The Chemical Analysis of Argonne Premium Coal Samples

Edited by Curtis A. Palmer
U.S. Geological Survey Bulletin 2144

Arsenic, antimony, and selenium by flow injection or continuous flow-hydride generation-atomic absorption spectrometry

By Philip L. Hageman and Eric Welsch

PRINCIPLE

Geologic samples are digested using a multiacid procedure in an open Teflon vessel. At the end of the digestion period, arsenic, antimony, and selenium are reduced to oxidation states, +3, +3, and +4, respectively. Sodium borohydride is added to the solution resulting in rapid formation of the hydrides as illustrated by:

3 NaBH₄ + 4H₃AsO₃ ® 4 AsH₃(g) + 3 H₃BO₃ + 3 NaOH
3 NaBH₄ + 4H₃SbO₃ ® 4 SbH₃(g) + 3 H₃BO₃ + 3 NaOH
3 NaBH₄ + 4H₂SeO₃ ® 4 H₂Se(g) + 3 H₃BO₃ + 3 NaOH

The gaseous hydrides are stripped from the analytical stream and transported with inert gas to the atomizer (a heated quartz furnace) of the atomic absorption spectrophotometer. For selenium, the quartz furnace is heated by an air acetylene flame to 2000°C; the arsenic and antimony furnace is electrically heated to 900 and 1,000° C respectively. Concentrations of the elements are determined using calibration standards in solutions of similar matrix.

INTERFERENCES

Interferences usually associated with atomic absorption analysis are negligible, but incomplete recoveries of the elements from the digest solution may yield low analytical results. Incomplete recoveries are principally due to:

1. Concentration of certain transition and heavy metals (e.g. Cu, Fe, Ni, and Sn) of more than 500 ppm in the sample compete with As, Sb, and Se, for available NaBH₄. This competition may result in insufficient NaBH₄ for completion of the hydride-forming reaction.
2. Concentrations of one or more of the hydride forming elements in excess of 1,000 ppm. Competing hydride elements deplete the oxygen supply in the furnace which is needed to convert hydrides to ground state elements.
3. Interference of hydride formation by incompletely digested organic material.
4. Possible volatility losses of the analyte in an organic rich matrix due to organometallic compounds.
5. Coprecipitation of the hydride elements if a metal is reduced to the metallic form, as is seen with Ag or Au.

Problems one and two are generally of minimal concern in environmental samples because the probability of high concentrations of these elements is quite low. More often, interference problems occur in mineral studies, but can be resolved by dilution of the sample solution. This dilution will raise the appropriate detection limits. Special care should be taken to ensure that all the organic material in organic-rich sample is thoroughly and rapidly digested (i.e., oxidized) to enable the reaction to reach completion and to avoid loss through volatilization.

SCOPE

The hydride generation-atomic absorption spectrophotometric method (HG-AAS) described herein is useful for the determination of As, Sb, and Se, in a variety of geochemical samples. The optimum concentration ranges without sample dilution for these elements in various sample media are as follows:

Above these ranges, the options of sample dilutions versus alternative techniques, e.g. energy dispersive X-ray fluorescence for selenium, should be considered. One day is required to complete digestion of 40 samples. The analyses of 40 samples requires about 1.5 hours of instrument time for each element.

APPARATUS

• Standard laboratory hot plate with a 30x60-cm heating surface
• 2.5-cm-thick x 25-cm-wide x 50-cm-long aluminum heating block with 34-mm holes drilled through in a 5x10 matrix
• Thick-walled, 30-mL Teflon bottles, #0201 T from Savillex Corp., Minnetonka, Minnesota
• 125-mL Erlenmeyer flasks with refluxers
• 60 mL plastic bottles with screw tops
• Gilson 212b autosampler
• Perkin Elmer 4100 AA with FIAS 400, AS 90 autosampler, PC controller and printer for arsenic and antimony determinations
• Perkin Elmer 2380 AA with Varian hydride generator Model V6A76 and strip chart recorder, for selenium determination.

