## 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 fl ame techniques without extraction or sample concentration. This increase in sensitivity results from an increase in atom density within the furnace as compared to fl ame atomic absorption. Many elements can be determined at concentrations as low as 1.0 g/L. An additional advantage of electrothermal atomic absorption is that only a very small volume of sample is required.

The electrothermal technique is used only at concentration levels below the optimum range of direct fl ame atomic absorption because it is subject to more interferences than the fl ame procedure and requires increased analysis time. The method of standard additions may be required to insure validity of data. Because of the high sensitivity of this technique, it is extremely susceptible to contamination; extra care in sample handling and analysis may be required.

2. Principle



### Electrothermal atomic absorption spectroscopy is based on the same principle as direct fl ame 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 vaporabsorbs monochromatic radiation from the source. A photoelectric detector measures the intensity of transmitted radiation. The inverse of the transmittance is related logarithmically to the absorbance, which is directly proportional to the number density of vaporized ground-state atoms (the Beer-Lambert law) over a limited concentration range.

3. Interferences



### Electrothermal atomization determinations may be subject to signi fi cant interferences from molecular absorption as well as chemical and matrix effects. Molecular absorption may occurwhen 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 arc can correct for background up to absorbance levels of about 0.8. Continuum lamp intensity diminishes at long wavelengths and use of continuum background correction is limited to analyticalwavelengths below 350 nm. Zeeman effect background correctors can handle background absorbance up to 1.5 to 2.0. TheSmith-Hieftje correction technique can accommodate background absorbance levels as large as 2.5 to 3.0 (see Section3111A.3). Both Zeeman and Smith-Hieftje background corrections are susceptible to rollover (development of a negative absorbance-concentration relationship) at high absorbances. The rollover absorbance for each element should be available in the manufacturer ’ s literature. Curvature due to rollover should become apparent during calibration; dilution produces a morelinear calibration plot. 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 modi fi cation can be useful in minimizing interferenceand increasing analytical sensitivity. Determine need for a modi fi er by evaluating recovery of a sample with a known addition. Recovery near 100% indicates that sample matrix does not affect analysis. Chemical modi fi ers generally modify relative volatilities of matrix and metal. Some modi fi ers enhance matrix removal, isolating the metal, while other modi fi ers inhibit metalvolatilization, allowing use of higher ashing/charring temperatures and increasing ef fi ciency of matrix removal. Chemical modi fi ers are added at high concentration (percent level) and can lead to sample contamination from impurities in the modi fi er solution. Heavy use of chemical modi fi ers may reduce the useful life (normally 50 to 100 firings) of the graphite tube. Some speci fi c chemical modi fi ers and approximate concentrations are listed in Table 3113:I.

Addition of a chemical modi fi er directly to the sample before analysis is restricted to inexpensive additives (e.g. phosphoric acid). Use of palladium salts for matrix modi fi cation normally requires methods of co-addition, in which sample and modi fi erare added consecutively to the furnace either manually or, preferably, with an automatic sampler. Palladium salts (nitrate is preferred, chloride is acceptable) are listed in Table 3113:I as a modi fi er for many metals. The palladium solution (50 to 2000 mg/L) generally includes citric or ascorbic acid, which aids reduction of palladium in the furnace. Citric acid levels of 1 to2% are typical. Use of hydrogen (5%) in the coolant gas (available commercially as a mixture) also reduces palladium, eliminating need for organic reducing acids. C AUTION 

### : Do not mix hydrogen and other gases in the laboratory; hydrogen gas is very fl ammable

— handle with caution . Use low levels of palladium (50 to 250 mg/L) for normal samples and higher levels for complex samples. Addition of excess palladium modi fi er may widen atomization peaks; in such cases peak area measurements * Approved by Standard Methods Committee, 1999. Joint Task Group: 20th Edition

—Raymond J. Lovett (chair), David J. Kaptain. ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY (3113)/Introduction 3-25

### may provide higher quality results. The recommended mode of modi fi er use is through co-addition to the furnace of about 10  L of the palladium (or other) modi fi er solution. Palladium may not be the best modi fi er in all cases and cannot be recommended unconditionally. Test samples requiring a modi fi er first with palladium; test other modi fi ers only if palladium is unsuccessful or to minimize modi fi er cost. See Section 3113B.3 for preparation of modi fi er solution.

