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Cascade impactor aerosol samples for PIXE and PESA analyses
Nuclear Instruments and Methods 181 (1981) 499-502 North-Holland Publishing Company
CASCADE IMPACTOR AEROSOL SAMPLES FOR PIXE AND PESA ANALYSES * S. BAUMAN, P.D. HOUMERE and J.W. NELSON Departments of Physics and Oceanography, Florida State University, Tallahassee, FL 32306, U.S.A.
Single orifice impactors have been modified to fractionate a full range of aerosol aerodynamic diameters into eight closed and two open ended intervals (>16, 16-8, 8-4, 4 - 2 , 2-1, 1.0-0.5, 0.50-0.25, 0.12-0.06 and <0.06/~m). Experimental measurements with a methylene blue aerosol are in reasonable agreement with the theoretical 50% cut-point diameters. Outdoor and industrial-environment samples collected from TaUahassee "and Reading, PA, respectively, show wide variations in the mass distributions of 15 elements measured by PIXE and proton elastic scattering analyses (PESA). The ten intervals provide adequate resolution to characterize the aerosol in a region of great environmental interest.
1. Introduction
For laboratory tests of the design, the aerosol used for impaction was obtained from a 1.2 X 10 -3 M solution of methylene blue (particle density 1.26 g/cm 3) [4] by a Sierra Instruments model 7330 fluid atomization aerosol generator. Air-blast atomization and inertial impaction produce a monodisperse aerosol between 0.03 /~m and 1 /ira diameter which is directed into a chamber containing the impactor. A flow of 1 1/min of air through the impactor is maintained with a vacuum pump and the methylene blue particles are impacted onto glass cover slides mounted into each impactor stage. Loaded slides were sputter coated with tantalum in preparation for electron microscopy. The samples were viewed with a scanning electron microscope at about 7500 magnification and polaroid photographs taken of their geometric centers. Only single, round particles were selected for evaluation, eliminating coagulated and otherwise deformed particles. The chosen population exhibits a high degree of spherical symmetry as shown in fig. 1, a picture of methylene blue particles on stage L2 taken at 31,500 magnification. The particle images are ocularly measured to an accuracy o f +0.005 /~m with a 12 times magnifier containing a measuring reticle. As shown in the histograms of fig. 2, these stages exhibit a desirable fractionization of the aerosol. The mean value of the diameter for stage L1 is 0.11/~m for 114 particles and that of L2 is 0.17/~m for 120 particles. If the generator output were linear and no bounce and reintrainment occurred, it is expected that these values would approach the values of 0.09/~m and 0.18 p m respectively. Of interest is the sharpness of the lower edge
Inertial impaction has been extensively utilized to collect air particulate matter in aerodynamically fractionated samples. Samples from single circularorifice types are well suited to ion-beam analysis techniques with microgram quantities impacted in diameters of the order of a few millimeters. Additional low pressure and afterfilter stages have been developed for use with commercially available impactors [1,2] which permit division into a full range of eight closed and two open ended intervals (from < 0 . 0 6 to > 1 6 /2m) for which the cut-points are one half those of the previous stage. Both laboratory and field results are presented to validate the cut-off diameters of the low pressure stages and to demonstrate the overall utility of this device.
2. Impactor design and tests Following the work o f Hering, Flagan and Friedlander [3], two additional low pressure stages were developed for use with a similar single-orifice impactor. In the interests of modest pumping requirements for field operation, the vacuum pump specification was relaxed to 650 mm (25.6 in.) of Hg vacuum at 1 1/min to allow the use of a diaphragm pump. The complete nine stages plus afterfilter form a compact unit 24 cm in length and 5.9 cm in diameter with a mass of one kilogram [2]. * Supported in part by EPA Grant 806162 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland
499
xII E. AEROSOL SAMPLERS
500
S. Bauman et al.
I Cascade impactor aerosol samples f o r PIXE and PESA
Fig. 1. Methylene blue particles at 31500X on impactor stage L2.
of the distribution which indicates stage cut-points in the desired regions of 0.6 #m and 0 . 1 2 / l m . Up to the present, an afterfilter suitable for electron
70
I
I
I
I
1
I
t
I
I
I
I
I
microscopy has not been found so that a similar study at and below 0.06 #m could be performed. Within experimental accuracy the new low pressure impactor stages appear to function as expected.