REAGENTS

• Deionized water (DI)
• Nitric acid, HNO3 ‘INSTRA-ANALYZED’ grade
• Hydrochloric acid, HCl ‘INSTRA-ANALYZED’ grade
• Perchloric acid, HClO4 ‘INSTRA-ANALYZED’ grade
• Sulfuric acid, H2SO4 ‘INSTRA-ANALYZED’ grade
• Hydrofluoric acid, HF reagent grade
• Ascorbic acid, C6HO6 reagent grade
• Potassium iodide, KI reagent grade
• Sodium borohydride, NaBH4 reagent grade
• Sodium hydroxide, NaOH reagent grade

6 N HCl solution: Dilute ‘INSTRA-ANALYZED’ grade HCL suitable for trace metals analysis, with an equal volume of DI water. The use of the ‘INSTRA-ANALYZED’ grade or HCl of similar purity is advised throughout the procedure.

Sodium borohydride solution: For As and Sb dissolve 0.5 g NaOH and 2.0 g NaBH₄ in DI water and dilute to 1 L in a volumetric flask. For Se dissolve 3.5 g NaBH₄ and 5 g NaOH in DI water and dilute to 1 L. All solutions should be made weekly and kept refrigerated between analyses.

Potassium iodide-ascorbic acid solution: Dissolve 100 g KI in DI water. Add 50 g C₆HO₆. Dilute to 1 L with DI water. Stable for at least 2 weeks.

Saturated persulfate: Dissolve sufficient K₂S₂2O₈ into one liter of DI so that crystals remain and no more will go into solution.

Arsenic and antimony standard solutions: Commercially prepared As and Sb standards are used to make a 10 ppm stock solution in 10 percent HCl. The 10 ppm stock is used to prepare 20, 40, and 80 ppb working standards by transferring 0.2 mL, 0.4 mL, and 0.8 mL aliquots to three 100-mL volumetric flasks. To these add 50 mL of 6 M HCl, 20 mL of KI/C₆HO₆ solution, and enough DI water to bring the volume to 100 mL. The working standards are stable for at least 1 week and should be refrigerated between analyses.

Selenium standard solutions: A commercially prepared selenium stock is used to make a 10 ppm standard in 10 percent HCl. From this 0.05, 0.10, and 0.20 mL aliquots are transferred to three 100 mL volumetric flasks and brought to volume with 50 mL 6 M HCl, 4 mL H₂SO₄, and DI water. Important note: for water analysis, do not add H₂SO₄ to standard solutions. These standards should be stable for at least 1 week and kept refrigerated between analyses.

SAFETY PRECAUTIONS

The principal hazards associated with the technique deal primarily with the decomposition of the samples and the use of concentrated acids. Most dangerous is HF which inflicts painful and lasting bone and neural damage. Gloves, goggles or safety glasses, and a laboratory coat should be used whenever handling chemical reagents. Extra care should be taken in the dispensing of this acid and all equipment used in this operation should be rinsed thoroughly afterward. A salve such as calcium gluconate or magnesium sulfate should be prominently located in the laboratory and applied if an HF burn is detected. A chemical exhaust hood should be used for the digestion procedure and over the atomic absorption instrument due to the evolution of toxic hydrides and HCl vapors. There is a danger of H₂ ignition and flashback if the inert carrier gas is not turned on in advance. Review the CHP and MSDS for further information.

