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STANDARD METHODS: FOR THE EXAMINATION OF WATER AND WASTEWATER

18TH EDITION 1992

Prepared and published jointly by:
AMERICANPUBLIC HEALTH ASSOCIATION
AMERICAN WATER WORKS ASSOCIATION
WATER ENVIRONMENT FEDERATION

Joint Editorial Board
Arnold E. Greenberg, APHA,Chairman
Lenore S. Clesceri, WEF
Andrew D. Eaton, AWWA

Managing Editor
Mary Ann H. Franson

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American Public Health Association
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      Standard methods for the examination of water and wastewater.
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3113 METALS BY ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY*

3113 A. Introduction

1. Applications

Electrothermal atomic absorption permits determination of most metallic elements with sensitivities and detection limits from 20 to 1000 times better than those of conventional flame techniques without extraction or sample concentration. This increase in sensitivity results from the increased residence time of the ground-state atoms in the optical path, which is many times that of conventional flame atomic absorption. Many elements can be determined at concentrations as low as 1.0 µg/L. An additional advantage of electrothermal atomic absorption is that only a very small volume of sample is required.

* Approved by Standard Methods Committee. 1989.

Use the electrothermal technique only at concentration levels below the optimum range of direct flame atomic absorption because it is subject to more interferences than the flame procedure and requires increased analysis time. The method of standard additions often is required to insure validity of data. Because of the high sensitivity of this technique. exercise great care to avoid contamination errors.

2. Principle

Electrothermal atomic absorption spectroscopy is based on the same principle as direct flame atomization but an electrically heated atomizer or graphite furnace replaces the standard burner head. A discrete sample volume is dispensed into the graphite sample tube (or cup). Typically. determinations are made by heating the sample in three or more stages. First, a low current heats the tube to dry the sample. The second, or charring stage, destroys organic matter and volatilizes other matrix components at an intermediate temperature. Finally, a high current heats the tube to incandescence and, in an inert atmosphere, atomizes the element being determined. Additional stages frequently are added to aid in drying and charring, and to clean and cool the tube between samples. The resultant ground-state atomic vapor absorbs monochromatic radiation from the source. A photoelectric detector measures the intensity of transmitted radiation, which is inversely proportional to the quantity of ground-state atoms in the optical path over a limited range.

3. Interference

Electrothermal atomization determinations are subject to significant interferences from molecular absorption as well as chemical and matrix effects. Molecular absorption may occur when components of the sample matrix volatilize during atomization, resulting in broadband absorption. Several background correction techniques are available commercially to compensate for this interference. A continuum source such as a deuterium are can correct for background up to absorbance levels of 0.8 or so. Zeeman effect background correctors can handle background absorbencies up to 1.5 to 2.0. The Smith-Hieftje correction technique can accommodate background absorbance levels as large as 2.5 to 3.0 (see Section 3111A.3). Use background correction when analyzing samples containing high concentrations of acid or dissolved solids and in determining elements for which an absorption line below 350 nm is used.

Matrix modification can be useful in minimizing interference. Accomplish this by adding various chemicals to the sample. Alternatively, program a modern autosampler to add matrix modifiers directly to the sample in the furnace chamber. Some matrix modifiers reduce volatility of the element being determined or increase its atomization efficiency by changing its chemical composition. This permits use of higher charring temperatures to volatilize interfering substances and increases sensitivity. Other matrix modifiers increase volatility of the matrix. Specific matrix modifiers are listed in Table 3113:I.

Temperature ramping. i.e., gradual heating, can be used to decrease background interferences and permits analysis of samples with complex matrices. Ramping permits a controlled, continuous

increase of furnace temperature in any of the various steps of the temperature sequence. Use ramp drying for samples containing mixtures of solvents or for samples with a high salt content (to avoid spattering). Samples that contain a complex mixture of matrix components sometimes require ramp charring to effect controlled, complete thermal decomposition. Ramp atomization may minimize background absorption by permitting volatilization of the element being determined before the matrix. This is especially applicable in the determination of such volatile elements as cadmium and lead.

Use standard additions to compensate for matrix interferences. When making standard additions, determine whether the added species and the element being determined behave similarly under the specified conditions. [See Section 3113B.4d2)].