Temperature ramping, i.e., gradual heating, can be used todecrease 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. Ramp drying is used for samples containing mixtures of solvents or for samples with a high salt content (to avoid spattering). If spattering is suspected, develop drying ramp by visual inspection of the drying stage, using a mirror. Samples that contain a complex mixture of matrixcomponents sometimes require ramp charring to effect controlled, complete thermal decomposition. Ramp atomization may minimize background absorption by permitting volatilization ofthe element being determined before the matrix. This is especially applicable in the determination of such volatile elements as cadmium and lead. Use of time-resolved absorbance pro fi les (available on most modern instruments) greatly aids method development. Changes in atomization, notably the element peakappearance time and magnitude of background and metal absorbances, can be monitored directly.

Improve analysis by using a graphite platform, inserted into the graphite tube, as the atomization site. The platform is not heated as directly by the current fl owing through the graphite tube; thus the metal atomizes later and under more uniform conditions.

Use standard additions to compensate for matrix interferences. When making standard additions, determine whether the added metal and that in the sample behave similarly under the speci fi ed conditions. [See Section 3113B.4 d2)]. In the extreme, test every sample for recovery (85 to 115% recovery desired) to determine if standard addition is needed. Test every new sample type for recovery. Recovery of only 40 to 85% generally indicates that standard addition is required. Often, as long as the samples are from sources of consistent properties, a representative recovery can be used to characterize the analysis and determine the necessity of standard addition. Test samples of unknown origin or of complex composition (digestates, for example) individually for metal recovery. Ideally, chemical modi fi ers and graphite platforms render the sample fi t to be analyzed using a standard analytical calibration curve. Always verify this assumption; however, a properly developed method with judicious use of chemical modi fi ers should eliminate the necessity for standard addition in all but the most extreme samples.

Chemical interaction of the graphite tube with various elements to form refractory carbides occurs at high charring andatomization 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.

4. Sensitivity, Detection Levels, and Optimum Concentration Range



### Estimated detection levels and optimum concentration ranges are listed in Table 3113:II. These values may vary with thechemical 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 fl ow rate of the purge gas or by using gas interrupt during atomization. Note, however, that these techniques also will increase the effects ofany interferences present. Sensitivity can be decreased by diluting the sample, reducing sample volume, increasing purge-gas fl ow, 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 TABLE 3113:I. P OTENTIAL M ATRIX M ODIFIERS FOR ELECTROTHERMAL ATOMIC ABSORPTION S PECTROMETRY *

Modi fi er Analyses for Which Modi fi er May Be Useful 1500 mg Pd/L 1000 mg

Mg(NO 3 ) 2/L 1 Ag, As, Au, Bi, Cu, Ge, Mn, Hg, In, Sb, Se, Sn, Te, Tl 500

–2000 mg Pd/L reducing

agent2 † Ag, As, Bi, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sb 5000 mg Mg(NO 3 ) 2/L 1 Be, Co, Cr, Fe, Mn, V 100

–500 mg Pd/L 2 As, Ga, Ge, Sn 50 mg Ni/L 2 As, Se, Sb 2% PO4 3 1000 mg Mg (NO 3 ) 2 /L 1 Cd, Pb
*Assumes 10  L modi fi er/10  L sample. † Citric acid (1

–2%) preferred; ascorbic acid or H 2 acceptable. TABLE 3113:II. D ETECTION LEVELS AND C ONCENTRATION RANGES FOR ELECTROTHERMAL ATOMIZATION ATOMIC ABSORPTION S PECTROMETRY

