« PreviousContinue »
of sulfur dioxide at levels of interest can be prepared using permeation tubes. In the systems for generating these atmospheres, the permeation tube emits 80, gas at a known, low, constant rate, provided the temperature of the tube is held constant (+0.1° C.) and provided the tube has been accurately calibrated at the temperature of use. The so, gas permeating from the tube is carried by a low flow of inert gas to a mixing chamber where it is accurately diluted with 50,-free air to the level of interest and the sample taken. These systems are shown schematically in Figures A2 and A3 and have been described in detail by O'Keeffe and Ortman (9), Scaringelli, Frey, and Saltzman (10), and Scaringelli, O'Keeffe, Rosenberg, and Bell (11).
18.104.22.168 Preparation of Standard Atmospheres. Permeation tubes may be prepared or purchased. Scaringelli, O'Keeffe, Rosenberg, and Bell (11) give detalled, explicit directions for permeation tube callbration. Tubes with a certified permeation rate are avallable from the National Bureau of Standards. Tube permeation rates from 0.2 to 0.4 ug./minute, inert gas flows of about 50 ml./ minute, and dilution air flow rates from 1.1 to 15 Uters/minute conveniently give standard atmospheres containing desired levels of so, (25 to 390 mg./m.; 0.01 to 0.15 p.p.m. SO.). The concentration of 80, in any standard atmosphere can be calculated as follows:
erence conditions. P =Tube permeation rate, rg./minute. RA=Flow rate of dilution air, liter/minute
at reference conditions. R:=Flow rate of inert gas, Uter/minute at
reference conditions. 22.214.171.124 Sampling and Preparation of Callbration Curve. Prepare a series (usually six) of standard atmospheres containing SO, levels from 25 to 390 ug. SO,/m.. Sample each atmosphere using similar apparatus and taking exactly the same alr volume as will be done in atmospheric sampling. Determine absorbances as directed in 7.2. Plot the concentration of So, in ug./m.: (x-axis) against A-A, values (y-axis), draw the straight line of best fit and determine the slope. Alternatively, regression analysis by the method of least squares may be used to calculate the slope. Calculate the reciprocal of the slope and denote as Bg.
8.3 Sampling Efficiency. Collection efficiency is above 98 percent; eficiency may fall off, however, at concentrations below 25 eg./m... (12, 13)
9.1 Conversion of Volume. Convert the volume of air sampled to the volume at ref
erence conditions of 25° C. and 760 mm. Hz. (On 24-hour samples, this may not be possible.)
9.2.1 When sulfite solutions are used to prepare calibration curves, compute the con. centration of suifur dioxide in the sample:
(A-Ao) (103) (B.) ug. SO2/m.=
760 mm. Hg, liters.
For 30-minute and 1-hour samples,
D = 1.
For 24-hour samples, D=10. • 9.2.2 When So, gas standard atmospheres are used to prepare callbration curves, compute the sulfur dioxide in the sample by tho following formula:
SO2, Mg./m.s=(A-Ao) XB,
9.2.3 Conversion of mg./m.' to p.p.m.=II desired, the concentration of sulfur dioxide may be calculated as p.p.m. SO, at reference conditions as follows:
p.p.m. So,=ug. So,/m." X 3.82 x 1010. References. (1) West, P. W., and Gaeke, G. C., "Fixa
tion of Sulfur Dioxide as Sulfitomercurate III and Subsequent Colorimetric Determination", Anal. Chem.
28, 1816 (1956). (2) Ephraims, F., “Inorganic Chemistry,"
p. 562, Edited by P.C.L. Thorne and E. R. Roberts, 5th Edition, Inter
science. (1948). (3) Lyles, G. R., Dowling, F. B., and Blanch
ard, V. J., "Quantitative Determination of Formaldehyde in Parts Per Hundred Million Concentration Level", J. Air Poll. Cont. Assoc. 15, 106
(1965). (4) Scarlngelli, F. P., Saltzman, B. E., and
Frey, S. A., "Spectrophotometric Determination of Atmospheric Sulfur Dioxide", Anal. Chem. 39, 1709 (1967).
