Literature Cited Ayers, C. W., Anal. Chem. Acta 15,77 (1956). Bock, F. G., Swain, A. P., Stedman, R. L., Cancer Res. 29, 584 (1969). Hanlon. D. P.. Watt, D. S.. Westhead. E. W.. Anal. Biochem. 16,225 (1966). ’ Hagopian, M., ENVIRON. Scr. TECHNOL. 3,567 (1969). Kertes. A. S.. Marcus. Y.. “Solvent Extraction Research.” Wiley, New York, 1969, pp 257-80. Morrison, G . H., Freiser, H., “Solvent Extraction in Analytical Chemistry,” Wiley, New York, 1957, pp 204 and 5. Pillsbury, H. C., Bright, C. C., O’Connor, K. J., Irish, F. W., J . Ass. Off. Anal. Chenz. 52,458 (1969). Ringbom, A., “Complexation in Analytical Chemistry,” Interscience, New York, 1963, pp 373 and 4.
Stedman, R. L., Chem. Rea. 68,153 (1968). Stedman, R. L., Miller, R. L., Chem. Znd. 1967,618. Swain, A. P., Cooper, J. E., Stedman, R. L., Cancer Res. 29,579 (1969). Wartman, W. B., Jr., Cogbill, E. C., Harlow, E. S., Anal. Chem. 31,1705 (1959). Weiss, W., Weiss, W. A., Arch. Enciron. Health 14, 682 (1967). Receiced,for reaiew February 11, 1970. Accepted July 13, 1970. This work was supported by a grant from the American Medical Association Education Research Fund and by Public Health Service Grant ES-00159. Presented at the Division of Analytical Chemistry, 159th Meeting ACS, Houston, Tex., February 1970.
COM M UN lCATl ON
An Improved Impactor for Aerosol Studies-Modified
Andersen Sampler
John Nan-Hai Hu Ethyl Corp., Ferndale, Mich. 48220 Reducing Particle Bounce-Off An Andersen sampler has been modified to extend its lower range from 0.56 to 0.17 p by operating the Andersen sampler at 3 ft3/min instead of the designed value of 1 ft3/min. The conventional method of coating the collection surface with grease or other materials did not effectively eliminate particle reflection. Effective operation was obtained by use of glassfiber filters as collection plates. The modified Andersen sampler was calibrated with a Royco Particle Counter (Model 200A) and monodispersed polystyrene particles. The wall loss was very low in the range of particle sizes studied.
C
ascade impactors of various designs have been used for aerosol studies for many years (Andersen, 1958; Brink, 1958; Gillespie and Johnstone, 1955; May, 1945; Mitchell and Pilcher, 1959; Ranz and Wong, 1952). Among these impactors, the Andersen sampler has the advantages of being simple, inexpensive, and rugged. However, it is disadvantageous in that the lowest stage constants (50% cutoff point of the last stage) are limited to about 0.5 p (aerodynamic equivalent) because of the flow-rate limitation. For example, the manufacturer of the Andersen sampler recommends operation at 1 ft3/min. Under this flow condition, its lowest stage constant, D50for Stage 6, is 0.56 p (Flesch et al. 1967). To extend the lower range of usefulness, many investigators have studied impactors operating under reduced pressure (Parker and Buchholz, 1968; Prins, 1965; Tomaides and McFarland, 1968). The stage constants of a n impactor can also be reduced by increasing the flow rate. This method has a further advantage in that the sampling time is reduced. However, McFarland and Zeller (1963) showed that particles begin to bounce off the collection surface when the sampling flow rate is too high. Coating the collection surface with sticky materials has been known to reduce bounce-off (Parker and Buchholz, 1968; McFarland and Zeller, 1963). However, Liu (1969) observed bounce-off even when the collection surface had been coated with materials reported to prevent bounce-off effectively.
To reduce the stage constants, we increased the flow rate of the Andersen sampler from 1 to 3 ft3/min. While calibrating the unit, we found that even with a thin layer of grease o n the collection plates, the collection efficiency increases with particle size, reaching a maximum at a certain size and begins to drop as the particle size increases. Theoretically, the collection efficiency should increase continuously with particle size if there is no bounce-off. Thus, the drop in collection efficiency at large particle size was clearly the result of bounceOff.