I
3. Environmental 60
STAGE L1
STAGE L2
5O "144
120
Particles
Particles
mu
-Z 40
o 13_
$ 5o
I
E
2 20 I0
o
i .06
I
I
-t__ i 1
.18
.30
I
[ I--I l .06
.18
--L_ i
i ] .30
DIAMETER ( ~ m )
Fig. 2. Histograms for particle diameters found on stages L1 and L2.
applications
Initial trials of the impactor were made in Tallahassee in June, 1978. Only the lower six stages with afterfilter were used to obtain twelve hour outdoor samples. Distributions for some of the elements as determined via PIXE are shown in figs. 3 and 4. The distribution of elements (A1, Ca, Fe) were chosen for fig. 3 to show the similarity in their mass distributions which likely indicates a similar origin. Alternatively, the similarity of mass distributions chosen in fig. 4 (Br and Pb) are typical of an automotive exhaust aerosol. The corresponding C1 distribution as shown in fig. 5a reveals a significant contribution on all stages. This can be contrasted to that of fig. 5b, an indoor sample from an industrial site in Reading, PA with mass concentrated on the afterfilter stage (<0.06 #m). Using the PESA technique to measure the N distribution, corresponding to the C1 distribu-
S. Bauman et al. / Cascade impactor aerosol samples for PIXE and PESA 0.8
I.C TLH 6 / 2 0 / 7 8
12 HR
~/~o.6
TLH
0.8
t3 :2 04
6/20/78
12HR
Fe
422ng
CI
~1~ °.~
~ o.2
465ng
0.4
i
AO.C
I
I
I
I
tT_ 0.2 0.0 r
0.4
CQ
I
0.06
I
o,2
025
05
,5
Aerodynamic
586ng
I
2!0
Diameter
4.0
8!0
,Lo
(/xm)
~.0-
,,'- 0.2
b
i
0.o'
0.8-
CI 4 2 4 n g / m
AI 198ng
0.4
_i
I
0.06
012
3 KBI2
10/50/78
0.4-
L~ 0.2 0.0
501
o25
b
05
,5
tT_ o . 2 -
20
4!0
~1o
,Lo
(,u.m) Fig. 3. PIXE mass distributions from impactor samples of outdoor aerosol. Aerodynamic
O.C
Diameter
0.06
,
~
0.12 0.25
I
015
tO
Aerodynamic
t
2.0
Diameter
[--I
4,0
6.0
,
16.0
(/j.rn)
Fig. 5. Chlorine mass distr~utions from an a) outdoor and b) industrial indoor aerosol.
1.0
0"8 f
TLH
6/20/78
tion of fig. 5b, yields the result shown in fig. 6. This distribution is biased toward the larger particles due to the inability to measure N above the high background due to the Milipore filter. The constancy of
12HR
l-'~ 0.6 Br
51ng F.O
r{i
u~ O.2
I
I
0.8 iI
I
~
I
Pb 72 ng
0.6
N
KBI 5 10/30/78
0.4 u~0.2
I¸ I
[
o.oi i
4 0 8!o 16.0 Aerodynamic D i a m e t e r (#m) Fig. 4. PIXE mass distributions from impactor samples of outdoor aerosol. 0.06
0.12 0.25
0.5
1.0
2.0
o.o6
o.12 0.25
o!5
Aerodynamic
i.o
2.o
Diameter
41o (/zm)
eJ.o
J 16.0
Fig. 6. Nitrogen distribution from proton scattering analysis of impactor samples. XII E. AEROSOL SAMPLERS
502
S. Bauman et al. / Cascade impactor aerosol samples for PIXE and PESA
with a minimum amount of sample collection. These varied examples of aerosol element distributions illustrate the utility and need for this impactor design.
i.o
0.8
:~ 0 . 6
Se 9 6 n g / m
3 KBI 3 10/30/78
4. Conclusion
g,
j-
0.4
?:, ,t- 0 . 2
I
0.0 0.8
?
!
I
I
t
I
i
1
Se 3 3 n g / m 3 KBI2 1 0 / 3 0 / 7 8
<~[:~ 0.6' o
The authors acknowledge the assistance o f Mr. William Miller for electron microscope operation and photography.
0.4 "u o t2
Ion beam analyses of single orifice impactors with added low pressure stages appear sufficient to characterize the mass distribution of the respirable aerosol. For many purposes further fractionation of the mass distribution of the ambient air particulate matter appears unnecessary.
0.2
°°
--1
J
0.06 o.L2 0.25 0.5 Aerodynamic
,
Lo
,
2.0
,
410 8.o
,
16.0
Diameter ( u . m )
Fig. 7. Selenium distributions for two adjacent eight hour periods in an indoor sample.
the distribution of Se from an arc furnace source for two adjacent eight hour periods is seen in fig. 7. Such distributions permit environmental impact assessment
References [1] PIXE International Corporation, P.O, Box 2235, Tallahassee, FL 32304, U.S.A. [2] Delron Inc., Columbus, Ohio, U.S.A. [3] S.V. Hering, R.C. Flagan and S.K. Friedlander, Env. Sci. Tech. 12 (1978) 667. [4] Handbook on Aerosols, TID-26608, National Technical Information Service, Springfield, VA U.S.A. (1976).