(5) Pate, J. B., Ammons, B. E., Swanson, (10) Scaringelli, F. P., Frey, 8. A., and SaltzG, A., Lodge, J. P., Jr., “Nitrite In
man, B. E., "Evaluation of Teflon terference in Spectrophotometric De
Permeation Tubes for Use with Sulfur termination of Atmospheric Sulfur
Dioxide", Amer. Ind. Hygiene ASSOC. Dioxide", Anal. Chem. 37, 942 (1965).
J. 28, 260 (1967). (6) Zurlo, N. and Griffini, A. M., “Measure- (11) Scaringelli, F. P., O'Keeffe, A. E., Rosen. ment of the 80, Content of Air in the
berg, E., and Bell, J. P., "Preparation Presence of Oxides of Nitrogen and
of Known Concentrations of Gases Heavy Metals", Med. Lavoro, 53, 330
and Vapors with Permeation Devices (1962).
Callbrated Gravimetrically". Anal. (7) Scaringelli, F. P., Elfers, L., Norris, D.,
Chem. 42, 871 (1970). and Hochheiser, S., "Enhanced Sta (12) Urone, P., Evans, J. B., and Noyes, C. M., bility of Sulfur Dioxide in Solution",
"Tracer Techniques in Sulfur DiAnal. Chem. 42, 1818 (1970).
oxide Colorimetric and Conductio(8) Lodge, J. P. Jr., Pate, J. B., Ammons,
metric Methods”, Anal Chem. 37, 1104 B. E. and Swanson, G. A., “Use of
(13) Bostrom, C. E., “The Absorption of Su. Cont. Assoc. 16, 197 (1966).
fur Dioxide at Low Concentrations (9) O'Keeffe, A. E., and Ortman, G. C.,
(p.p.m.) Studied by an Isotopic “Primary Standards for Trace Gas
Tracer Method”, Intern. J. Air Water Analysis”. Anal. Chem. 38, 760 (1966).
Poll. 9, 33 (1965).
APPENDIX B-REFERENCE METHOD FOR THE
1.1 Air is drawn into a covered housing and through a filter by means of a high-flowrate blower at a flow rate (1.13 to 1.70 m.81 min.; 40 to 60 ft./min.) that allows suspended particles having diameters of less than 100 um. (Stokes equivalent diameter) to pass to the filter surface. (1) Particles within the size range of 100 to 0.1mm, diameter are ordinarlly collected on glass aber filters. The mass concentration of suspended particulates in the ambient air (ug./m.) 18 computed by measuring the mass of collected particulates and the volume of air sampled.
1.2 This method is applicable to measurement of the mass concentration of suspended particulates in ambient air. The size of the sample collected is usually adequate for other analyses.
2. Range and Sensitivity.
2.1 when the sampler is operated at an average flow rate of 1.70 m.$/min. (60 1t./ min.) for 24 hours, an adequate sample will bo obtained even in an atmosphere having concentrations of suspended particulates as low as 1 ug./m.'. II particulate levels are unusually high, a satisfactory sample may be obtained in 6 to 8 hours or less. For determination of average concentrations of suspended particulates in ambient air, a standerd sampling period of 24 hours 18 recommended.
2.2 Weights are determined to the nearest milligram, airflow rates are determined to the nearest 0.03 m.°/min. (1.0 ft./min.), times are determined to the nearest 2 minutes, and mass concentrations are reported to the nearest microgram per cubic meter.
3.1 Particulate matter that is olly, such AS photochemical smog or wood smoke, may block the filter and cause a rapid drop in airflow at & nonuniform rate. Dense fog or high humidity can cause the filter to become too wet and severely reduce the airflow through the filter.
