University of Massachusetts at Amherst Department of Chemistry

James Kearns Tyson Research Group

Department of Chemistry, University of Massachusetts701 Lederle Graduate Research Tower

710 North Pleasant Street, Amherst, MA 01003-9306, USA

Field Portable Methods for the Determination of Arsenic

in Environmental Samples

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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

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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

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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.

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The Challenges of Laboratory Instruments:

(1) High Cost (2) Materials and Maintenance (3) Trained Technician

Is There a Need for Field Portable Instruments?

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.

Kinniburgh, D.G.; Kosmus,W. Talanta, 2002, 58, 165-180.

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.”

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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.

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The Gutzeit Method Chemistry

Zn(0) Zn2+ + 2e-

The Formation of Arsine (AsH3)

2H+ + 2e- H2

As(III) + 3e- As(0)

As(0) + 3e- +3H+ AsH3

AsH3 (g) + 3HgBr2 (aq) As(HgBr)3 (aq) + 3HBr

AsH3 (g) + 3AgNO3 (s) AsAg3 (s) + 3HNO3

The reaction of Arsine

Brindle, I. D. “Vapour-generation analytical chemistry: from Marsh to multimode sample-introduction system” Analytical Bioanalytical Chemistry 388, 2007, 735-741.

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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.

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Molybdenum Blue Chemistry

Molybdate reacts with the +5 species of P, As, Sb, and Bi.

12 MoO42- + AsO4

3- + 24H+ → AsMo12O403-+ 12H2O

Chemical Reaction: for formation of molybdenum blue

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.

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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.

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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)

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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)

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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

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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.

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The Research or Kinniburgh and Kosmus

Kinniburgh, D.G., Kosmus, W. Talanta, 2002, 58, 165-180

Thompson, M. Howarth, R.J. Analyst, 1976, 690

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Current Analytical Precision with the Gutzeit Method

Kinniburgh, D.G., Kosmus, W. Talanta, 2002, 58, 165-180

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

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The Standard Plot of Color Versus Concentration

Concentration of As(III) g L-1

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The Determination of Standard Deviation in Concentration

Concentration of As (III)

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The Values so and k at 24 Hours of Reaction Time

Concentration As (III) g L-1 Standard Deviation in Concentration

10 2.382725 4.147850 6.5894

As(III) g L-1

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Results: Table of So and K Values at Different Times

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Results: Graphs Comparing Time and Temperature

Concentration of As (III) g L-1

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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.

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