« 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 So, 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).
126.96.36.199 Preparation of Standard Atmospheres. Permeation tubes may be prepared or purchased. Scaringelli, O'Keeffe, Rosenberg, and Bell (11) give detailed, explicit directions for permeation tube calibration. Tubes with a certified permeation rate are available from the National Bureau of Standards. Tube permeation rates from 0.2 to 0.4 Mg./minute, inert gas flows of about 50 ml./ minute, and dilution air flow rates from 1.1 to 15 liters/minute conveniently give standard atmospheres containing desired levels of so, (25 to 390 ug./m.3; 0.01 to 0.15 p.p.m. SO,). The concentration of So, in any standard atmosphere can be calculated as follows:
erence conditions. P =Tube permeation rate, ug./minute. Ro=Flow rate of dilution air, liter/minute
at reference conditions. Ri=Flow rate of inert gas, liter/minute at
reference conditions. 188.8.131.52 Sampling and Preparation of Calibration Curve. Prepare a series (usually six) of standard atmospheres containing SO2 levels from 25 to 390 ug. SO,/m.3. Sample each atmosphere using similar apparatus and taking exactly the same air volume as will be done in atmospheric sampling. Determine absorbances as directed in 7.2. Plot the concentration of So, in ug./m.3 (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; efficiency may fall off, however, at concentrations below 25 ug./m.3. (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. Hg. (On 24-hour samples, this may not be possible.)
760 t+273 VR=Volume of air at 25° C. and 760 mm.
Hg, liters. V =Volume of air sampled, üters. P =Barometric pressure, mm. Hg. t =Temperature of air sample, °C. 9.2 Sulfur Dioxide Concentration.
9.2.1 When sulfite solutions are used to prepare calibration curves, compute the concentration of suifur dioxide in the sample:
(A-A0) (103) (Bs) ug. SO2/m.3=
VR A =Sample absorbance. A,=Reagent blank absorbance. 103= Conversion of liters to cubic meters. VR =The sample corrected to 25° C. and
760 mm. Hg, liters. B, = Calibration factor, wg./absorbance
unit. D = Dilution factor.
For 30-minute and 1-hour samples,
For 24-hour samples, D=10. 9.2.2 When SO2 gas standard atmospheres are used to prepare calibration curves, compute the sulfur dioxide in the sample by the following formula:
SO2, ug./m.3=(A–Ao) XBg
9.2.3 Conversion of ug./m.s to p.p.m.=If 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.3 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 Lev. el", J. Air Poll. Cont. Assoc. 15, 106
(1965). (4) Scaringelli, 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,
G. A., Lodge, J. P., Jr., “Nitrite Interference in Spectrophotometric Determination of Atmospheric Sulfur
Dioxide”, Anal. Chem. 37, 942 (1965). (6) Zurlo, N. and Grifini, A. M., “Measure
ment of the so, Content of Air in the Presence of Oxides of Nitrogen and Heavy Metals", Med. Lavoro, 53, 330
(1962). (7) Scaringelli, F. P., Elfers, L., Norris, D.,
and Hochheiser, S., "Enhanced Stability of Sulfur Dioxide in Solution”,
Anal. Chem. 42, 1818 (1970). (8) Lodge, J. P. Jr., Pate, J. B., Ammons,
B. E. and Swanson, G. A., “Use of Hypodermic Needles as Critical Orifices in Air Sampling,” J. Air Poll.
Cont. Assoc. 16, 197 (1966). (9) O'Keeffe, A. E., and Ortman, G. C.,
“Primary Standards for Trace Gas Analysis”. Anal. Chem. 38, 760 (1966).
(10) Scaringelli, F. P., Frey, S. A., and Saltz
man, B. E., "Evaluation of Teilon Permeation Tubes for Use with Sulfur Dioxide", Amer. Ind. Hygiene Assoc.
J. 28, 260 (1967). (11) Scaringelli, F. P., O'Keeffe, A. E., Rosen
berg, E., and Bell, J. P., "Preparation of Known Concentrations of Gases and Vapors with Permeation Devices Calibrated Gravimetrically”, Anal.
Chem. 42, 871 (1970). (12) Urone, P., Evans, J. B., and Noyes, C. M.,
"Tracer Techniques in Sulfur Dioxide Colorimetric and Conductiometric Methods", Anal Chem, 37, 1104
(1965). (13) Bostrom, C. E., “The Absorption of Sul
fur Dioxide at Low Concentrations (p.p.m.) Studied by an Isotopic Tracer Method", Intern. J. Air Water Poll. 9, 33 (1965).
APPENDIX B-REFERENCE METHOD FOR THE conditions the error in the measured aver
DETERMINATION OF SUSPENDED PARTICULATES age concentration may be in excess of +50 IN THE ATMOSPHERE (HIGH VOLUME percent of the true average concentration, deMETHOD)
pending on the amount of reduction of air
flow rate and on the variation of the mass 1. Principle and Applicability.
concentration of dust with time during the 1.1 Air is drawn into a covered housing
24-hour sampling period. (4) and through a filter by means of a high-flow
5. Apparatus. rate blower at a flow rate (1.13 to 1.70 m.3/
5.1 Sampling. min.; 40 to 60 ft.3/min.) that allows sus
5.1.1 Sampler. The sampler consists of pended particles having diameters of less
three units: (1) the faceplate and gasket, than 100 um. (Stokes equivalent diameter)
(2) the filter adapter assembly, and (3) the to pass to the filter surface. (1) Particles
motor unit. Figure B1 shows an exploded within the size range of 100 to 0.1um. diame
view of these parts, their relationship to each ter are ordinarily collected on glass fiber fil
other, and how they are assembled. The ters. The mass concentration of suspended
sampler must be capable of passing environparticulates in the ambient air (ug./m.3) is
mental air through a 406.5 cm. (63 in.?) computed by measuring the mass of collected
portion of a clean 20.3 by 25.4 cm. (8- by particulates and the volume of air sampled.
