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Field Portable Methods for the Determination of Arsenic in Environmental Samples

Field Portable Methods for the Determination of Arsenic in Environmental Samples. James Kearns Tyson Research Group Department of Chemistry, University of Massachusetts 701 Lederle Graduate Research Tower 710 North Pleasant Street, Amherst, MA 01003-9306, USA. Presentation Outline.

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Field Portable Methods for the Determination of Arsenic in Environmental Samples

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  1. Field Portable Methods for the Determination of Arsenic in Environmental Samples James Kearns Tyson Research Group Department of Chemistry, University of Massachusetts 701 Lederle Graduate Research Tower 710 North Pleasant Street, Amherst, MA 01003-9306, USA

  2. Presentation Outline • 1. Research Goals, the Arsenic Problem, and Field Kits. • 2. The Chemical Methods • The Gutzeit Method: Hydride Generation of Arsenic. • The Molybdenum Blue Method. • 3. Experimental • Project 1: 24 Hour Field Kit Sensitivity • Project 2: Measuring Arsenic in Soils with the Gutzeit Method • Project 3: Silver Nitrate as a Detection Reagent for the Gutzeit Method • Project 4: Molybdenum Blue and the Detection of Arsenic with Cameras • Project 5: Flow Injection and the Determination of Arsenic • Project 6: The Stoichiometry of Heteropolyacids • 4. SNPs Research • 5. Future Work • 6. Questions

  3. What is the Size of the Arsenic Problem? Millions of people worldwide are chronically exposed to arsenic through drinking water, including 35—77 million people in Bangladesh. Argos, M. et al. The Lancet, Early Online Publication, 2010

  4. The Goals of this PhD. Research Project Goal: To develop a more reliable field portable chemical method to measure arsenic in environmental samples at, or below, 10 µg L-1 (ppb). Areas of investigation: (1) Improvement in Gutzeit methodology for water and soil testing with digital image analysis and use of silver nitrate as a reagent (2) Optimization of the molybdenum blue chemistry (3) The single nucleotide polymorphism study to understand the health consequences of arsenic exposure.

  5. The Challenges of Laboratory Instruments: Is There a Need for Field Portable Instruments? (1) High Cost (2) Materials and Maintenance (3) Trained Technician The Current Reliability of Field Portable Methods: “Accurate, fast measurement of arsenic in the field remains a technical challenge. Technological advances in a variety of instruments have met with varying success. However, the central goal of developing field assays that reliably and reproducibly quantify arsenic has not been achieved” Melamed, D. Anal. Chim. Acta,2005, 532, 1-13. The Need for Field Kits: “The only feasible approach (for the measurement of the tube wells, which are estimated to be more than 10 million) is through the use of field kits.” Kinniburgh, D.G.; Kosmus,W. Talanta, 2002, 58, 165-180.

  6. The Gutzeit Test Reaction 1 (aq): arsenite + zinc + acid produces AsH3 ,which rises into head space of reaction container. Reaction 2 (g): AsH3 reacts with mercuric bromide impregnated test strip. Measurement: Yellow-brown color produced after set time is compared with preprinted chart.

  7. The Gutzeit Method Chemistry Zn(0)  Zn2+ + 2e- The Formation of Arsine (AsH3) The reaction of Arsine AsH3 (g) + 3HgBr2 (aq)  As(HgBr)3 (aq) + 3HBr AsH3 (g) + 3AgNO3 (s) AsAg3 (s) + 3HNO3 2H+ + 2e- H2 As(III) + 3e- As(0) As(0) + 3e- +3H+ AsH3 Brindle, I. D. “Vapour-generation analytical chemistry: from Marsh to multimode sample-introduction system” Analytical Bioanalytical Chemistry 388, 2007,735-741.

  8. Molybdenum Blue Method Ammonium molybdate, sulfuric acid, a reducing agent and a catalyst are combined; the molybdate forms an inorganic polymer, which is then reduced and turns from yellow to blue.