PROCEDURE (rock, soil, and sediment)

1. Weigh 0.25 g sample (<80-mesh) into a 30-mL Teflon vessel, add 9 mL HNO₃ and 0.25 mL of 10 percent HCl. Allow to stand for 3 hours.
2. Add 2 mL HClO₄, 2 mL H₂SO₄, 10 mL HF and heat overnight at 125°C.
3. Cool, add 25 mL 6 N HCl and let stand for half an hour.
4. Transfer the sample solution to a 60-mL polyethylene bottle and bring up to 55 g with DI water.
5. Approximately 8 mL of the solution is decanted into 13x100-mm test tubes for selenium analysis and another 8 mL is mixed with 2 mL of KI-C₆HO₆ solution in 17x100-mm test tubes and allowed to stand for 1 hour before arsenic or antimony analysis.
6. Arsenic and antimony are determined by means of a Perkin Elmer-4100AA and FIAS-400 hydride system while selenium is determined using a Varian hydride generation system which is joined with a Perkin Elmer-2380AA.
7. Sample peaks are compared to standard peaks recorded on a strip chart recorder for selenium while the 4100 software does the data reduction mathematically for arsenic and antimony.

PROCEDURE (coal and plant)

1. Weigh a 0.1 g sample of coal or a 1.0 g sample of plant material into a 125 mL Erlenmeyer flask.
2. Add 20 mL HNO₃, 2 mL H₂SO₄, and let stand overnight.
3. Then add 3 mL of HClO₄, insert refluxers, and heat at about 175°C for 30 min.
4. Remove refluxers and continue to heat to dense white fumes.
5. See step three of the rock procedure.

PROCEDURE (water and extracts)

1. Weigh 10 g liquid sample into a 30-mL Teflon vessel.
2. Add 1 mL of saturated K₂S₂O₈ and let stand for 1 hour.
3. Add 1 mL conc HCl and heat at 110°C with watch glass in place.
4. Remove watch glass after 1 hour and continue heating for roughly 2½ to 3 hr or until the volume is reduced to somewhere between 2 and 5 mL.
5. Add another 2 mL conc HCl, replace the watch glass, and heat for another hour.
6. Cool, add 25 mL 6 N HCl and let stand for half an hour.
7. Transfer to 60-mL polyethylene bottles with distilled water, and bring to a weight of 20 g.

OPERATING CONDITIONS

The analyte content of the digest solution is determined using the instrumental operating conditions shown in table 1.

ASSIGNMENT OF UNCERTAINTY

The analytical results for As, Sb, and Se in selected reference materials, duplicate samples, and method blanks are summarized in table 2.

BIBLIOGRAPHY

Aruscavage, Philip, 1977, Determination of arsenic, antimony, and selenium in coal by atomic absorption spectrometry with a graphite tube atomizer: U.S. Geological Survey, Journal of Research, v. 5, no. 4, p. 405-408.

Briggs, P.H., and Crock, J.G., 1986, Automated determination of total selenium in rocks, soils, and plants: U.S. Geological Survey Open-File Report 86-40.

Crock, J.G., and Lichte, F.E., 1982, an improved method for the determination of arsenic and antimony in geologic materials by automated hydride generation-atomic absorption spectroscopy: Analytica Chimica Acta 144, p. 223-233.

Guo, T., Erler, W., and Schulze, H., The determination of arsenic, selendium, and antimony in fly ash using flow injection hydride AAS: Applied Atomic Spectroscopy no. 4.5E.

Harms, T., 1988, Branch of Geochemistry, Oral communication to J.G. Crock, In-house value by fluorimetry: U.S. Geological Survey, Denver, Colo.

National Institute of Standards and Technology, 1976, 1979, and 1992, Certificate of analysis: U.S. Department of Commerce, Gaithersburg, Maryland.

Perkin-Elmer Technical Summary, Perkin-Elmer FIAS-200 flow injection system for atomic spectroscopy: Order No. TSAA-10.

Potts, P.J., Tindle, A.G., and Webb, P.C., 1992, Geochemical reference material compositions: CRC Press Inc., Boca Raton, Fla., 313 p.

Varian Associates, VGA-76 vapor generation accessory: Operation Manual, Publication no. 85-100577-00, March 1984.

Water Resources Division, Statement of Analysis: U.S. Geological Survey, Denver Colo.

Wilson, S., May 1994, Branch of Geochemistry oral communication to editor: U.S. Geological Survey, Denver, Colo.

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