Threshold Limit Values,” American Conference of Governmental Industrial Hygienists, 1962. Conftance, J. D., Power, 114 (2), 56-7 (February 1970). Davis, P. 0. A. L., Moore, D. J . , Air Water Pollut. Int. J . , 8, 515-33 (1964). Evans, B. H., “Natural Air Flow Around Buildings,” Texas Engineering Experiment Station, Research Report No. 59, 1957. Fan, L., Brooks, N. H., “Dilution of Waste Gas Discharges from Campus Buildings,” Keck Laboratory Tech. Memo 68-1, California Institute of Technology, 1968. Halitsky, J., J . Air Pollut. Contr. Ass., 12, 74-80 (1962). Halitsky, J., Trans. ASHRAE, 69,464-84 (1963). Halitsky, J., Amer. Ind. Hyg. Ass. J., 26, 106-16 (1965).
Lester, D., Greenberg, L. A., Arch. Ind. Hyg. Occup. Med., 2. 348-9 (1950). Lidwell, O.‘M., Lovelock, J. E., J . Hyg., 44, 326-32 (1946). Lovelock, J. E., Lipsky, S. R., J. Amer. Chem. SOC.,82, 431-3 (1960). Munn, R. E., Cole, A. F. W., Atmos. Environ., 1,33-43 (1967). Saltzman, B. E., Coleman, A. I., Clemons, C. A., Anal. Chem., 38,753-8 (1966). Turk, A,, Edmonds, S. M., Mark, H. L., ENVIRON.SCI. TECHNOL., 2,44-8 (1968). Received for review August 27, 1971. Accepted December 16, 1971. This work was supported in part by a grant from the California Institute of Technology President’s Fund.
Aerosol Density Measurements Using a Modified Spiral Centrifuge Aerosol Spectrometer Owen R. MOSS,^ Harry J. Ettinger, and James R. Coulter Los Alamos Scientific Laboratory, University of California, Los Alamos, NM 87544
A spiral centrifuge aerosol spectrometer using a partial Archimedean spiral has been built and calibrated to define aerosol aerodynamic size and particle density. This instrument can define aerodynamic diameters from 0.1-10 pm with a resolution of 5%. Particle aerodynamic size, a function of particle density, shape, and size, must be determined to estimate potential inhalation hazards and when calibrating field air samplers, such as impactors and cyclones. Initially, the instrument was used to define aerodynamic size characteristics of test aerosols used for calibrating size-separating air samplers and aerosols of major air pollution importance. This instrument has measured individual densities of 3.83, 2.53, and 1.37 g/cm3for spherical particles of fly ash, iron oxide, and methylene blue-uranine, respectively.
I
t is necessary to describe aerosol aerodynamic properties to define inhalation hazards accurately and to predict performance of air-cleaning equipment. Previous work using the conifuge has been reported by Sawyer and Walton (1950), Keith and Derrick (1960), Tillery (1967), and Moss (1971). Stober and Flachsbart (1969) combined the spiral channel concept with the separating qualities of the conifuge by rotating a spiral channel cut in a horizontal plane. This paper describes the design and calibration of a modified, spiral centrifuge aerosol spectrometer (spiral), and use of the instrument to measure the density of spherical fly ash, iron oxide, and methylene blue-uranine particles.
