Environ. Sci. Technol. 1982, 16,633-635
NOTES Nonideal Collection Characteristics of a Cascade Impactor with Various Collection Substrates Edward B. Barr," George J. Newton, and Hsu-Chi Yeh Inhalation Toxicology Research Institute, Lovelace Biomedical and Environmental Research Institute, Inc., Albuquerque, New Mexico 87185 ~~
rn A series of collection substrates was used to evaluate the dependence of collection efficiency on substrate type on one stage of a seven-stage cascade impactor. Data obtained were used to determine which substrates yielded the best collection characteristics for cascade impactors. Stainless steel, silver membrane filters, and cellulose acetate membrane filters as cascade impactor collection substrates showed stage collection characteristics that closely fit theoretical predictions. Fiber-type substrates showed the most deviation from the theoretical efficiency curve. Introduction Cascade impactors are instruments that are widely used to determine size distribution of aerosols. One of the important factors that affect the accuracy of impactor data in describing an aerosol is the type of substrate used to collect aerosol particles. Glass-fiber filter media and grease-coated substrates are often used as collection surfaces. However, glass-fiber filters distort the collection characteristics (1, 2). Coated substrates are not always practical for collecting aerosols that are to be analyzed for inorganic or organic components because the coating may interfere with the chemical analysis of the aerosols. This paper describes the calibration of a cascade impactor with oleic acid particles using ten different uncoated substrates to determine the effect of the substrate on collection effciency. Materials and Methods The third stage of a seven-stage stainless steel hightemperature, high-pressure Lovelace multijet cascade impactor (3) was used at ambient temperature and pressure to evaluate each substrate type. The performance of this cascade impactor was previously evaluated (3). Table I lists impactor dimensions and characteristics. The third stage was chosen as a representative stage of the impactor. The impactor test section also included the first and second stages to take into account the effect of these stages on the collection efficiency of stage 3. Stages 4 through 7 were removed from the impactor. Test System. A schematic of the experimental setup is shown in Figure 1. Test aerosols of oleic acid were produced by using a Berglund-Liu vibrating-orifice monodisperse aerosol generator (Model 3050 Thermosystems, Inc., St. Paul, MN). A positive displacement pump (Model RP-SY, Fluid Metering Inc., Oyster Bay, NY) was used to feed a solution of oleic acid in isopropyl alcohol to the generator. Particles were dried and diluted with filtered air at a flow rate of 100 L/min. The size of the remaining nonvolatile oleic acid droplets was calculated from the 0013-936X/82/09 16-0633$01.25/0
Table I. High-Temperature/High-Pressure Lovelace Multijet Cascade Impactor: Dimensions and Characteristics effective cutoff diam (ECD), m stage no. of jet diam no. jets (N) (W), cm calcda exptlb 1 2 3 4
5 6 7
3 3 3 5 7 10 20
0.633 0.506 0.399 0.231 0.170 0.117 0.058
8.8 6.3 4.4 2.5 1.9
1.3 0.6
9.1 6.1 4.2
2.4 1.8 1.3 0.6
Calculated ECD = 1.1769 X 104(N1)W3/Q)1'2, where of air = 1.84 X P, W = jet diameter (cm), Q = (flowrate) = 275 cm3/s, T = Newton, e t al., 20 'C, and pressure = 62.5 mmHg. 1981.
N = number of jets, 1) = viscosity
volumetric concentration of the oleic acid in isopropyl alcohol, the vibrating frequency of the orifice, and the solution flow rate through the orifice ( 4 ) . Monodisperse aerosols with aerodynamic diameters between 2.2 and 5.6 pm were used in this study. The calculated particle size is accurate to 2 % , and the geometric standard deviation of the aerosol is very close to unity. Aerosol passed through a 20-mCi s6Kr discharger, which reduced aerosol charge to near Boltzmann equilibrium, into a mixing chamber (15 L). A 16.5-L/min aerosol sample was pulled through the test fixture with the remaining aerosol exhausted through a filter. The test fixture consisted of two sampling probes and the impactor test section. Aerosol concentration was determined by a Climet Model 208A optical particle counter (Climet Instruments, Redlands, CA), which sampled before and after the impactor test section. The length and radius of curvature of sampling probes were chosen to be the same so that similar losses within the probes can be expected. Without the impactor in the test line, variation in particle concentrations between probes was minimal. The ratio of upstream to downstream concentration ranged between 0.98 and 1.12 with a mean and standard deviation equal to 1.05 f 0.05. All data were taken with the first three stages loaded with the substrate type being tested and with the impactor operating at a flow rate of 16.5 L/min. Collection efficiency was calculated for each particle size by determining number concentration at upstream and downstream; counting duration was 1 min. The collection efficiency was calculated as efficiency, % = (1 - p ) X 100% (1) where p = downstream concentration/upstream concentration.
