in certain regions of the plume, therefore, allows separation of the sample spectra from the spectra of the atmospheric species, to aid in the identification of the sample material. The persistence of the neutral atom lines over the ion lines of the sample material should also help to minimize line interferences for analytical purposes when time resolving methods are used. Because a significant amount of energy from the laser beam is absorbed by the plume, a multispike laser pulse would seem to offer a good way to further excite the sample species, once they have entered the plume. Reproducible miltispike laser pulses, however, are difficult to achieve at this time and the reproducible single-spike laser pulse appears to be the best choice for quantitative analysis. For techniques which further excite the plasma by an electrical discharge, it is worthwhile to note the occurrence of an atmospheric plasma 2 mm above the target prior to the arrival of the target species. The plasma could cause breakdown of the electrode gap prior to the arrival of the species of analytical interest, resulting in wasted energy and undesired spectra and background radiation. External control of the
firing time of the electrical discharge relative to the firing of the laser is suggested. In addition to affecting the spectra, the absorption of the laser beam by the plume probably affects the amount of sample that is vaporized during an analysis. The extent of absorption of the laser beam in the plume, and therefore the distribution of its energy between the plume and the target, depends upon the ease by which electrons can be ejected from the target material to initiate inverse bremsstrahlung absorption and the ease by which electrons can be produced in the atmosphere above the target. Matrix effects which depend upon the atmosphere as well as the sample material should therefore be significant when lasers with these power densities are used for spectrochemical analysis. They would surely be significant if bound-free transitions play an important role in the absorption of the light in the plume.
RECEIVED for review October 21, 1968. Accepted February 3, 1969. Presented in part at the Mid-American Symposium on Spectroscopy, Chicago, Ill., June 1966.
Determination of Atmospheric Concentrations of Sulfuric Acid Aerosol by Spectrophotometry, Coulometry, and Flame Photometry F. P. Scaringelli and K. A. Rehme U.S. Department of Health, Education, and Welfare, National Air Pollution Control Administration, 4676 Columbia Parkway, Cincinnati, Ohio 45226
Heretofore, nonspecific and insensitive methods were used to measure sulfuric acid aerosols in the atmosphere. We have developed a precise, sensitive, and highly selective method for determination of these toxic pollutants. The aerosols are collected by impaction or filtration, which separates the sulfuric acid from sulfur dioxide. The sulfuric acid is decomposed at controlled temperatures under a stream of nitrogen to separate the acid from other sulfates. The liberated sulfur trioxide in nitrogen is converted to sulfur dioxide by reaction with hot copper. The sulfur dioxide produced is analyzed either spectrophotometrically, coulometrically, or flame photometrically. The method can measure sulfuric acid in the presence of 10 to 100 times as much sulfur dioxide and other sulfates. Ammonium sulfate interferes quantitatively with the determination of sulfur acid. We do not, however, consider this a serious interference, because this compound forms in the atmosphere from sulfuric acid and gaseous ammonia. The relative standard deviation with standard samples is 5% or less. The sensitivities of the method for colorimetric, coulometric, and flame photometric analysis are 0.3, 0.03, and 0.003 pg of H2S04per sample.
AEROSOLS of sulfuric acid are believed to be at least partly responsible for the smog disasters of London, Meuse Valley, Donora, and New York. Therefore, there is a need for an accurate measure of these toxic pollutants. The source of these pollutants is the burning of fuels that contain sulfur compounds. Although the primary sulfur product of combustion of these compounds is sulfur dioxide, from 1 to 10% is further oxidized to sulfur trioxide, depending on the conditions in the furnace. The sulfur trioxide rapidly reacts
with moisture to produce sulfuric acid aerosols. Additional quantities of sulfuric acid aerosols are produced in the atmosphere by photochemical and catalytic reactions (1-13). Because of the toxicity of these aerosols, a method of measurement that is more specific and more sensitive than any in current use is needed. Ideally, the method must be capable of efficiently collecting these aerosols in the respirable through the submicron size range and of determining these aerosols in the presence of 10 to 100 times as much sulfur dioxide and ~~
~
~~~
~
(1) R. E. Waller, A. G. F. Brooks, and J. Cartwright, Intern. J . Air & Water Pollution, 7, 773 (1963). (2) C. E. Junge and T. G. Ryan, Quart. J. Roy. Meteorol. SOC.,84, 46 (1958). (3) C. F. Goodeve, Trans. Faraday SOC.,32,1218 (1936). (4) J. H. Coste and G. B. Courtier, ibid., 32, 1198 (1936). (5) J. Firket, ibid., 32, 1192 (1936). (6) Ministry of Health, “Mortality and Morbidity During the London Fog of December, 1952,” Reports on Public Health and Medical Subjects No. 95, H. M. Stationery Office, London,
