Organometallic species arsenic

The application of these techniques has led to the discovery of a number of organometallic species of arsenic, tin, and antimony in the marine environment. Germanium has not been observed to form organometallic compounds in nature. Some aspects of the geochemical cycles of these elements which have been elucidated by the use of these methods are discussed. 

Arsenic can exist in several oxidation states, as both inorganic and organometallic species, and in dissolved and gaseous phases (Table I). Dissolved arsenic species can adsorb to suspended solids and be carried down to the sediments in an aquatic system. Since gaseous arsenic compounds can form, arsenic can be removed from the sediments as dissolved gas or in gas bubbles (e.g. CH ). Thus, arsenic can cycle within aquatic ecosystems and this cyclic behavior has been reviewed by Ferguson and Gavis (1 ) and Woolson 2). In any given system, it is necessary to understand the behavior of a variety of different arsenic compounds as well as a variety of environmental compartments in order to totally characterize the cyclic behavior of this element. 

A considerable number of organometallic species of arsenic, antimony and bismuth have been detected in the natural environment in different manners. A number of these are nonmethyl compounds which have entered the environment after manufacture and use [e.g. butyltin and phenyltin compounds for antifouling paints on boats, and arsanilic acid (Figure 2, 5) and phenylarsonic acids (Figure 2, 6-8) for animal husbandry]. Only a few methyl compounds are now manufactured and used (e.g. methyltin compounds for oxide film precursors on glass and methylarsenic compounds for desiccants or defoliants). 

A range of chromatographic techniques coupled to element specific detectors has been used in speciation studies to separate individual organometallic species (e.g., butyltins, arsenic species) and to separate metals bovmd to various biomolecules. The combination of a chromatographic separation with varying instrumental detection systems are commonly called coupled, hybrid, or hyphenated techniques (e.g., liquid chromatography inductively coupled plasma-mass spectrometry (LC-ICP-MS), gas chromatography-atomic absorption spectroscopy (GC-AAS)). The detection systems used in coupled techniques include MS, ICP-MS, atomic fluorescence spectrometry (AFS), AAS, ICP-atomic emission spectrometry (ICP-AES), and atomic emission detection (AED). 

For organometallic species, the analytical challenge involves quantitative extraction of the species from the solid phase without appreciable transformation to other forms. Various liquid extractants have been used such as methanolic hydrochloric acid (extraction of organotins). Strong alkali digests (KOFI or tetramethylammonium hydroxide) will dissolve most tissue without breakdown or transformation of arsenic species. As an alternative to liquid extraction, methylmercury species may be extracted from solids using steam distillation. Specific reaction conditions will affect the recovery of various species and rigorous validation of extraction procedures is therefore recommended. 

Among the most powerful tools in the synthetic organic chemists arsenal are organometallic compounds and metal-mediated transformations. The literature involving organometallic intermediates has literally exploded in the past 30 years, and the rate of publication in this area continues to accelerate. While many transformations involve heterogeneous conditions (metal-solution interface), the majority of these reactions involve discrete organometallic species. 

The application of high temperatures to increase the speed of HPLC separation extends to ion chromatography and to inorganic analysis. Le et al. [18,19] reported a 50% reduction in analysis time when a number of selenium and arsenic species including inorganic forms, organometallics, and compounds with amino acids and sugars were analyzed at 70°C. 

The initial decomposition of Ga(CH3)3 involves the loss of methyl radicals (114-121). These methyl radicals can subsequently react with H2 or the arsenic source, such as AsH3, abstract H from an organometallic or hydrocarbon species, or recombine. The reaction with H2 leads to H radicals that can react with the parent organometallic compound to accelerate its decomposition. The following are some of the reactions ... 

Lee, J.S. and Nriagu, J.O. (2007) Stability constants for metal arsenates. Environmental Chemistry, 4(2), 123-33. Lehr, C.R., Polishchuk, E., Radoja, U. and Cullen, W.R. (2003) Demethylation of methylarsenic species by Mycobacterium neoaurum. Applied Organometallic Chemistry, 17(11), 831-34. 

Santosa, S.J., Wada, S., Mokudai, H. and Tanaka, S. (1997) The contrasting behaviour of arsenic and germanium species in seawater. Applied Organometallic Chemistry, 11(5), 403-14. 

Over the past several years, the area of gas-phase transition metal ion chemistry has been gaining increasing attention from the scientific community [1-16]. Its appeal is manifold first, it has broad implications to a spectrum of other areas such as atmospheric chemistry, corrosion chemistry, solution organometallic chemistry, and surface chemistry secondly, an arsenal of gas phase techniques are available to study the thermochemistry, kinetics, and mechanisms of these "unusual" species in the absence of such complications as solvent and ligand... 

Capillary electrophoresis (CE) provides high resolution for separation of chemical compounds. Separations of metal ions, of metal ions in different oxidation states and of organometallic compounds are all possible with appropriate CE conditions. This technique is being investigated for speciation. Since sample volumes in CE are generally very small, a detector capable of very low detection limits is desirable. Thus, ICP-MS has been combined with CE to provide a means for studying metal speciation. CE-ICP-MS procedures have been described for the separations of platinum species (Michalke and Schramel, 1996), selenium species (Kumar et al., 1995 Michalke and Schramel, 1996) and arsenic species (Magnuson et al., 1997). Detection limits were about 1 mgl 1 (platinum species) and 10 and 24 pg for Sclv and Scvl, respectively. An application of CE-ICP-MS to platinum species in soils is described in Section 15.8.6. 

The separation of organomercury was conducted by using a SB-methyl-100 capillary column and pure CO2 as the mobile phase. FID and atomic fluorescence were used for detection. The same column was also used for separation of mercury, arsenic, and antimony species using carbon dioxide as the mobile phase. A chelating reagent, bis(trifluoroethyl)dithiocarbamate, was used in this case to convert the metal ions to organometallic compounds before the separation. The detection limit of FID was 7 and 11 pg for arsenic and antimony, respectively. 

Arsenic chemistry is complex, involving a variety of oxidation states, both as anionic and cationic species, and both inorganic and organometallic compounds. Of these, III and V are the most common oxidation states. The oxidation states of arsenic change easily and reversibly. As(III) is commonly encountered as the arsenite ion, H2ASO3 . Arsenious acid is a weak acid, p/Cai = 9.2, pK. 2 = 13. 

Devalla S and Feldmann J (2003) Detetmination of lipid-soluble arsenic species in seaweed-eating sheep from Orkney. Applied Organometallic Chemistry 17 906-912. 

Many analytes may form volatile species with other elements in the sample, for example, halides, or be present in compounds that exhibit high-vapor pressures at relatively low temperatures (mercury, arsenic, selenium, organometallics). Such compounds may be volatilized and swept from the tube, in molecular form, prior to the atomization step. These losses can be dealt with by adding a large excess of a reagent (a modifier) to change, in situ, the thermochemical behavior of the analyte and the matrix. 

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