Dimethylarsenate

Chemical Designations - Synonyms Hydroxydimethylarsine Oxide Dimethylarsenic Acid Ansar SilvisarSlO Chemical Formula (CH3)2AsOOH. 

As(III), As(V), monomethyl arsonic acid, and dimethylarsenic acid, which gives a quick and reliable estimation of exposure to inorganic arsenic. 

Inorganic arsenicals such as arsenic trioxide, sodium arsenite, lead arsenate, calcium arsenate and Paris Green have been used for many years as soil sterilants. Organic arsenical herbicides, in which the organic group is bonded directly to the arsenic atom, have been used extensively for post-emergence control of weeds in cotton. Several of the more important herbicides are sodium cacodylate (monosodium dimethylarsenic acid) and sodium salts of methane arsonic acid. The latter compounds exist in two principal forms the monosodium salt (MSMA) at pH6.4 and the disodium salt (DSMA) at pH10.2. 

Maher [13] has described a procedure for the determination of inorganic arsenic, monomethylarsenic and dimethylarsenic in marine organisms and estuarine sediments. The arsenic species are isolated by solvent extraction, separated by ion-exchange chromatography and selectively determined by arsine generation. Recoveries of spikes of 5 and lOpg of arsenic taken through the whole procedure were 92-96%. 

Prepare a 100 ng mpi mixed standard solutions of arsenic by serial dilution of 1000 pg mpi stock solutions of dimethylarsenic (DMA), monomethylarsenic (MMA), arsenic(III) and arsenic(V)... 

The use of arsenic and its organic derivatives as herbicides, pesticides, and wood preservatives has been increasing steadily each year. Large quantities of arsenical compounds are manufactured by the chemical industry and eventually find their way into the environment (l ). About seventy percent of these chemicals are inorganic in form and the rest are organoarsenicals (2). Of the organoarsenicals, the most important species from the point of view of use and health effects are monomethylarsonic acid (MMA), dimethylarsenic acid (DMA), and p-aminophenylarsonic acid (p-APA). 

This provides a route to 1,2-dihydro-arsenins and -antimonins. With antimonin the 1,2-dihydroantimonin was intermixed with the 1,4-analogue. It could not be isolated pure and was studied using NMR spectroscopy with pure 1,4-dihydroantimonin (prepared as shown) used as a standard. With arsenin the intermediate 1,2-dihydroarsenin forms a methiodide which can be reversibly deprotonated to l,l-dimethylarsen(V)in (111), shown by NMR to exist principally in the ylide form (112). 

Hirata, M Mohri, T Hisanaga, A. and Ishinishi, N. (1989) Conversion of arsenite and arsenate to methylarsenic and dimethylarsenic compounds by homogenates prepared from livers and kidneys of rats and mice. Applied Organometallic Chemistry, 3(4), 335-41. 

Odanaka et al. [2] has reported the application of gas chromatography with multiple ion detection after hydride generation with sodium borohy-dride to the determination of methylarsenic and dimethylarsenic compounds, trimethylarsenic oxide and inorganic arsenic in soil and sediments. Recoveries in spiking experiments were 100-102% (methylarsenic and dimethylarsenic compounds and inorganic arsenic) and 72% (trimethylarsenic oxide). 

In September 1966 The New York Times published a report that cacodylic acid, an organic arsenic containing compound, was also being used in Vietnam.49 Cacodylic acid was a defoliant that was also toxic to man. According to the Merck Index, cacodylic acid was a dimethylarsenic acid containing 54.29 per cent arsenic, and was extremely poisonous.50 Seventy grams would kill the average 150-lb man if administered subcutaneously. 

Henry and Thorpe [14] separated monomethylarsonic acid, dimethylarsenic acid, As(III) and As(IV) on an ion exchange column from samples of pond water receiving fly ash from a coal-fired power station. They then determined these substances by differential pulse polarography. The above four arsenic species were present in non saline water systems. Moreover, a dynamic relationship exists whereby oxidation-reduction and biological methylation-dimethylation reactions provide the pathways for the intercoversions of the arsenicals. 

Elemental arsenic is used in alloys of lead and copper, in semiconductors, and as an additive to the grid metal in storage batteries. Arsenic compounds have been used as pesticides and wood preservatives [4]. Although arsenic is most well known for its use as a poison, some evidence suggests that trace amounts may be essential for good health [5]. Inorganic forms of arsenic are more toxic than the organic forms arsenite, As (III), is the most toxic form of the element, followed by arsenate, As (V), monomethylarsenate, and dimethylarsenate. 