England. (7) H. Collumbine, R. E. Pattle, and F. Burgess, “Toxicity of Fog,”
7th International Congress of Comparative Pathology, Lausanne, Switzerland, 1955. (8) H. H. Schrenk, H. Heimann, G. D. Clayton, and W. M. Gefafer, U. S. Pub. Health Bull., 306, 162 (1949). (9) E. R. Gerhard and H. F. Johnstone, Ind. Eng. Chem., 47, 972 (1955). (10) M. D. Thomas, Intern. J. Air & Water Pollution, 6, 443 (1962). (11) H. F. Johnstone and D. R. Coughanowr, Ind. Eng. Chem., 50, 1169 (1958). (12) N. A. Renzetti and G. J. Doyle, Intern. J. Air Pollution, 2, 327 (1960). (13) M. 0. Amdur, ibid., 1,170 (1959). VOL. 41, NO. 6, MAY 1969
707
other sulfates. Sensitivity of the method should be in the microgram-per-cubic-meter range. Heretofore, nonspecific, relatively insensitive methods were used; namely, collection of the aerosol on filters and titration with standard alkali, or double titration with sodium tetraborate (1, 14, 15). Thomas and Ivie developed a manual and an automatic method, which are more sensitive than the above methods, but are nonspecific. These methods are based on collection of the aerosol by sonic impaction and detection with a conductivity cell (16). All of the methods mentioned measure only the "effective" hydrogen ion concentration and are subject to positive interferences from other ionizable materials. Ammonia and other alkaline materials reduce the response for sulfuric acid. Alternatively, the total atmospheric sulfates are determined : these measurements give only an estimate of the degree of sulfuric acid pollution and do not differentiate between sulfuric acid and other sulfates (17-19). We have developed a sensitive, highly selective method for determining sulfuric acid aerosols by isothermally decomposing the acid to sulfur trioxide under a stream of nitrogen. The liberated sulfur trioxide is converted to sulfur dioxide over hot copper. The sulfur dioxide produced is determined either spectrophotometrically, coulometrically, or flame photometrically. A similar technique has been used to determine total sulfur in metals and other materials, but at elevated temperatures (20-28). EXPERIMENTAL
The apparatus shown in Figure 1 was assembled according to the instructions detailed in the recommended procedure (see next section). To optimize the temperature for reduction of the sulfur trioxide, the temperature of the copper was increased in steps of 50 "C. At each temperature interval, replicate portions of standard sulfuric acid solutions were inserted into the combustion tube, which was kept at 600 "C to ensure rapid decomposition of the sample. Samples of sulfuric acid were prepared by delivering 10 pg of sulfuric acid with a microliter syringe to a copper boat. Excess water was removed at 110 "C prior to placing the boat into the combustion tube. When the coulometer indicated no further increase in response with higher temperatures, the temperature was considered optimum for conversion of sulfur trioxide to sulfur dioxide. To determine decomposition temperatures of the various sulfates, the copper was kept at 500 "C and the temperature of the combustion tube was manually programmed starting at (14) P. P. Mader, W. J. Hamming, and A. Bellin, ANAL.CHEM., 22, 1181 (1950). (15) B. T. Commins, Analysf, 88, 364 (1963). (16) M. D. Thomas, J. 0. Ivie, J. N. Abersold, and R. H. Hendricks, IND. ENG. CHEM., ANAL.ED., 15, 287 (1943). (17) R . J. Bertolacini and J. E. Barney 11, ANAL.CHEM.,29, 281 (1957). (18) N. Zurlo, Med. Lueoro, 53, 325 (1962). (19) M. V. Alekseeva and K. A. Bustueva, Gigiena i Sunir., 4, 13 (1954). (20) L. Acs and S.Barabas, ANAL.CHEM., 36,1825 (1964). (21) K. E. Burke and C. M. Davis, ibid., 34, 1747 (1962). (22) S. Barabas and J. Kaminski, ibid., 35, 1702 (1963). (23) D. R.Buerman and C. E. Melvan, ibid., 34, 319 (1962). (24) C. Bloomfield, Analyst, 87, 586 (1962). 27, 1970 (25) D. B. Hagerman and R. A. Faust, ANAL.CHEM., (1955). (26) R. P. Larsen, L. E. Ross, and N. B. Ingber, ibid., 31, 1596 (1959). (27) J. H. Sen Gupta, ibid., 35, 1971 (1963). (28) R. Bandi, E. G. Buyok, and W. A. Straub, ibid., 38, 1485 (1966). 708
ANALYTICAL CHEMISTRY
COULOMETER STAINLESS STEEL /-BALL JOINT 12/5
F PD
ZIRCON CERAMIC rCOMBUSTlON TUBE BREACH CONNECTOR [CONOFLOW
//
II
II
REGULATOR
IH
ALL GLASS MIDGET IMPING WITH SOCKET JOINT
LSILICONE SEPTUM
k 73;
Figure 1. Experimental design
room temperature. First, dilute solutions of various sulfates were prepared in water. Aliquots were delivered to a copper boat and dried at 110 "C. In turn, each sample was introduced into the sampling stream and the temperature was increased at a rate of 10 to 20 "C per minute. The temperature at which the coulometer indicated an initial response was recorded as the decomposition temperature of the salt. Sulfuric acid aerosols were generated in the pg/ma range with the apparatus shown in Figure 2. Dry clean air was passed over fuming sulfuric acid to produce an aerosol by the reaction of sulfur trioxide with residual moisture. The aerosol was then diluted to the appropriate concentration range with dry air. The concentrations of sulfuric acid in the chamber were determined by collecting the aerosol on glass-fiber filters or by impacting on copper discs. Determinations