size and filtration conditions. Therefore. it is complementary to the other filter types, rather than competitive a i t h them. A subsequent paper will illustrate the application of the unique properties of this new filter medium. Acknowledgrnerit
The authors are indebted to 6. Jech, Czechoslovak Academy of Sciences, who suggested this research. Literature Cited Adzumi, H., Bull. Chem. SOC.Japan 12, 292-303 (1937). Fleischer, R. L., Price, P. B., Walker. R. M., Science 149, 383-93 (1965). Frank, E. R., Lodge, J. P., Jr.. Spurn);, K. R.. Inter/Micro68, Chicago, Ill., June 10-14, 1968. Fuchs, N . A., “Mechanics of Aerosols.” p. 227, Pergamon Press, Oxford, 1964. Gormley. P. G., Kennedy, M., Proc. Roy. Irish Acrrd. SSA, 163-9 (1949). Hampl. V., Spurnf, K. R., Collection Czech. Chem. Commun. 31, 1152-61 (1966). Natanson, G . L., Dokl. Akad. Nauk U S S R 112, 100-3 (1957).
Pich, J.. Collection Czech. Cherti. C ‘ w i i / i i u n . 29, 2223-7 (1964). Price, P. B.. Walker, R. M., J . Appl. P h y ~ .33, 3407-12 (1962). Spurn?, K. R., Fortschr.-Ber. V D I - Z . 3(17), 1-70 (1967). Spurn?, K. R.. Zentr. Biol. Aerosol Forsch. 12, 369-407 (1965). Spurn$, K. R.. Zentr. Biol. Aerosol F o r x h . 12/13, 3-56 (1965/66). Spurn?, K. R., Hampl, V., Collection Czech. Che//r.Commun. 30, 507-14 (1965). Spurny, K. R.. Hampl. V., Collection Czech. Chem. Commun. 32, 4190-6 (1967). Spurn?. K. R., Lodge. J. P.. Jr., Atmospheric Environ., 2, 429-40 (1968). Spurny, K. R., Pich, J.. Collection Czech. Chem. Comni~in. 30, 2276-87 (1965). Spurng. K. R., Pich. J., Staub 24, 250-6 (1964). Twomey, S . , Bull, Obs. Puy de Doiiie 10, 173-80 (1962). ZQvigka, F., “Kinetic Theory of Gases.” Akadeniicke, Prague, 1951. Recei1,en‘ for review October 20, 1967. Accepted Noi3emDer 4 . 1968. National Center for Atriiospheric Research opernted by the University Corpornrion f o r Atmospheric ReseLirch under sponsorship of the National Science Foundation.
Aerosol Filtration by Means of Nuclepore Filters Aerosol Sampling and Measurement KvGtoslav R. Spurny,l James P. Lodge, Jr., Evelyn R. Frank, and David C. Sheesley Laboratory of Atmospheric Sciences, National Center for Atmospheric Research, Boulder, Colo. 50301
The structure of Nuclepore filters differs greatly from that of all other filter materials, and the filtration properties differ correspondingly. The filter has been evaluated in a variety of applications; its properties suggest new uses. In some applications the Nuclepore is superior, in others inferior. to other filters; on the whole. the new material is complementary to membrane filters. It is especially suited €or collecting samples for light and electron microscopy, short-term gravimetric sampling, and a new approach to particle size determination. It has advantages for use in tape samplers of the AIS1 type. It is inferior to membrane filters in total retentivity.
D
uring the 20 years in which membrane filters have been generally available, a large body of techniques has been developed for their use in the study of aerosols Visitor, Atmospheric Chemistry Program. NCAR. 1966/ 1967. Permanent affiliation, Czechoslovak Academy of Sciences, Institute of Physical Chemistry, Prague. 464 Environmental Science & Technology
(Millipore Corp., 1968; Spurn$, 1965, 1966. 1967). The substantially different properties of Nuclepore filters (Spurnf, Lodge, et al., 1969) raised questions as to their competitive use for the same sorts of tests as the membranes. Therefore. a series of exploratory investigations was made to delineate applications in which each type of filter was superior. The previous work (Spurn?, Lodge. et al., 1969) has shown that membrane filters are more retentive; hence. they are best for high-efficiency, Ion-flow air cleaning. Lodge and Swanson (1964) provided suggestive data on the efficiency of membrane filters for removing condensation nuclei. A few experiments for sampling and culturing airborne microbiota and for determining the ice nucleus content of air suggested that the new filter is at least as satisfactory as membrane. However, neither study was pursued to the point of making definitive measurements. A limited number of tests n.ere made with the “filtration particle size separator” (Figure 1 ) . Definite, though crude, size classification was observed. The device apparently can be used to detect deviations from a normally stable size distribution; however, in the absence of a good quantitative theory
Figure 1. Schematic of filtration particle size separator Main aerosol stream enters through a narrow (15 X 2 mm.) orifice and moves vertically down to filter F , . Portions of stream are diverted to other filters in accordance with particle size and air flow. In a typical experiment, the following pore diameters and flow rates were used: F1, 0.5 pn., 0.8 liter/min.; F?, 0.8 Mm., 1.2 liters/mln.; Fa, 1.0 urn., 1.5 litershin.; F4, 2.0 pm.. 1.8 liters] min.
w
Aerosol
-c
G
D
Air
Figure 3. Detail of filter holder, F, in Figure 1 Face and cross-sectional views
flowmeter and at a pressure drop measured by gage. p . The assembly was supported by a stand, S. At the end of the desired sampling time, the screw collar, C, was loosened enough to disengage a detent and permit a new filter to be rotated into the active position. Up to eight 10-mm. filter disks can be accommodated in the stainless steel holder. A number of holders can be loaded in the laboratory, placed in airtight containers, and carried into the field. This arrangement greatly decreased sample contamination and obviated handling loose filters outside the laboratory.