Element Wavelength
nm Estimated Detection

Level 

g/L Optimum Concentration Range 

g/L
Al 309.3 3 20

–200 Sb 217.6 3 20

– 300 As 193.7 1 5

– 100 Ba 553.6 2 10

–200 Be 234.9 0.2 1

– 30 Cd 228.8 0.1 0.5

– 10 Cr 357.9 2 5

– 100 Co 240.7 1 5

– 100 Cu 324.7 1 5

– 100 Fe 248.3 1 5

– 100 Pb* 283.3 1 5

– 100 Mn 279.5 0.2 1

– 30 Mo 313.3 1 3

– 60 Ni 232.0 1 5

– 100 Se 196.0 2 5

– 100 Ag 328.1 0.2 1

–25 Sn 224.6 5 20

– 300 *The more sensitive 217.0-nm wavelength is recommended for instruments with background correction capabilities. 3-26 METALS (3000)

### as a reducing agent, thereby aiding in producing more groundstate atoms. Pyrolytically coated graphite tubes can increasesensitivity for the more refractory elements and are recommended. 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 signi fi cant interference reduction with improved sensitivity. Sensitivity changes with sample tube age. Discard graphite tubes when signi fi cant variations in sensitivity or poor reproducibility are observed. The use of high acid concentrations, brine samples, and matrix modi fi ers often drastically reduces tube life. Preferably use the graphite platform in such situations.
5. References

1. PERKIN -ELMER C ORP . 1991. Summary of Standard Conditions for

Graphite Furnace. Perkin-Elmer Corp., Norwalk, Conn. 2. R OTHERY , E., ed. 1988. Analytical Methods for Graphite Tube Atomizers. Varian Techtron Pty, Ltd., Mulgrave, Victoria, Australia. 

### 6. Bibliography

FERNANDEZ , F.J. & D.C. M ANNING . 1971. Atomic absorption analyses of metal pollutants in water using a heated graphite atomizer. Atomic Absorption Newsletter 10:65. S EGAR , D.A. & J.G. G ONZALEZ . 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. B ARNARD , 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. D UCE . 1976. Matrix effects and their control during the fl ameless 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. M ARTIN , T.D. & J.F. KOPP . 1978. Methods for Metals in Drinking Water. U.S. Environmental Protection Agency, Environmental Monitoring and Support Lab., Cincinnati, Ohio. H YDES , D.J. 1980. Reduction of matrix effects with a soluble organicacid in the carbon furnace atomic absorption spectrometric determination of cobalt, copper, and manganese in seawater. Anal. Chem. 52:289. S OTERA , J.J. & H.L. KAHN . 1982. Background correction in AAS. Amer. Lab. 14:100. S MITH , S.B. & G.M. H IEFTJE . 1983. A new background-correction method for atomic absorption spectrometry. Appl. Spectrosc. 37: 419. GROSSER , Z. 1985. Techniques in Graphite Furnace Atomic Absorption Spectrophotometry. Perkin-Elmer Corp., Ridge fi eld, Conn. S LAVIN , W. & G.R. C ARNICK . 1985. A survey of applications of the stabilized temperature platform furnace and Zeeman correction. Atomic Spectrosc. 6:157. B RUEGGEMEYER , 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. It is also applicable to analysisof bismuth, gallium, germanium, gold, indium, mercury, tellurium, thallium, and vanadium, but precision and accuracy data are not yet available.

2. Apparatus



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

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

. 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 suf fi cient thermalenergy 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 tominimize oxidation of the furnace tube and to prevent the formation of metallic oxides. Use graphite tubes with platforms to minimize interferences and to improve sensitivity.

d. Readout: See Section 3111A.6 c
. e. Sample dispensers: Use microliter pipets (5 to 100  L) or an automatic sampling device designed for the speci fi c instrument.

f. Vent: See Section 3111A.6 f

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

h. Membrane fi lter apparatus: Use an all-glass fi ltering device and 0.45-  m or smaller-pore-diameter membrane fi lters. For trace analysis of aluminum, use polypropylene or TFE devices.