of the aerosol in the chamber were made alternately by the new method, by titration with 10-3N sodium hydroxide, and by a conductivity device, which was assembled in our laboratory. Recommended Procedure. APPARATUS.Assemble the gas train as shown in Figure 1, consisting of the following: a cylinder of pure nitrogen; a two-stage regulator; a purification trap containing molecular sieve; a flow-regulating device; a breech connector with adapter for the rapid insertion of sample; a zirconium oxide combustion tube, which is attached to the breech, sealed, and made gas-tight by filling the annular space between the tube and the breech with HighPyseal Cement (Fisher Scientific Co., Pittsburgh, Pa.); a silicone septum, open-barrel type; and a stainless-steel tube, 8 inches long and l/,e-inch 0.d. ending in either a stainlesssteel ball joint or a '/*-inch X l/le-inch Swagelok reducer. Loosely pack the tapered end of the combustion tube with degreased copper turnings (washed with acetone) to a depth of 3 inches. One of the following detectors is required: spectrophotometer and midget impinger ; microcoulometer with recorder, Dohrmann or equivalent; or flame photometric detector (FPD), with recorder, Melpar SO2 Analyzer, Model SR 1100-1, or equivalent. REAGENTS.Use a stock solution of standardized 0.03N sulfuric acid, 1.5 g/l. COLLECTION OF SAMPLE.To determine the total concentration of sulfuric acid in the atmosphere, collect the aerosol on 1-inch circular filters of glass fiber, Gelman Type "A" or equivalent. These filters are devoid of any organic material and have a minimum retention efficiency of 9 9 . 7 z for particles larger than 0.3 micron as measured by the Dioctyl Phthalate Penetration test. Wash filters thoroughly in batch quantities with 1 acetic acid, distilled water, ethanol, and acetone, Dry with clean air and store in a petri dish or desiccator. Before use, check filters for uniformity by holding them to the light. Set up sampling train consisting of, in order, a stainless-steel filter holder, flowmeter, flow control device, temperature and vacuum gauge, and air pump. Calibrated hypodermic needles, 13- or 15-gauge, can be used
Figure 2. Aerosol generator A . Pressure regulator B. Silica gel C. Activated carbon
D. Rotameters E. Fuming sulfuric acid flask, S 28/5 F. Humidifying flask, S 28/5 G. Three-foot condenser
AIRLINE
E
H. Distillation trap used as a mixer I. Water manometer J. Waste exhaust tube K. Sampling tube connection L. Five-gallon carboy
as critical orifices to control the rate of flow. Insert a filter into the holder; start vacuum pump and timer. It is desirable to collect 5 to 50 pg of sulfuric acid for the colorimetric analysis and 1 to 10 pg for instrumental procedures. For example, at a concentration of 5 pg/m3 and a flow rate of 10 liters/min, collect the sample for 100 minutes. Alternatively, a portion of a Hi-Vol filter that is 0.001 of its total area can be used for the analyses. This filter of glass fiber is commonly used to collect particulate matter from the atmosphere. Fold the filter and punch or cut out, with a cork borer or hole-puncher, the required sample. Measure the internal diameter of the cutting instrument with a vernier caliper. If the size distribution of the aerosol is desired, impact it on a copper disc or copper planchet with a single-jet multi-stage impactor. Cut the disc to fit the dimensions of the impactor. Oxide coatings can be removed by heating the copper with a flame until it glows, then dropping the hot copper in methanol. Store in petri dishes. Before use, condition copper under a stream of nitrogen at 600 “C. The Andersen (29) sampler can be used if replicate analyses for each stage are required. The impacted spots can be cut from the copper disc and combined, or analyzed individually. ANALYSIS.Insert the combustion tube into a furnace that can maintain a temperature of 400 “C. Adjust the flow of nitrogen through the system to 200 ml/min, and check for leaks with soap solution in the usual manner. Bring the combustion tube to 400 “C, and the reaction tube containing copper to 500 “C. Condition the tube by saturating the system with sulfuric acid. By means of a microliter syringe, place 25 p1 of the standard sulfuric acid solution, 1.5 g/l. on a copper disk. Evaporate the excess water by placing the disc in a beaker, which is immersed into a bath of 70% phosphoric acid at 110 “C. Then, for convenience, place the disc in a copper boat. Quickly open the breech cap,.insert the boat into the hot zone of the combustion tube, and immediately close the breech. Repeat the conditioning operation at least six times or until the efficiency of conversion is optimized at 80 to 85%. If the coulometer is to be used, it can serve as an indicator for optimum conditions. If the flame photometer is used, condition the system until maximum response is obtained. The system is now ready for calibration or for actual samples. Reconditioning of the system is required for each startup or for large temperature changes. However, a 25-pl aliquot of the standard solution usually is sufficient to restore optimum conditions. Select one of the detection systems. For colorimetric detection, connect a midget impinger (with ball joint)
(29) A. A. Andersen, J. Bacteriol., 76,471 (1958).
