review of the single and sequential chemical extraction procedures that have been more widely applied to determine the plant and the human bioavailability of PTMs from contaminated soil and their presumed geochemical forms. Examples of complementary use of chemical and instrumental techniques and applications of PTMs speciation for risk and remediation assessment are illustrated. 

In addition to the need to monitor known problematic compounds, newer compounds are being identified as potential threats to humans and as such need to be monitored in the atmosphere. For example, researchers reported (10) that several chemical and instrumental analyses of HPLC fractions provided evidence for the presence of /V-nitroso compounds in extracts of airborne particles in New York City. The levels of these compounds were found to be approximately equivalent to the total concentrations of polycyclic aromatic hydrocarbons in the air. Since 90% of the N-nitroso compounds that have been tested are carcinogens (10), the newly discovered but untested materials may represent a significant environmental hazard. The procedure involved collecting samples of breathable, particulate matter from the air in New York City. -These samples were extracted with dichloro-methane. Potential interferences were-removed by sequential extractions with 0.2 N NaOH (removal of acids, phenols, nitrates, and nitrites) and 0.2 N H2S04 (removal of amines and bases). The samples were then subjected to a fractional distillation and other treatments. Readers interested in the total details should consult the original article (10). Both thin-layer chromatography (TLC) and HPLC were used to separate the compounds present in the methanolic extract. 

For example, the speciation analysis of metal ions in sediments can be performed by sequential extraction, whereby each of the fractions obtained shows a different form of association of the metal in the soil matrix. As mentioned in Chapter 1, an accepted modem procedure is ... 

Enzymatic extraction carried out using in vitro models of the gastrointestinal tract is cheaper, faster, more reproducible, and ethically easier than the alternative of studies involving people and animals. Estimation of nutrient bioavailability (including trace elements, both essential and toxic) is particularly important for nutritionists, pharmacists, and toxicologists. Application of sequential procedures allows analyte fractionation (metals usually), but enzyme selectivity allows leaching of certain speciation forms of the determined elements. Table 6.9 gives examples of application of enzymatic extraction procedures for trace element analysis and speciation analysis [71, 72]. 

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. 

Sediment samples on the other hand may be treated in different ways, depending on the information required. Water contained in sediments can be removed and analyzed by the same methods as for aqueous samples, but speciation information on the solid fraction is more difficult to acquire. Series of sequential extractions are already employed to acquire the information needed to understand the cycling of As in sediments, for example, on water-soluble, phosphate-exchangeable, organically-bound and residual phases in such media." Literature on extraction procedures for detection, hyphenated by chromatographic techniques, is much more recent extractions of... 

This book presents the basic techniques in the organic chemistry laboratory with the emphasis of doing the work correctly the first time. To this end, examples of what can go wrong are presented with admonishments, often bordering on the outrageous, to forestall the most common of errors. This is done in the belief that it is much more difficult to get into impossible experimental troubles once the student has been warned of the merely improbable ones. Complicated operations, such as distillation and extraction, are dealt with in a straightforward fashion, both in the explanations and in the sequential procedures. 

It is possible to exaggerate any differences by repeating a manipulation many times, for instance, in sequential solvent extractions. Paper chromatography is an example of a separation procedure in which a large number of partition equilibria occur between an organic solvent and the water in the cellulose. 

A convenient and frequently used procedure is sequential solvent extraction. A first step, with dichloromethane, for example, will extract flavonoid aglycones and less polar material. A subsequent step with an alcohol will extract flavonoid glycosides and polar constituents. 

Metals such as Fe, Cu, Zn, Pb, Hg, Se, As, Co, Cd, Cr and Ni have been extracted using an acid solution (usually HNO,) in a focused open-vessel system for periods ranging from 3 min for soil samples [68] to 50 min for coal (where metals are much more strongly retained) [46] the system of Fig. 5.8A allowed the sample to be brought into contact with fresh solvent in four cycles, which resulted in the relatively short extraction time achieved. One clear example of the dramatically reduced extraction times afforded by microwaves is the extraction of Pb, Zn and Cu from calcareous soil [277], where the sequential procedure used proved considerably faster than conventional extraction (2 h versus 20 h). 

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