Separation and preconcentration of elements

The present Section provides a discussion of the following separation and preconcentration methods solvent extraction, precipitation and co-precipitation with collectors, volatilization, and methods based on the use of ion-exchangers and other sorbents. These methods are used not only with spectrophotometry, but also in conjunction with other methods of determination. 

The extraction process and extractive methods for separation and preconcentration of elements are described in several monographs and reviews [5,6]. 

Solvent extraction separation is based on differences in the solubilities of elements and their compounds between two immiscible liquid phases. Usually, the initial phase is an aqueous solution and the second phase is an organic solvent, immiscible with water. Some properties of the more common organic solvents are listed in Table 1.1. The ion to be extracted into the non-aqueous phase should first be transformed into an uncharged species. 

Stripping ( re-extraction , back-extraction , or scrubbing ) involves bringing the element from the organic extract back into the aqueous phase. 

The extraction efficiency, i.e., the degree of transfer of the speeies from the aqueous to the organic phase, is defined in terms of the distribution- (or extraction-) coefficient, (D). The quantity D is the ratio of total concentration (i.e., the concentration of all the existing forms) of the element in the organic phase (Sco) to the total concentration in the aqueous phase (2cw) in the aqueous phase, at equilibrium 

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Another study showed that PEI can also serve as a useful polymeric agent for separation and preconcentration of elements [43],... 

Recent applications of ion-exchange and chelating resins for separation and preconcentration of elements prior to the spectrophotometric detection have been described [36-41]. 

Sorption on membranes immobilized with azorhodanine reagents and sulfonitrophenol M [42], fibres of poly(acrylonitrile)-carboxylated polyethylene-polyamine [43] and a filter-paper containing chemically attached hexamethylenediamino groups [44] have recently been employed for separation and preconcentration of elements. 

Geckeler, K.E., Bayer, E., Spivakov, B.Y., Shkinev, VM. Vorob Eva, G.A. (1986) Liquid-phase polymer-based retention, a new method for separation and preconcentration of elements. Analytica ChimicaActa, 189, 285-292. 

Separation and preconcentration of PGMs prior to detection are generally required in the analysis of complex environmental samples because of the low concentrations to be determined and large excess of matrix elements interfering with detection [34-36, 65, 66]. Liquid chromatography (LC) using ion-exchange... 

Inorganic ion exchangers have not been applied widely, but in some cases they can be useful in separation and preconcentration of trace elements. They include hydrous oxides and acid salts of multivalent metals, heteropoly acids (e.g., phosphomolybdenic acid), sparingly soluble ferrocyanides, and synthetic aluminosilicates (zeolites) [198,199]. 

Palagyi S and Braun T (1992) Separation and preconcentration of trace elements and inorganic species on solid polyurethane foam sorbents. In Alfassi ZB and Wai CM, eds. Preconcentration Techniques for Trace Elements, pp. 363-400. CRC Press,... 

Terada K (1994) Separation and preconcentration of trace elements. In Alfassi ZB, ed. Determination of trace elements, pp. 109-144. VCH Verlags-gesellschaft mbH, Weinheim. 

The application of the Chelex 100 resin separation and preconcentration, with the direct use of the resin itself as the final sample for analysis, is an extremely useful technique. The elements demonstrated to be analytically determinable from high salinity waters are cobalt, chromium, copper, iron, manganese, molybdenum, nickel, scandium, thorium, uranium, vanadium, and zinc. The determination of chromium and vanadium by this technique offers significant advantages over methods requiring aqueous final forms, in view of their poor elution reproducibility. The removal of sodium, chloride, and bromide allows the determination of elements with short and intermediate half-lives without radiochemistry, and greatly reduces the radiation dose received by personnel. This procedure was successfully applied in a study of... 

Chemical separation of matrix and preconcentration of analytes is used to avoid matrix effects, clogging effects on the cones and disturbing interferences of analyte ions with polyatomic ions of matrix elements and plasma gases. A trace/matrix separation method is required for ultratrace analysis. 

For analysis of solutions, ICP-mass spectrometry (ICP-MS) is very promising (Houk et al., 1980 Houk, 1986 Bacon et al., 1990). Recent advances in separation and preconcentration techniques are discussed by Horvath et al. (1991). Bacon et al. (1990) report that although ICP-MS is a multi-element technique, recent papers tend to concentrate on a small number of target elements. With isotope dilution mass spectrometry (IDMS), detection limits are further reduced (Heumann, 1988) IDMS is also suitable for accurate speciation in very low concentration levels of elements (Heumann, 1990). For the direct analysis of solid samples, glow discharge mass spectrometry (GD-MS) (Harrison etal., 1986) is of interest. Tolg (1988) has suggested that a substantial improvement in the absolute detection power of GD-MS, as applied to micro analysis, can be expected, at least in comparison with the ICP as ion source. 

ICP-MS dominates the field of environmental assay most metallic and amphoteric elements are susceptible of analysis, and it is often a great convenience (to say nothing of being relatively economical) to be able to assay for all elements of interest in a single analysis. Another feature of ICP-MS, however, has been exploited perhaps even more tellingly than multielement analysis. This is that the sample introduction system lends itself to a wide variety of enhancement schemes, in part because the sample is introduced to the instrument at atmospheric pressure and in part because samples are most often in a water-based (dilute acid) medium. These attributes combine to allow various separation and preconcentration schemes to be implemented on-line or nearly so. 

It is little wonder, then, that analysts working in environmental laboratories were amongst the first to grasp the importance of Walsh s benchmark papers.3,4 Hitherto the complex nature of their typical sample matrix and the nature of the elements they were interested in often resulted in the need for complex and time consuming separation and preconcentration by skilled chemists prior to the determination step. Suddenly they were apparently being offered a very sensitive and almost universally applicable technique where the only sample preparation needed seemed to be sample dissolution. Eventually problems did start to surface, as is invariably the case, but as Chapter 3 will show, few have proved insuperable. 

Chapters 1-4 provide the characteristics of the separation and preconcentration methods solvent extraction, flotation, coprecipitation with collectors, volatilization, ion exchange etc. These chapters deal also with the fundamentals of spectrophotometry, spectrophotometric methods of analysis, and most important chromogenic reagents. Chapters 5-57 have been devoted to individual elements or groups of related elements. 

Separation and preconcentration procedures recently employed for spectrophotometric determination of particular elements in various materials are discussed in Appendices to Chapters 5-57. 

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