containing 10 ml of tetrachloromercurate solution. Insert sample and collect the sulfur dioxide from the gaseous effluent for 10 minutes. Similarly, run one or more blanks and one or more control samples for each series of determinations to check the accuracy of the conversion factor. Blanks and controls should also be analyzed with the other detectors. After collecting the sulfur dioxide from each sample, determine the concentration according to the method of Scaringelli et al. (30). For coulometric determination, follow the procedure outlined by the Dohrmann Manual using high gain, low sensitivity (6 or 10 ohms), and the sulfur cell (31). Allow the titration to go to completion; less than 10 minutes is normally required. Response of the instrument is rapid, but the titration is not completed rapidly because the capacity of the system is usually exceeded. A trapezoidal curve, modified by portions of a normal curve at the beginning and end, appears on the chart. This phenomenon appears to be caused by polarization and depolarization of the electrodes. Compute the area under the curve after subtracting the area of the blank produced by the volatility of the iodine in the cell. A ball and disc integrator is convenient and satisfactory for measurement of these areas. For flame photometric detection, follow the procedure detailed in the manual for the Melpar SOz Analyzer Model No. SR 1100-1 (32). Reduce the amount of light supplied to the photomultiplier tube in order to maintain linearity and to guard against saturation of the detector. Apply black tape with sharp edges to the face of the interference filter to produce a slit approximately 3 mm by 2 mm. Operate the electrometer to provide sensitivities of 1.6 pA full scale, attenuator settings of 16 X lo5. Adjust the flow rate of nitrogen through the combustion tube to 175 ml/min. Use a flow rate of 40 ml/min for oxygen. First, press ignitor and then turn on the valve that controls the hydrogen to ignite the flame. Adjust the flow rate of hydrogen to 200 mlimin. Caution: If the flame goes out, turn off the flow of hydrogen gas and allow the gases to sweep out all traces of hydrogen before pressing the ignitor button again. Maintain temperatures of 100 “C or above for the flame housing and for the exit port to avoid condensation or adsorption problems. If condensation occurs in the flame housing, losses of sulfur dioxide will occur. If water from the exhaust gases condenses in the exit port, momentary interruptions in the gas stream will result in spurious spikes on the recorder. (30) F. P. Scaringelli, B. E. Saltzman, and S. A. Frey, ANAL. CHEM., 39, 1709 (1967).
