Measurement of arsenic in water and soil based on gas-phase chemiluminescence

Arsenic occurs widely in nature and is a known human carcinogen. Developmental, immunological, and neurological defects are linked with chronic exposure to arsenic in drinking water. The United States Environmental Protection Agency (US EPA) prescribed safe limit is 10 μg/L. Standard atomic spectrometry based methods are expensive. Field wet techniques require large amounts of acid, other reagents and paper strips impregnated with toxic mercury and lead compounds. This dissertation presents a new, fast, safe, affordable automated system configurable for laboratory or field use. Arsenic in the sample is chemically or electrochemically reduced to arsine that reacts with ozone atop a photomultiplier tube, producing chemiluminescence. Direct chemical, electrochemical, and liquid chromatography methods are described.

The first method uses sodium borohydride for the reduction of arsenic. Differential determination of arsenate and arsenite is based on the different pH dependence on their conversion to arsine. At pH ≤1, both arsenate and arsenite are quantitatively converted. At pH 4-5, only arsenite is converted. Under these conditions, limit of detection (LOD) is 0.05 and 0.09 μg/L for total arsenic and arsenite, respectively, with a 3-mL water sample. The relative standard deviation for 3 determinations was 1.2 and 2.1% for 1 μg/L total arsenic and arsenite respectively. The arsenic concentrations in this dissertation are all based on that of elemental arsenic.

The Electrochemical method uses a Platinum screen anode and stainless steel cathode in two compartments, separated by a Nafion membrane. Arsenite is selectively reduced on a stainless steel cathode while a cadmium-coated cathode reduces both forms. The limit of detection is 1.5 and 4 μg/L for arsenite and total arsenic respectively with a 2-mL water sample. The relative standard deviation for 3 determinations was 2.6 and 4.5% for 10 μg/L arsenite and total arsenic respectively. This environment-friendly method uses only re-usable sulfuric acid electrolyte, air, water and electricity but requires further development.

Arsenite, arsenate, dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) are separated on anion-exchange column using carbonate and hydroxide eluents. Separated species are photolytically oxidized by UV-light, converting organic species to their respective inorganic forms. Subsequent online reaction with acid and borohydride produces arsine, detected by CL. For arsenite, arsenate, MMA and DMA the LOD is 0.4, 0.2, 0.5 and 0.3 μg/L respectively for a 100-μL injected sample. The relative standard deviation for 3 determinations was 3.5, 2.8, 2.2, and 4.1% for 10 μg/L of each of arsenite, arsenate, MMA, and DMA respectively.

The system has been tested successfully on water and soil samples, and can be adapted for matrices such as biological samples and body fluids. There are no significant practical interferences