Anal. Chem. 2005, 77, 4765-4773
Speciation-Capable Field Instrument for the Measurement of Arsenite and Arsenate in Water Kei Toda,* Takashi Ohba, and Mari Takaki
Department of Environmental Science, Faculty of Science, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan Sathrugnan Karthikeyan and Shizuko Hirata
National Institute of Advanced Industrial Science and Technology (AIST) Chugoku, 2-2-2 Hiro-Suehiro, Kure, 737-0197, Japan Purnendu K. Dasgupta
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Hydride generation to form arsine and in-line preconcentration of the arsine into an alkaline KMnO4 receiver followed by molybdenum blue (MB) colorimetric determination of the arsenate formed is proposed for the highly sensitive and separate measurement of total inorganic As and As(III). Reduction of As to AsH3 is carried out by NaBH4; when the reduction is carried out at pH 1, all the inorganic As is reduced to AsH3, and when carried out at pH 7, only As(III) is reduced. Reductions at the two different pH levels are carried out in two different arsine generators simultaneously using constant addition of NaBH4 with solenoid pumps. The AsH3 is collected by individual porous membrane diffusion scrubbers filled with a stationary solution of KMnO4, and the contents of the two scrubbers sequentially enter a flow analysis stream. MB is formed by merging with a ammonium molybdate-ascorbic acid reagent, passing through a heated reactor, and is then measured by a LED/photodiode-based absorbance detector. Robustness was confirmed for total As using three types of certified natural water samples. Speciation analysis data from well water samples analyzed by this method agree well with HPLCICPMS measurements in a different laboratory. The system has been successfully applied to field measurements of As(III) and As(V), where levels were significantly below 1 µg/L. For a 20-mL sample, the limits of detection (LODs) for this inexpensive instrument are 0.3 µg/L for both As(III) and total As. When an 80-mL sample is analyzed, LODs are 0.07 µg/L As(III) and 0.09 µg/L total As. The general approach should be applicable to many other analyte species of interest that can be isolated from the matrix by the formation of a suitable volatile compound that can be recaptured. The presence of arsenic in drinking water has reached calamitous proportions in many parts of Asia.1,2 Consumption of * Corresponding author: (e-mail)
[email protected]. 10.1021/ac050193e CCC: $30.25 Published on Web 06/30/2005
© 2005 American Chemical Society
even low levels of As over a long period causes a multitude of diseases.3 In natural waters, As is dominantly present in the more toxic inorganic forms, and of these, arsenite (As(III)) is much more toxic than arsenate (As(V)).4-6 For this reason, speciation of As oxidation states is desirable. The source of the waterborne As, the 20th most abundant element in the earth’s crust,7 is largely natural, being derived from the oxidation of arsenopyrite sediments in shallow aquifers and by desorption from ferric hydroxide minerals in aquifer sediments under reducing conditions.8,9 A large number of kits, mostly based on the chromogenic Gutzeit reaction,10 have been used for water testing in Bangladesh. There is significant controversy as to how well these kits work.11,12 In Bangladesh and India, the World Health Organization (WHO) recommends a maximum level in drinking water of 50 µg of As/L whereas it recommends a standard of 10 µg of As/L for developed nations.13 Setting aside any rationale for this strange double standard,14 obviously the difficulty of using such kits at the 10 (1) Dasgupta P. K., Ed. Arsenic. Analytical chemistry and beyond; Talanta 2002, 58, 1-235. (2) Chakraborti, D., Hussam, A., Alauddin, M., Eds. J. Environ. Sci. Health 2003, A38, 1-305 (Special issue on arsenic.). (3) Karim, M. M. Water Res. 2000, 34, 304-310. (4) Jain, C. K.; Ali, I. Water Res. 2000, 34, 4304-4312. (5) Tabacova, S.; Hunter, E. S., III; Gladen, B. C. Toxicol. Appl. Phamacol. 1996, 138, 298-307. (6) The World Health Organization. WHO Environmental Health Criteria 18: Arsenic; IPCS International Programme on Chemical Safety, WHO: Geneva, 1981; pp 43-49, 88-122. (7) Ahmed, F. A. It’s elemental: The Periodic Table-Arsenic; Chem. Eng. News special issue, 2003. http://pubs.acs.org/cen/80th/arsenic.html. (8) Nickson, R.; McArthur, J. M.; Burgess, W.; Ahmed, K. Z.; Ravenscroft, P.; Rahman, M. Nature 1998, 395, 338-338. (9) Kondo, H.; Ishiguro, Y.; Ohno, K.; Nagase, M.; Toba, M.; Takagi, M. Water Res. 1999, 33, 1967-1972. (10) Kinniburgh, D. G.; Kosmus, W. Talanta 2002, 58, 165-180. (11) Rahman, M. M.; Mukherjee, D.; Sengupta, M. K.; Chowdhury, U. K.; Lodh, D.; Chanda, C. E.; Roy, S.; Selim, M.; Quamruzzaman, Q.; Milton, A. H.; Shahidullah, S. M.; Rahman, M. T.; Chakraborti, D. Environ. Sci. Technol. 2002, 36, 5385-5394. (12) van Geen, A.; Cheng, Z.; Seddique, A. A.; Hoque, M. A.; Gelman, A.; Graziano, I. H.; Ahsan, H.; Parvez, F.; Ahmed, K. M. Environ. Sci. Technol. 2005, 39, 299-303. (13) Richardson, S. D. Anal. Chem. 2003, 75, 2831-2857.
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µg/L As level will be much greater. In the United States, new regulations require public water systems to have
