Speciation of inorganic arsenic by hydride generation atomic absorption spectroscopy Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v405sd46w

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  • A sensitive technique for the determination of As(III) and As{V) at μ/L and sub-μg/L concentrations from a single sample aliquot has been developed. This technique is based on the hydride generation atomic absorption (HGAA) spectrophotometry. The hydrides of the arsenic species are generated in a reaction vessel by mixing the sample solution with NaBH₄. By controlling the pH of the reaction mixture, the hydrides of As (III) and As(V) are sequentially produced. The hydrides are swept out of the solution with hydrogen and directed to the atomization cell. A flame-in-a-tube technique is employed for atomization. The hydrogen/hydride mixture passes through a hydrogen/air flame where hydrogen radicals convert the arsine to atomic arsenic. The atomic vapor then passes into a 20-cm long quartz observation tube where the atomic absorbance of arsenic is measured at 193.7 nm. The flame-in-a-tube technique was found to produce more consistent results than atomization based on electro-thermal heating of a quartz tube to 900° C. The parameters affecting the atomic absorption signal of arsenic that are independent of the oxidation state of arsenic were optimized using a 10 μg/L As(III) standard in 0.15 M citrate buffer. Optimization studies of the parameters that affect the speciation of inorganic arsenic were conducted on standards containing 10 μg/L cf both As(III) and As(V) in a 0.15 M citrate buffer. The optimized speciation procedure is based on measuring the peak absorbance signal due to As(III) in a 1-mL sample in 0.15 M citrate after an initial injection of 0.05 mL of 20% (w/v) NaBH₄. The pH of the remaining solution is lowered to zero by addition of 0.4 mL of concentrated HC1. The second injection of 0.1 mL of 20% (w/v) NaBH₄ results in a peak whose peak absorbance signal is due to As(V). The method developed for the speciation of inorganic arsenic was applied to water and biological (hair) samples for the determination of As(III) and As(V). As expected from previous studies, only pentavalent arsenic was found in both water samples--0.1 μg/L in river water and 0.8 μg/L in tap water. Since only As(III) accumulates in the body tissues, a KOH digestion was employed on the hair sample to prevent the oxidation of the arsenic in the sample to the pentavalent form. In the hair sample, only the trivalent form was found at a concentration of 2.6 μg/g of hair. The accuracy of the technique was tested through the use of NBS standard orchard leaves (SRM #1573). After drying the leaves in a muffle furnace at 450° C and dissolving the leaf ash in HC1, the diluted sample was analyzed for arsenic concentration by the developed procedure. A concentration of 10.6 μg As per gram of orchard leaves was found and agrees well with the NBS listed concentration of 10 ± 1μg/g of dried leaves. For a 1-mL sample aliquot, the detection limits for As (III) and As(V) are 0.06 and 0.2 μg/L, respectively. The atomic absorption sensitivities for As(III) and As(V) are 0.2 and 0.4 μg/L, respectively. The time of analysis per sample or standard is approximately 1 min.
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