Chemical interaction of the graphite tube with various elements to form refractory carbides occurs at high charring and atomization temperatures. Elements that form carbides are barium, molybdenum, nickel, titanium, vanadium, and silicon. Carbide formation is characterized by broad, tailing atomization peaks and reduced sensitivity. Using pyrolytically coated tubes

for these metals minimizes the problem. For the analysis of aluminum, thorium-treated L’vov platforms provide sharper peaks at low concentrations and enhanced charring stability.

4. Sensitivity, Detection Limits, and Optimum Concentration Range

Estimated detection limits and optimum concentration ranges are listed in Table 3113:11. These values may vary with the chemical form of the element being determined, sample composition or instrumental conditions.

For a given sample, increased sensitivity may be achieved by using a larger sample volume or by reducing flow rate of the purge gas or by using gas interrupt during atomization. Note however, that these techniques also will increase the effects of any interferences present. Sensitivity can be decreased by diluting the sample, reducing sample volume, increasing purge-gas flow, or using a less sensitive wavelength. Use of argon, rather than nitrogen as the purge gas generally improves sensitivity and reproducibility. Hydrogen mixed with the inert gas may suppress chemical interference and increase sensitivity by acting as a reducing agent, thereby aiding in producing more groundstate atoms. Using pyrolytically coated graphite tubes can increase sensitivity for the more refractory elements. The optical pyrometer/maximum power accessory available on some instruments also offers increased sensitivity with lower atomization temperatures for many elements.

Using the Stabilized Temperature Platform Furnace (STPF) technique, which is a combination of individual techniques, also offers significant interference reduction with improved sensitivity. Sensitivity changes with sample tube age. Discard graphite tubes when significant variations in sensitivity or poor reproducibility are observed. The use of high acid concentrations brine samples, and matrix modifiers often drastically reduces tube life. Preferably use the L’vov platform in such situations.

5. Reference

1. SLAVIN. W. 1984. Graphite Furnace AAS—A Source Book. Perkin-Elmer Corp. Norwalk. Conn.

6. Bibliography

FERNANDEZ, F.J. & D.C. MANNING 1971. Atomic absorption analyses of metal pollutants in water using a heated graphite atomizer. Atomic Absorption Newsletter 10:65.

SEGAR. D. A. & J.G. GONZALEZ 1972. Evaluation of atomic absorption with a heated graphite atomizer for the direct determination of trace transition metals in sea water. Anal, Chim, Acta 58:7.

BARNARD, W.M. & M.J. FISHMAN. 1973. Evaluation of the use of heated graphite atomizer for the routine determination of trace metals in water. Atomic Absorption Newsletter 12:118.

KAHN. H.L. 1973. The detection of metallic elements in wastes and waters with the graphite furnace. Int. J. Environ. Anal. Chem. 3:121.

WALSH, P.R., J.L. FASCHING & R.A. DUCE. 1976. Matrix effects and their control during the flameless atomic absorption determination of arsenic, Anal. Chem. 48:1014.

HENN, E.L. 1977. Use of Molybdenum in Eliminating Matrix Interferences in Flameless Atomic Absorption. Spec. Tech. Publ. 618. American Soc. Testing & Materials, Philadelphia, Pa.

MARTIN. T.D. & J.F. KOPP. 1978. Methods for Metals in Drinking Water. U.S. Environmental Protection Agency. Environmental Monitoring and Support Lab. Cincinnati. Ohio.

HYDES, D.J. 1980. Reduction of matrix effects with a Soluble organic acid in the carbon furnace atomic absorption spectrometric determination of cobalt, copper, and manganese in seawater, Anal. Chem. 52:289.

SOTERA, J.J. & H.L. KAHN. 1982. Background correction in AAS. Amer. Lab. 14:100.

SMITH, S.B. & G.M. HIEFTIE. 1983. A new background-correction method for atomic absorption spectrometry. Appl, Spectrose. 37:419.

GROSSER, Z. 1985. Techniques in Graphite Furnace Atomic Absorption Spectrophotometry. Perkin-Elmer Corp., Ridgefield, Conn.

SLAVIN, W. & G.R. CARNICK. 1985. A survey of applications of the stabilized temperature platform furnace and Zeeman correction. Atomic Spectrose. 6:157.

BRUEGGEMEYER, T. & F. FRICKE. 1986. Comparison of furnace & atomization behavior of aluminum from standard & thorium-treated L’vov platforms. Anal. Chem. 58:1143.