CASCADE IMPACTOR AEROSOL SAMPLES FOR PIXE AND PESA ANALYSES * S. BAUMAN, P.D. HOUMERE and J.W. NELSON Departments of Physics and Oceanography, Florida State University, Tallahassee, FL 32306, U.S.A.
Single orifice impactors have been modified to fractionate a full range of aerosol aerodynamic diameters into eight closed and two open ended intervals (>16, 16-8, 8-4, 4 - 2 , 2-1, 1.0-0.5, 0.50-0.25, 0.12-0.06 and <0.06/~m). Experimental measurements with a methylene blue aerosol are in reasonable agreement with the theoretical 50% cut-point diameters. Outdoor and industrial-environment samples collected from TaUahassee "and Reading, PA, respectively, show wide variations in the mass distributions of 15 elements measured by PIXE and proton elastic scattering analyses (PESA). The ten intervals provide adequate resolution to characterize the aerosol in a region of great environmental interest.
1. Introduction
For laboratory tests of the design, the aerosol used for impaction was obtained from a 1.2 X 10 -3 M solution of methylene blue (particle density 1.26 g/cm 3) [4] by a Sierra Instruments model 7330 fluid atomization aerosol generator. Air-blast atomization and inertial impaction produce a monodisperse aerosol between 0.03 /~m and 1 /ira diameter which is directed into a chamber containing the impactor. A flow of 1 1/min of air through the impactor is maintained with a vacuum pump and the methylene blue particles are impacted onto glass cover slides mounted into each impactor stage. Loaded slides were sputter coated with tantalum in preparation for electron microscopy. The samples were viewed with a scanning electron microscope at about 7500 magnification and polaroid photographs taken of their geometric centers. Only single, round particles were selected for evaluation, eliminating coagulated and otherwise deformed particles. The chosen population exhibits a high degree of spherical symmetry as shown in fig. 1, a picture of methylene blue particles on stage L2 taken at 31,500 magnification. The particle images are ocularly measured to an accuracy o f +0.005 /~m with a 12 times magnifier containing a measuring reticle. As shown in the histograms of fig. 2, these stages exhibit a desirable fractionization of the aerosol. The mean value of the diameter for stage L1 is 0.11/~m for 114 particles and that of L2 is 0.17/~m for 120 particles. If the generator output were linear and no bounce and reintrainment occurred, it is expected that these values would approach the values of 0.09/~m and 0.18 p m respectively. Of interest is the sharpness of the lower edge
Inertial impaction has been extensively utilized to collect air particulate matter in aerodynamically fractionated samples. Samples from single circularorifice types are well suited to ion-beam analysis techniques with microgram quantities impacted in diameters of the order of a few millimeters. Additional low pressure and afterfilter stages have been developed for use with commercially available impactors [1,2] which permit division into a full range of eight closed and two open ended intervals (from < 0 . 0 6 to > 1 6 /2m) for which the cut-points are one half those of the previous stage. Both laboratory and field results are presented to validate the cut-off diameters of the low pressure stages and to demonstrate the overall utility of this device.
2. Impactor design and tests Following the work o f Hering, Flagan and Friedlander [3], two additional low pressure stages were developed for use with a similar single-orifice impactor. In the interests of modest pumping requirements for field operation, the vacuum pump specification was relaxed to 650 mm (25.6 in.) of Hg vacuum at 1 1/min to allow the use of a diaphragm pump. The complete nine stages plus afterfilter form a compact unit 24 cm in length and 5.9 cm in diameter with a mass of one kilogram [2]. * Supported in part by EPA Grant 806162 0 0 2 9 - 5 5 4 X / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © North-Holland
499
xII E. AEROSOL SAMPLERS
500
S. Bauman et al.
I Cascade impactor aerosol samples f o r PIXE and PESA
Fig. 1. Methylene blue particles at 31500X on impactor stage L2.
of the distribution which indicates stage cut-points in the desired regions of 0.6 #m and 0 . 1 2 / l m . Up to the present, an afterfilter suitable for electron
70
I
I
I
I
1
I
t
I
I
I
I
I
microscopy has not been found so that a similar study at and below 0.06 #m could be performed. Within experimental accuracy the new low pressure impactor stages appear to function as expected.
