Other selective extraction methods

In recent years the use of ion-exchange membranes for the selective extraction of heavy metals from soil suspensions and solutions have been developed, for example, for Cd, Cr, Ni and Pb (Liang and Schoenau, 1996). Extracts of Cd, Cu and Pb by this method were shown to be better correlated with wheat seedling uptake than chemical extraction with 0.1 moll-1 HC1, 0.01 moll-1 CaCl2 or 0.005moll-1 DTPA (Lee and Zhang, 1994). 

Extraction procedures have also been developed for the determination of the anionic species in soils of elements such as sulfur which are important as binding sites for metals as well as for its own mobility and availability (Cordos et al., 1995). The important biosignificant element selenium has similarly received attention (Blaylock and James 1993 Seby et al., 1997) and procedures for the speciation of phosphorus have been developed (Vaz et al., 1992 Chapman et al., 1997). 

Sophisticated analytical procedures for the speciation of chromium (Prokisch et al., 1995) and platinum (Michalke et al., 1997) in the soil solution or soil extracts have also been described. 

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Other selective extraction methods applied to some pesticides, such as molecularly imprinted polymer extraction, could be extended to organophosphoms compounds. 

In addition to their use in the functional speciation role, selective extraction methods have been used to target element species in soil, or elements bound to, or associated with, particular soil phases or compounds. Examples include the use of extractants to release, for determination, metals on exchange sites, or metals bound or associated with soil iron or manganese oxyhydroxides or with soil organic matter. Most of these extractants are, however, less specific than intended and may extract species from other phases. Such extractants, however, are commonly, and conveniently, designated by their target species, e.g. extractable metal species or carbonate-bound species, but should more strictly be regarded as examples of speciation in which the species are operationally defined, i.e. by the method used to isolate them. 

Hydrometallurgical recovery processes operate on a broader variety of waste products and fi equently recover other metals in addition to cadmium. They generally employ dissolution by acid treatment followed by selective extraction methods such as precipitation or ion exchange to separate the products. The economics in the... 

Logsdail (1983) considered that the main advantages of the technique over other solvent extraction methods were that it was more versatile, suitable for the extraction of heat-sensitive materials, highly selective, extracted low-volatile components easily and allowed easy recovery of solvent. 

Separation and Purification of Isomers. 1-Butene and isobutylene caimot be economically separated into pure components by conventional distHlation because they are close boiling isomers (see Table 1 and Eig. 1). 2-Butene can be separated from the other two isomers by simple distHlation. There are four types of separation methods avaHable (/) selective removal of isobutylene by polymeriza tion and separation of 1-butene (2) use of addition reactions with alcohol, acids, or water to selectively produce pure isobutylene and 1-butene (3) selective extraction of isobutylene with a Hquid solvent, usuaHy an acid and (4) physical separation of isobutylene from 1-butene by absorbents. The first two methods take advantage of the reactivity of isobutylene. Eor example, isobutylene reacts about 1000 times faster than 1-butene. Some 1-butene also reacts and gets separated with isobutylene, but recovery of high purity is possible. The choice of a particular method depends on the product slate requirements of the manufacturer. In any case, 2-butene is first separated from the other two isomers by simple distHlation. 

Although on-line sample preparation cannot be regarded as being traditional multidimensional chromatography, the principles of the latter have been employed in the development of many on-line sample preparation techniques, including supercritical fluid extraction (SFE)-GC, SPME, thermal desorption and other on-line extraction methods. As with multidimensional chromatography, the principle is to obtain a portion of the required selectivity by using an additional separation device prior to the main analytical column. 

Principles and Characteristics A good extraction method, in addition to quantitative recoveries, ensures a high degree of selectivity. The demands for quantitative and selective extraction are difficult to meet simultaneously. Quantitative extraction with recoveries being independent of the matrix requires a strong extraction fluid. On the other hand, selective extraction is usually associated with fairly mild extraction conditions where only the components of interest are dissolved... 

The use of the Hildebrand solubility parameter approach to aid solvent selection with a few simple experiments, starting from the liquid solvents used in traditional extraction methods, limits the efforts needed in method development. As for other extraction... 