3113 B. Electrothermal Atomic Absorption Spectrometric Method

1. General Discussion

This method is suitable for determination of micro quantities of aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel, selenium, silver, and tin.

2. Apparatus

a. Atomic absorption spectrometer: See Section 3111A. 6a. The instrument must have background correction capability.

b. Source lamps: See Section 3111A.6d.

c. Graphite furnace: Use an electrically heated device with electronic control circuitry designed to carry a graphite tube or cup through a heating program that provides sufficient thermal energy to atomize the elements of interest. Furnace heat controllers with only three heating steps are adequate only for fresh waters with low dissolved solids content. For salt waters, brines, and other complex matrices, use a furnace controller with up to seven individually programmed heating steps. Fit the furnace into the sample compartment of the spectrometer in place of the conventional burner assembly. Use argon as a purge gas to minimize oxidation of the furnace tube and to prevent the formation of metallic oxides. Use graphite tubes with L vov platforms to minimize interferences and to improve sensitivity.

d. Readout: See Section 3111A.6c.

e. Sample dispensers: Use microliter pipets (5 to 100 µL) or an automatic sampling device designed for the specific instrument.

f. Vent: See Section 3111A. 6f.

g. Cooling water supply: Cool with tap water flowing at 1 to 4 L/min or use a recirculating cooling device.

h. Membrane filter apparatus: Use an all-glass filtering device and 0.45-µm membrane filters. For trace analysis of aluminum use device of polypropylene of TFE devices.

3. Reagents

a. Metal-free water: See Section 3111B.3c.

b. Hydrochloric acid, HCI.1 + 1 and cone.

c. Nitric acid. HNO₃, 1 + 1 and cone.

d. Matrix modifiers:

1) Ammonium nitrate, 10% (w/v): Dissolve 100 g NH₄NO₃ in water. Dilute to 1000 mL with water.

2) Ammoniun phosphate. 40%: Dissolve 40 g (NH₄)₂HPO₄ in water. Dilute to 100 mL with water.

3) Calcium nitrate. 20 000 mg Ca/L: Dissolve 11.8 g Ca(NO₃)₂·4H₂O in water. Dilute to 100 mL with water.

4) Nickel nitrate. 10 000 mg Ni/L: Dissolve 49.56 g Ni(NO₃)₂·6H₂O in water. Dilute to 1000 mL with water.

5) Phosphoric acid. 10% (v/v): Dilute 100 mL cone H₃PO₄ to 100 mL with water.

For preparation of other matrix modifiers see references or follow manufacturer’s instructions.

e. Stock metal solutions: Refer to sections 3111B and 3114.

f. Chelating resin: 100 to 200 mesh^* purified by heating at 60°C in 10N NaOH for 24 h. Cool resin and rinse 10 times each with alternating portions of 1N HCI. metal-free water. In NaOH. and metal-free water.

g. Metal-free seawater (or brine): Fill a 1.4-cm-1D × 20-cm-long borosilicate glass column to within 2 cm of the top with purified chelating resin. Elute resin with successive 50-mL portions of 1N HCI, metal-free water. 1N NaOH and metal-free water at the rate of 5 mL/min just before use. Pass salt water or brine through the column at a rate of 5 mL/min to extract trace metals present. Discard the first 10 bed volumes (300 mL) of cluate.

4. Procedures

a. Sample pretreatment: Before analysis, pretreat all samples as indicated below. Rinse all glassware with 1 + 1 HNO₃ and water. Carry out digestion procedures in a clean dust-free laboratory area to avoid sample contamination. For digestion of trace aluminum, use polypropylene of TFE utensils to avoid leachable aluminum from glassware.

1) Dissolved metals—See Section 3030B. For samples requiring arsenic and/or selenium analysis add 3 mL 30% hydrogen peroxide and an appropriate concentration of nickel before analysis. For all other metals no further pretreatment is required except for adding an optional matrix modifier (see Table 3113:1).

2) Total recoverable metals (Al, Sb, Ba, Be, Cd, Cr, Co, Cu, Fe, Pb, Mn, Mo, Ni, Ag, and Sn)—NOTE: Sb and Sn are recovered unless HCI is used in the digestion. See Section 3030D. Quantitatively transfer digested sample to a 100-mL volumetric flask, add an appropriate amount of matrix modifier (if necessary, see Table 3113:1), and dilute to volume with water.

