Sequential extraction sequence

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. 

The SEDEX method (Ruttenberg, 1992) is a multistep selective, sequential extraction method that separately quantifies five distinct sedimentary phosphorus reservoirs on the basis of their chemical reactivity to a sequence of solvents ... 

Figure 8 Sequence of extractants and extraction conditions that make up the SEDEX sequential extraction method for quantifying different forms of phosphorus in marine sediments (after Ruttenherg, 1992).

Shuman (1982, 1985), working on the speciation of Cu, Mn, Fe and Zn, modified the sequence of the extraction as exchangeable, oxidizable (organically bound), easily reducible (Mn oxides-bound), moderately reducible (amorphous Fe oxides-bound), strongly reducible (crystalline Fe oxides-bound), and residual. The low percentages of the oxidizable fractions obtained following the sequential extraction scheme of Shuman (1985) as compared to those obtained following the scheme of Tessier et al. (1979) were attributed to the lack of selectivity of the method used (Charlatchka et al, 1997). 

M HNO3 adjusted to pH 2 at 85 °C for 1 and 2 h in sequence, followed by 5 mL of a mixture of 0.8 M NH4AC and 20% HNO3 for 30 min). The soil sample was simultaneously measured for the total and five fractions, including the residue after the sequential extractions of copper, cadmium and lead in triplicate. The recoveries of the sequential extraction as the sum of five fractions were between 92.1 and 113.8% for the three metals. 

Fractionated Extraction For the sequential extraction (Fig 1) a Soxhlet was used. Diethyl ether, ethanol and hot water was the solvent sequence selected. The extraction time for each procedure was defined by at least SO cycles and the temperature was the boiling point of each solvent. The values used for the extraction were 56 cycles (at 36 C), 52 (at 78 C) and 52 (at 98... 

D. monetaria (25.6 kg dry weight) was extracted in sequence with methylene chloride (16 h.) and ethanol (16 h.) in a Soxhlet apparatus. The solvent was removed in vacuo to yield 211.1 g of methylene chloride extract (fraction A, Figure 2) and 174.4 g of extract ethanol (fraction B, Figure 2). Fraction A was sequentially extracted with hot hexane (fraction Al, Figure 2) and toluene (fraction A2, Figure 2). Fraction B was extracted with ether. The ether-insoluble fraction (72.2 g) (fraction C, Figure 2) was... 

Although during the first years of development of sequential extraction almost every researcher had his own sequence, made on the basis of more or less free selection of extraction instruments and order of their use, recently there has been relative unification of applied models, and hence there is a possibility of appropriate comparison of findings made by various authors. Since selectivity of certain extractors is already relatively well known, and so are the effects of secondary reactions, re-adsorption and other obstructing occurrences, the authors seldom use - in essence inadequate - notions such as exchangeable phase , easily reducible phase , etc. but more frequently use notions such as ammonium-acetate phase , hydroxylamine phase , etc. Therefore, isolated metal fractions are ascribed to applied extraction instruments, naturally with clearly defined conditions for extraction, which have been more or less standardized for each extraction level. In order to ascribe fractions of heavy metals, defined in this manner, to a certain substrate, one has to know to what extent were the most widely spread substrates susceptible to the effect (breakdown, i.e. dissolving) of applied extraction instruments. 

Extraction of hemiceUulose is a complex process that alters or degrades hemiceUulose in some manner (11,138). Alkaline reagents that break hydrogen bonds are the most effective solvents but they de-estetify and initiate -elimination reactions. Polar solvents such as DMSO and dimethylformamide are more specific and are used to extract partiaUy acetylated polymers from milled wood or holoceUulose (11,139). Solvent mixtures of increasing solvent power are employed in a sequential manner (138) and advantage is taken of the different behavior of various alkaUes and alkaline complexes under different experimental conditions of extraction, concentration, and temperature (4,140). Some sequences for these elaborate extraction schemes have been summarized (138,139) and an experimenter should optimize them for the material involved and the desired end product (102). 

Below is a review of the chemical extractants and their extraction order, in sequence, in the selective sequential dissolution procedures (Table 4.5). 

To sequence an entire polypeptide, a chemical method devised by Pehr Edman is usually employed. The Edman degradation procedure labels and removes only the amino-terminal residue from a peptide, leaving all other peptide bonds intact (Fig. 3-25b). The peptide is reacted with phenylisothiocyanate under mildly alkaline conditions, which converts the amino-terminal amino acid to a phenylthiocarbamoyl (PTC) adduct. The peptide bond next to the PTC adduct is then cleaved in a step carried out in anhydrous trifluo-roacetic acid, with removal of the amino-terminal amino acid as an anilinothiazolinone derivative. The deriva-tized amino acid is extracted with organic solvents, converted to the more stable phenylthiohydantoin derivative by treatment with aqueous acid, and then identified. The use of sequential reactions carried out under first basic and then acidic conditions provides control over... 

So far we have not learned anything from the NOE data except to assign all of the protons. This is an essential first step, but the real goal is to extract distances between specific pairs of protons from the NOE data. Sequential NOEs are not very useful because we already know that an amino acid residue is close in space to its nearest neighbor in the sequence. The real mother lode of the NOESY data consists of the medium-range (i -> i + 2, i —> i + 3 and i -> i + 4) and long-range connectivities. These define the secondary structure... 

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