A Pulse-Charging, Pulse-Precipitating Electrostatic Aerosol Sampler Benjamin Y. H. Liu and Avtar C. Verma Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minn. 55455
ELECTROSTATIC PRECIPITATORS with the wire-in-tube (1-4) or point-to-plane (5-7) geometry, or some variation thereof, have been used extensively for the collection of aerosol samples. Similar devices based upon electrical charging and precipitation have also been used for the classification of aerosol particles (8-12) and for the in situ concentration and size distribution measurement of particles (8). However, none of these devices is entirely satisfactory for collecting aerosol samples for concentration and size distribution measurement by means of a n optical or a n electron microscope, inasmuch as representative samples of aerosols are not always easily obtainable. Attempts to obtain a uniform and representative aerosol sample by means of electrostatic precipitation were first made by Adley (13) who described a wire-in-tube electrostatic sampler whose outer collecting tube was in reciprocating motion t o uniformly distribute the particles over the collecting area. Recently, Liu et al. (14) introduced the pulse-precipitating principle in the design of a two-stage electrostatic aerosol sampler t o obtain a uniform aerosol deposit over a large collecting area. This paper presents improvements in the design of this sampler and describes a new charger based upon the pulse-charging principle. Performance characteristics of this pulse-charging-pulse-precipitating (PCPP) electrostatic aerosol sampler are also presented. Sampler Operation. The PCPP electrostatic aerosol sampler is a two-stage electrostatic precipitator consisting of a charging section and a precipitating section arranged as in Figure 1. A steady aerosol flow of 2.6 liters per minute is maintained in the sampler by a suction pump connected t o the outlet of the sampler. Particles are charged as they flow through the charging section and precipitated in the precipitating section by the high precipitating voltage pulses. Various types of collecting surfaces can be placed in the precipitating section of the sampler for collecting aerosol particles. Suitable collecting surfaces include glass microscope slides and cover slips, electron microscope grids coated with Formvar, carbon or other films, flat metal plates and any other flat electrically conducting or nonconducting surface. Although precipitation of particles occurs over the entire precipitating (1) M. Robinson, ANAL.CHEM., 33, 109 (1961). (2) G. L. Rounds and H. J. Matoi, Ibid., 27, 828 (1955). (3) S. C. Stern, D. R. Steele, and 0. E. A. Bolduan, A. M . A . Arch, Itid. Health, 18, 30 (1958). (4) D. A. Fraser, I d . Hygiene Quart. 17, 75 (1956). (5) P. E. Morrow and T. T. Mercer, Ind. Hygiene J., 25, 8 (1964). (6) C. E. Billings and L. Silverman, J. Air Pollution Control Assoc., 12,586 (1962). (7) P. C. Reist, Proc. 9th AEC Air Cleaning Conf., 2, 613, Conf.660904, USAEC, Div. Tech. Inf. Extension, Oak Ridge, Tenn. (1967). (8) K. T. Whitby, and W. E. Clark, Tellus, 18, 573 (1966). (9) H. H. Yoshikawa, G. A. Swartz, J. T. MacWaters, and W. L. Fite, Rea. Sei. Insfr., 27,359 (1956). (10) G . Langer, J. Appl. Phys., 32, 955 (1961). (11) G. Langer, Rev. Sei. Znsfr. 33, 83 (1962). (12) G . Langer, Intern. J . Air Water Pollution, 8, 167 (1964). (13) F. E. Adley, Am. Ind. Hyg. Assoc. J . , 19, 75 (1958). (14) B. Y. H. Liu, K. T. Whitby, and H. S. Yu,Reu. Sci. Instr., 38,100(1967).
