Selection of Extractants

Three different extractants were discussed for the feasibility study on calcareous soil, namely EDTA, DTPA and mixed acid ammonium acetate/EDTA. 

EDTA (ethylenediaminetetraacetic acid) extracts of soils tend in general to correlate well with plant contents, in particular with the plant-available fraction for Cd, Cu, Ni, Pb and Zn [208-210], EDTA (0.05 mol at pH 7 was used in the certification of the two soils mentioned above [197]. This test is assumed to extract both carbonate-bound and organically-bound fractions of metals and was hence considered to be suitable for calcareous soil analysis. 

Mixed acid ammonium acetate/EDTA reagent introduced by Lakanen and Ervio [211] was found to provide good correlation with wheat and maize contents for Cu and Zn [212]. This method was discarded as there is some evidence that EDTA at pH 5.5 can precipitate Cr, Pb and Zn, as was observed in polluted sediments [213]. Moreover, the benefits of supplementing the acid ammonium acetate did not seem worth the more complicated procedure. 

DTPA (diethylenetriaminepentaacetic acid) extracts have been extensively studied, particularly as a Zn diagnostic procedure for calcareous soils [210,214,215]. DTPA (0.005 mol L at pH 7.3 was thought to be a possibility for the determination of extractable Cd, Cu, Fe and Mn, but would be less suitable for Cr and Ni. This procedure was stressed to be more complicated than the EDTA one and was recognized to be often misused [201] in addition, DTPA extracts less than EDTA which might lead to sensitivity problems. This method was, however, retained for the feasibility study owing to its high degree of acceptance. EDTA was the method of preference as it was extensively tested in previous studies and will be soon adopted as an ISO Standard. 

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In on-line extraction the process is coupled directly ( hyphenated ) to the analytical technique used for further analysis of the extract (either spectroscopy or, more frequently, chromatography, because of the limited selectivity of extraction). Common examples include SFE-GC, SFE-SFC, SFE-HPLC, SFE-FTIR,... 

Soxhlet extraction is well established, and generally exhaustively extracts all additives. The selection of extraction solvent can make large differences to the extraction time. The generally long extraction times followed by concentration steps may determine losses of volatile or thermally labile components. Because this form of extraction is one of the oldest and still widely used in industry, it is the standard to which many of the newer extraction technologies (which are likely to determine future applications) are referred. However, it should be realised that extraction mechanisms may be different, and thus comparisons are sometimes irrelevant. 

Principles and Characteristics Because of the limited selectivity of extraction, a chromatographic analysis is almost always needed. Recently, a fair amount of progress has been made regarding the front end of the total analysis procedure, namely the integration of sample preparation (this being the analytical bottleneck) and separation. The idea behind such systems is to perform sample extraction, cleanup and concentration as an integral part of the analysis in a closed system. Scheme 7.2 shows the main procedures related to sample preparation for chromatographic analysis. 

Efficiency of Extraction. Selectivity of Extraction. Extraction Systems. Extraction of Uncharged Metal Chelates. Methods of Extraction. Applications of Solvent Extraction. 

The selectivity of extractive separations of metal ions M" is commonly described by the separation factor (SF), defined as the ratio of the distribution ratios, [Eq. (4.3)], of the ions in the same system. In the case of two metal ions A" and Z" extracted from the same aqueous solution by an acidic bidentate extractant HE, with no synergist in the system, one obtains ... 

However, even this simplified formula does not justify the use of the ratio of stability constants of the extracted complexes as the only measure of selectivity of extractive separations. Such a widely used approach is obviously based on an implicit assumption that the partition constants of neutral complexes ML of similar metal ions are similar, so that their ratio should be close to unity. This is, however, an oversimplification because we have shown that the ifoM values significantly differ even in a series of coordi-natively saturated complexes of similar metals [92,93]. Still stronger differences in the values have been observed in the series of lanthanide acetylacetonates, due to different inner-sphere hydration of the complexes (shown earlier), but in this case, self-adduct formation acts in the opposite direction [100,101] and partly compensates the effect of the differences in. Tdm on S T(see also Fig. 4.15). Such compensation should also be observed in extraction systems containing coordinatively unsaturated complexes and a neutral lipophilic coextractant (synergist). 

A simple way to increase the selectivity of extraction techniques is to derivatize the carbonyl compounds. 

As is seen from the presented data, ILs are true designer solvents. The modification of their constituents gives solvents with very different properties, namely, efficiency and selectivity of extraction of various solutes. In the near future, it should have important implications, as more and more finely tuned solvents are likely to appear for a variety of tasks. 

