US EPA Method 7A - Determination Of Nitrogen Oxide Emissions From Stationary Sources (Ion Chromatographic Method)
NOTE: This method does not include all of the specifications (e.g., equipment and supplies) and procedures (e.g., sampling and analytical) essential to its performance. Some material is incorporated by reference from other methods in this part. Therefore, to obtain reliable results, persons using this method should have a thorough knowledge of at least the following additional test methods: Method 1, Method 3, Method 5, and Method 7.
|Nitrogen oxides(NOx), as NO2, including:
Nitric oxide (NO)
Nitrogen dioxide (NO2)
This method is applicable for the determination of NOx emissions from stationary sources.
Adherence to the requirements of this method will enhance the quality of the data obtained from air pollutant sampling methods.
A grab sample is collected in an evacuated flask containing a dilute sulfuric acid-hydrogen peroxide absorbing solution. The nitrogen oxides, excluding nitrous oxide (N2O), are oxidized to nitrate and measured by ion chromatography.
Biased results have been observed when sampling under conditions of high sulfur dioxide concentrations (above 2000ppm).
5.1 This method may involve hazardous materials, operations, and equipment. This test method may not address all of the safety problems associated with its use. It is the responsibility of the user of this test method to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to performing this test method.
The following reagents are hazardous. Personal protective equipment and safe procedures are useful in preventing chemical splashes. If contact occurs, immediately flush with copious amounts of water at least 15 minutes. Remove clothing under shower and decontaminate. Treat residual chemical burns as thermal burns.
Irritating to eyes, skin, nose, and lungs.
Rapidly destructive to body tissue. Will cause third degree burns. Eye damage may result in blindness. Inhalation may be fatal from spasm of the larynx, usually within 30 minutes. May cause lung tissue damage with edema. 3 mg/m3 will cause lung damage in uninitiated. 1 mg/m3 for 8 hours will cause lung damage or, in higher concentrations, death. Provide ventilation to limit inhalation. Reacts violently with metals and organics.
Same as in Method 7, Section 6.1.
Same as in Method 7, Section 6.2, except the stirring rod and pH paper are not needed.
For the analysis, the following equipment and supplies are required. Alternative instrumentation and procedures will be allowed provided the calibration precision requirement in Section 10.1.2 and audit accuracy requirement in Section 11.3 can be met.
Class A; 1-, 2-, 4-, 5-ml (two for the set of standards and one per sample), 6-, 10-, and graduated 5-ml sizes.
50-ml (two per sample and one per standard), 200-ml, and 1-liter sizes.
To measure to within 0.1 mg.
The ion chromatograph should have at least the following components:
An anion separation or other column capable of resolving the nitrate ion from sulfate and other species present and a standard anion suppressor column (optional). Suppressor columns are produced as proprietary items; however, one can be produced in the laboratory using the resin available from BioRad Company, 32nd and Griffin Streets, Richmond, California. Peak resolution can be optimized by varying the eluent strength or column flow rate, or by experimenting with alternative columns that may offer more efficient separation. When using guard columns with the stronger reagent to protect the separation column, the analyst should allow rest periods between injection intervals to purge possible sulfate buildup in the guard column.
Capable of maintaining a steady flow as required by the system.
Capable of measuring the specified system flow rate.
Compatible with the output voltage range of the detector.
Unless otherwise indicated, it is intended that all reagents conform to the specifications established by the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available; otherwise, use the best available grade.
Same as Method 7, Section 7.1.
Same as Method 7, Section 7.1.1.
The following reagents and standards are required for analysis:
Same as Method 7, Section 7.1.1.
Dry an adequate amount of sodium nitrate (NaNO3) at 105 to 110 C (221 to 230 F) for a minimum of 2 hours just before preparing the standard solution. Then dissolve exactly 1.847 g of dried NaNO3 in water, and dilute to l liter in a volumetric flask. Mix well. This solution is stable for l month and should not be used beyond this time.
Dilute 5 ml of the standard solution to 200 ml with water in a volumetric flask, and mix well.
Weigh 1.018 g of sodium carbonate (Na2CO3) and 1.008 g of sodium bicarbonate (NaHCO3), and dissolve in 4 liters of water. This solution is 0.0024 M Na2CO3/0.003 M NaHCO3. Other eluents appropriate to the column type and capable of resolving nitrate ion from sulfate and other species present may be used.