3.2 Glass-fiber Alters are comparatively Insensitive to changes in relative humidity, but collected particulates can be hygroscopic. (2)
4. Precision, Accuracy, and Stability.
4.1 Based upon collaborative testing, the relative standard deviation (coefficient of variation) for single analyst variation (repeatability of the method) is 3.0 percent. The corresponding value for multilaboratory variation (reproducibility of the method) is 3.7 percent. (3)
4.2 The accuracy with which the sampler measures the true average concentration depends upon the constancy of the airflow rate through the sampler. The airflow rate is affected by the concentration and the nature of the dust in the atmosphere. Under these
conditions the error in the measured average concentration may be in excess of +50 percent of the true average concentration, depending on the amount of reduction of airflow rate and on the variation of the mass concentration of dust with time during the 24-hour sampling period. (4)
5.1.1 Sampler. The sampler consists of three units: (1) the faceplate and gasket, (2) the filter adapter assembly, and (3) the motor unit. Figure B1 shows an exploded view of these parts, their relationship to each other, and how they are assembled. The sampler must be capable of passing environmental air through & 406.5 cm. (63 in.') portion of a clean 20.3 by 25.4 cm. (8- by 10-in.) glass-fiber filter at & rate of at least 1.70 m./min. (60 ft./min.). The motor must be capable of continuous operation for 24hour periods with input voltages ranging from 110 to 120 volts, 50-80 cycles alternato ing current and must have third-wire safety ground. The housing for the motor unit may be of any convenient construction 60 long as the unit remains airtight and leakfree. The life of the sampler motor can be extended by lowering the voltage by about 10 percent with a small "buck or boost" transformer between the sampler and power outlet.
5.1.2 Sampler Sheltet. It is important that the sampler be properly Installed in a suitable shelter. The shelter is subjected to extremes of temperature, humidity, and all types of air pollutants. For these reasons the materials of the shelter must be chosen carefully. Properly painted exterior plywood or heavy gauge aluminum serve well. The sampler must be mounted vertically in the shelter so that the glass-fiber filter is parallel with the ground. The shelter must be provided with a roof so that the filter is protected from precipitation and debris. The internal arrangement and configuration of a suitable shelter with a gable roof are shown in Figure B2. The clearance area between the main housing and the roof at its closest point should be 580.5+193.5 cm. (90+30 in.'). The main housing should be rectangular, with dimensions of about 29 by 36 cm. (1142 by 14 in.)...
5.1.3 Rotameter. Marked in arbitrary units, frequently 0 to 70, and capable of being calibrated. Other devices of at least comparable accuracy may be used.
5.1.4 Orifice Calibration Unit. Consisting of a metal tube 7.6 cm. (3 in.) ID and 15.9 cm. (644 in.) long with a static pressure tap 5.1 cm. (2 in.) from one end. See Figure B3. The tube end nearest the pressure tap is flanged to about 10.8 cm. (444 in.) OD with & male thread of the same size as the inlet end of the high-volume air sampler. A single metal plate 9.2 cm. (35% in.) in diameter and 0.24 cm. (332 in.) thick with a central orifice 2.9 cm. (148 in.) in diameter is held in place at the air inlet end with a female threaded ring. The other end of the tube is fanged to hold a loose female threaded coupling, which screws onto the inlet of the sampler. An 18bole metal plate, an integral part of the unit, is positioned between the orifice and sampler to simulate the resistance of a clean glassfiber filter. An orifice callbration unit is shown in Figure B3.
5.1.5 Differential Manometer. Capable of measuring to at least 40 cm. (16 in.) of vater.
5.1.6 Positive Displacement Meter. Callbrated in cubic meters or cubic feet, to be used as a primary standard.
5.1.7 Barometer. Capable of measuring ato mospheric pressure to the nearest mm.
5.2.1 Pilter Conditioning Environment. Balance room or desiccator maintained at 15' to 35oC. and less than 50 percent relative humidity.
