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Evaluation of a commercial electrostatic aerosol sampler
Atmospheric Environment Pergamon Press 1971. Vol. 5, pp. 65-66. Printed in Great Britain.
TECHNTCAL EVALUATION
NOTE
OF A COMMERCIAL ELECTROSTATIC SAMPLER
AEROSOL
Abstract-The collection efficiency of a commercial electrostatic aerosol sampler has brtzn measured at flow rates of 4 and 6 1. min-’ with monodisperse 0.500, 1.099, 1.947 pm dia. aerosols. EfEciencies were in the range of SO-90 per cent and sample collection was O&Y slightly biased with respect to particle size. Aerosol deposition was uniform on glass microscope slides but not on Miliipore filters. Aerosol deposition on the coated side of electron microscope grids was equal to deposition on the glass slides but was one-half less on the uncoated side. This discrepancy was tentatively attributed to preferential deposition on the grid wires. INTRODUCTION ELECTROSTATIC precipitation is used quite of!en in aerosol research to collect particles for counting and sizing. Recently, a commercial, two-stage electrostatic sampler* (consisting of separate charging and precipitating sections) based upon earlier experimental designs Ltu (1967,1968) became a&able. This instrument was evaluated here for collection efficiency as a function of particle size and for uniformity of aerosol deposition on each of several types of collecting surfaces (glass slides, htilipore filters and electron microscope grids). EXPERIMENTAL Details of the aerosol generating system used in this experiment have been described, RMBERG (1970). Test aerosols were electrically neutral, mono-disperse 0.500 and 1.099 pm dia. poi>Jtyrene latex and centrifuged monodisperse 1.947 pm dia. polyvinyitoluene. The experimental arrangement for collecting the aerosols is shown in FIG. 1. Aerosol streams entered both the electrostatic aerosol SAMPLE LINE TO MILLIPORE FlLTE
0 FLOWMETER ACUUM PUMP
ELECTROSTATIC AEROSOL SAMPLER FIG. 1. mperimental
STAGE
arrangement
AND
STAGE
for collecting aerosols.
sampler and the bypass system at either 4 or 6 1. min- I. A sampling fine to a Miilipore filter was provided in the bypass system to check the relative proportion of single latex particles to agglomerates and also to measure the number concentration by microscopy. Two capillary flowmeters and vacuum pumps were used to balance flow rates. Microscopic observation of the Miilipore filters revealed that all the test aerosols contained at feast 90 per cent and in most cases 95 per cent or more, single particles. Loss of aerosol in the precipitator by impaction or turbulent diffusion was found to be negligible by comparing the numbs concentrations of input and output aerosols at flow rates from 4 to 8 1. min-’ with no voltage appkd to the sampler. SULTS
The efficiency of the precipitator was determined by comparing the number concentration of particles collected on the glass slide to the number concentration found on a GS Miliipore sample. l
Model 3100, Therm0 Systems, Inc., St. Paul, Minnesota, 65 A.%5:1--E
U.S.A.
D. RIMBERGand D. KEAFER
66
The results are shown in the table along with the manufacturer’s stated efficiencies for uranine aerosols. Collection efficiencies are comparable with the manufacturer’s values although somewhat higher in all cases. The differences are significant, since the estimated error of efficiencies obtained with the latex aerosols is only iS per cent. The lower etBciencies given by the manufacturer may possibly be attributed to structural di!Terences between the prototype with which they were obtained and the commercial sampler used in the present evaluation. Deposition of test aerosols on glass slides was uniform, the same particle density being observed with the light microscope in randomly selected areas. Deposition was nonuniform on untreated GS Mill&m tIlten, but uniform deposition was obtained on filters that were first exposed to alpha radiation, indicating that the inherent electrostatic charge on Millipore filters was the cause of the uneven deposit. The deposition on the uncharged filters also was equal to that on the glass slides on which they were mounted.
Efficiency ( “/o) Particle Dia. &m) 1.947 1.099 0.500
Experimental* 4 1. mill-’ 6 1. mm-’ 80 a5 89
93 81 88
Manufacturert 4.2 1. min” 79 78 76
l Latex particles. t Uranine particles.
Uniformity of deposition on carbon-coated electron microscope grids mounted on glass slides was checked with a 1.099 pm aerosol. The number of particles deposited per unit area of the grid and of the gfass slide was measured with light and electron microscopy. Grids were mounted with both coated and uncoated sides exposed to the aerosol. Deposition on the coated sides of the grids was equivalent to deposition on the glass slides, while the uncoated sides contained about 50 per cent less particies. Electrical grounding of the grids did uot change the particle distribution. Samples obtained from grids mounted directly on the collection stage rather than on a glass slide showed the same discrepancy between coated and uncoated sides. This discrepancy was tentatively attributed to particle buildup on the copper grid wires. Further evaluation of the sampler is planned using both larger and smaller aerosols. D. RIMBERG D. KEAFER
Health cmd Safety Laboratory U.S. Atomic Energy Commission New York, N.Y. 10014, U.S.A.
REFERENCES LN B. Y. H., WWBY K. T. and Yu H. H. S. (1967) Electrostatic aerosol sampler for light and electron microscopy. Rev. Scient. Instun. J$ lOO-102. LN B. Y. H. and VERMAA. C. (1968) A pulse-charging, pulse-precipitating electrostatic aerosol sampler. Anrdyt. Chem. 40,843-846. RJMBEROD. and THOMASJ. W. (1970) Comparison of partick size of latex aerosols by optical and gravity settling methods. .I. Cuffoid Inter&s Sci. 32, 101-105.