3. Reagents



### a. Metal-free water: See Section 3111B.3 c
. b. Hydrochloric acid, HCl, 1 1 and conc. c. Nitric acid, HNO 3 

### , 1 1 and conc. d. Matrix modi fi er stock solutions: 1) Magnesium nitrate , 10 000 mg Mg/L: Dissolve 10.5 g Mg(NO 3 

### ) 2 

### 6H 2 

### O in water. Dilute to 100 mL. ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY (3113)/Electrothermal Atomic Absorption Spectrometric Method 3-27

### 2) Nickel nitrate , 10 000 mg Ni/L: Dissolve 4.96 g Ni(NO 3 

### ) 2 

### 6H 2 

### O in water. Dilute to 100 mL.

3) Phosphoric acid, 10% (v/v): Add 10 mL conc H 3 

### PO 4 

### to water. Dilute to 100 mL.

4) Palladium nitrate , 4000 mg Pd/L: Dissolve 8.89 g Pd(NO 3 

### ) 2 

### H 2 

### O in water. Dilute to 1 L.

5) Citric acid, 4%: Dissolve 40 g citric acid in water. Dilute to 1 L.

N OTE 

### : All of the modi fi er solutions recommended in Table 3113:I can be prepared with volumetric combination of theabove solutions and water. For preparation of other matrix modi fi ers, see references or follow manufacturers ’ instructions.

e. Stock metal solutions: Refer to Sections 3111B and 3114. f. Chelating resin: 100 to 200 mesh* puri fi ed by heating at 60 ° C in 10N NaOH for 24 h. Cool resin and rinse 10 times each with alternating portions of 1 N HCl, metal-free water, 1 N NaOH, and metal-free water.

g. Metal-free seawater (or brine): Fill a 1.4-cm-ID  20-cmlong borosilicate glass column to within 2 cm of the top with puri fi ed chelating resin. Elute resin with successive 50-mL portions of 1 N HCl, metal-free water, 1 N 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 eluate.
4. Procedures



### a. Sample pretreatment: Before analysis, pretreat all samples as indicated below. Rinse all glassware with 1 1 HNO 3 

### andwater. Carry out digestion procedures in a clean, dust-free laboratory area to avoid sample contamination. For digestion of trace aluminum, use polypropylene or 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/100 mL sample and an appropriate volume of nickel nitrate solution (see Table 3113:I) before analysis. For all other metals no further pretreatment is required except for adding an optional matrix modi fi er.

2) Total recoverable metals (Al, Sb, Ba, Be, Cd, Cr, Co, Cu, Fe, Pb, Mn, Mo, Ni, Ag, and Sn)

—N OTE 

### : Sb and Sn are not recovered unless HCl is used in the digestion. See Section 3030D. Quantitatively transfer digested sample to a 100-mL volumetric fl ask, add an appropriate amount of matrix modi fi er (see Table 3113:I), and dilute to volume with water.

3) Total recoverable metals (As, Se)

—Transfer 100 mL of shaken sample, 1 mL conc HNO 3 

### , and 2 mL 30% H 2 

### O 2 

### 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:I), and dilute to volume in a 100-mL volumetric fl ask with water.Substitution of palladium is uneconomical. Nickel may be deleted if palladium is co-added during analysis. 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 data collection system. 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 whensample has a high level of dissolved solids. Background correction normally is not necessary at wavelengths longer than 350 nm. If background correction above 350 nm is needed deuterium arc background correction is not useful and other types must be used.