Same as Method 7, Section 7.3.8.
Same as in Method 7, Section 8.1.
Same as in Method 7, Section 8.2, except delete the steps on adjusting and checking the pH of the sample. Do not store the samples more than 4 days between collection and analysis.
|Section||Quality Control Measure||Effect|
|10.1||Ion chromatograph calibration||Ensure linearity of ion chromatograph to standards|
|11.3||Audit sample analysis||Evaluate analytical technique, preparation of standard|
Prepare a series of five standards by adding 1.0, 2.0, 4.0, 6.0, and 10.0 ml of working standard solution (25 g/ml) to a series of five 50-ml volumetric flasks. (The standard masses will equal 25, 50, 100, 150, and 250 g.) Dilute each flask to the mark with water, and mix well. Analyze with the samples as described in Section 11.2, and subtract the blank from each value. Prepare or calculate a linear regression plot of the standard masses in g (x-axis) versus their peak height responses in millimeters (y-axis). (Take peak height measurements with symmetrical peaks; in all other cases, calculate peak areas.) From this curve, or equation, determine the slope, and calculate its reciprocal to denote as the calibration factor, S.
If any point on the calibration curve deviates from the line by more than 7 percent of the concentration at that point, remake and reanalyze that standard. This deviation can be determined by multiplying S times the peak height response for each standard. The resultant concentrations must not differ by more than 7 percent from each known standard mass (i.e., 25, 50, 100, 150, and 250 g).
Calibrate according to manufacturer's specifications prior to initial use.
Calibrate against a mercury barometer.
Calibrate dial thermometers against mercury-in-glass thermometers.
Calibrate mechanical gauges, if used, against a mercury manometer such as that specified in Section 6.1.6 of Method 7.
Calibrate against standard weights.
11.1.1 Note on the analytical data sheet, the level of the liquid in the container, and whether any sample was lost during shipment. If a noticeable amount of leakage has occurred, either void the sample or use methods, subject to the approval of the Administrator, to correct the final results. Immediately before analysis, transfer the contents of the shipping container to a 50-ml volumetric flask, and rinse the container twice with 5 ml portions of water. Add the rinse water to the flask, and dilute to the mark with water. Mix thoroughly.
11.1.2 Pipet a 5-ml aliquot of the sample into a 50-ml volumetric flask, and dilute to the mark with water. Mix thoroughly. For each set of determinations, prepare a reagent blank by diluting 5 ml of absorbing solution to 50 ml with water. (Alternatively, eluent solution may be used instead of water in all sample, standard, and blank dilutions.)
11.2.1 Prepare a standard calibration curve according to Section 10.1.1. Analyze the set of standards followed by the set of samples using the same injection volume for both standards and samples. Repeat this analysis sequence followed by a final analysis of the standard set. Average the results. The two sample values must agree within 5 percent of their mean for the analysis to be valid. Perform this duplicate analysis sequence on the same day. Dilute any sample and the blank with equal volumes of water if the concentration exceeds that of the highest standard.
11.2.2 Document each sample chromatogram by listing the following analytical parameters: injection point, injection volume, nitrate and sulfate retention times, flow rate, detector sensitivity setting, and recorder chart speed.
Same as Method 7, Section 11.4.
Carry out the calculations, retaining at least one extra significant figure beyond that of the acquired data. Round off figures after final calculations.
Calculate the sample volume Vsc (in ml), on a dry basis, corrected to standard conditions, using Equation 7-2 of Method 7.
12.2.1 Calculate the sample concentration C (in mg/dscm) as follows:
C = (H)(S)(F)(104)/Vsc Eq. 7A-1
|H||=||Sample peak height, mm.|
|S||=||calibration factor, g/mm.|
|F||=||Dilution factor (required only if sample dilution was needed to reduce the concentration into the range of calibration), dimensionless.|
|104||=||1:10 dilution times conversion factor of: (mg/103 g)(106 ml/m3).|
12.2.2 If desired, the concentration of NO2 may be calculated as ppm NO2 at standard conditions as follows:
ppm NO2 = 0.5228 C   Eq. 7A-2
The analytical range of the method is from 125 to 1250 mg NOx/m3 as NO2 (65 to 655 ppmv), and higher concentrations may be analyzed by diluting the sample. The lower detection limit is approximately 19 mg/m3 (10 ppmv), but may vary among instruments.
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