Figure 2. Field sampling equipment using 1-pm. Suclepore filters in rotating holder F Air A enters through filter, passes through rotameter at pressure measured by gage p , and on to pump P. Apparatus rtabilized by stand S
of the operation of the device, there is no way to use it to deterniine a size distribution from first principle5. Smiipliiig of Aerosols
Since the flow rate-pressure drop characteristics of Nuclepore are similar to those of membrane filters (Spurn?, Lodge. er a!., 1969), the same equipment can be used interchangeably for either. In addition, for reasons covered later. it is possible to use smaller, less expensive Nuclepore disks for many purposes. For sampling in the field. the multiple holder shown in Figures 2 and 3 was developed. Air, A , was drawn through the active filter, F , by pump. P , at a rate measured by the
Analysis of Collected Samples As was shown by Spurn9 et al. (1969), Nuclepore filters are uniform in weight and virtually nonhygroscopic. Since they are also light in weight (ca. 1 mg. per c m . ) . they can be weighed before and after sampling on a microbalance such as the Cahn electrobalance. Samples as small as 10 pg. can thus be weighed with acceptable accuracy. permitting shorter sampling periods than have previously been possible with established gravimetric methods. The 10-mm. filters mentioned above are suitable for this purpose. The high and uniform transparency of the Nuclepore material suggested that optical densitometry might be employed as an index of quantity of particles collected. The manufacturer furnished a trial quantity of the fiber in tape form for tests on both synthetic and natural aerosols nith a Gelman Model 23000 tape sampler and Model 14101 densitometer. The higher light transmission of the filter material necessitated modifying the densitometer; a 4-mm. diaphragm was interposed in the light path to permit adjustment of the instrument to 100% transmission mith clean filter in place. Figure 4 shou s the relation between collected mass of particles and optical density for two test aerosols: platinum oxide and selenium. one intensely black, the other red. As with all such relationships. this one follows a power law rather than the Beer-Lambert law, presumably because at Volume 3, Number 5, May 1969 465
I.o
0.5
I
I
I
-
-
>
;
;
0.2
z w
n 4
u
-/-2
0.05
0
-
O . I i-/ ‘ A
0.02 /
0.01
0
.
/
I
I
10
20
small loadings the particles form a single iayei, ar mg:rrrry higher concentrations the probability of particle overlap rises, and at heavy loadings the thickness of a continuous particle layer is being increased. Only this last condition could he expected to obey the Beer-Lambert equation. As is the case with the usual paper tape sampling, an explicit, general formula relating optical density to filter mass loading does not exist; however, calibrations can be made and used for any specific aerosol. The primary advantage is the uniformity of the filter, which makes it possible to attach meaning to very small optical densities. In this sense, the method is definitely more sensitive than
I 30
ure 7. Another sample of naturaI : lama Cubes of sea ssll p m100
Figure 6. Electron micrograph of replica of particles collected from atmosphere in vicinity of Pana,ma Canal
Figure 8. Visualization of filtration mechanisms
Rounded particles appear to be ammoniiurn sulfalei irregular ‘lbundle” is probably biological in origin
Polydisperse later parlide~on Z - ~ I . pore diameter Nudepore, showing bolb interception and impaction
466 Environmental Science & Technolog:Y
those using paper or membrane filter tapes. The use of Nuclepore, on the other hand, seems to have little o r no advantage if filters are evaluated by reflectance. If microscopy of the collected particles is contemplated, the use of Nuclepore is clearly advantageous. Paper is a poor substrate for microscopy, and tape samplers, even those designed for membrane filters (Polydorova, Spurnq, et a/., 1965) generally require a scrim-reinforced membrane. Nuclepore. on the other hand, is durable enough
to withstand any sort of handling that can he given to paper. Another advantage of Nuclepore is the ease of performing subsequent tests on the collected material. If precise data are needed on collected weight, the uniform thickness makes it possible to punch out a standard area and weigh it. Alternatively, excised portions of the tape can be subjected to many of the tests discussed below. If an arbitrary measure of aerosol concentration is sufficient-and the above densitometric method is clearly arhitrary-consideration should be given to measuring the change in pressure drop across a continuously aspirated filter. The data of Spurn9 et a / . (1969) show that Nuclepore is sensitive in this respect. If it were calibrated for a particular aerosol. this method could be made both sensitive and precise. Collected samples, or portions of them, can he analyzed after solution by any chemical method of sufficient sensitivity. In addition. the filter is free of metallic imourities. so und issolved samples can he analyzed hy neutron activatiim . -.. maieriai . . . .IS transparent, . . large . . .particies .. can Since me niter be counted easily, sized, and examined morphologically under the optical microscope. Its only disadvantage is its considerable tirefringeme, which somewhat limits its use :^Le n:--,,.. --"A ^ C I'^ ..I&-in polarized IialLL LLllcLu~cuyy. rlLlnlry, lllu3L LIIc: ullldmicrochemical techniques for individual particle analysis
.
