Development of a Gas-phase Chemiluminescence Detection System for the Measurement of Arsenic in Environmental and Biological Samples

Abstract

Arsenic in groundwater used for drinking is now regarded as one of the most serious health hazards of this decade. Long term exposure to low concentrations of arsenic has been reported to cause cancer of the skin, bladder and other internal organs. Also, various ionic forms of arsenic are known to be very toxic to most microorganisms. Measurement of trace arsenic at parts per billion (ppb) levels in environmental and biological samples is a key component for the mitigation and understanding of this crisis. This M.S. Thesis Project deals with the development of an analytical technique for the measurement of arsenic based on a gas-phase chemiluminescence reaction between ozone (O3) and arsine (AsH3) as the detection principle. The approach is capable of analyzing arsenic concentration in a 1.0 mL sample in one minute. The determination of As(III) and As(V) was achieved based on the different pH dependence and the reducibility of these species to arsine gas by sodium borohydride (NaBH4). The intense chemiluminescence formed in a reflective glass reaction cell from the reaction of AsH3 – O3 is detected by a sensitive photomultiplier tube (PMT). The signal is further amplified, digitalized and recorded with a complete data acquisition computer controlled system. The limit of detection (LOD) is 0.146 μg/L (ppb or 146 ppt) for total arsenic concentration in 10 determinations. To validate the performance of the gas-phase chemiluminescence based arsenic analyzer, results were compared with Hydride Generation Atomic Absorption Spectroscopy (HG-AAS) and Atomic Fluorescence Spectroscopy (AFS) techniques. The chemiluminescence detection system was also coupled with a flow injection system to enhance its efficiency. Sequential procedures including direct chemical and data analysis methods and step by step development of the analyzer are described in depth. The system has been effectively tested using standard and unique field water samples from several regions of Ethiopia.