I
3. Environmental 60
STAGE L1
STAGE L2
5O "144
120
Particles
Particles
mu
-Z 40
o 13_
$ 5o
I
E
2 20 I0
o
i .06
I
I
-t__ i 1
.18
.30
I
[ I--I l .06
.18
--L_ i
i ] .30
DIAMETER ( ~ m )
Fig. 2. Histograms for particle diameters found on stages L1 and L2.
applications
Initial trials of the impactor were made in Tallahassee in June, 1978. Only the lower six stages with afterfilter were used to obtain twelve hour outdoor samples. Distributions for some of the elements as determined via PIXE are shown in figs. 3 and 4. The distribution of elements (A1, Ca, Fe) were chosen for fig. 3 to show the similarity in their mass distributions which likely indicates a similar origin. Alternatively, the similarity of mass distributions chosen in fig. 4 (Br and Pb) are typical of an automotive exhaust aerosol. The corresponding C1 distribution as shown in fig. 5a reveals a significant contribution on all stages. This can be contrasted to that of fig. 5b, an indoor sample from an industrial site in Reading, PA with mass concentrated on the afterfilter stage (<0.06 #m). Using the PESA technique to measure the N distribution, corresponding to the C1 distribu-
S. Bauman et al. / Cascade impactor aerosol samples for PIXE and PESA 0.8
I.C TLH 6 / 2 0 / 7 8
12 HR
~/~o.6
TLH
0.8
t3 :2 04
6/20/78
12HR
Fe
422ng
CI
~1~ °.~
~ o.2
465ng
0.4
i
AO.C
I
I
I
I
tT_ 0.2 0.0 r
0.4
CQ
I
0.06
I
o,2
025
05
,5
Aerodynamic
586ng
I
2!0
Diameter
4.0
8!0
,Lo
(/xm)
~.0-
,,'- 0.2
b
i
0.o'
0.8-
CI 4 2 4 n g / m
AI 198ng
0.4
_i
I
0.06
012
3 KBI2
10/50/78
0.4-
L~ 0.2 0.0
501
o25
b
05
,5
tT_ o . 2 -
20
4!0
~1o
,Lo
(,u.m) Fig. 3. PIXE mass distributions from impactor samples of outdoor aerosol. Aerodynamic
O.C
Diameter
0.06
,
~
0.12 0.25
I
015
tO
Aerodynamic
t
2.0
Diameter
[--I
4,0
6.0
,
16.0
(/j.rn)
Fig. 5. Chlorine mass distr~utions from an a) outdoor and b) industrial indoor aerosol.
1.0
0"8 f
TLH
6/20/78
tion of fig. 5b, yields the result shown in fig. 6. This distribution is biased toward the larger particles due to the inability to measure N above the high background due to the Milipore filter. The constancy of
12HR
l-'~ 0.6 Br
51ng F.O
r{i
u~ O.2
I
I
0.8 iI
I
~
I
Pb 72 ng
0.6
N
KBI 5 10/30/78
0.4 u~0.2
I¸ I
[
o.oi i
4 0 8!o 16.0 Aerodynamic D i a m e t e r (#m) Fig. 4. PIXE mass distributions from impactor samples of outdoor aerosol. 0.06
0.12 0.25
0.5
1.0
2.0
o.o6
o.12 0.25
o!5
Aerodynamic
i.o
2.o
Diameter
41o (/zm)
eJ.o
J 16.0
Fig. 6. Nitrogen distribution from proton scattering analysis of impactor samples. XII E. AEROSOL SAMPLERS
502
S. Bauman et al. / Cascade impactor aerosol samples for PIXE and PESA
with a minimum amount of sample collection. These varied examples of aerosol element distributions illustrate the utility and need for this impactor design.
i.o
0.8
:~ 0 . 6
Se 9 6 n g / m
3 KBI 3 10/30/78
4. Conclusion
g,
j-
0.4
?:, ,t- 0 . 2
I
0.0 0.8
?
!
I
I
t
I
i
1
Se 3 3 n g / m 3 KBI2 1 0 / 3 0 / 7 8
<~[:~ 0.6' o
The authors acknowledge the assistance o f Mr. William Miller for electron microscope operation and photography.
0.4 "u o t2
Ion beam analyses of single orifice impactors with added low pressure stages appear sufficient to characterize the mass distribution of the respirable aerosol. For many purposes further fractionation of the mass distribution of the ambient air particulate matter appears unnecessary.
0.2
°°
--1
J
0.06 o.L2 0.25 0.5 Aerodynamic
,
Lo
,
2.0
,
410 8.o
,
16.0
Diameter ( u . m )
Fig. 7. Selenium distributions for two adjacent eight hour periods in an indoor sample.
the distribution of Se from an arc furnace source for two adjacent eight hour periods is seen in fig. 7. Such distributions permit environmental impact assessment
References [1] PIXE International Corporation, P.O, Box 2235, Tallahassee, FL 32304, U.S.A. [2] Delron Inc., Columbus, Ohio, U.S.A. [3] S.V. Hering, R.C. Flagan and S.K. Friedlander, Env. Sci. Tech. 12 (1978) 667. [4] Handbook on Aerosols, TID-26608, National Technical Information Service, Springfield, VA U.S.A. (1976).