Trimethyl arsenic

Odanake et al. [1] have reported the application of gas chromatography with multiple ion detection after hydride generation with sodium borohydride to the determination of mono and dimethyl arsenic compounds, trimethyl arsenic oxide and inorganic arsenic in soil and sediments. Recoveries in spiking experiments were 100-102% (mono and dimethyl arsenic compounds and inorganic arsenic) and 72% (trimethyl arsenic oxide). 

In the Challenger mechanism or scheme, oxidative methylation of arsenicals containing As(III) produces a methylated product that contains As(V). Because methylation is oxidative, As(V) must be reduced to trivalency before it can be methylated. Hence, the pathway for the formation of mono-, di-, and trimethylated arsenic species consists of alternating oxidation and reduction reactions (Chapter 2). 

Information concerning the methylated form of arsenic in the environment has been scant until recently. Braman and Foreback detected traces of the following arsenicals in natural water, bird eggshells, seashells and human urine dimethylarsinic acid, methylarsonic acid, arsenate and arsenite. They suggested that dimethylarsinic acid was the major and ubiquitous form of arsenic and that methylarsonic acid was also present, but in lower concentration. Edmonds and Francesconi reported the presence of di- and trimethylated arsenic in marine fauna. 

Arsenate was orally administered (1 mg As/kg to rate, mice and hamsters). Urine samples were collected at 24 h intervals and were analysed. Inorganic, mono-, di- and trimethylated arsenic compounds were detected as urinary metabolites among which the lalst one was detected as a new metabolite. These data indicate that inorganic arsenic may be biomethylated up to the trimethylated stage in these animals. 

A similar result was obtained from an experiment in which another food chain (phytoplankton C. vulgaris-graieT Moina sp.-carnivorous guppy Poecilia sp. adult) was examined. Figure 4 shows the relative concentration of arsenic species accumulated and demonstrates that the relative concentration of dimethyl arsenic and trimethyl arsenic compounds dramatically increased successively with an elevation in the trophic level. 

These compounds, combining metal-to-carbon and metal-to-azide bonds, are abundant in the right-hand half of the periodic table (groups VIII to VIA). Most of them, like triethyllead(rV) azide (g), are covalent, while some, like trimethyl-arsenic(V) azide (h), form ions. Most of these compounds do not explode, indi-... 

The extraction efficiency of arsenic from soil and sediments is low (11,119). Because most analytical techniques for determining arsenic species are performed on water-based solutions of the analytes (extractable arsenic), the results obtained for soil and sediments represent only a small proportion of the total arsenic present. Of this extractable portion, the inorganic species arsenate and arsenite dominate (1), although methylarsonate and dimethylarsinate are also found as natural constituents in some soils (43). These four arsenicals are also commonly found in sediments or in the interstitial water (porewater) of the sediments (45,134,135), and a trimethylated arsenic species, possibly trimethylarsine oxide, has also been detected in some sediment porewater samples (135). 

The unknown compound tri-t-butyl arsenate has now been produced by transesterification of trimethyl arsenate with a large excess of dry t-butyl alcohol, but neither transesterification nor other conventional approaches gave the interesting, sterically hindered arsenite (Ph3CO)3As. The method successfully used was the reaction between arsenic trichloride and Ph3CONa in ether. 

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