wall. The aerosol is collected on a 6-ft-long foil placed along the outer edge of the channel. The LASL spiral was built mainly to incorporate a deeper channel. This change stemmed from the extensive theoretical calculations done by Stober at the University of Rochester (Stober and Flachsbart, 1969). Other modifications were in the following areas: (1) the spiral channel was cut in forged, high-strength aluminum rather than titanium; (2) the outer coolant housing was omitted [modifications 1 and 2 were based on Stober’s experimental work demonstrating that the deposition patterns obtained from sampling monodisperse aerosols were not distinct at rotational speeds higher than 6000 rpm, and that at lower speeds the titanium channel and coolant housing were not necessary. These changes were fortuitous since recently, Raabe (1971) has observed that the rotating head had less tendency to heat up with the coolant housing removed.]; (3) channel shape was altered slightly to that of an &-AEROSOL
AEROSOL INLET BEARING ASSEMBLY
LAMINATOR COLLECTION FOIL
CENTER INLET
Design
SPIRAL CHANNEL HOUSING
Figure 1 is a schematic drawing of the Los Alamos Scientific Laboratory (LASL) spiral. There are two airflows into the rotating head. Clean air for the laminating air layer enters at a controlled rate through the main bearing housing. The test aerosol is drawn through the aerosol inlet bearing assembly and the center inlet, entering the spiral channel near its central
LOWER SUPPORT PLATE
-LAMINATING -MAIN
To whom correspondence should be addressed. 614 Environmental Science & Technology
AIR INLET
-+ TOTAL AIR FLOW OUTLET BEARING HOUSING
Figure 1. LASL spiral centrifuge aerosol spectrometer
Archimedean spiral for the last 900’ of rotation; (4) to seal the rotating head so that an aerosol could be drawn into the channel, a top inlet bearing assembly was made [the seal (Magnetic Seal Corp., West Barrington, R.I.) was provided by a carbon ring riding flat against a highly polished metal surface held in place by magnetic forces; (5) the top plate and channel housing surfaces were milled to produce a good metal-tometal seal; and (6) slip-fitting spacers were incorporated into the main bearing housing to reduce the leakage of air across the bearings. New center inlets were also designed in an attempt to improve the instrument’s size-separating capabilities and minimize aerosol deposition prior to the channel entry point. Figure 2 shows three different inlets and the resulting deposition patterns on the foils. These patterns were obtained under identical sampling conditions using 0.312-pm diam Polystyrene Latex (PSL) spheres. Inlet S2 introduces the aerosol into the spiral channel along the center 25 mm of the inner wall. Inlet S7 completely surrounds the aerosol with clean laminated air at the point of mixing of the two airstreams. By removing the aerosol from the top and bottom of the channel, extremely distinct deposition patterns are obtained even though edge effects must exist. Inlet SO simply introduces the aerosol into the channel without any entrance port. Estimates of collection efficiencies as a function of particle size for inlet S2 and S7 were calculated on the basis of the expected particle travel toward the outer wall of the inlet. The necessary equations come directly from Stokes’ law assumptions and classical mechanics and are similar to those discussed by Stober and Flachsbart (1969). At 1000 rpm, inlets S2 and S7 introduce losses of approximately 10% for particles with aerodynamic diameters, D,, of 4 pm. Inlet SO introduces a similar error for D, of 18 pm. At 3000 rpm, comparable losses occur at 2 and 9 pin. D, is the linear dimension given t o a particle to describe its terminal settling velocity and is the diameter of a upit density sphere that, under the same force field and in the same medium, has the same terminal settling velocity (Walton, 1954). In contrast, the Stokes’ diameter, D,,of a particle is the diameter of a sphere with equivalent density that has the same D,. Inlets S2 and S7 are useful for aerodynamic shape factor and particle density measurements for small particles, where the spread of the deposition pattern for particles of the same size must be small. For larger aerosols, the simplified inlet, SO, can be used where the sharpness of the deposition patterns of identical particles must be sacrificed for more accurate information on the total distribution. Calibration PSL spheres of density 1.057 g/cm3 (Pugh and Heller, 1957) were used to calibrate this instrument. The aerosol was passed through a drying column, operated at 90°C, and a tritium deionizer (Soong, 1968) into a mixing and dilution chamber. The aerosol was sampled from the chamber at 0.44 l./min. Total flow, aerosol plus laminating air, for all of the calibrations was 15.4 l./min. This flow was used so that the air velocity in the channel would equal the velocity in Stober’s channel for total flows of 10 l./min. For each calibration run, the location of the singlet and aggregate deposition patterns was noted (Figure 2 ) . The run was then repeated with electron microscope grids appropriately placed along the center line of the 179-cm-long foil. After sizing the singlets, their aerodynamic diameter was estimated and plotted against the location of the grid from the center end of the foil. Knowing the density and Stokes’ diam-
DEPOSIT ION PATTERNS
INLET
1
s2 DOUBLETSI
\ SINGLETS
I
S7 TRIPLETS IN A ROW
TRIPLETS IN A C L U S T E R /
I
-4 B
so
--- DUSTY
AIR C L E A N AIR
Figure 2. Comparing three center inlet designs under identical sampling conditions for 0.312 pm Polystyrene Latex spheres
2 1 a LLI
001 0” 0
* A
,
D, MEASURED FROM SINGLETS D, CALCULATED FROM AGGREGATES ,
18
,
!