Select proper inert- or sheath-gas fl ow. In some cases, it is desirable to interrupt the inert-gas fl ow during atomization. Suchinterruption results in increased sensitivity by increasing residence time of the atomic vapor in the optical path. Gas interruption also increases background absorption and intensi fi es interference effects, but modern background correction methodsusually eliminate these problems. Consider advantages and disadvantages of this option for each matrix when optimizing analytical conditions.

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 and provide enough time and temperature for complete evaporation without boiling or spattering.

Select atomization temperature by determining the lowest temperature providing maximum sensitivity without signi fi

cantly eroding precision. Optimize by a series of successivedeterminations at various atomization temperatures using a standard solution giving an absorbance of 0.2 to 0.5.

The charring temperature must be high enough to maximize volatilization of interfering matrix components yet too low tovolatilize the element of interest. With the drying and atomization temperatures set to their optimum values, analyze a standardsolution 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 signi fi cant drop in sensitivity. Plot charring temperature versus sample absorbance: the optimum charring temperature is the highest temperature without reduced sensitivity. Verify optimization with major changes in sample matrix.

c. Instrument calibration: Prepare standard solutions for instrument calibration by diluting metal stock solutions. Prepare standard solutions fresh daily.

Prepare a blank and at least three calibration standards in theappropriate concentration range (see Table 3113:II) 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 (¶ 3 g ) as the standard solution diluent. In addition, add the same concentration of matrix modi fi er (if required for sample analysis) to the standard solutions.

Inject a suitable portion of each standard solution, in order ofincreasing concentration. Analyze each standard solution in triplicate to verify method precision. * Chelex 100, or equivalent, available from Bio-Rad Laboratories, Richmond, CA. 3-28 METALS (3000)

### Construct an analytical curve by plotting the average peakabsorbances 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% to 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. Usually add modi fi er immediately after the sample, preferably using an automatic sampler or a micropipet. Some methods require modi fi er to beinjected before the sample. Use the same volume and concenTABLE 3113:III. INTERLABORATORY S INGLE -ANALYST PRECISION D ATA FOR ELECTROTHERMAL ATOMIZATION M ETHODS 1 Single-Analyst Precision % RSD

Element Concentration 

g/L Lab Pure Water Drinking Water Surface Water Ef fl uent

1 Ef fl uent 2 Ef fl uent 3
Al 28 66 108 70

— — 66 125 27 35 24

— — 34 11 000 11

— — 22

— — 58 300 27

— — 19

— — 460 9

— —— 30

— 2 180 28

— —— 4

— 10.5 20 13 13 13 56 18 230 10 18 13 21 94 14 As 9.78 40 25 15 74 23 11 227 10 6 8 11 15 6 Ba 56.5 36 21 29 59 23 27 418 14 12 20 24 24 18 Be 0.45 18 27 15 30 2 11 10.9 14 4 9 7 12 12 Cd 0.43 72 49 1 121 35 27 12 11 17 22 14 11 15 Cr 9.87 24 33 10 23 15 10 236 16 7 11 13 16 7 Co 29.7 10 17 10 19 24 12 420 8 11 13 14 9 5 Cu 10.1 49 47 17 17

— 30 234 8 15 6 21

— 11 300 6

— —— 11

— 1 670 11

— —— 6

— Fe 26.1 144 52 153

— — 124 455 48 37 45

— — 31 1 030 17

— — 30

— — 5 590 6

— — 32

— —

370 14

— —— 19

— 2 610 9

— —— 18

— Pb 10.4 6 19 17 21 19 33 243 17 7 17 18 12 16 Mn 0.44 187 180

— —— 275 14.8 32 19

— —— 18 91.0 15

— — 48

— — 484.0 4

— — 12

— — 111.0 12

— —— 21

— 666.0 6

— —— 20

— Ni 26.2 20 26 25 24 18 9 461.0 15 11 9 8 11 4 Se 10.0 12 27 16 35 41 13 235.0 6 6 15 6 13 14 Ag 8.48 10

