STUDIES IN THE ATOMIC SPECTROMETRIC DETERMINATION AND SPECIATION OF ARSENIC IN ENVIRONMENTAL SAMPLES

The application of work described in this dissertation, the determination of total and arsenic species in water and rice samples, is of considerable societal importance as large numbers of the citizens of many countries around the world are exposed to potentially harmful concentrations of these carcinogenic compounds by daily consumption. The field of analytical chemistry is crucial to support the operation of any treatment to decrease arsenic concentrations as well as any regulations and legislation regarding arsenic in food and the environment. The current goals of research in trace arsenic measurements and speciation are to increase knowledge of the subject and to improve upon current methods by enhancing the figures of merit, such as accuracy and reproducibility, while balancing with the cost of analysis. The topics described in this dissertation were investigated primarily through the use of hydride generation atomic fluorescence spectrometry (HG-AFS), as a cost- effective alternative to inductively coupled plasma mass spectrometry (ICP-MS). Inductively coupled plasma optical emission spectrometry (ICP-OES) and ICP-MS were also used. A cryogenic trapping (CT) procedure for preconcentration was developed; and a preconcentration factor of 12 was achieved using HG-CT-ICP-OES. An anion-exchange column was introduced into a HG-AFS system for the simultaneous preconcentration and speciation of inorganic arsenic in natural water samples. The column consisted of a glass tube of 60 mm length and 4 mm id, containing approximately 0.8 g of Amberlite IRA-400 resin. The instrument was modified so that it operated on an external hydrogen supply. The detection limits were 13 ng L-1 and 15 ng L-1 for As(III) and As(V), respectively. The method was evaluated by the analysis of spiked natural waters. The recoveries for 0.5 and 1 μg L-1 As(III) were 88-112%; the recoveries for 0.5 and 1 μg L-1 As(V) were 94-112%. This method was also validated by the accurate analysis of a seawater certified reference material, NASS-6, which contains 1.43 ± 0.12 μg L-1 (total arsenic). A method was developed for the speciation of arsenic by reversed-phase ion-pair chromatography HG- AFS. The detection limits obtained for of four arsenic species [As(III), As(V), DMA and MMA] are comparable to those obtained ICP-MS detection. The phenomenon of the lack of precision in the determination of arsenic in rice was investigated by developing a method for the determination of total arsenic in individual rice grains. Two types of rice were analyzed: the average arsenic concentrations for type I and type II rice samples are 532 μg kg-1 and 255 μg kg-1, respectively; the standard deviation of the arsenic concentration in single rice grain are 757 and 489 μg kg-1, respectively. Based on the results, the recommended sample size was at least 47 g and 25 g for type I and type II rice, respectively, based on an assumption that a confidence limit of 10% the mean arsenic concentration value was desired. Recommendations for future work including the further improvement and application of the developed methods are also made in the final chapter