As a consequence, the selectivity of extraction of first transition series dications does not follow the Irving Williams order when these reagents are used in base metal recovery. The bis(2-ethylhexyl)ester of phosphoric acid (D2EHPA) shows [1] a preference Zn2+ > Cr21 > Mn2+ > Fe2+ > Co2+ > Ni2+ V2+ which is exploited in the recovery of zinc from primary sources. [69] M2+ ions which form tetrahedral complexes and M3+ ions which show a preference for octahedral donor sets give neutral complexes with 4 1 and 6 1 D2EHPA metal stoichiometries respectively,... 

Efficiency of extraction. Selectivity of extraction. Extraction systems. Extraction of uncharged metal chelates. Methods of extraction. Applications of solvent extraction. 

Ensuring high-quality analytical performance in trace analysis, if separation of sample components by extraction is indispensable, requires implementation of the appropriate extraction method and establishment of suitable operational parameters to ensure a high efficiency of extraction. Selection of extraction conditions is crucial for quantitative recovery of analyte, or at least for sufficient effectiveness. If an aqueous solution is one of the extraction phases, problems such as complex-ation, hydrolysis, and solvation can play an important role. Extraction of elements from aqueous to organic phase often requires selection of appropriate ligands and control of pH. 

Dean JR and Xiong G. Extraction of organic pollutants from environmental matrices Selection of extraction technique. Trends Anal. Chem. 2000 19 553-564. 

The commercially available fibers include polydi-methylsiloxane (PDMS 100, 30, and 7 pm), PDMS-divinylbenzene (PDMS-DVB 65 pm), polyacrylate (PA 85 pm), carboxen-PDMS (CAR-PDMS 75 and 85 pm), Carbowax-DVB (CW-DVB 65 and 75 pm), Carbowax-templated resin (CW-TPR 50 pm), and DVB-CAR-PDMS (50/30 pm). The type of fiber used affects the selectivity of extraction. In general, polar fibers are used for polar analytes, and nonpolar types are used for nonpolar analytes. Selectivity toward target analytes and interferences can be enhanced by surfaces common to affinity chromatography. Fibers can be reused up to 100 analyses or more depending on the sample matrix, on the care of the analyst, and on the applications for which used. 

In fractional extraction, the feed material is extracted in two or more stages. The selectivity for essentied oils, fatty oils, and resins is controlled in each stage throu selection of extraction pressure, temperature, or cosolvent addition. With the first extraction stage at subcrltical tenperatures, sensitive essential oils are removed at mild conditions and at slx>rt processing times. 

Although a proper choice of solvent improves the selectivity of extraction, a large number of closely related steroids, chromogenic substances, and other nonspecific materials are also extracted with the steroids. Removal of such contaminants, especially those that will interfere in the final estimation, is important when the antibody specificity used to measure the steroid is relatively low. With high specificity and high affinity antibodies, this becomes less of an issue and is now more the rule than the exception. 

From the observation that the partitioning behavior of each protein was not affected by the presence of the others (38, 49), Goklen and Hatton resolved a mixture of cytochrome-c, ribonuclease-A and lysozyme, three low molecular weight proteins comprised in the range 12.4 - 14.3 kDa. With the same reversed micellar phase, Woll et (49) showed that the selectivity of extraction between ribonuclease-A and concanavalin-A could be modulated by vaiying the surfactant concentration a 40% enhancement... 

In addition to the equilibrium reached after 12 h of incubation, two equilibrium-like states were observed during the first 60 min of extraction [66]. It was proposed that the equilibrium-like shape of the extraction profile, found during the first 30 min of extraction, represents the equilibrium between the readily available free 2-cyclopentyl-cyclopentanone on the surface of the polyamide 6.6 powder, the headspace and the SPME-fiber. The second equilibrium-like state probably represents equilibrium for the 2-cyclopentyl-cyclopentanone originally present inside the polyamide 6.6 powder, at a close distance to the outer surface. The equihbrium-like shape of the recovery profile may lead to an erroneous selection of extraction conditions, which in turn would lead to erroneous quantitation. The long equilibrium time reflects the need for rapid methods to estimate the volatile content under non-equihbrium conditions. 

Small drops lead to more transfer area and better extraction, but to slower settling and less capacity. Thus, selection of extraction equipment frequently involves a compromise choice balancing efficiency against capacity. 

This chapter has identified the main extraction techniques used for the extraction of organic analytes from solid matrices, e.g. soil. The main purpose of each technique has been to remove the analyte from the matrix as effectively as possible. In some instances, e.g. supercritical fluid extraction, some attempt is made to achieve selectivity of extraction by altering the operating conditions, such as temperature and pressure, or by the addition of an organic modifier. In other situations, the sole purpose of the extraction technique is to remove the analyte from the matrix under the strongest possible conditions without any concern for selectivity. The selectivity in these circumstances results exclusively from the method of analysis which follows. It should also be borne in mind, depending on the level of contamination of the sample, that further extract pre-concentration and/or clean-up may be required prior to analysis in order to achieve trace level analytical results. The methods used for effective pre-concentration are described later in Chapter 10. 

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