(31) Dohrmann Instrument Co., “Operating Instruction for the Microcoulometer,” 990 Varian Street, San Carlos, Calif. (32) Melpar, Inc., “Instruction Manual,” Falls Church, Va. VOL. 41, NO. 6, MAY 1969
a
709
11100
4
0
0
1
M 3
/
1
=t 2-
/
20
0 L
t
STD. O N OF S L O P I : 0.021 STD. DIV. OF INTIRCLPT: 0.22 R I L D E V . OF POINTS:I.S%
5'
I / V Figure 3. Sulfate decomposition temperatures
i
Ib
1;
210
5:
o:
5:
1 1 4jo
H2SO4 lMIN,yg
Figure 4. Colorimetric calibration with sulfuric acid The photomultiplier tube responds rapidly to changes in ultraviolet light produced by sulfur compounds in the hydrogen flame. The electrometer measures the changes in current through the photomultiplier tube and produces a peak on the recorder. The area under this peak is a linear function of the amount of sulfuric acid in the sample. Normally, no regeneration of the copper is required for periods of 3 months or longer. With highly contaminated samples, regeneration is necessary whenever the conversion factor drops sharply. Regenerate the copper in place by passing a stream of 5 z hydrogen in nitrogen or other inert gas at a rate of 50 ml/min for 1 hour at a temperature of 300 "C. CALIBRATION. For coulometry and for flame photometry, accurately transfer with a micro-syringe onto preconditioned copper discs graduated amounts (2,4,6,8,10 pl) of the standard sulfuric acid solution. For colorimetry, larger graduated amounts are required (5, 10, 15, 20, 25 pl). Remove the excess water in each sample at 110 "C. Avoid prolonged heating. Determine the concentration of sulfuric acid as described in the analytical procedure. RESULTS AND DISCUSSION
The initial decomposition temperatures of various sulfates, which were obtained by the described system or were reported by Ostroff et al. ( 3 3 , are shown in Figure 3. These results were obtained by manually programming the temperature at a rate of 10 to 20 "C per minute. No response is obtained from dry samples of sulfate that appear above the isothermal operating temperature of 400 "C. The compounds that appear below the line will respond quantitatively as sulfuric acid. Very high temperatures, above 1200 "C, are required to decompose the common alkaline metal sulfates, potassium and sodium sulfate. Zinc sulfate has an initial decomposition temperature of 425 "C, which is closest to the operating temperature. At this temperature the SO2 is barely detectable by the coulometer. Programming the temperature is useful as a research method, but it is not practical as a routine procedure. Heating and cooling the furnace increases the analysis time substantially. Under the conditions of the procedure, the only interference that could be present in substantial amounts is ammonium sulfate. In all likelihood, this compound forms in the atmosphere by the reaction of sulfuric acid and gaseous ammonia. Ammonium sulfate was reported to be a pulmonary irritant (34). (33) A. G . Ostroff and R. T. Sanderson,J. Inorn. Nuclear Chem.,. 9,. 45 (1959). (34) M. 0. Amdur and M. Corn, Amer. Ind. H y g . Assoc. J., 24, 326 (1963). ~
710
ANALYTICAL CHEMISTRY
Calibration curves were obtained with standard sulfuric acid solutions by colorimetric, coulometric, and flame photometric detection. The calibration curve for the colorimetric detection system is shown in Figure 4. The slope of the curve indicates a conversion efficiency of 87.5 % for the colorimetric analyses. The relative deviation at 23.0 pg of sulfuric acid for a series of calibration runs obtained on different days was 2.2 The efficiency factor is the ratio obtained by dividing the amount of sulfuric acid found (computed from the sulfite standard) by the amount of sulfuric acid added (computed from the sulfuric acid standard). The calibration curve for coulometric detection, Figure 5, indicated an efficiency of 82.1% and a relative standard deviation of 2.97% at 8.4 pg of sulfuric acid. The efficiency factor for coulometry is the ratio obtained by dividing the amount of sulfuric acid found, computed from Faraday's law, by the amount of standard sulfuric acid added. The difference in the values of the slopes can be explained partly by the efficiency of collection. The efficiency is better in the tetrachloromercurate solution than in the iodine solution of the sulfur cell because the sulfur dioxide evolves in highly localized concentrations exceeding the capacity of the latter. A typical calibration curve determined on a particular day with flame photometric detection is shown in Figure 6. A relative standard deviation of 1.05 at 6.0 pg of sulfuric acid was obtained. Day-to-day variations were much greater, but this would not be a problem if at least two standards were determined daily. The recommended procedure actually calls for a standard every 10 samples. The plot of pg of sulfuric acid against area, current times time, gives a linear response under these conditions. The area under the curve must be determined because the widths of the peaks vary in proportion to the speed with which the heat is transferred to the sample. Results of the determination of H2S04produced by the aerosol generator in the microgram-per-cubic-meter range indicated that the titration procedure did not provide sufficient precision or sensitivity. A conductivity device gave better reproducibility than the titration procedure, but the newly recommended method gave better precision than either of these methods. In determination of the size distribution of the aerosols, the recommended method is superior because of its specificity and sensitivity and because chemical manipulations are eliminated. Copper disks, conditioned as described, gave better collection efficiencies than other inert metals--e.g., platinum.
z.
