Method 7740 (SW-846-7740) 

Selenium (Atomic Absorption, Furnace Technique) 

Revision 0, September 1986 

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METHOD 7740 

SELENIUM (ATOMIC ABSORPTION. FURNACE TECHNIQUE) 

1.0 SCOPE AND APPLICATION 

1.1 Method 7740 is an atomic absorption procedure approved for 

determining the concentration of selenium in wastes, mobility-procedure extracts, 

soils, and ground water. All samples must be subjected to an appropriate 

dissolution step prior to analysis. 

2.0 SUMMARY OF METHOD 

2.1 Prior to analysis by Method 7740, samples must be prepared in order 

to convert organic forms of selenium to inorganic forms, to minimize organic 

interferences, and to convert samples to suitable solutions for analysis. The 

sample-preparation procedure varies, depending on the sample matrix. Aqueous 

samples are subjected to the acid-digestion procedure described in this method. 

Sludge samples are prepared using the procedure described in Method 3050. 

2.2 Following the appropriate dissolution of the sample, a representative 

aliquot is placed manually or by means of an automatic sampler into a graphite 

tube furnace. The sample aliquot is then slowly evaporated to dryness, charred 

(ashed), and atomized. The absorption of lamp radiation during atomization will 

be proportional to the selenium concentration. 

2.3 The typical detection limit for this method is 2 ug/L. 

3.0 INTERFERENCES 

3.1 Elemental selenium and many of its compounds are volatile; therefore, 

samples may be subject to losses of selenium during sample preparation. Spike 

samples and relevant standard reference materials should be processed to 

determine if the chosen dissolution method is appropriate. 

3.2 Likewise, caution must be employed during the selection of 

temperature and times for the dry and char (ash) cycles. A nickel nitrate 

solution must be added to all digestates prior to analysis to minimize 

volatilization losses during drying and ashing. 

3.3 In addition to the normal interferences experienced during graphite 

furnace analysis, selenium analysis can suffer from severe nonspecific absorption 

and light scattering caused by matrix components during atomization. Selenium 

analysis is particularly susceptible to these problems because of its low 

analytical wavelength (196.0 nm). Simultaneous background correction is required 

to avoid erroneously high results. High iron levels can give overcorrection with 

deuterium background. Zeeman background correction can be useful in this 

situation. 

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3.4 If the analyte is not completely volatilized and removed from the 

furnace during atomization, memory effects will occur. If this situation is 

detected, the tube should be cleaned by operating the furnace at full power at 

regular intervals in the analytical scheme. 

3.5 Selenium analysis suffers interference from chlorides (>800 mg/L) and 

sulfate (>200 mg/L). The addition of nickel nitrate such that the final 

concentration is 1% nickel will lessen this interference. 

4.0 APPARATUS AND MATERIALS 

4.1 250-mL Griffin beaker. 

4.2 10-mL volumetric flasks. 

4.3 Atomic absorption spectrophotometer: Single- or dual-channel, 

single- or double-beam instrument with a grating monochromator, photomultiplier 

detector, adjustable slits, a wavelength range of 190-800 nm, and provisions for 

simultaneous background correction and interfacing with a strip-chart recorder. 

4.4 Selenium hollow cathode lamp, or electrodeless discharge lamp (EDL): 

EDLs provide better sensitivity for the analysis of Se. 

4.5 Graphite furnace: Any graphite furnace device with the appropriate 

temperature and timing controls. 

4.6 Strip-chart recorder: A recorder is strongly recommended for furnace 

work so that there will be a permanent record and so that any problems with the 

analysis, such as drift, incomplete atomization, losses during charring, changes 

in sensitivity, etc., can easily be recognized. 

4.7 Pipets: Microliter with disposable tips. Sizes can range from 

5 to 1,000 uL, as required. 

5.0 REAGENTS 

5.1 ASTM Type II water (ASTM 01193): Water should be monitored for 

impurities. 