— — 15 27 16 56.5 14

— — 7 16 23

0.45 27 166 48

—— — 13.6 15 4 10

—— — ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY (3113)/Electrothermal Atomic Absorption Spectrometric Method 3-29

### tration of modi fi er for all standards and samples. Dry, char, andatomize 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) orpeak area of the sample is greater than absorbance (concentration) or peak area of the most concentrated standard solution,dilute sample and reanalyze. If very large dilutions are required, another technique (e.g., fl ame AA or ICP) may be more suitable for this sample. Large dilution factors magnify small errors on fi nal calculation. Keep acid background and concentration of matrix modi fi er (if present in the solutions) constant. Dilute the sample in a blank solution of acid and matrix modi fi ers.

TABLE 3113:IV. INTERLABORATORY O VERALL PRECISION D ATA FOR ELECTROTHERMAL ATOMIZATION M ETHODS 1
Overall Precision % RSD

Element Concentration 

g/L Lab Pure Water Drinking Water Surface Water Ef fl uent

1 Ef fl uent 2 Ef fl uent 3
Al 28 99 114 124

— — 131 125 45 47 49

— — 40 11 000 19

— — 43

— — 58 300 31

— — 32

— — 460 20

— —— 47

— 2 180 30

— —— 15

— 10.5 37 19 22 50 103 39 230 26 16 16 17 180 21 As 9.78 43 26 37 72 50 39 227 18 12 13 20 15 14 Ba 56.5 68 38 43 116 43 65 418 35 35 28 38 48 16 Be 0.45 28 31 15 67 50 35 10.9 33 15 26 20 9 19 Cd 0.43 73 60 5 88 43 65 12 19 25 41 26 20 27 Cr 9.87 30 53 24 60 41 23 236 18 14 24 20 14 20 Co 29.7 13 26 17 18 21 17 420 21 21 17 18 13 13 Cu 10.1 58 82 31 32

— 74 234 12 33 19 21

— 26 300 13

— —— 14

— 1 670 12

— —— 13

— Fe 26.1 115 93 306

— — 204 455 53 46 53

— — 44 1 030 32

— — 25

— — 5 590 10

— — 43

— —

370 28

— —— 22

— 2 610 13

— —— 22

— Pb 10.4 27 42 31 23 28 47 243 18 19 17 19 19 25 Mn 0.44 299 272

——— 248 14.8 52 41

——— 29 91.0 16

— — 45

— — 484.0 5

— — 17

— — 111.0 15

— —— 17

— 666.0 8

— —— 24

— Ni 26.2 35 30 49 35 37 43 461.0 23 22 15 12 21 17 Se 10.0 17 48 32 30 44 51 235.0 16 18 18 17 22 34 Ag 8.48 23

— — 16 35 34 56.5 15

— — 24 32 28

0.45 57 90 368

——— 13.6 19 19 59

——— 3-30 METALS (3000)

### Proceed to ¶ 5 a below. 2) Method of standard additions

—Refer to ¶ 4 c above. The method of standard additions is valid only when it falls in thelinear 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.

TABLE 3113:V. INTERLABORATORY RELATIVE ERROR D ATA FOR ELECTROTHERMAL ATOMIZATION M ETHODS 1
Relative Error %

Element Concentration 

g/L Lab Pure Water Drinking Water Surface Water Ef fl uent

1 Ef fl uent 2 Ef fl uent 3
Al 28.0 86 150 54

— — 126 125.0 4 41 39

— — 30 11 000.0 2

— — 14

— — 58 300.0 12

— — 7

— — 460.0 2

— —— 11

— 2 180.0 11

— —— 9

— Sb 10.5 30 32 28 24 28 36 230.0 35 14 19 13 73 39 As 9.78 36 1 22 106 13 16 227.0 3 7 10 19 6 13 Ba 56.5 132 54 44 116 59 40 418.0 4 0 0 13 6 60 Be 0.45 40 16 11 16 10 15 10.9 13 2 9 7 8 8 Cd 0.43 58 45 37 66 16 19 12.0 4 6 5 22 18 3 Cr 9.87 10 9 4 2 5 15 236.0 11 0 9 13 5 8 Co 29.7 7 7 1 6 3 13 420.0 12 8 8 11 5 18 Cu 10.1 16 48 2 5