  9. Molybdenum Blue Chemistry Molybdate reacts with the +5 species of P, As, Sb, and Bi. Chemical Reaction: for formation of molybdenum blue 12 MoO42- + AsO43- + 24H+ → AsMo12O403-+ 12H2O Matsunaga, H.; Kanno, C.; Toshishige, M. Suzuki, T.M. Talanta, 2005, 66, 1287-1293. Analytes which react with the molybdenum blue chemistry The Stages: of molybdenum blue formation 1. Complex only reacts in a solution containing arsenic (V). 2. After reduction, the complex’s Max is near 850nm.

  10. The “Molybdenum” Blue Complex 1. Arsenate + molybdate + acid + reducing agent gives blue color due to formation of heteropoly species containing both Mo (IV) and Mo (VI). 2. Octahedral subunits form the structure. 3. Arsenic substitutes for a molybdenum or trapped in the interior of the larger polymer. Gouzerh, P.; Proust, A. Main-group element, organic, and organometallic derivatives of polyoxometalates. Chem. Rev. 1998, 98, 77.

  11. Color Measurement and Tristimulus Colorimetry • Photons come in different wavelengths • According to tristimulus colorimetry theory, the human eye interacts with three regions of the electromagnetic spectrum • Detection methods measure light using tristimulus theories Konica Minolta, the essentials of imaging web site http://www.konicaminolta.com/instruments/knowledge/light/concepts/08.html, (accessed August, 2010)

  12. Reflectance Spectroscopy • I = I0*e-kx • Reflectance spectroscopy operates according to Beer’s Law • Where I is observed light • I0 is the original light intensity • The value k is the absorption coefficient specific for that substance at a specific wavelength. • The value x is the distance the photons travel through the substance USGS, about reflectance spectroscopy website, http://speclab.cr.usgs.gov/aboutrefl.html, (accessed August, 2010)

  13. Quantification of Molecules Using Reflectance Spectroscopy • I = I0*e-kx • These methods used a tristimulus scanner • I and Io are known, the precision of the emission of incident wavelengths and their detection have to be further established • The value k is the absorption coefficient specific for that substance at a specific wavelength. The k value is not known because the reaction products are not homogeneous or characterized • The value x is the distance the photons travel through the substance because the thickness of the mercuric bromide is not known and are not uniform • The USGS reflectance spectroscopy places samples on glass, this experiment uses white plastic

  14. Project 1: Improving Field Kit Sensitivity using Digital Image Analysis • Time: five replicate measurements at 20, 30, 40 minutes and 24 hours at the concentrations of 10, 25, 50, 100, 250, 500 µg L-1 (ppb). • Temperature: five replicate measurements at 35° C and the concentrations of 10, 25, 50, 100, 250, 500 µg L-1 (ppb). • Determinations of (1) s (standard deviation of field kit measurements), (2) S0 (Standard deviation at zero concentration), (3) k (constant relative error) with time and temperature variations plus scanner use. • The Red, Green and Blue values were measured using computer software.

  15. The Research or Kinniburgh and Kosmus Thompson, M. Howarth, R.J. Analyst, 1976, 690 Kinniburgh, D.G., Kosmus, W. Talanta, 2002, 58, 165-180

  16. Current Analytical Precision with the Gutzeit Method Kinniburgh, D.G., Kosmus, W. Talanta, 2002, 58, 165-180

  17. Results: Tables of Standard Deviation Values and Mean Blue Values at Different Times

  18. The Standard Plot of Color Versus Concentration Blue Pixel Count Concentration of As(III) g L-1

  19. The Determination of Standard Deviation in Concentration Blue Pixel Value Concentration of As (III)

  20. The Values so and k at 24 Hours of Reaction Time Standard Deviation of Concentration As(III) g L-1

  21. Results: Table of So and K Values at Different Times

  22. Results: Graphs Comparing Time and Temperature Blue Pixel Value Concentration of As (III) g L-1

  23. Conclusions of DIA Experiment • 1. Scanning improves precision compared to naked eye determination at 20 minutes. • Naked eye (Kinniburg) k = 0.3 and So = 7. • This method produced k = 0.2 and So = 2.6. • 2. Increasing the reaction time from 20 minutes to 40 minutes also decreases k (for 20, 30, and 40 minutes) and increases So(for 20,30, and 40 minutes). • 3. Running the reaction at 35°C produces results similar to 40 minutes and 24 Hours.

  24. Conclusions of DIA Experiment Table of s Values at 10 and 50 g L-1

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