Environmental samples, arsenic species commonly

Arsenic Species Commonly Found in Environmental Samples... 

The use of different combinations of solvents has been used to extract a variety of metal(loid) species from a range of different sample types. The majority of As species, including the two common oxidation states, are highly polar and very water soluble. The most popular extraction methods are therefore based on using water or mixtures of methanol and water in various proportions, and these have been applied to both marine and terrestrial samples. The extraction of arsenic species from biological and environmental samples has been reviewed. Research has focused mostly on the so-called water-soluble species, and very few methods have been developed for the other As-containing fraction termed lipid soluble. ... 

For the determination of arsenic compounds, three major steps must be taken into consideration. The arsenic species must be extracted from the sample (unfortunately in situ determination of arsenic compounds is not possible at environmental concentrations). During the extraction step, the arsenic compounds must not change or decompose chemically. Therefore, the extraction step should be as mild as possible and almost all the arsenic present in a sample must be extracted (51). A combination of various extractants is often necessary to reach all the arsenic. Polar organic solvents or water are commonly used for these purposes. 

Figure 1 Oxidation states of volatile and soluble arsenic transformed by biological systems. pKi represents the equilibrium between the protonated cation and neutral forms of the hydroxyl group of dimethylarsinic acid. pKj represents the equilibrium between the neutral form and anionic form (2). Occasionally referred to as arsenous acid (2). HO—As=0 has also been referred to as arsenious acid (52). The term arsenite often refers to any of the As(lll) oxides. The species will be a function of the solute and the pH. Given that the pKa of arsenious acid is 9.2 (52), the protonated form is most likely the predominant form in aqueous environmental and physiological samples below pH 9. Structure commonly used when discussing arsenite in biological transformations, but is most likely not the predominant form in aqueous solutions. Monosodium methane arsonic acid (MSMA) is the commercially available sodium salt.

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