Selected Extraction Methods

During selection of the extraction methods to include in this chapter we considered the popularity of the method, maturity, simplicity of application in laboratory routine, cost, and prospects for development by further modification. New promising extraction modifications can be found in numerous current review papers. We 

The term solid-phase extraction was introduced by personnel of the J. T. Baker Company in 1982. The method consists of retention of the analytes from a liquid or gaseous sample to a solid stationary phase and subsequent removal of analytes using an appropriate eluent. The main purpose of SPE is isolation and preconcentration of compounds of interest or sample clean-up and simplification of the matrix. Application of this sample preparation technique also allows extract fractionation. As a result of significant reduction in the volume of organic solvents used, high recovery, and the possibility of process automation, SPE is a good alternative for conventional liquid-liquid extraction. According to their affinity for the compound of interest, stationary phases are classified as follows  

Many novel sorbents have been invented for simplification and improvement of 

Silica gel modified with carbon chains, including the most popular, -Cig (octadecylsilica), in which alkyl groups have 18 atoms of carbon, are usually applied in reversed phase separations. Different alky or aryl groups are used for modification of silica, such as -C2, -C4, -Cg, -CN, and -NH2. All these functional groups are hydrophobic therefore, retention of analytes is a result of nonpolar-nonpolar attractive forces or dispersion forces. Sorbents of silica gel modified with carbon chains are stable over a narrow pH range. 

For example, Sephadex adsorbent is used for separation of low and high molecular weight molecules. 

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Another way of applying the selective extraction method directly on the initial solution is to produce a solution of low acidity. This can be achieved by using the hydrofluoride method for fluorination and decomposition of raw material. As was discussed in Paragraph 8.2.2, the raw material is fluorinated by molten ammonium hydrofluoride yielding soluble complex fluorides of ammonium and tantalum or niobium. The cake obtained following fluorination is dissolved in water, leading to a solution of low initial acidity that is related for the most part to the partial hydrolysis of complex fluoride compounds. The acidity of the solution is first adjusted to ensure selective tantalum extraction. In the second step, the acidity of the raffinate is increased to provide the necessary conditions for niobium extraction. 

Prior work has been concentrated mostly on the isolation of distinct NSO compound classes from soluble organic matter. Often, selective extraction methods are applied as described for the isolation of, for example, phenols, [39,40] fatty acids [41] and basic nitrogen compounds such benzoquinolines... 

The most useful chemical species in the analysis of arsenic is the volatile hydride, namely arsine (AsH3, bp -55°C). Analytical methods based on the formation of volatile arsines are generally referred to as hydride, or arsine, generation techniques. Arsenite is readily reduced to arsine, which is easily separated from complex sample matrices before its detection, usually by atomic absorption spectrometry (33). A solution of sodium borohydride is the most commonly used reductant. Because arsenate does not form a hydride directly, arsenite can be analyzed selectively in its presence (34). Specific analysis of As(III) in the presence of As(V) can also be effected by selective extraction methods (35). 

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. 

Pellegrino, C., P. Massanisso, and R. Morabito. 2000. Comparison of twelve selected extraction methods for the determination of butyl- and phenyltin compounds in mussel samples. Trends Anal. Chem. 19 97-106. 

The pH of the steam distillate was 6.2 in all cases. The total flavor isolate (by SDE) of plain cashews had, on the whole, a strong pungent and green aroma, reminiscent of the cashew nut testa and cashew shell, whereas the flavor isolates from roasted samples had the characteristic mildly nutty aroma also. The flavor fractions obtained by selective extraction method gave some information about the chemical nature of compounds responsible for the characteristic flavor notes. Accordingly, the basic fraction of roasted nuts, (both oven-and oil-roasted), had the typical nutty aroma associated with pyrazine. compounds. The basic fraction of plain cashews did not have any characteristic flavor in particular. 

Preliminary GC analysis revealed that there are more number of peaks in roasted samples compared to plain cashews. Also selective extraction method was found to be slightly superior to the SDE method under the conditions of the experiment adopted in this study. However, the compounds in oven-roasted and oil-roasted samples did not differ much, qualitatively and quantitatively. In total, 26 compounds have been identified in plain cashews and 3 compounds in roasted samples. The identified peaks constituted 70 percent of the total peaks registered in GC analysis of the individual samples. The descriptive flavor profile of the eluting peaks of the plain and roasted samples were studied. Since the flavor isolate from oven roasted cashew nuts contained the flavor components of plain cashews also and since analysis showed that there was not much difference between the flavor constituents of oven-roasted and oil-roasted samples, the aromagram of the oven-roasted cashew nuts (SDE) was taken as representative. Fig.l gives the GC profile of oven roasted cashew nuts and the sensory properties of the numbered peaks are included in Table III. 

The MMA homopolymer and the unreacted PVA are removed from the reaction product by the selective extraction method. The grafted branch PMMA is separated from the backbone by oxidative cleavage of all 1,2-glycols of PVA (about 2 mole%)29 Molar masses of the isolated graft copolymers and the separated branches are osmo-metrically determined, after acetylation of hydroxyl groups in benzene. The chemical composition of the graft copolymers is determined from the saponification value of the acetylated sample. 

Sheppard, M. I., and Stevenson, M. (1997). Critical evaluation of selective extraction methods for soils and sediments. In Contaminated Soils (R. Prost, ed.). Proceedings of 3rd International Conference on the Biogeochemistry of Trace Elements, May 15-19, 1995, pp. 69-97. INRA, Paris, France. 

Table 9.1 Selected extraction methods for flavanones from citrus species. 

D. Dong, X. Hua, Y. Li, and Z. Li, Lead adsorption to metal oxides and organic material of freshwater surface coating determined using a novel selective extraction method, Environl. Pollut. 119 (2002) pp. 317-321. 

Selective extraction methods using two-phase solvent systems may also be used to enrich fatty acids. Solvent systems such as dimethyl formamide, hexane, and ethylene glycol can form a two-phase system that effectively partitions sunflower oil TAGs rich in linoleic acid from those depleted in linoleic acid (111). TAGs partitioned in this way may contain up to 84.7% linoleic acid. This method would not likely be used in industry because the magnitude of the losses are usually unacceptable. 

The use of water-immiscible solvents for increase of sensitivity led to the development of the selective extraction method by Dean and his... 

Interference by Pd is prevented by prior separation with dimethylglyoxime, either as extractant or as precipitant. In the selective extraction method, Pd does not interfere since its dithizonate is formed rather slowly [Pd(HDz)2 is grey-green]. Before mercury is determined, gold and platinum should also be separated, e.g., by selective reduction to the elements. 

Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

To understand the relevance of selective extraction methods for soil studies. 

Table 5.7 Selective-extraction methods which are diagnostic of plant uptake [26]. Reprinted from Sci. Total Environ., 178, Ure, A. M., Single extraction schemes for soil analysis and related applications , 3-10, Copyright (1996), with permission from Elsevier...

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... 

Determination, offline GC-FID or FPD-P recoveries (1) >95% of hydrocarbons (2) quantitative extraction of phosphonates (3) phosphonic acids global recoveries are close to 80%, a loss of about 20% occurring during the derivatization process successive implementation of three extraction steps giving a selective extraction method... 

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

Selective extraction method of polycyclic aromatic compounds from tobacco smoke condensate and quantitative determination by HPLC-UV and fluorescence detection ... 

The separation of the mineral matter from the binder solution is conducted with various ways, depending on the selected extraction method. 

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