36
54
72
90
,
I
\
108
\
I
126
144
162
L, LENGTH FROM THE CENTER END OF THE FOIL ( c m )
Figure 3. Calibration curves at 1000 and 3000 rpm for a total airflow of 15.41. min
eter, D,, of the particle, the aerodynamic diameter can be estimated. Since these experiments deal with spherical particles, D , is identical to the observed projected area diameter, D , (diameter of a circle which has the same area as the image of the particle). The relationship between D, and D, for spherical particles is :
d i d C D , = dCDa
(1)
Particle density is p, and C, and C, account for particles of size D , and D , no longer seeing a continuous viscous medium (Green and Lane, 1964) or (Davies, 1945), and are functions of particle diameter and barometric pressure. For the calibration experiments, the projected area diameter, D,, of the particle was measured. Calculating C, and multiplying terms on the left side of Equation 1 gives dzD,.D, was calculated from this value either by a simple iterative fit program or by plotting vs. D,for the average barometric pressure of the experimental area. Figure 3 shows the calibration curves for 1000 and 3000 rpm at 15.4 l./min total flow down the channel. Once the singlets of a particular monodisperse calibration aerosol were sized and assigned an aerodynamic diameter, the aggregates provided supplemental calibration points (Stober and Flachsbart, 1969; Stober et al., 1969). The points indicated by triangles in Figure 3 were obtained from the location of the aggregate deposition
./ED,
Volume 6, Number 7, July 1972 615
patterns. These points fill in the calibration curve where monodisperse singlet sizes are not readily available. Aerosol Density Experiments The LASL spiral was used to measure the individual particle density for spherical particles of fly ash, iron oxide (Fe203), and methylene blue-uranine (mixed at a 4 :1 wt ratio). Fly Ash. A sample of fly ash particles obtained from the mechanical collector of a power plant was observed to contain large, hollow particles around 50 pm in diam. These particles were surrounded and often filled, when broken, with smaller spheres. It was not clear if the small particles were also hollow. An expected density of 3.4 f 0.7 g/cm3 for a solid fly ash particle was calculated based on an average chemical composition suggested by Stern (1968). If the smaller particles are hollow, their density should be much less. The fly ash was packed into a Wright Dust Feed and generated at an output of approximately 0.04 cm3 of packed dust/ min diluted by 14 l./min of clean air. This aerosol was then sampled from the mixing chamber through the tritium deionizer into the spiral. In all cases, carbon substrate electron microscope grids were fastened along the centerline of the foil with a drop of Carboline Series K clear vinyl. After the airborne particles were sampled, the grids were removed and photographed with an electron microscope, and the collected particles sized with a Zeiss TGZ-3 Particle Size Analyzer. All of the distributions measured on the grids had geometric standard deviations, cg's, less than 1.1. This indicated that the aerosol had been separated into monodisperse segments along the collection foil. The density of the individual spherical particles can be calculated from Equation l by plotting the aerodynamic diamagainst the corrected Stokes eter corrected for slip, diameter, d C D , . Slope of the line force fitted through zero is the square root of the density. This is shown in Figure 4 and Table I. Correlation coefficient for this fit was greater than 0.9. Density calculated is 3.83 g/cm3 for fly ash spheres between 0.4 and 2.9 pm in diam. This implies that the particles in the size range measured were solid, and their density independent of size. Iron Oxide. Spherical iron oxide particles have been used as test aerosols for in vitro lung deposition studies (Schlesinger and Lippmann, 1971). Submicron