David et al. [184] have shown that cool on-column injection and the use of deactivated thermally stable columns in CGC-FID and CGC-F1D-MS for quantitative determination of additives (antistatics, antifogging agents, UV and light stabilisers, antioxidants, etc.) in mixtures prevents thermal degradation of high-MW compounds. Perkins et al. [101] have reported development of an analysis method for 100 ppm polymer additives in a 500 p,L SEC fraction in DCM by means of at-column GC (total elution time 27 min repeatability 3-7 %). Requirements for the method were (i) on-line (ii) use of whole fraction (LVI) and (iii) determination of high-MW compounds (1200 Da) at low concentrations. Difficult matrix introduction (DMI) and selective extraction can be used for GC analysis of silicone oil contamination in paints and other complex analytical problems. 

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

One popular method of separating an analyte species from a complicated liquid sample is the technique known as liquid-liquid extraction or solvent extraction, first mentioned in Chapter 2. In this method, the sample containing the analyte is a liquid solution, typically a water solution, that also contains other solutes. The need for the separation usually arises from the fact that the other solutes, or perhaps the original solvent, interfere in some way with the analysis technique chosen. An example is a water sample that is being analyzed for a pesticide residue. The water may not be a desirable solvent and there may be other solutes that may interfere. It is a selective dissolution method—a method in which the analyte is removed from the original solvent and subsequently dissolved in a different solvent (extracted) while most of the remainder of the sample remains unextracted, i.e., remains behind in the original solution. 

Ideally, the pollutants to be determined should be removed from the matrix as completely as possible with a minimum amount of the other non-target components. This type of selectivity was certainly anticipated from supercritical fluid extraction. However, trace organic pollutants cover a wide range of polarity, volatility, and molecular size, making selective extraction very difficult to achieve. Currently the most popular extraction methods are Soxhlet [191,400, 402-404], blending [189, 408, 409, 411-455], liquid column extraction and ultrasonic extraction [456], and more recently supercritical fluid extraction [386,456-463]. 

Phosphate must be applied as fertilizer to the soil. Ideally it is added in quantities sufficient to guarantee optimal yields, but not in excess in order to avoid P transportation into other compartments of the ecosystem. The amount added should be based on an accurate estimation of the plant-available fraction of P already present in a soil.This is an old and difficult task and a large number of extraction methods have been used since intensive land use was practised. Recently methods have been worked out in which a strip of filter paper impregnated with an Fe oxide (2-line ferri-hydrite) is dipped into a soil suspension and the amount of P adsorbed by the paper is taken as being plant-available (Sissingh,1988 Van der Zee et ah, 1987 Sharpley, 1993 Sharpley et ah,1994 Kuo and Jellum, 1994 Myers et ah 1997). Anion and cation resins extracted more P from four heavily fertilized soils than from goethite (Delgado Torrent, 2000). Other oxyanions adsorbed by soil Fe oxides are silicate, arsenate, chromate, selenite ( ) and sulphate. Adsorption of sulphate led to a release of OH ions and was substantially lowered once the Fe oxides were selectively removed (Fig.16.17). 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

Solvents used here for a general liquid-liquid extraction method were selected from Snyders solvent selectivity triangle. As extraction liquids have to be composed of mixtures of three solvents which may enter into maximum interaction with the analyte, three solvents had to be selected that represent a wide variety of selective interactions. In addition, the solvents should be sufficiently polar to ensure quantitative extraction. Besides selectivity and polarity requirements, the solvents should also meet a few other criteria, mainly for practical reasons they should not be miscible with water, have low boiling points (for relatively fast evaporation procedures) and have densities sufficiently different from the density of water, for pure solvents as well as for selected binary or ternary mixtures of solvents. 

This method can be used when the enantiomers of interest are not coeluting with other compounds in the sample and when accurate quantitative information is not the highest priority of the analysis. The sample will have been prepared by an extraction method selected from those in unitgi.i and should have a concentration of 50 to 100 ppm. The identity of the components of the sample should be known from gas chromatography-mass spectrometry (GC-MS) together with their retention indices on the achiral stationary phase. Additional sample cleanup procedures may be needed to ensure the optimum results that are evaluated below ... 

For a compound to contribute to the aroma of a food, the compound must have odor activity and volatilize from the food into the head-space at a concentration above its detection threshold. Since aroma compounds are usually present in a headspace at levels too low to be detected by GC, headspace extraction also requires concentration. SPME headspace extraction lends itself to aroma analysis, since it selectively extracts and concentrates compounds in the headspace. Some other methods used for sample preparation for aroma analysis include purge-and-trap or porous polymer extraction, static headspace extraction, and solvent extraction. A comparison of these methods is summarized in Table Gl.6.2. 

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