Sequential extraction procedures applications

This chapter considers methods of trace element speciation, and their application to soils, that involve selective chemical extraction techniques. It will be concerned firstly with extraction by single selective reagents and secondly with the development and application of sequential extraction procedures for soils and related materials. Sequential extraction procedures for sediments are discussed in depth in Chapter 11. Speciation in the soil solution and modelling aspects of its interaction with soil solid phases are comprehensively covered in Chapter 9 and will not be considered here. 

Table 10.4 Application of sequential extraction procedures to reference materials...

McGrath, D. (1996) Application of single and sequential extraction procedures to polluted and unpolluted soils. Sci. Total Environ., 178, 37-44. 

Ma, Y., and Uren, N. (1998). Transformation of heavy metals added to soil—Application of a new sequential extraction procedure. Geoderma 84, 157—168. 

The other popular sequential extraction procedure is the protocol proposed by the Community Bureau of Reference, Commission of the European Community (known as the BCR protocol). The method was proposed on the basis of interlaboratory smdies undertaken in order to harmonize conditions for soil and sediment sample analysis. Based on the research data, in 1992 it was stated that application of EDTA or acetic acid solution is appropriate and sufficient for elimination of the bioaccessible fi action of metals from soil samples [62]. In the case of other samples, best results were achieved after application of a three-stage procedure with the following extractants ... 

Wisotzky, F., and Cremer, N. (2003b). Sequential extraction procedure in columns, 2 Application of a new method. Environ. Geol. 44, 805-810. 

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. 

Particularly significant recent developments include the production of these standard protocols, the application of chemometric procedures, and attempts to bring together results of sequential extraction and biotoxicity testing (e.g. Wang et al., 1998) in order to produce information on bioavailability. 

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. 

There are many review articles concerning application of USE in food technology [57] and for isolation of bioactive substances from herbs and other plant materials [58], as well as leaching of heavy metals from environmental and industrial samples [59]. Application of ultrasound during sequential extraction of trace elements significantly shortens the whole procedure however, for satisfactory efficiency it is necessary to increase the temperature and modify the matrix. 

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]. 

Rauret, G., Ldpez-Sanchez, J. F., Sahuquillo, A., Barahona, E., Lachica, M., Ure, A., Davidson, C. M., Gomez, A., Luck. D., Bacon, J., Yli-Halla, M., Muntau H., and Quevauviller, Ph. (2000). Application of a modified BCR sequential extraction (three-step) procedure for the determination of extractable trace metal contents in a sewage sludge amended soil reference material (CRM 483), complemented by a three-year stability study of acetic acid and EDTA extractable metal content. J. Environ. Monit. 2, 228-233. 

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. 

Another popular and selective extraction technique widely used in bioanalysis is solid phase extraction (SPE). SPE is a separation process utilizing the affinity of the analytes to a solid stationary phase. By manipulating the polarity and pH of the mobile phase, the analytes of interest or undesired impurities pass through stationary phase sequentially according to their physical and chemical properties. For a SPE procedure, a wash step refers to the elution of the unwanted impurities which are discarded and the elution step refers to the elution of the analytes of interest which are collected. While the fundamental remains the same in decades, the continuing invention and introduction of new commercial stationary phases and accessory devices have boosted the application of SPE in bioanalysis and many other fields. 

Bismuth chloride (16 mg, 0.05 mmol) and dry dichloromethane (3 ml) were placed in a 50-ml two-necked flask under nitrogen. An aldehyde (1.1 mmol) and a silyl enol ether (1.0 mmol) were added sequentially and the resulting reaction mixture was stirred at room temperature. The reaction was quenched with methanol/1 M HCl (10/1, 0.1 ml), water (10 ml) is added, and the product was extracted with dichloromethane (10 ml) three times. After drying, the solvent was removed in vacuo to leave a crude product, which was purified on thin layer chromatography to afford the corresponding aldol in good yield. A similar procedure was also applicable to the Michael-type reaction [91BCJ990]. 

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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