901
+
U.811X
STO. OEV. OF SLOPE: 0.0104 STO. OEV. OF INTERCEPT: 0.046 REL. DEV. OF POINTS: 4.5 %
H2SO4 TIIKEN,pg
Figure 5. Coulometric calibration with sulfuric acid It is possible to improve the procedure further by selecting a more reactive metal and adjusting the temperature of the combustion tube to the decomposition temperature of the metal sulfate formed. The coulometric and the flame photometric detection systems are more sensitive and more rapid than the colorimetric system; however, they are less specific for sulfur dioxide. Reducing compounds, which may be present, can distill into the coulometric detector and react with iodine. The flame photometer, the most expensive instrument, responds to all sulfur compounds. Analysis time per sample with the coulometer and flame photometer is 10 minutes or less, whereas the colorimetric procedure requires an additional half-hour for each set of 10 samples or less. Waxy materials that were found in a few samples coated the electrode and slowed the titration considerably. Frequent cleaning of the electrode was necessary. The limits of sensitivity of the coulometer and the FPD for sulfuric acid are about 0.03 pg and 0.003 pg, respectively; however, the precision of the method in this range is poor because of the possibility of contamination. Interferences. Filters of fiber glass are not entirely satisfactory for the collection of sulfuric acid aerosols. Although these filters can trap aerosols of submicron size effectively, they contain residual alkali that creates a sulfuric acid demand. Appreciable losses, 1 to 20 pg per 2.58 cmz of filter, occur if standard solutions of sulfuric acid are placed on these filters and analyzed according to the procedure. With thoroughly washed filters, losses are reduced considerably to 1 to 2 pg per 2.58 cm2. Fortunately, this is less of a problem with samples from the field, for the aerosols contact only a small area of the filter and during the dynamic process of collection, particulate material forms a mat that covers these alkaline sites. Furthermore, sulfur dioxide, usually present in 10 to 100 times greater concentration in the atmosphere than the acid aerosols, neutralizes these alkaline substances by forming sulfates. Although this reaction is a serious interference in other methods for the determination of sulfuric acid as sulfate, it is not a problem in the new method, which is so highly specific that these sulfates, Na2S04, and other metal sulfates, are not detected. When air streams containing sulfur dioxide were passed through a clean filter, no sulfuric acid was found. The use of double filters in an attempt to correct for the oxidized sulfur dioxide actually overcompensates for this interference. Active sites on the first filter are covered by particulate mate-
I
I
I
I
I
I
1
:I g
y:-U.l08
I
50
.-e
Zi4O
t
11
20
10 -
00
I
I
2
4
6 8 H2SO4 T u , pg
I
10
J
12
Figure 6 . Flame photometric calibration with sulfuric acid rial and therefore are not available for reaction; the backup filter contains no accumulation of material and provides a larger surface area for the conversion of sulfur dioxide to sulfates. When they are available, quartz or metal filters of suitable porosity may be the collector of choice for sulfuric acid aerosols. Membrane filters or filters made of organic material are unsuitable. They decompose in the combustion tube, producing white clouds of dense smoke in the absence of oxygen. This smoke does not appear to interfere appreciably with the colorimetric analyses, but it does interfere with the other two detectors. With the coulometer, oxidizable substances in the cloud react with iodine to give a positive response. With the photometric detector, a reduced signal with long tailing is obtained, which cannot be entirely explained with existing data. In the recommended procedure the operating temperature was reduced to 400 "C to minimize the possibility of reducing other sulfates by carbonaceous material, which is present on most Hi-Vol filters. Carbon will reduce sulfates at high temperatures when the system is oxygen-deficient. Several experiments were performed to investigate this problem as a possible source of interference. Samples of organic soluble extract, 2 mg, from Hi-Vol filters were analyzed. This quantity represented 500 times the concentrations of material normally found in a cubic meter of air. The interference was Milligram quantities of ascorbic insignificant, 1 to acid and glucose also produced negligible interference, equivalent to 0.6 pg and 0.3 pg of sulfuric acid, respectively. Milligram quantities of sodium sulfate, which normally give no response, in the presence of either ascorbic acid or glucose gave responses of 1.5 or less. Even in typical high pollution areas, the total quantity of particulate matter taken for analysis is 700 pg or less. Because the quantities tested are far in excess of that found in most air samples, no significant interference is expected from carbonaceous material. Table I lists concentrations of sulfuric acid and total sulfates
2z.