5.2 Concentrated nitric acid (HNO,): Acid should be analyzed to 

determine levels of impurities. If a method blank made with the acid is <MDL, 

the acid can be used. 

5.3. Hydrogen peroxide (30%): Oxidant should be analyzed to determine 

levels of impurities. If a method blank made with the oxidant is <MDL, the 

oxidant can be used. 

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5.4 Selenium standard stock solution (1,000 mg/L): Either procure a 

certified aqueous standard from a supplier and verify by comparison with a second 

standard, or dissolve 0.3453 g of selenious acid (actual assay 94.6% H 2 Se03 , 

analytical reagent grade) or equivalent in Type II water and dilute to 200 mL. 

5.5 Nickel nitrate solution (5%): Dissolve 24.780 g of ACS reagent grade 

Ni(NO 3 ) 2 .6H 2 0 or equivalent in Type II water and dilute to 100 mL. 

5.6 Nickel nitrate solution (1%): Dilute 20 mL of the 5% nickel nitrate 

to 100 mL with Type II water. 

5.7 Selenium working standards: Prepare dilutions of the stock solution 

to be used as calibration standards at the time of the analysis. Withdraw 

appropriate aliquots of the stock solution, add 1 mL of concentrated NNO 3 , 2 mL 

of 30% H 2 0 2 , and 2 mL of the 5% nickel nitrate solution. Dilute to 100 mL with 

Type II water. 

5.8 Air: Cleaned and dried through a suitable filter to remove oil, 

water, and other foreign substances. The source may be a compressor or a 

cylinder of industrial-grade compressed air. 

5.9 Hydrogen: Suitable for instrumental analysis. 

6.0 SAMPLE COLLECTION, PRESERVATION, AND HANDLING 

6.1 All samples must have been collected using a sampling plan that 

addresses the considerations discussed in Chapter Nine of this manual. 

6.2 All sample containers must be prewashed with detergents, acids, and 

Type II water. Plastic and glass containers are both suitable. 

6.3 Special containers (e.g., containers used for volatile organic 

analysis) may have to be used if very volatile selenium compounds are to be 

analyzed. 

6.4 Aqueous samples must be acidified to a pH of <2 with nitric acid. 

6.5 Nonaqueous samples shall be refrigerated, when possible, and analyzed 

as soon as possible. 

7.0 PROCEDURE 

7.1 Sample preparation: Aqueous samples should be prepared in the manner 

described in Steps 7.1.1 to 7.1.3. Sludge-type samples should be prepared 

according to Method 3050. The applicability of a sample-preparation technique 

to a new matrix type must be demonstrated by analyzing spiked samples and/or 

relevant standard reference materials. 

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7.1.1 Transfer 100 mL of well-mixed sample to a 250-mL Griffin 

beaker; add 2 mL of 30% H 2 0 2 and sufficient concentrated HNO 3 to result in 

an acid concentration of 1% (v/v). Heat for 1 hr at 95°C or until the 

volume is slightly less than 50 mL. 

7.1.2 Cool and bring back to 50 mL with Type II water. 

7.1.3 Pipet 5 mL of this digested solution into a 10-mL volumetric 

flask, add 1 mL of the 1% nickel nitrate solution, and dilute to 10 mL 

with Type II water. The sample is now ready for injection into the 

furnace. 

7.2 The 196.0-nm wavelength line and a background correction system must 

be employed. Follow the manufacturer's suggestions for all other spectrophotometer parameters. 

7.3 Furnace parameters suggested by the manufacturer should be employed 

as guidelines. Because temperature-sensing mechanisms and „temperature 

controllers can vary between instruments or with time, the validity of the 

furnace parameters must be periodically confirmed by systematically altering the 

furnace parameters while analyzing a standard. In this manner, losses of analyte 

due to overly high temperature settings or losses in sensitivity due to less than 

optimum settings can be minimized. Similar verification of furnace parameters 

may be required for complex sample matrices. 