10-in.) glass-fiber filter at a rate of at least 1.2 This method is applicable to measurement of the mass concentration of suspended
1.70 m.3/min. (60 ft.3/min.). The motor must
be capable of continuous operation for 24particulates in ambient air. The size of the
hour periods with input voltages ranging sample collected is usually adequate for other analyses.
from 110 to 120 volts, 50–60 cycles alternat
ing current and must have third-wire safety 2. Range and Sensitivity.
ground. The housing for the motor unit 2.1 When the sampler is operated at an
may be of any convenient construction so average flow rate of 1.70 m.3/min. (60 ft.3/
long as the unit remains airtight and leakmin.) for 24 hours, an adequate sample will
free. The life of the sampler motor can be be obtained even in an atmosphere having
extended by lowering the voltage by about concentrations of suspended particulates as
10 percent with a small "buck or boost" low as 1 ug./m.3. If particulate levels are transformer between the sampler and power unusually high, a satisfactory sample may be outlet. obtained in 6 to 8 hours or less. For deter
5.1.2 Sampler Shelter. It is important mination of average concentrations of sus
that the sampler be properly installed in a pended particulates in ambient air, a stand
suitable shelter. The shelter is subjected to sampling period of 24 hours is
extremes of temperature, humidity, and all recommended.
types of air pollutants. For these reasons 2.2 Weights are determined to the near- the materials of the shelter must be chosen est milligram, airflow rates are determined to carefully. Properly painted exterior plywood the nearest 0.03 m.3/min. (1.0 ft.3/min.),
or heavy gauge aluminum serve well. The times determined to the nearest 2
sampler must be mounted vertically in the minutes, and mass concentrations are re
shelter so that the glass-fiber filter is paralported to the nearest microgram per cubic lel with the ground. The shelter must be meter.
provided with a roof so that the filter is pro3. Interferences.
tected from precipitation and debris. The 3.1 Particulate matter that is oily, such internal arrangement and configuration of as photochemical smog or wood smoke, may a suitable shelter with a gable roof are shown block the filter and cause a rapid drop in in Figure B2. The clearance area between the airflow at a nonuniform rate. Dense fog or main housing and the roof at its closest high humidity can cause the filter to become point should be 580.5 +193.5 cm. (90+30 too wet and severely reduce the airflow in.). The main housing should be rectanguthrough the filter.
lar, with dimensions of about 29 by 36 cm. 3.2 Glass-fiber filters are comparatively (1112 by 14 in.). insensitive to changes in relative humidity, 5.1.3 Rotameter. Marked
arbitrary but collected particulates can be hygro- units, frequently 0 to 70, and capable of scopic. (2)
being calibrated. Other devices of at least 4. Precision, Accuracy, and Stability.
comparable accuracy may be used. 4.1 Based upon collaborative testing, the 5.1.4 Orifice Calibration Unit. Consisting relative standard deviation (coefficient of
of a metal tube 7.6 cm. (3 in.) ID and 15.9 variation) for single analyst variation (re- cm. (614 in.) long with a static pressure tap peatability of the method) is 3.0 percent.
5.1 cm. (2 in.) from one end. See Figure The corresponding value for multilaboratory
B3. The tube end nearest the pressure tap is variation (reproducibility of the method) is
flanged to about 10.8 cm. (414 in.) OD with 3.7 percent. (3)
a male thread of the same size as the inlet 4.2 The accuracy with which the sampler
end of the high-volume air sampler. A single measures
metal plate 9.2 cm. (35/8 in.) in diameter and depends upon the constancy of the airflow 0.24 cm. (332 in.) thick with a central orifice rate through the sampler. The airflow rate is 2.9 cm. (17 in.) in diameter is held in place affected by the concentration and the nature at the air inlet end with a female threaded of the dust in the atmosphere. Under these ring. The other end of the tube is flanged to
hold a loose female threaded coupling, which screws onto the inlet of the sampler. An 18hole 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 calibration unit is shown in Figure B3.
5.1.5 Differential Manometer. Capable of measuring to at least 40 cm. (16 in.) of water.
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 atmospheric pressure to the nearest mm.
5.2.1 Filter Conditioning Environment. Balance room or desiccator maintained at 15° to 35°C. and less than 50 percent relative humidity.
5.2.2 Analytical Balance. Equipped with a weighing chamber designed to handle unfolded 20.3 by 25.4 cm. (8- by 10-in.) filters and having a sensitivity of 0.1 mg.
5.2.3 Light Source. Frequently a table of the type used to view X-ray films.
5.2.4 Numbering Device. Capable of printing identification numbers on the filters.
6.1 Filter Media. Glass-fiber filters having & collection efficiency 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 suspended particulates, (5) although some other medium, such as paper, may be desirable for some analyses. If 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. Procedure. 7.1 Sampling.
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 face. plate from the filter holder. Install a 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 filter from sticking. During inclement weather the sampler may be removed to a protected area for filter change. Close 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 filter lengthwise so that only surfaces with collected particulates are 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 calibration.
7.2 Analysis. Equilibrate the exposed filters for 24 hours in the filter 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 calibrated against actual airflow with the orifice calibration unit. Before the orifice calibration unit can be used to calibrate the rotameter, the orifice calibration unit itself must be calibrated against the positive displacement primary standard.
8.1.1 Orifice Calibration Unit. Attach the orifice calibration 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 calibration 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 time periods. Record the reading on the differential manometer at each airflow. The different constant airflows are obtained by placing &