z
VOL. 41, NO. 6, MAY 1969
711
~~
Table 11. Field Samples from Philadelphia and Chicago
Table I. Field Samples from Major Cities and Suburbs
Sample Major cities Akron Atlanta Boise Charlotte Chicago Denver Detroit Hartford Houston Huntsville Lexington Los Angeles Newark New Orleans New York
Omaha Pittsburgh St. Louis San Francisco Seattle Tucson Wichita Suburbs Delaware Idaho S. Carolina
Founda as HtSOa, mlm3
Total sulfate: pg/m3
~
Foun& as
HzS04,
Sample
z
6.70 6.15 1.02 5.37 2.59 2.48 1.15 6.19 7.38 3.44 7.79 5.63 6.27 1.52 15.58 0.89 6.29 6.71 1.26 3.07 0.92 0.80
14.7 9.9 0.8 10.2 5.2 3.9 1.8 7.7 18.1 8.2 10.8 11.0 7.1 7.2 51.9 2.2 9.5 16.4 3.3 6.4 1.5 2.9
45.6 62.1 127.5 52.6 49.8 63.6 63.9 80.4 40.8 42.0 72.1 51.2 88.3 21.1 30.0 40.5 66.2 40.9 38.1 48.0 61.3 27.6
3.16 0.45 2.35
6.9 0.5 5.8
45.8 90.0 40.5
Total sulfate: bg/m3
HzSOa,
z
P-15351
51.1 52.2
44.8
114.0 116.5
P-15353
46.3 44.8
48.1
96.3 93.1
P-17951
9.64 10.08
18.5
52.1 54.5
P-17957
4.70 4.64
17.4
27.0 26.7
P-17975
25.5 27.4
30.6
83.3 89.5
P-36755
11.4
19.8
57.6
C-08624
13.7
18.2
75.3
8.4
27.9 28.7
C-08692
2.34 2.41
C-33310
23.6 22.8
30.0
78.7 76.0
(2-33662
14.7 15.8
25.0
58.8 63.2
13.4
33.1 32.9
36.5
49.9 49.3
(2-61113
By coulometry. *As determined by Air Quality and Emission Data Program (methyl thymol blue procedure).
H2SOa, pdm
a
C-61135
4.43 4.41 18.2 18.0
Bv coulometry. As determined' by Air Quality and Emission Data Program (methyl thymol blue procedure). a
b
in air samples obtained in 22 major cities and a few suburban stations in the U.S.A. These Hi-Vol samples were supplied to us by the Air Quality and Emission Data Division of the National Air Pollution Control Administration, together with their analyses of total sulfates by the methylthymol blue procedure. Our analyses for sulfuric acid were performed by the recommended procedure with the microcoulometer. With field samples, all filters were used as received and not prewashed. These results, therefore, may incorporate a slight error because of the residual alkali present in the filter. In the methyl thymol blue procedure, strips of filter 3/4 inch by 7 inches are taken for the analysis. These filters were of the flash-fired type, which has no detectable sulfate; other filters may have up to 4 mg of sulfate per 80 square inches of filter. If precise quantitative data are desired, the total sulfate must be multiplied by 1.021, because the concentration of total sulfate was computed on the basis of 96 as the formula weight whereas the concentration of sulfuric acid is based on 98. Except for the sample from Boise, Idaho, the sulfuric acid concentration represents 20 to 90% of the total sulfates. A correlation exists in that samples with high concentration of sulfuric acid usually give high values of total sulfates. But no fixed numerical value can be applied to total sulfates to obtain a reliable indication of the amount of sulfuric acid in the atmosphere. Therefore, there is no fixed relationship between concentrations of total sulfates and sulfuric acid. Table I1 lists the analyses of samples collected in Philadelphia and in Chicago, shown as P and C numbers, respectively. These samples, supplied to us from the previously mentioned source, should be typical of high pollution 712
ANALYTICAL CHEMISTRY
areas. Again, the proportion of sulfuric acid varies between 26 and 96.3% of the total sulfate. Our analyses of sulfuric acid are presented in duplicate to indicate the expected reproducibility of the recommended method. Except for two values, the reproducibility is within 4.5 %. The precision is very good considering that aliquot portions, 0.001 of the total area of the Hi-Vol filter, were cut out by hand. During the early stages of the method's development, small squares were cut from the filter with a scalpel and the area was measured with a vernier caliper. In later analyses, better prevision was obtained by folding the filter and cutting out circles with a cork-borer. The internal diameter of the borer was measured accurately with the caliper. In most cases, but not always a sharp rise in sulfuric acid concentration is accompanied by a sharp rise in total sulfate. In a few cases, the sulfuric acid concentration exceeded the total sulfate. This could be caused by errors in sampling techniques or by analytical errors in either method. Errors in sampling can readily occur, because the analysis for total sulfate calls for a longitudinal strip amounting to about 0.1 of the filter whereas the new procedure uses 0.001 of the filter cut from the center. Replicate analyses taken in the center away from the edges showed good agreement. In the methylthymol blue procedure, an ion exchange column is used to remove interference from cations. If the recovery from this column is reduced, then a lower value results. Alternatively, a positive interference may exist on some filters-e.g., from organosulfur compounds-with the new procedure. In any
event, this anomaly, in which the sulfuric acid concentration exceeds the total sulfate, occurs in less than 2x of the more than 100 samples analyzed. Stability. Some field samples on filters of glass fiber were reanalyzed after 43 days. The results indicated a decay rate between 0.6 and 1.66z per day. Because these samples were several weeks old when first analyzed, it would be difficult to extrapolate to zero time. We do not know whether the decay rate is a linear function. It may be possible to stabilize the sample by treating it with gaseous ammonia after collection. In any event, fresh samples should be analyzed to eliminate this phenomenon as a source of error. In conclusion, the recommended method is more sensitive, more reproducible, and more specific than the methods in general use for this toxic pollutant. It can provide accurate and specific measurements for particle size distribution of
these aerosols in a short time interval and can differentiate between sulfuric acid and inert sulfates. ACKNOWLEDGMENT
The authors express their thanks to George Morgan of the Air Quality and Emission Data Program, Standards and Criteria Development, National Air Pollution Control Administration, for supplying us with Hi-Vol Filters and for analyses of the total sulfates; and to J. P. Bell for assistance in the installation of the flame photometric instrument, RECEIVED for review December 17,1968. Accepted February 17, 1968. Presented in part before the Division of Water, Air and Waste Chemistry, ACS San Francisco, April 1968. Mention of commercial products does not constitute endorsement by the Public Health Service.