7.4 Inject a measured uL-aliquot of sample into the furnace and atomize. 

If the concentration found is greater than the highest standard, the sample 

should be diluted in the same acid matrix and reanalyzed. The use of multiple 

injections can improve accuracy and help detect furnace pipetting errors. 

7.5 Analyze all EP extracts, all samples analyzed as part of a delisting 

petition, and all samples that suffer from matrix interferences by the method of 

standard additions. 

7.6 Run a check standard after approximately every 10 sample injections. 

Standards are run in part to monitor the life and performance of the graphite 

tube. Lack of reproducibility or significant change in the signal for the 

standard indicates that the tube should be replaced. 

7.7 Duplicates, spiked samples, and check standards should be analyzed 

every 20 samples. 

7.8 Calculate metal concentrations: (1) by the method of standard 

additions, (2) from a calibration curve, or (3) directly from the instrument's 

concentration read-out. All dilution or concentration factors must be taken into 

account. 

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8.0 QUALITY CONTROL 

8.1 All quality control data should be maintained and available for easy 

reference or inspection. 

8.2 Calibration curves must be composed of a minimum of a blank and three 

standards. A calibration curve should be made for every hour of continuous 

sample analysis. 

8.3 Dilute samples if they are more concentrated than the highest. 

standard or if they fall on the plateau of a calibration curve. 

8.4 Employ a minimum of one blank per sample batch to determine if 

contamination or any memory effects are occurring. 

8.5 Verify calibration with an independently prepared check standard 

every 15 samples. 

8.6 Run one spike duplicate sample for every 10 samples. A duplicate 

sample is a sample brought through the entire sample preparation and analytical 

process. 

8.7 The method of standard additions (see Method 7000, Section 8.7) shall 

be used for the analysis of all EP extracts, on all analyses submitted as part 

of a delisting petition, and whenever a new sample matrix is being analyzed. 

9.0 METHOD PERFORMANCE 

9.1 Precision and accuracy data are available in Method 270.2 of Methods 

for Chemical Analysis of Water and Wastes. 

9.2 The data shown in Table 1 were obtained from records of state and 

contractor laboratories. The data are intended to show the precision of the 

combined sample preparation and analysis method. 

10.0 REFERENCES 

1. Methods for Chemical Analysis of Water and Wastes, EPA-600/4-82-055, 

December 1982, Method 270.2. 

2. Gaskill, A., Compilation and Evaluation of RCRA Method Performance Data, 

Work Assignment No. 2, EPA Contract No. 68-01-7075, September 1986. 

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TABLE 1. METHOD PERFORMANCE DATA 

Sample Preparation Laboratory 

Matrix Method Replicates 

Emission control dust 3050 14, 11 ug/g 

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METHOD 7740 

SELENIUM (ATOMIC ABSORPTION, FURNACE METHOD) 

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7.1 

Type 

of sample for 

sample 

preparation 

Sludge-type 

samples 

Aqueous 

samples 

V 

7.1.1 Transfer 

portion of sample 

to beaker; add 30% 

H202 and conc. 

HNO3 ; heat. 

7.1.2 Cool; 

bring to 

volume. 

7.1.3 Pipet digested 

solution into flask; 

add nickel nitrate 

solution; dilute. 

7.1 Prepare 

sample according 

to Method 3050. 

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II HHH [ -TT 
7.4 Dilute 

sample and 

reanalyze. 

Stop 

7.2 Set 

instrument 

parameters. 

7.3 Periodically 

check validity 

of furnace 

parameters. 

7.4 Inject sample 

into furnace; 

atomize. 

7.5 Analyze by 

method of 

standard 

addition. 

7.6 Run check 

standard after 

10 sample 

injections. 

7.7 Routinely 

analyze duplicates, 

spiked samples, 

and check 

standards. 

7.8 Calculate 

metal 

concentrations. 

METHOD 7740 

SELENIUM (ATOMIC ABSORPTION, FURNACE METHOD) 

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