Chemical separation Speciation

Instrumental Quantitative Analysis. Methods such as x-ray spectroscopy, oaes, and naa do not necessarily require pretreatment of samples to soluble forms. Only reUable and verified standards are needed. Other instmmental methods that can be used to determine a wide range of chromium concentrations are atomic absorption spectroscopy (aas), flame photometry, icap-aes, and direct current plasma—atomic emission spectroscopy (dcp-aes). These methods caimot distinguish the oxidation states of chromium, and speciation at trace levels usually requires a previous wet-chemical separation. However, the instmmental methods are preferred over (3)-diphenylcarbazide for trace chromium concentrations, because of the difficulty of oxidizing very small quantities of Cr(III). 

Many different separation and detection systems have been used for speciation. For example, size fractionation and ultra-filtration have been used for separation with the separated species then being determined by neutron activation (Tanizaki et al., 1992). These physico-chemical separation processes are, however, time consuming and the species have to be collected and then determined separately. Although the techniques are invaluable for certain types of speciation where the interaction of the species with colloids and sediments is important, hybrid or coupled techniques are usually preferred. 

Speciation Due to the fact that transactinide nuclei are detected after chemical separation via their nuclear decay, the speciation cannot be determined. Currently, the speciation in all transactinide chemistry experiments has to be inferred by carefully studying the behavior of lighter, homologue elements. The chemical system must be chosen in such a manner that a certain chemical state is probable and stabilized by the chemical environment. 

Pre-irradiation chemical separations followed by activation analysis are also performed for speciation studies. Speciation cannot be done after irradiation, since the chemical form can be changed due to recoil effects under the drastic conditions of nuclear reactions. This type of analysis is called chemical or molecular activation analysis (CAA). 

Speciation studies about the chemical nature of trace elements have recently attracted much attention. Chemically separated fractions can be analyzed by NAA (both INAA and RNAA). Out of the increasing number of publications, two examples are mentioned here, one for biological investigations and the other for environmental investigations. 

Today it has become clear that the effect of trace elements in living systems, in food, and in the environment depends on the chemical form in which the element enters the system and the final form in which it is present. The form, or species, clearly governs its biochemical and geochemical behaviour. lUPAC (the International Union for Pure and Applied Chemistry) has recently set guidelines for terms related to chemical speciation of trace elements (Templeton et al. 2000). Speciation, or the analytical activity of measuring the chemical species, is a relatively new scientific field. The procedures usually consist of two consecutive steps (i) the separation of the species, and (2) their measurement An evident handicap in speciation analysis is that the concentration of the individual species is far lower than the total elemental concentration so that an enrichment step is indispensable in many cases. Such a proliferation of steps in analytical procedure not only increases the danger of losses due to incomplete recovery, chemical instability of the species and adsorption to laboratory ware, but may also enhance the risk of contamination from reagents and equipment. 

Speciation involves a number of discrete analytical steps comprising the extraction (isolation) of the analytes from a solid sample, preconcentration (to gain sensitivity), and eventually derivatisation (e.g. for ionic compounds), separation and detection. Various problems can occur in any of these steps. The entire analytical procedure should be carefully controlled in such a way that decay of unstable species does not occur. For speciation analysis, there is the risk that the chemical species can convert so that a false distribution is determined. In general, the accuracy of the determinations and the trace-ability of the overall analytical process are insufficiently ensured [539]. 

Chemical and physical properties of the contaminant should also be investigated. Solubility in water (or other washing fluids) is one of the most important physical characteristics. Hydrophobic contaminants can be difficult to separate from the soil particles and into the aqueous washing fluid. Reactivity with wash fluids may, in some cases, be another important characteristic to consider. Other contaminant characteristics such as volatility and density may be important for the design of remedy screening studies and related residuals treatment systems. Speciation is important in metal-contaminated sites. 

The development of sensitive and rapid analytical schemes for the extraction and separation of inorganic tin and organic tin compounds and their chemical speciation products from water, sediments, and biological materials (WHO 1980 Reuhl and Cranmer 1984 Hall and Pinkney 1985 Laughlin and Linden 1985 Thompson et al. 1985 Blunden and Chapman 1986 USPHS 1992). 

Although Se and S are similar chemically, their redox speciation is different enough so that decoupling of Se from S can occur. This is illustrated in the Eh-pH stability diagrams for Se and S given in Figure 1. Under moderately reducing conditions, Se is stable as Se(I V) or Se(0), whereas S(tV) is not stable at all, and S(0) is stable only under a restricted set of conditions. Thus, Se may be separated from S if it is precipitated as Se(0), for example. 

Rosenberg, E. (2003). The potential of organic (electrospray- and atmospheric pressure chemical ionization) mass spectrometric techniques coupled to liquid-phase separation for speciation analysis. /. Chromatogr. A 1000, 841 — 889. 

Several different types of chromatography have been coupled with atomic spectrometric detectors. Most applications involving chromatography coupled with atomic spectrometry yield speciation data, i.e. they separate different chemical forms of an analyte. 

The guiding principles for the selection or development of speciation procedures are similar to those recommended for other forms of chemical analysis. For example, the initial step should be careful definition of the problem, including listing of the analytical specifications (e.g. type of analysis, concentration range, potential sources of error). This step can be followed by selection of a suitable measurement procedure, nomination of a selective separation procedure (if required) and organisation of the total protocol. 

In water studies it is standard practice to filter the sample soon after collection, usually through a 0.45p,m membrane disc (made of cellulose acetate, cellulose nitrate or polycarbonate). This process arbitrarily divides the sample components into soluble and insoluble fractions, but as shown in Table 2.3, the average size of different chemical species varies widely, and some differentiation between species can be obtained through using filter media of different pore sizes. For example, fully dissolved compounds can be separated from finer colloidal forms by using gel filtration and dialysis, and sub-division of the total content into fractions based on particle or molecular size (see Section 2.3) has been used for speciation of elements in waters. 

HPLC units have been interfaced with a wide range of detection techniques (e.g. spectrophotometry, fluorimetry, refractive index measurement, voltammetry and conductance) but most of them only provide elution rate information. As with other forms of chromatography, for component identification, the retention parameters have to be compared with the behaviour of known chemical species. For organo-metallic species element-specific detectors (such as spectrometers which measure atomic absorption, atomic emission and atomic fluorescence) have proved quite useful. The state-of-the-art HPLC detection system is an inductively coupled plasma/MS unit. HPLC applications (in speciation studies) include determination of metal alkyls and aryls in oils, separation of soluble species of higher molecular weight, and separation of As111, Asv, mono-, di- and trimethyl arsonic acids. There are also procedures for separating mixtures of oxyanions of N, S or P. 

The speciation or determination of the chemical nature of an element involves two major processes separation of mixtures and identification of their components. In some cases, adequate identification of the nature of chemical species may be obtained from separation data alone, whereas complete quantitative determination of molecular species requires an initial separation followed by purification of individual compounds. 

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