After investigating several methods for reducing particle bounce-off, we discovered that placing a glass-fiber filter (Gelman Type A) on top of each collection plate greatly reduced bounce-off. With this method, we were able to efficiently run the Andersen sampler at a flow rate of 3 ft3/min without appreciable particle bounce-off. The effectiveness of the glass-fiber filters in minimizing bounce-off probably results from their porous, fibrous structure. When a particle hits a flat surface and bounces, the direction of bounce is upward. When a fibrous filter is used as the collection surface, chances are that a particle will hit a cylindrical fiber, bounce downward, and eventually be collected by other fibers. A particle that bounces upward also has a good chance of being collected by the surrounding fibers if the fiber it initially hits is lower than the surrounding fibers. A question that might be asked is: Does the rough surface of the glass-fiber filter cause any diffusional losses of very fine particles? The answer is no. We have fed 0.23-p particles into the modified Andersen sampler running at 3 ft3/min. At this rate, we did not expect any collection of particles of this size in the first few stages, and we found this to be so. In addition to eliminating bounce-off effectively, the glassfiber filters o n top of the collection plates greatly simplify sample handling, especially for chemical analysis. The modified method could be tested at higher flow rates to further reduce the stage constants and the sampling time. This is particularly important for dilute samples, such as atmospheric aerosols. Volume 5, Number 3, March 1971 251
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Monodispersed Polystyrene Particles f r o m Atomizer
Two 90 m m millipore f i l t e r s i n parallel
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Figure 1. Experimental setup for calibration of modified Andersen sampler
Calibration The experimental setup for calibration of the modified Andersen sampler is shown in Figure 1. Monodispersed polystyrene latex particles ranging from 0.23 t o 1.95 p and a Royco Particle Counter (Model 200A) were used. Particles generated from a Royco atomizer were passed through a mixing chamber, then fed directly into the sampler. Makeup air was needed because the generator was not capable of delivering 3 ft3/min of aerosol. Taps were put on the stages of the sampler for pressure measurement and for drawing samples for the Royco Particle Counter. The flow through the Andersen sampler was first set a t 3 ft3/min by adjusting a valve between the sampler and the pump. This resulted in a pressure drop of 9.4-in. H 2 0 across Taps 1 and 6. We used this pressure drop to set the flow rate for subsequent tests.
The following calibration procedure was used : 1. The collection plates were first removed. Particles of a known size (0.23, 0.36, 0.56, 0.8, 1.1, and 1.95 p) were fed into the sampler, and the flow rate was adjusted to 3 ft3/min. Royco samples were taken from Taps 1 and 2 to determine whether there was any loss of particles on Nozzle Plate 1 (Figure 1). This procedure was repeated for the successive pairs of taps. As shown in Figure 1, the procedure can be used through Stage 5 (Taps 5 and 6). To determine the loss of particles on Nozzle Plate 6, we had to take Stage 2 out of the sequence and put it below Stage 6. Because the holes in Nozzle Plate 2 are large enough, this change did not upset the flow through the Andersen sampler. 2. The collection plates (glass-fiber filters supported by glass plates) were then installed in the sampler. Particles of the same sizes as used in Step 1 were fed into the unit, and the flow was adjusted to 3 ft3/min. Royco samples were taken from each successive pair of taps down to Stage 5 . For Stage 6, the procedure described in Step 1 was used. 3. From each corresponding pair of counts obtained for all particle sizes (0.23 to 1.95 p used in this study) with and without the collection plates, we were able to calculate the collection efficiencies for each stage as a function of particle size. For example, when feeding 0.56-p particles without collection plates, the average Royco counts were 7000 and 7031 for samples taken from Taps 5 and 6, respectively-meaning that there was no loss of 0.56-p particles on Nozzle Plate 5 . With particles of the same size and with the glass-fiber filter collection plates, the respective average counts were 4787 and 819. The difference in counts at Tap 5 with and without collection plates (4787 cs. 7000) is a result of different aerosol concentrations. From these data, the collection efficiency of Stage 5 for 0.56-11 particles was calculated to be (4787 - 819)/ 4787 = 83 %. By the same method, we obtained the collection efficiencies for each stage as a function of particle size. The calibration curves obtained by this method are shown in Figure 2. Since the method was limited by both the lower limit of the Royco Counter (about 0.2 p) and the availability of the largest monodispersed latex particles (1.95 p), the calibration was limited to the size range of 0.23 to 1.95 p. As a result, we only have complete calibration curves for Stages 3, 4, and 5 , and partial curves for Stages 2 and 6. However, by