@ 1982 American Chemical Society
Environ. Sci. Technol., Vol. 16, No. 9, 1982 633
VIBRATING ORIFICE MONODISPERSE AEROSOLGENERATOR (’JOMAG)
MIXING CHAMBER
Table 11. Experimental Values of (e50)1/z and (*w Y / ( *
10)1/2
substrate type
1
1
IMPACTOR TEST S L O N
SAMPLE PROBE TO CLIMET PARTICLE COUNTER
theoretical (1)ceramic fibera ( 2 ) Gelman AE glass fiber ( 3 ) Whatman G F / A fiber (4)Whatman 41 fiber ( 5 ) Ghia Zylon fiber, 10 p m ( 6 ) stainless steel ( 7 ) Millipore type VC, 0.1 pm (8)Millipore type AA, 0.8 pm ( 9 ) Selas silver membrane, 0.8 pm (10)Nuclepore, 0.1pm
1
SAMPLE PROBE TO CLIMET PARTICLE COUNTER
Flgure 1. Schematic of calibration system for cascade impactor showing the vibratlnporifice monodisperseaerosol generator (VOMAG), 85Krcharge neutralizer, sampling chamber, pre-test-section sampling probe, cascade impactor test section, and post-test-section sampling probe.
1
80
g
80-
2 -
w
0 40U
*
( 50 )I1’ 0.328 0.214 ~t 0.009 0.266 t 0.011 0.275 t 0.012 0.295 c 0.012 0.306 t 0.013
0.313 c 0.013 0.328 t 0.014
(* (* 1.14 1.75b
1.48b 1.36b 1.47 1.22b 1.19 1.18b
0.014
1.17
0.331 f 0.015
1.16
0.352 t 0.014
1.23
0.330
i
Fiberfrax ceramic fiber paper, grade 970-7H, (Carborundum Co., Niagra Falls, NY). (*90)1/2/(* could n o t be determined because efficiency curve did n o t reach either 90% or 10%values. The ratios used were (1) (*5 d 1 ’ 2 / ( * 2 5 ) 1 ’ a $ ( 2 ) (*9D)1/2/(*27.5)1/2, (3) (*8 L 5 ) 1 / z / (*z8.5)1’2,.(5)( * 9 0 ) 1 ’ 2 / / ( * 2 5 ) 1 ’ 2 , and ( 7 ) (*81)1’z/(* l,,,4)1’z. These ratios were taken as an estimate of (* lo)l’z where the subscript indicates the value of at that percent efficiency from the curves in Figure 2. a
*
U
w -
ferences in collection efficiencies of tested substrates can be attributed to differences in the physical characteristics of the substrates such as filter porosity, fiber diameter, Ll 7 substrate thickness, and surface texture. Results with solid Od.18 ’ ’ ’ 0.24 ’ ’ ’ ’ 0.40 0.42 aerosols may differ significantly from those obtained by using oleic acid aerosols. Figure 2. Calibration of stage 3 of Lovelace multljet cascade impactor One measure of the desirability of a substrate as a showing (*)l/* vs. efficiency. Curves 1-10 correspond to (1) ceramic collection surface is the sharpness of the efficiency curve. filter (0),(2) Gelman glass fiber type AE filter (A),(3) Whatman GF/A The ratio of (*Y2values at 90% and 10% efficiency was filter (O),(4) Whatman 41 flker (O),(5) ohia Zylon filter (A),(6) stainless steel (.), (7) Millipore type VC filter, (8) Millipore type AA filter, (9) Selas used by May (6) as a test of the sharpness of cutoff. The silver filter, and (10) Nuclepore filter substrates ( X ) . The dotted line approaches unity, the more closer the ratio !\kw)1/2/ (910)1/2 represents Marple’s theoretical curve. Data points are not shown for accurately a simple plot of cumulative collected mass vs. curves 7-9. stage ECD values reflects the actual aerosol size distriratios bution. Table I1 shows the range of (9w)1/2/(*lo)1/2 Results for the ten substrates tested. A (990)’/2/(910)1/2 ratio of Data were collected for ten different substrates and the