3) Total recoverable metals (As. Se)—Transfer 100 mL of Shaken sample. 1 mL cone HNO₃ and 2 mL 30% H₂O₂ to a clean, acid-washed 250-mL beaker. Heat on a hot plate without allowing solution to boil until volume has been reduced to about 50 mL. Remove from hot plate and let cool to room temperature. Add an appropriate concentration of nickel (See Table 3113:1). and dilute to volume in a 100-mL volumetric flask with water. Simultaneously prepare a digested blank by substituting water for sample and proceed with digestion as described above.

b. Instrument operation: Mount and align furnace device according to manufacturer’s instructions. Turn on instrument and strip-chart recorders. Select appropriate light source and adjust to recommended electrical setting. Select proper wavelength and set all conditions according to manufacturer’s instructions, including background correction. Background correction is important when elements are determined at short wavelengths or when sample has a high level of dissolved solids. In general, background correction is usually not necessary at wavelengths longer than 350 nm. Above 350 nm deuterium are background correction is not useful and other types must be used.

Select proper inter-or sheath-gas flow. In some cases it is desirable to interrupt the inert-gas flow during atomization. Such interruption results in increased sensitivity by increasing residence time of the atomic vapor in the optical path. Gas interruption also increases background absorption and intensities interference effects. Consider advantages and disadvantages of this option for each matrix when optimizing analytical conditions.

* Chelex 100. or equivalent. available from Bio-Rad Laboratories. Richmond. Calif.

To optimize graphite furnace conditions, carefully adjust furnace temperature settings to maximize sensitivity and precision and to minimize interferences. Follow manufacturer’s instructions.

Use drying temperatures slightly above the solvent boiling point to provide enough time and temperature for complete evaporation without boiling or spattering.

The charring temperature must be high enough to maximize volatilization of interfering matrix components yet too low to volatilize the element of interest. With the drying and atomization temperatures set to their optimum values, analyze a standard at a series of charring temperatures in increasing increments of 50 to 100°C. When the optimum charring temperature is exceeded, there will be a significant drop in sensitivity. Plot charring temperature versus sample absorbance: the optimum charring temperature is the highest temperature without reduced sensitivity.

Select atomization temperature by determining the temperature providing maximum sensitivity without significantly eroding precision. Optimize by a series of successive determinations at various atomization temperatures using a standard solution giving an absorbance of 0.2 to 0.5.

c. Instrument calibration: Prepare standards for instrument calibration by dilution of the metal stock solutions. Prepare standards fresh daily.

Prepare a blank and at least three calibration standards in the appropriate concentration range (See Table 3113:11) for correlating element concentration and instrument response. Match the matrix of the standard solutions to those of the samples as closely as possible. In most cases, this simply requires matching the acid background of the samples. For seawaters or brines, however, use the metal-free matrix (¶ 3g) as the standard solution diluent. In addition, add the same concentration of matrix modifier (if required for sample analysis) to the standard solutions.

Inject a suitable portion of each standard solution. In order of increasing concentration. Analyze each standard solution in triplicate to verify method precision.

Construct an analytical curve by plotting the average peak absorbencies, or peak areas of the standard solution versus concentration on linear graph paper. Alternatively, use electronic instrument calibration if the instrument has this capability.

d. Sample analysis: Analyze all samples except those demonstrated to be free of matrix interferences (based on recoveries of 85%-115% for known additions) using the method of standard additions. Analyze all samples at least in duplicate or until reproducible results are obtained. A variation of ≤ 10% is considered acceptable reproducibility. Average replicate values.

1) Direct determination—Inject a measured portion of pretreated sample into the graphite furnace. Use the same volume as was used to prepare the calibration curve. Dry, char, and atomize according to the preset program. Repeat until reproducible results are obtained.

Compare the average absorbance value or peak area to the calibration curve to determine concentration of the element of interest. Alternatively read results directly if the instrument is equipped with this capability. If absorbance (or concentration) or peak area of the most concentrated sample is greater than absorbance (concentration) or peak area of the standard, dilute sample and reanalyze. If very large dilutions are required, another technique (e.g., flame AA or ICP) may be more suitable for this sample. Large dilution factors magnify small errors on final calculation. Keep acid background and concentration of matrix modifier (if required) constant. If sample is diluted with water, add acid and matrix modifier to restore the concentration of both to the original. Alternatively, dilute the sample in a blank solution of acid and matrix modifiers.

Proceed to ¶ 5a below.