iron oxide aerosols are used in this lab for test purposes. Density of these small spheres was measured to see if it was equal to the 2.55 g/cm3 as measured by Spertell and Lippmann (1971) for larger (>I pm) spherical particles. Starting with a solution of iron chloride, we used a dialysis procedure to remove the chloride and leave a concentrated colloidal suspension of iron oxide (Raabe, 1970). Generation and sampling of the aerosol was identical to the procedure in the calibration runs with PSL. Figure 4 shows the plot of dcD, vs. d C D , for the iron oxide spheres. The data force fitted
dCD,,
Table I. Density Estimations from Figure 4 Particle composition
Projected area diam range, pm
1/Fho, ED,
Density, gicm3
Methylene blue-uranine (4 :1 by wt) spheres Iron oxide spheres Fly ash spheres
0.6-2.2 0.12-0.8 0.4-2.9
1.17 1.59 1.96
1.37 2.55 3.83
616 Environmental Science & Technology
i
I
fis~S, THE CORRECTED
I
I
I
I
STOKES DIAMETER ( p m )
Figure 4. Curves estimating density of fly ash, iron oxide, and methylene blueuranine spheres
through zero with a correlation coefficient greater than 0.9. This gave a density of 2.53 g/cm3, independent of size, for spheres between 0.12 and 0.8 pm diam (Figure 4 and Table I). Methylene Blue/Uranine. A spinning disk aerosol generator was used at LASL to produce 1-9 pm monodisperse methylene blue-uranine (MB/U) aerosols to calibrate respirable dust samplers (Ettinger et al., 1970). In addition, a submicron methylene blue-uranine aerosol was generated from 1-10 wt % solutions in ethanol using a Pen-i-Sol nebulizer. Analytical procedures were identical with those previously described for PSL calibration experiments and fly ash density measurements. Force fitting the data shown in Figure 4 gave a constant density of 1.37 g/cm3for spheres of MB/U between 0.6 and 2.2 pm in diam. Correlation coefficientfor this fit was >0.9. Discussion The LASL spiral design is a versatile, dependable research instrument. Our machine is somewhat easier to operate than the original Stober design and has excellent resolution. When used for shape factor studies or aerodynamic diameter measurements, the resolution is about 5 % for a given D,. This can be improved under extremely careful experimental conditions. For routine experimental conditions the resolution is influenced both by the electron microscopy in terms of reproducing magnifications on the final prints (this has been measured to be f 3 x in our lab) and the fact that all the pictures for sizing cannot be taken at the center of the electron microscope grid. Figure 4 and Table I summarize the density measurements made on spherical particles. Over the size range measured, the density for fly ash, iron oxide, and methylene blue-uranine spheres was 3.83, 2.53, and 1.37 g/cm3, respectively. In each case density was independent of particle size. Acknowledgment Thanks are given to Werner Stober for his helpful suggestions and the use of his original design drawings which formed
the basis for our modified spiral. We also thank Ronald G . Stafford of LASL for his assistance in the preparation of the iron oxide aerosol and George W. Royer of LASL for generating the methylene blue-uranine aerosol with the LASL spinning disk generator. Literature Cited Davies. C. M.. Proc. Phvs. SOC..57. 259-79 (1945). ~. Ettinger, H. J:, Partridge, J. E.’, Royer, G . -W., J . Amer. Ind. Hyg. ASS.,31, 537-45 (1970). Green. H. L.. Lane. W. R.. “Particulate Clouds,” 2nd ed., pp 69-70, E. & F.’N. Spon Ltd., London, 1964.’ Keith, C. H., Derrick, J. C., J . Colloid Sci., 15, 340-56 (1960). Moss, 0. R., J . Amer. Ind. Hyg. Ass., 32, 221-9 (1971). Pugh, T. L., Heller, W., J . ColloidSci., 12, 173-80 (1957). Raabe, 0. G., Fission Product Inhalation Laboratories, Lovelace Foundation for Medical Education and Research, Albuquerque, N.M., personal communication, 1970-71. Sawyer, K. F., Walton, W. H., J . Sci. Instrum., 27, 272-6 (1950).