.1
-.
"
"'sualizatiou
o
...:-..-"----.
2 aemsol amund
-for Nucleopore with 8-pm. ,irtial efficiencies pore diame-y...yy..y
ter, as a function of particle radius and face velocity q
- - - D1ffuSion Impartion
Aslimed density of partlclles 21 g./cc. 1. q = 0.1 cm./sec. 2. q = 0.5 cm_/EOC. 3. q = 2.0 em./sec. 4. D = 7.5 em./rec.
I = f (E,qI 0.i
0.001
uwes for par. to larger psre 10
(1.2
0.l
/
/
os
ID
' Luml
50
10.0
Volume 3, Number 5, May 1969 461
0.8
c
06
1
04
c
3ZC
t 001 0.001
I
I
0.01
I
I
0 10
I
/
1.0
I
10.0
r [flml Figure
12. Aerosol size discrimination by filters in series
Numbered curves show computed collection efficiency as a function of particle radius for particles of density 2 g./cc. on 2-pm. pore diameter Nuclepore at indicated face velocities, q
-._._ Resultant efficiency of series 1. q = 0.5 cm./sec. 2. q = 10 cm./sec. 3. q = 35 cm./sec.
developed by Lodge and his coworkers (Lodge, 1959) appear to give similar results on either membrane or Nuclepore, although the entire series of methods has not yet been tested on Nuclepore. Spurn); et al. (1969) noted that Nuclepore is an elegant material for collecting particles for electron microscopy. Silicon monoxide replicas of the surface usually enclose and “extract” the collected particles for subsequent examination. provided only that the particles are not soluble in chloroform. Even if they do dissolve. their morphology is preserved. This is probably the case with the latex spheres of Figure 5 . Figures 6 and 7 show natural particles collected in a remote field site in Panama (Pate, Lodge. et d . , 1966). The opacity of most of the “particles” demonstrates that these are not empty replicas, but that the collected material is still present. Figures 8 and 9 show the delicacy of the technique, and provide a striking visualization of the competing mechanisms of filtration. Particle Size Determination In addition to direct microscopy, the Nuclepore filters of larger pore sizes can be used to measure particle size.
468
Environmental Science & Technology
Figures 10 and 11 show the family of efficiency curves for a filter having a pore diameter of 8 p n . and an aerosol density of 21 grams per cc. The curves correspond to the separate impaction and diffusion terms as well as to different face velocities, which were computed from the equations of Spurn? et al. (1969). (As noted, the interception term is small.) The figures show that the penetration maximum can be shifted by changing the face velocity. Any sort of particle detector working on the aerosol stream before and after passage through the filter can thus yield a measure of size distribution. Nuclepores in series can make even finer discriminations than when used singly. Figure 12 shous the computed total efficiency curves for three identical filters of ?-pm. pore diameter in series, operating at three different face velocities (achieved by adjusting the filter diameter). The dot-dashed line is the resulting over-all eficiency curve. Evidently a nearly monodisperse portion can be extracted from a polydisperse aerosol stream M ith such a filter series, although pore clogging will limit the duration of the operation. Conclusions A large number of exploratory experiments have indicated that Nuclepore filters have unique properties which commend them for use in practical aerosol studies. Nuclepore and membrane filters are far more complementary than competitive. The present studies need to be carried further to work out optimum methods for using the new filter material in each application. Literature Cited Lodge, J. P., Jr., Nubila 11, 58-65 (1959). Lodge. J. P., Jr., Swanson, G. A., J. Rrch. Ar/iios(i/icreiqiies 1, 17-8 (1964). Millipore Corp., “Millipore Bibliography,“ May 196s. Pate, J. B.. Lodge, J. P., Jr.. Science 153,408-10 (1966). Polydorova, M., Spurn?. K. R.. Paspa, D.. Benak. F., Intern. J . Air Water Pollution 9, 23-5 ( 1965). Spurn?, K. R., Zentr. Biol. Aerosol Forsch. 12, 369-407 (1965). Spurn?, K. R.. Zentr. Biol. Aero.ro1 Forscli. 12/13. 3-56 ( 1965-66). Spurn?. K. R., Zentr. Biol.Aerosol Forscli. 13, 398 ( 1967). Spurn?, K. R., Lodge, J. P.. Jr.. Frank. E. R., Sheesley, D. C, ENVIRON. SCI.TECHNOL. 3, 353-64 ( 1969). Received for review October 20, 1967. Accepted November 4, 1968. The Center for Atmospheric Research is operated by the University Corporation for Atrnospheric Research under sponsorship of the Natioiinl Science Foundn tio n.