TECHNTCAL EVALUATION
NOTE
OF A COMMERCIAL ELECTROSTATIC SAMPLER
AEROSOL
Abstract-The collection efficiency of a commercial electrostatic aerosol sampler has brtzn measured at flow rates of 4 and 6 1. min-’ with monodisperse 0.500, 1.099, 1.947 pm dia. aerosols. EfEciencies were in the range of SO-90 per cent and sample collection was O&Y slightly biased with respect to particle size. Aerosol deposition was uniform on glass microscope slides but not on Miliipore filters. Aerosol deposition on the coated side of electron microscope grids was equal to deposition on the glass slides but was one-half less on the uncoated side. This discrepancy was tentatively attributed to preferential deposition on the grid wires. INTRODUCTION ELECTROSTATIC precipitation is used quite of!en in aerosol research to collect particles for counting and sizing. Recently, a commercial, two-stage electrostatic sampler* (consisting of separate charging and precipitating sections) based upon earlier experimental designs Ltu (1967,1968) became a&able. This instrument was evaluated here for collection efficiency as a function of particle size and for uniformity of aerosol deposition on each of several types of collecting surfaces (glass slides, htilipore filters and electron microscope grids). EXPERIMENTAL Details of the aerosol generating system used in this experiment have been described, RMBERG (1970). Test aerosols were electrically neutral, mono-disperse 0.500 and 1.099 pm dia. poi>Jtyrene latex and centrifuged monodisperse 1.947 pm dia. polyvinyitoluene. The experimental arrangement for collecting the aerosols is shown in FIG. 1. Aerosol streams entered both the electrostatic aerosol SAMPLE LINE TO MILLIPORE FlLTE
0 FLOWMETER ACUUM PUMP
ELECTROSTATIC AEROSOL SAMPLER FIG. 1. mperimental
STAGE
arrangement
AND
STAGE
for collecting aerosols.
sampler and the bypass system at either 4 or 6 1. min- I. A sampling fine to a Miilipore filter was provided in the bypass system to check the relative proportion of single latex particles to agglomerates and also to measure the number concentration by microscopy. Two capillary flowmeters and vacuum pumps were used to balance flow rates. Microscopic observation of the Miilipore filters revealed that all the test aerosols contained at feast 90 per cent and in most cases 95 per cent or more, single particles. Loss of aerosol in the precipitator by impaction or turbulent diffusion was found to be negligible by comparing the numbs concentrations of input and output aerosols at flow rates from 4 to 8 1. min-’ with no voltage appkd to the sampler. SULTS
The efficiency of the precipitator was determined by comparing the number concentration of particles collected on the glass slide to the number concentration found on a GS Miliipore sample. l
Model 3100, Therm0 Systems, Inc., St. Paul, Minnesota, 65 A.%5:1--E
U.S.A.
D. RIMBERGand D. KEAFER
66
The results are shown in the table along with the manufacturer’s stated efficiencies for uranine aerosols. Collection efficiencies are comparable with the manufacturer’s values although somewhat higher in all cases. The differences are significant, since the estimated error of efficiencies obtained with the latex aerosols is only iS per cent. The lower etBciencies given by the manufacturer may possibly be attributed to structural di!Terences between the prototype with which they were obtained and the commercial sampler used in the present evaluation. Deposition of test aerosols on glass slides was uniform, the same particle density being observed with the light microscope in randomly selected areas. Deposition was nonuniform on untreated GS Mill&m tIlten, but uniform deposition was obtained on filters that were first exposed to alpha radiation, indicating that the inherent electrostatic charge on Millipore filters was the cause of the uneven deposit. The deposition on the uncharged filters also was equal to that on the glass slides on which they were mounted.
Efficiency ( “/o) Particle Dia. &m) 1.947 1.099 0.500
Experimental* 4 1. mill-’ 6 1. mm-’ 80 a5 89
93 81 88
Manufacturert 4.2 1. min” 79 78 76
l Latex particles. t Uranine particles.
Uniformity of deposition on carbon-coated electron microscope grids mounted on glass slides was checked with a 1.099 pm aerosol. The number of particles deposited per unit area of the grid and of the gfass slide was measured with light and electron microscopy. Grids were mounted with both coated and uncoated sides exposed to the aerosol. Deposition on the coated sides of the grids was equivalent to deposition on the glass slides, while the uncoated sides contained about 50 per cent less particies. Electrical grounding of the grids did uot change the particle distribution. Samples obtained from grids mounted directly on the collection stage rather than on a glass slide showed the same discrepancy between coated and uncoated sides. This discrepancy was tentatively attributed to particle buildup on the copper grid wires. Further evaluation of the sampler is planned using both larger and smaller aerosols. D. RIMBERG D. KEAFER
Health cmd Safety Laboratory U.S. Atomic Energy Commission New York, N.Y. 10014, U.S.A.
REFERENCES LN B. Y. H., WWBY K. T. and Yu H. H. S. (1967) Electrostatic aerosol sampler for light and electron microscopy. Rev. Scient. Instun. J$ lOO-102. LN B. Y. H. and VERMAA. C. (1968) A pulse-charging, pulse-precipitating electrostatic aerosol sampler. Anrdyt. Chem. 40,843-846. RJMBEROD. and THOMASJ. W. (1970) Comparison of partick size of latex aerosols by optical and gravity settling methods. .I. Cuffoid Inter&s Sci. 32, 101-105.