5.22 Analytical Balance. Equipped with & weighing chamber designed to handle unfolded 20.3 by 25.4 cm. (8- by 10-in.) diters and having a sensitivity of 0.1 mg.
6.2.3 Light Source. Frequently a table of the type used to view X-ray films.
5.2.4 Numbering Device. Capable of print ing identification numbers on the filters.
6.1 Filter Media. Glass-aber Alters having a collection emciency of at least 99 percent for particles of 0.3 um. diameter, as measured by the DOP test, are suitable for the quantitative measurement of concentrations of sus pended particulates, (5) although some other medium, such as paper, may be desirable for some analyses. I a more detailed analysis is contemplated, care must be exercised to use filters that contain low background concentrations of the pollutant being investigated. Careful quality control is required to determine background values of these pollutants.
7.1.1 Filter Preparation. Expose each filter to the light source and inspect for pinholes, particles, or other imperfections. Filters with visible imperfections should not be used. A small brush is useful for removing particles. Equilibrate the filters in the filter conditioning environment for 24 hours. Weigh the filters to the nearest milligram; record tare weight and filter identification number. Do not bend or fold the filter before collection of the sample.
7.1.2 Sample Collection. Open the shelter, loosen the wing nuts, and remove the faceplate from the filter holder. Install & numbered, preweighed, glass-fiber filter in position (rough side up), replace the faceplate without disturbing the filter, and fasten securely. Undertightening will allow air leakage, overtightening will damage the spongerubber faceplate gasket. A very light application of talcum powder may be used on the sponge-rubber faceplate gasket to prevent the alter from sticking. During inclement weather the sampler may be removed to a protected area for filter change. Closo the roof of the shelter, run the sampler for about
5 minutes, connect the rotameter to the nipple on the back of the sampler, and read the rotameter ball with rotameter in a vertical position. Estimate to the nearest whole number. If the ball is fluctuating rapidly, tip the rotameter and slowly straighten it until the ball gives a constant reading. Disconnect the rotameter from the nipple; record the initial rotameter reading and the starting time and date on the filter folder. (The rotameter should never be connected to the sampler except when the flow is being measured.) Sample for 24 hours from midnight to midnight and take a final rotameter reading. Record the final rotameter reading and ending time and date on the filter folder. Remove the faceplate as described above and carefully remove the filter from the holder, touching only the outer edges. Fold the älter lengthwise so that only surfaces with collected particulates are in contact, and place in a manila folder. Record on the folder the filter number, location, and any other factors, such as meteorological conditions or razing of nearby buildings, that might affect the results. If the sample is defective, void it at this time. In order to obtain a valid sample, the high-volume sampler must be operated with the same rotameter and tubing that were used during its callbration.
7.2 Analysis. Equilibrate the exposed filters for 24 hours in the Alter conditioning environment, then reweigh. After they are weighed, the filters may be saved for detailed chemical analysis.
7.3.1 Sampler Motor. Replace brushes before they are worn to the point where motor damage can occur.
7.3.2 Faceplate Gasket. Replace when the margins of samples are no longer sharp. The gasket may be sealed to the faceplate with rubber cement or double-sided adhesive tape.
7.3.3 Rotameter. Clean as required, using alcohol.
8.1 Purpose. Since only a small portion of the total air sampled passes through the rotameter during measurement, the rotameter must be callbrated against actual airflow with the orifice calibration unit. Before the orifice calibration unit can be used to callbrate the rotameter, the orifice calibration unit itself must be callbrated against the positive displacement primary standard.
8.1.1 Orifice Calibration Unit. Attach the orifice callbration unit to the intake end of the positive displacement primary standard and attach a high-volume motor blower unit to the exhaust end of the primary standard. Connect one end of a differential manometer to the differential pressure tap of the orifice callbration unit and leave the other end open to the atmosphere. Operate the high-volume motor blower unit so that a series of different, but constant, airflows (usually six) are obtained for definite timo periods. Record the reading on the differential manometer at each airflow. The different constant airflows are obtained by placing a