The speciation of arsenic compounds, using IPC and ICP-MS as a detector, has received much attention [33-39]. Beauchemin et al. [33,34] used IPC for the analysis of arsenic species in dogfish muscle. The ion-pair reagent was 10 iM sodium dodecylsulfate in a 5% methanol, 2.5% glacial acetic acid mobile phase at pH 2.5 with a C18 column. The toxic inorganic species, As (III) and As (V), as well as the less toxic organoarsenic species, monomethylarsenic (MMA), dimethylarsenic (DMA), arsenobetaine (AB), and arsenocholine (AC), were separated. AB was the dominant species and constituted 84% of the total arsenic concentration with a detection limit of 300 pg (as As). 

Further work by the Caruso group [52] used MLC for arsenic speciation studies with detection by ICP-MS. As (III), As (V), dimethylarsenic (DMA), and monomethylarsenic (MMA) were separated (Fig. 10.8). Cetyltrimethylammonium bromide (0.05 M) was used to form the micelles along with a mobile phase of 10% propanol and 0.02 M borate as buffer. Dirty samples such as urine were analyzed easily by this method of LC. 

Mandal, B.K., Ogra, Y., Suzuki, K.T. (2001). Identification of dimethylarsenous and monomethylarsenous acids in human urine of arsenic affected areas in West Bengal, India. Chem. Res. Toxicol. 14 371-8. 

COTTON AIDE HC DILIC DIMETHYLARSENIC ACID DIMETHYLARSENIC ACID DMAA ERASE... 

Arsenic in aquatic environments is usually more concentrated in sediments and pore water than in the overlying water column (Ahmann et al., 1997 Smedley and Kinniburgh, 2002 Williams, 2001). The most abundant forms of arsenic are arsenate [As(V)] and arsenite [As(III)], but methylated forms can occur in mine-impacted environments (i.e., methylarsenic acid and dimethylarsenic acid) (Smedley and Kinniburgh, 2002) see Chapter 9.02. The principal pathway of arsenic toxicity is through dietary exposure to sediment and suspended particulates by fish, followed by human consumption. Environmental exposure to arsenic is a causal factor in human carcinogenous and other related health issues. Chronic exposure symptoms in humans include hyperkeratosis, hyperpigmentation, skin malignancies, and peripheral arteriosclerosis. Water provides the dominant pathway for arsenic exposure in humans (Williams, 2001). 

Dimethylarsenic chloride, Cacodyl trichloride, CH3)2AsClj,i crystallises in plates or columns from ether, fuming in air. At 40° to 50° C. the trichloride froths and decomposes into methyl chloride and methyldichloroarsine. It is prepared by passing chlorine into a solution of cacodyl chloride in carbon disulplude, or by adding powdered cacodylic acid in small quantities to phosphorus pentachloride mixed with dry ether. 

Howard and Arbab-Zavar have described a technique for the determination of inorganic As(III) and As(V), methylarsenic and dimethylarsenic species which is based on the trapping of arsines and selective volatilization into a heated quartz atomizer tube situated in the optical path of an atomic-absorption spectrometer. Improved reproducibility is obtained by the use of a continuous flow reduction stage and detection limits are approximately 0.25 ng. For a typical sample volume of 10 ml this corresponds to a detection limit of 0.025 ngmU of arsenic. Interference effects encountered by earlier workers were investigated and depression of results was observed in the presence of... 

The carrier-gas flow rate influences both the sensitivity and resolution of the method affecting the steady-state arsenic concentration in the atomizer cell and the rate of utilization. The effect on sensitivity of varying carrier-gas flow rate is demonstrated in Figure 2. The peak height for As(III) or As(V), methyl and dimethylarsenic decreased with increasing flow rate whereas instrument stability and resolution improved with increasing gas flow rates. The optimum flow rate is 150 ml min" F... 

FIGURE 2. Typical arsine signals under conditions of varying carrier-gas flow rates (A) ar-senic III) or arsenic(V), (B) methylarsenic, (C) dimethylarsenic. Reproduced from Reference 29 by permission of The Royal Society of Chemistry... 

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