Figure 2. Calibration curves; modified Andersen sampler (3 fta/min)
Particle S i z e , 252 Environmental Science & Technology
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particles of these sizes in the first five nozzle plates. There was about 10% loss of 0.23-p particles o n Nozzle Plate 6. The losses for larger particles were higher in this stage. However, this would not create any problem because most of the larger particles will be collected o n the preceding stages. To determine the extent of overall wall losses, we took Royco samples at Tap 1 and at the exit of the modified Andersen sampler with the glass-fiber filter collection plates installed for various sizes of particles. Since we had determined the collection eficiencies of all six stages with these particles, the counts at the exit of the sampler could be calculated from the measured counts at Tap 1. We compared the calculated counts at the exit of the sampler with the actual measured counts and estimated the wall losses as a function of particle size. The overall losses at 3 ft3/minwere about 10% for 0.23- and 0.36-p particles, and there was practically no loss of larger particles (0.56 to 1.95 p). We cannot predict the overall wall losses for polydispersed aerosols because the wall losses are a function of particle size. Overall loss should be determined individually in each specific application, especially for aerosols containing particles greater than 2 p , Acknowledgment The author thanks Charles E. Lynch for his help in carrying out the experiments.
0.2
0.15 I
2
3
4
5
6
Stoge Number
Figure 3. Stage constants, DLO’S of modified Andersen sampler
following the trend of the curves for Stages 3, 4, and 5 , we were able to extrapolate the curves for Stages 2 and 6 to obtain the 50% cutoff points, D5a, for these stages (dotted lines in Figure 2). The Dj0for Stage 1 was obtained by plotting the Djo’s us. stage number and extrapolating the curve to Stage 1 (Figure 3). This method of calibration (Hu, 1968) was used here to check the data obtained by Flesch et al. (1967) o n an Andersen sampler operating at 1 ft3/min; excellent agreement was obtained. Moreover, the slopes of the calibration curves shown in Figure 2 are about the same as those reported by Flesch et al. for the Andersen sampler operating at 1 ft3/min. This means that the increased flow rate in the modified Andersen sampler did not change the sharpness of separation, which gives us confidence in the calibration method. Wall Losses We have also used the Royco Particle Counter and the monodispersed latex particles to study wall losses. During the calibration, the average counts obtained above and below each of the first five nozzle plates were practically the same for all sizes of particles when the collection plates were not installed. This indicates that there was practically no loss of
Literature Cited Andersen, A. A., J . Bacleriol. 76,471-84 (1958). Brink, J. A,, Jr., Ind. Eng. Chem. 50,645 (1958). Flesch, J. P., Norris, E. H., Nugent, A. E., Jr., J . Anier. Znd. HJ’g.ASS.28,507-16 (1965). Gillespie, G . R., Johnstone, H. F., Clzem. Eng. Progr. 51, 74F (1955). Hu, J. N.-H., unpublished data, 1968. Liu, B. Y . H., Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota, St. Paul, Minn., personal communication, 1969. May, J. R., J . Sci. Instr. 22,187 (1945). McFarland, A. R., Zeller, H. W., “Study of a Large-Volume Impactor for High-Altitude Aerosol Collection,” Report No, 2391. Contract AT (11-1)-401 (TID-18624). . , , , AerosDace Research; April 1963. Mitchell, R. I., Pilcher, J. M., Ind. Eng. Chen7. 51, 1039 (1959). Parker. G. W.. Buchholz. H.. “Size Classification of Submicron Particles by a Low’-Pressure Cascade Impactor,” ORNL-4226, June 1968. Prins, M. P., “Submicron Particle Classifier Applicable for Airborne Virus Collection,” Report No. 2821 [Phase I1 Summary Progress Report, Contract DA 18-064-AMC229(A)], Applied Science Division, Litton Systems, Inc., December 1965. Ranz, W. E., Wong, J. B., Ind. Eng. Clzen7. 44,1371 (1952). Tomaides, M., McFarland, A. R., “Characterization of Chain Agglomerate Aerosols,” Final Report, Particle Laboratory Publication No. 130, Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota, November 1968. Receiaed for review April 2 , 1970. Accepted October 10, 1970.
Volume 5 , Number 3, March 1971 253
