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UBC Theses and Dissertations

Speciation of arsenicals Le, Xiao-Chun


Environmentally and biologically important organoarsenicals such as arsenobetaine, arsenocholine, arsenosugars, and the tetramethylarsonium ion do not form volatile hydrides on treatment with sodium borohydride, even though hydride generation is a commonly used analytical technique for arsenic. Consequently, it has been difficult to determine the concentration of these so called "hidden" arsenicals. To this end a method has been developed in which all arsenicals are decomposed completely to arsenate by using microwave oven heated solutions of potassium persulfate and sodium hydroxide. A portable hydride generator is described. Radioactive tracer studies show that fast reactions and high efficiencies (95%) are achieved by using the new generator. A system which combines flow injection analysis, on-line microwave oven digestion, the new hydride generator, and atomic absorption spectrometry (FIA/MD/HGAAS) is developed and shown to be capable of differentiating arsenite, arsenate, monomethylarsonic acid (MMAA), and dimethylarsinic acid (DMAA) from the "hidden" organoarsenic compounds such as arsenobetaine which are usually present in crab, shrimp, and other crustaceans. This system is capable of performing analysis at a sample throughput of 100-120 per hour. Calibration curves are linear from 5 to 200 ng ml⁻¹ of arsenic and the detection limit is 0.5 ng ml⁻¹ for 100 μ1 injection (0.05 ng of arsenic). The system can be coupled to a high performance liquid chromatograph (HPLC) and used as a detector for arsenic speciation studies. An alternative system coupling HPLC to an inductively coupled plasma mass spectrometer (ICPMS) is also investigated for the speciation of arsenicals commonly present in the environmental and biological systems. The analytical methods developed are successfully used to study the arsenicals present in marine animals, seaweeds, and human urine. Following human ingestion of crab and shrimp, which contain arsenobetaine as the major arsenic species, a fast urinary excretion of unchanged arsenobetaine is observed. No difference is found in either the excretion pattern or the excreted arsenic species in the urine of six volunteers who ingested either crab meat or shrimp. In contrast the arsenosugars present in macroalgae are metabolized and have a longer retention in the human body. When nine volunteers ingested a commercial seaweed product Nori, which contains an arsenosugar as the major arsenic species, both the urinary arsenic excretion pattern and the urinary arsenic species excreted varied from individual to individual. The present work also reveals the presence of arsenosugars in addition to arsenobetaine in marine bivalves. These findings shed light on some important aspects of arsenic biogeochemistry including the toxicological implications of the consumption of seaweed, the metabolism of arsenosugars, the biochemical pathway to arsenobetaine, and ultimately the cycling of arsenic in the marine environment.

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