— 15 234.0 8 7 0 4

— 19 300.0 4

— —— 21

— 1 670.0 6

— —— 2

— Fe 26.1 85 60 379

— — 158 455.0 43 22 31

— — 18 1 030.0 8

— — 8

— — 5 590.0 2

— — 12

— —

370.0 4

— —— 11

— 2 610.0 35

— —— 2

— Pb 10.4 16 10 17 1 34 14 243.0 5 15 8 18 15 29 Mn 0.44 332 304

— —— 556 14.8 10 1

— —— 36 91.0 31

— — 10

— — 484.0 42

— — 4

— — 111.0 1

— —— 29

— 666.0 6

— —— 23

— Ni 26.2 9 16 10 7 33 54 461.0 15 19 18 31 16 18 Se 10.0 12 9 6 36 17 37 235.0 7 7 0 13 10 17 Ag 8.48 12

— — 1 51 20 56.5 16

— — 8 51 22

0.45 34 162 534

—— — 13.6 3 12 5

—— — ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY (3113)/Electrothermal Atomic Absorption Spectrometric Method 3-31

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

Using linear graph paper, plot average absorbance or instrument response for the sample and the additions on the vertical axis against the concentrations of the added element on the horizontal axis, using zero as the concentration for the sample. Draw a straight line connecting the three points and extrapolate to zero absorbance. The intercept at the horizontal axis is thenegative of the element concentration in 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:



g metal/L  C  F


### where: C  metal concentration as read directly from the instrument or from the calibration curve,  g/L, and F  dilution factor. 

### b. Method of additions:



g metal/L  C  F


### where: C  metal concentration as read from the method of additions plot,  g/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 speci fi c quality control procedures to be followed during analysis. Although previous indicationswere 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 care when applying this method to the lower concentration ranges. Verify analyst precision at the beginning of each analytical run by making triplicate analyses. Verify autosampler precision by checking volumes (by weight) delivered by the autosampler at routinely used injection volume settings.

8. Reference

1. C OPELAND , T.R. & J.P. M ANEY . 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 fl ameless atomic absorption spectrophotometry using a modi fi ed graphite tube atomizer. Atomic Absorption Newsletter 12:158. Y ANAGISAWA , M., T. TAKEUCHI & M. S UZUKI . 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 fl ameless atomic absorption. Anal. Chem. 46:739. HENN , E.L. 1975. Determination of selenium in water and industrial ef

fl uents by fl ameless atomic absorption. Anal. Chem. 47:428. M ARTIN , T.D. & J.F. KOPP . 1975. Determining selenium in water, wastewater, sediment and sludge by fl ameless atomic absorption spectrometry. Atomic Absorption Newsletter 14:109. M ARUTA , T., K. M INEGISHI & G. S UDOH . 1976. The fl ameless atomic absorption spectrometric determination of aluminum with a carbon atomization system. Anal. Chim. Acta 81:313. C RANSTON , R.E. & J.W. M URRAY . 1978. The determination of chromium species in natural waters. Anal. Chim. Acta 99:275. H OFFMEISTER , 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. C ARRONDO , 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. C HAKRABARTI . 1979. Direct determination of tracesof molybdenum in synthetic sea water by atomic absorption spectrometry with electrothermal atomization and selective volatilization of the salt matrix. Anal. Chim. Acta 104:99. T IMINAGA , M. & Y. UMEZAKI . 1979. Determination of submicrogramamounts of tin by atomic absorption spectrometry with electrothermal atomization. Anal. Chim. Acta 110:55. 3-32 METALS (3000)