Schlesinger, R., Lippmann, M., paper presented at the Amer. Ind. Hyg. Ass. Conf. in Toronto, ONT, Canada (1971). Soong, An-Liang, MS thesis, University of Rochester, Rochester, NY (1968). Spertell, R. B., Lippmann, M., paper presented at the Amer. Ind. Hyg. Ass. Conf. in Toronto, ONT, Canada (1971). Stern, A. C., Ed., “Air Pollution,” Vol. 3, p 5 , Academic Press, New York, NY (1968). SCI. TECHNOL.. 3 (12). Stober. W.. Flachsbart., H.., ENVIRON. 1280-96 (1969). Stober, W., Brener, A., Blaschke, R., J . Colloid Interface Sci., 29 (4). 710-19 (1969). Stober,”W., Flachsbart, H., J . Aerosol Sci., 2 (2), 103-16 (1971). Tillery, M. I., “Assessment of Airborne Radioactivity,” pp 405-15, IAE Agency, Vienna (1967). Walton, W. H., Brit. J. Appl. Phys. Suppl., 3, S95 (1954). I
~
I,
Receiued for reuiew August 31, 1971. Accepted December 17, 1971. This work was performed under the auspices of the US. Atomic Energy Commission. Parts of this paper were presented at the American Industrial Hygiene Association Conference in Toronto, O N T , Canada (1971).
Dry Ashing of Airborne Particulate Matter on Paper and Glass Fiber Filters for Trace Metal Analysis by Atomic Absorption Spectrometry Thomas Y. Kometani’ Bell Laboratories, Murray Hill, NJ 07974
John L. Bove Department of Chemical Engineering, The Cooper Union for the Advancement of Science and Art, New York, NY 10003
Benjamin Nathanson, Stanley Siebenberg, and Martha Magyar2 Department of Air Resources, New York, NY 10003
Particulate air pollutants collected on paper filters can be dry ashed at 5OOOC without serious loss of trace metals by volatilization. Conversion of metal salts to sulfates by the addition of H2S04prior to dry ashing ensures virtually complete recovery of the metals tested. Losses reported during dry ashing of particulate matter collected on glass fiber filters are not necessarily ascribable to volatilization as has commonly been supposed. Metals such as Pb, Zn, Cu, and Cd react to varying extents with glass at high temperature to form insoluble metal silicates. Comparative studies of particulate matter collected on paper filters in New York City indicate that the results obtained by dry ashing compare favorably with those of accepted methods such as wet ashing and lowtemperature ashing. Good recoveries of Pb, Cu, Zn, and Cd from NYC samples were obtained by dry ashing at 5OOOC for 1 hr even without the prior use of H2S04.
P
articulate air pollutants are routinely collected on either glass fiber ( U S . Dept. of Health, Education and Welfare, 1962) or paper (Magyar et al., 1971) filter mats by means of a high-volume sampler (Jutze and Foster, 1967). The determination of trace metals in particulate matter by atomic absorption spectrometry (AAS)(Morgan and Homan, 1967;
West, 1968) usually involves the initial destruction of organic matter in the sample followed by acid dissolution of residues. Dry ashing at high temperatures, as a method of oxidizing organic matter on glass filters, has been ruled out by some investigators (Burnham et al., 1969, 1970; Kneip et al., 1970; Morgan and Homan, 1967) because of the observed losses of low-melting metals. The most frequent explanation offered for these losses has been the volatilization of metals at high temperatures. Consequently, samples have been oxidized by low-temperature ashing with an excited oxygen plasma (Gleit and Holland, 1962) at about 100°C or by wet ashing using a mixture of perchloric and nitric acids. It will be shown that the reported losses of metals during dry ashing of particulate matter on glass fiber filters are not due to volatilization but due to the formation of insoluble metal silicates. Furthermore, we show that particulate matter on paper filters can be dry ashed safely at 500°C without significant volatilization losses. Although the collection efficiency of a paper filter is not as good as that of a glass filter, the use of paper offers several important advantages over glass during sample preparation. Paper samples may be dry ashed at 5OOOC in a muffle furnace To w b m correspondence should be addressed. Present address, Long Island Laboratories, 3522 Linden Place, Flushing, N.Y. 11354. Volume 6, Number 7, July 1972 617
