Applications of sequential extraction

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...

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. 

Two experimental problems have been recognised in the application of sequential extractions for PTMs fractionation in soil and sediments. One is the PTMs readsorption and redistribution among phases during extraction, meaning that PTMs... 

The philosophy and limitations of sequential extraction are described in detail in Chapter 11, where the use of the approach in sediment research is also reviewed. Although originally developed for application to sediments, sequential extractions have since been applied successfully in many soil studies. Recent work... 

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. 

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]. 

Tab. 5 DDT related compounds quantified in the non-extractable organic matter of Teltow Canal sediment samples after application of sequential chemical degradation prodcedures (concentrations are given in pg/kg dry matter).

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. 

Advantages Low cost No grinding Broad applicability High b.p. solvent contamination of analyte Low investment Simple equipment Simultaneous extractions in series Low investment Simple equipment Rapid Economic solvent use Good reproducibility Low investment Simple equipment Economical Simple equipment Not traumatic Almost solvent free Concentrated analyte Rapid Low temperatures Rapid Automated Simultaneous extraction Low solvent use Rapid User friendly Automated Sequential extractions Not analyst labour intensive... 

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. 

Chang A.C., Page A.L., Wameke J.E., Grgurevic E. Sequential extraction of soil heavy metals following a sludge applications. J Environ Qual 1984 13 33-38. 

Hall G.E.M., Gauthier G., Pelchat J.C., Pelchate P., Vaive J.E. Application of a sequential extraction scheme to ten geological certified reference materials for the determination of 20 elements. J Anal At Spectrom 1996 11 787-796. 

Most of the applications of HPLC for protein analysis deal with the storage proteins in cereals (wheat, corn, rice, oat, barley) and beans (pea, soybeans). HPLC has proved useful for cultivar identihcation, protein separation, and characterization to detect adulterations (illegal addition of common wheat flour to durum wheat flour) [107]. Recently Losso et al. [146] have reported a rapid method for rice prolamin separation by perfusion chromatography on a RP POROS RH/2 column (UV detection at 230nm), sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE), and molecular size determination by MALDl-MS. DuPont et al. [147] used a combination of RP-HPLC and SDS-PAGE to determine the composition of wheat flour proteins previously fractionated by sequential extraction. 

A further area in which sequential extraction continues to be applied successfully is in assessment of the likelihood of mobilisation of metal contaminants from sediment-derived soil. When dredged sediment is used to reclaim land from the coastal margins or applied to arable soil to improve fertility, there is concern that potentially toxic elements accumulated under reducing conditions may be released on exposure to an oxygen-rich environment. Sequential extraction can be used to characterise the sediment prior to application, or to monitor changes in metal availability in the soil with time (e.g. Singh et al, 1998). 

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

Real, C., Barreiro, R. and Carballeira, A. (1994) The application of microwave heating in sequential extractions of heavy metals in estuarine sediments. Sci. Total Environ., 152, 135. 

Speciation of metal content in solids can be accomplished during the extraction process by subjecting the sample to successive extractions with progressively more aggressive solvents, or by extracting different subportions of sample with the different solvents. It has been shown that it is more difficult to obtain comparable results when using sequential extractions rather than individual extractions of subsamples with different extractants [124]. Some applications of extractions with different solvents are extraction with ... 

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. 

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. 

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

---