2) Method of standard additions—Refer to ¶ 4c above. The method of standard additions is valid only when it falls in the linear portion of the calibration curve. Once instrument sensitivity has been optimized for the element of interest and the linear range for the element has been established, proceed with sample analyses.

Inject a measured volume of sample into furnace device. Dry, char or ash, and atomize samples according to preset program. Repeat until reproducible results are obtained. Record instrument response in absorbance or concentration as appropriate. Add a known concentration of the element of interest to a separate portion of sample so as not to change significantly the sample volume. Repeat the determination.

Add a known concentration (preferably twice that used in the first addition) to a separate sample portion. Mix well and repeat the determination.

Plot averaged absorbance or instrument response for the sample and the two portions with known additions on the vertical axis with the concentrations of element added on the horizontal axis of linear graph paper. Draw a straight line connecting the three points and extrapolate to zero absorbance. The intercept at the horizontal axis is the concentration of the sample. The concentration axis to the left of the origin should be a mirror image of the axis to the right.

5. Calculations

a. Direct determination:

μg metal/L = C × F

where:

C   =   metal concentration as read directly from the instrument or from the calibration curve. μg/L, and

F   =   dilution factor.

b. Method of additions:

μg metal/L = C × F

where:

C   =   metal concentration as read from the method of additions plot, μg/L, and

F   =   dilution factor.

6. Precision and Bias

Data typical of the precision and bias obtainable are presented in Tables 3113:III, IV, and V.

7. Quality Control

See Section 3020 for specific quality control procedures to be followed during analysis. Although previous indications were

that very low optimum concentration ranges were attainable for most metals (see Table 3113:II). data in Table 3113:III using variations of these protocols show that this may not be so. Exercise extreme caution when applying this method to the lower concentration ranges. Verify analyst precision at the beginning of each analytical run by making triplicate analyses.

8. Reference

1. COPELAND. T.R. & J.P. MANEY. 1986. EPA Method Study 31: Trace Metals by Atomic Absorption (Furnace Techniques). EPA-600/S4-85-070. U.S. Environmental Protection Agency. Environmental Monitoring and Support Lab., Cincinnati. Ohio.

9. Bibliography

RENSHAW, G.D. 1973. The determination of barium by flameless atomic absorption spectrophotometry using a modified graphite tube atomizer. Atomic Absorption Newsletter 12:158.

YANGAGISWA, M., T. TAKEUCHI & M. SUZUKI 1973. Flameless atomic absorption spectrometry of antimony. Anal, Chim. Acta 64:381.

RATTONETTI, A. 1974. Determination of soluble cadmium, lead. silver and indium in rainwater and stream water with the use of flameless atomic absorption. Anal. Chem. 46:739.

HENN, E.L. 1975. Determination of selenium in water and industrial effluents by flameless atomic absorption. Anal. Chem. 47:428.

MARTIN, T.D. & J.F. KOPP.. 1975. Determining selenium in water wastewater, sediment and sludge by flameless atomic absorption spectrometry. Atomic Absorption Newsletter 14:109.

MARUTA, T., K. MINEGISHI & G. SUDOH, 1976. The flameless atomic absorption spectrometric determination of aluminum with a carbon atomization system. Anal. Chim. Acta 81:313.

CRANSTON, R.E. & J.W. MURRAY, 1978. The determination of chromium species in natural waters. Anal. Chim. Acta 99:275.

HOFFMEISTER, W. 1978. Determination of iron in ultrapure water by atomic absorption spectroscopy. Z. Anal. Chem. 50:289.

LAGAS. P. 1978. Determination of beryllium. barium. vanadium and some other elements in water by atomic absorption spectrometry with electrothermal atomization. Anal. Chim. Acta 98:261.

CARRONDO. M.J.T., J.N. LESTER & R. PERRY. 1979. Electrothermal atomic absorption determination of total aluminum in waters and waste waters. Anal. Chim. Acta 111:291.

NAKAHARA. T. & C.L. CHAKRABARTI. 1979. Direct determination of traces of molybdenum in synthetic sea water by atomic absorption spectrometry with electrothermal atomization and selective volatilization of the salt matrix. Anal. Chim. Acta 104:99.

TIMINAGA. M. & Y. UMEZAKI. 1979. Determination of submicrogram amounts of tin by atomic absorption spectrometry with electrothermal atomization Anal. Chim. Acta. 110:55.