A Comparative Study of Premixed and Turbulent Air-Hydrogen Flames in Atomic Fluorescence Spectrometry M. P. Bratzel, Jr., R. M. Dagnall,' and J. D. Winefordner Department of Chemistry, University of Florida, Gainesville, FIa. 32601
Turbulent diffusion and laminar premixed air-hydrogen flames are critically studied as atomizers for several elements measured by atomic fluorescence flame spectrometry with respect to the effect of aspirating aqueous and aqueous-organic mixture solvents, the effect of nebulizing gas on the quenching of fluorescence, the effect of exciting radiation scatter as a result of incomplete solvent and solute evaporation, and the effect of flame height and background. The turbulent flame system results generally in greater or comparable sensitivities and limits of detection to the premixed flame system for most elements. Because the turbulent flame does not cause an abnormally large scatter signal and is not very sensitive to choice of nebulizing gas and because total consumption nebulizer burners are simple and safer to use, the turbulent air-hydrogen flame produced is recommended for atomic fluorescence studies of elements which are appreciably atomized in such a flame-e.g., Cd, Ga, Fe, Pb, TI, and Sn.
THEOPTIMUM ATOM reservoir and nebulizer burner for atomic absorption spectrometric measurements has been found to be the long-path premixed flame and chamber nebulizer burner. Recent developments in this area have been directed toward the use of new gas mixtures, the development of more efficient nebulization techniques, and the design of the burner head to increase the number of elements which can be determined. On the other hand, turbulent flames with totalconsumption burners have been used with great success for atomic emission flame spectrometry. The optimum flame shape for atomic fluorescence spectrometry is a circular flame of relatively small diameter, but the most suitable means by which such a flame is obtained is not clear ( I , 2). The chamber-type nebulizer-burner system
has a low solution uptake rate in comparison with most total-consumption nebulizer burners, which may result in a loss of sensitivity and is often susceptible to explosive flashbacks. On the other hand, total-consumption nebulizer burners produce relatively large solvent droplets (especially in the lower flame regions) which may cause a background signal and noise due to random scattering of incident radiation from droplets in which solvent evaporation is incomplete or from the resulting solute particles when solute vaporization is incomplete (3). Also, the entrainment of considerable quantities of oxygen from the atmosphere result in flames which are not extremely reducing and may result in greater quenching effects than in premixed flames. Finally, the flame flicker noise associated with a turbulent flame is greater than with comparable premixed flames and is often the major noise which influences detection limits in atomic fluorescence spectrometry. The extreme simplicity and greater solution uptake rates of total consumption nebulizer burners are sufficient to warrant a more detailed examination of its capabilities with respect to atomic air-hydrogen flame fluorescence spectrometry. In this communication, it is shown that flames produced by a total consumption nebulizer burner are generally equivalent to or superior to air-hydrogen premixed flames produced by a chamber-type nebulizer burner. Measurements are made of the atomic fluorescence characteristics of a broad range of elements in both types of flames with special reference to absolute sensitivity, solution uptake rates, effect of aqueous and mixed organic solvents, effect of viscosity and surface tension, position of measurement in the flame, effect of quenching, incident light scattering, and magnitude and 1
(1) J. D. Winefordner and T. J. Vickers, ANAL.CHEM.,36, 161 (1964). (2) R. M. Dagnall, K. C. Thompson, and T. S. West, Anal. Chirn. Acta, 36, 269 (1966).
On leave from Imperial College, London, S.W. 7, U. K.
(3) M. P. Parsons and J. D. Winefordner, ANAL.CHEM., 38, 1595
(1966). VOL. 41,NO. 6, MAY 1969
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