section, the useful area over which the aerosol deposition is uniform consists of a much smaller 3- X 5-cm area within the dotted boundaries shown in Figure 1. Pulse Charger. The charger consists of a tungsten needle supported by a Teflon insulator in a standard 0.5-inch copper T fitting of the precision flareless type as shown in Figure 1. The tungsten needle is sharpened by a chemical etching technique t o a fine point with a radius of curvature estimated t o be approximately 5 microns. (Figure 2 shows that the corona starting voltage is approximately 1800 volts for positive corona; whereas for a negative corona the starting voltage is 1400 volts.) The voltage applied t o the needle is in the form of a 60 Hz ac of approximately 1300 volts rms superimposed upon a negative dc voltage of 1000 volts. The combined voltage wave form is shown in Figure 3. Because with this wave form the negative voltage on the needle exceeds the corona starting voltage during part of the cycle, a negative corona pulse is produced during each cycle. However, there is n o positive corona production, inasmuch as the positive voltage on the needle is kept below the corresponding starting voltage for positive corona. Aerosol particles are charged by pulses of negative corona as they pass through the charger. A high electric charge can be placed on particles, because the corona charging process occurs during that portion of the voltage cycle when the voltage on the needle, and hence the electric field in the charger, is a maximum. Further, the aerosol loss in the charge can be minimized, as the ac component of the applied voltage does not cause any precipitation of the particles in the charger while the dc component of the voltage, which causes the aerosol loss, can be kept relatively low. The complete voltage-current characteristics of this needlein-tube charger operated in this pulse-charging mode are shown in Figure 4. The average dc corona current is shown in the figure as a function of the rms value of the applied ac voltage for several values of the dc bias voltage levels. With increasing ac voltage the average dc corona current first rises rapidly and then more slowly. Finally, at sufficiently high ac voltages, the current actually begins to decrease indicating the start of positive corona. I n separate tests a dc corona current of 2 pA produced by 1300 volts rms ac with a - 1000 volts dc bias gave a sufficiently high electric charge on the particle without causing significant amounts of aerosol loss. Therefore, this has been chosen as the standard operating condition for the charger, and all performance tests reported here have been obtained under these conditions. A similar voltage-current characteristic for the charger and positive dc bias voltages are shown in Figure 5. Except for some differences in the absolute magnitude of the current, the general form of the curves is the same as similar curves for negative bias voltages. Pulse Precipitator. A uniformly deposited aerosol sample is obtained over a relatively large area in the precipitating section of the sampler by repeatedly filling the precipitating section with charged aerosol particles and precipitating these particles by the application of high precipitating voltage pulses. The precipitating voltage wave form and the electrical circuit used to produce it are shown in Figure 6. VOL. 40, NO. 4, APRIL 1968
843
CHARGING VOLTAGE
AEROSOLINLET
CHARGING VOLTAGE ~
V.-lOOO
BIAS
LNEOATIVE NEWTlVE CORONA CHARGING REGION
-z+*-
OR MICROSCOPE COMR SLIPS
1.5 SEC
PRECIPATATING VOLTAGE
?
/-ACRYLIC
PLASTIC
I
-SCREEN
COPPER
Figure 1. Pulse-charging, pulse-precipitating electrostatic aerosol sampler
That a representative and uniformly deposited sample of the charged particles can be obtained by the pulse precipitating principle can be understood by reference t o Figure 7. In Figure 7 particles with two different values of the electric mobility are shown to be precipitated onto a collecting surface of area A , which must be at a sufficient distance from the entrance to the precipitating section to avoid end effects. Because the volumes (represented by the two shaded regions in Figure 7) from which charged particles of these mobilities are collected by area A during each precipitating cycle are equal, no separation of particles according t o electric mobility, and hence according to particle size, can occur during the precipitation process. Further, because the volume of aerosol sampled during each precipitating cycle is equal to ( A ) X (h) as an examination of Figure 7 will show, the total aerosol volume sampled in n precipitating cycles is equal to ( n ) x 844
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ANALYTICAL CHEMISTRY
( A ) X (h), which is independent of the aerosol flow rate and depends only on the area, A , of the collecting surface and h, the vertical distance between the collecting surface and the top surface of the precipitating region. Thus, the sampler based upon the pulse-precipitating principle is capable of the determination of the absolute concentration of aerosol particles, when the efficiency of the charger is known. Performance of Sampler. The sampler was evaluated using monodisperse aerosol of uranine of 0.016-, 0.028-, 0.049-, 0.103-, 0.260-, 1.85-, and 6.3-micron diameter produced by an atomizer-impactor generator and a spinning disk generator previously described by Whitby et al. (15). During each test, six standard 22- X 22-mm microscope cover slips (15) K. T. Whitby, D. A. Lundgren, and C. M. Peterson, Intern. J. Air WaferPollution, 9, 263 (1965).
AC VOLTAGC, RYS VOLTS
Figure 4. Negative voltage-current characteristics of pulse charger
Figure 2. DC voltage-current characteristics of charger
‘-NEGATIVE
CORONA
AC WLTAGE,RYS
Figure 5. charger
Figure 3. Voltage wave form used on pulse charger showing the production of negative corona pulses
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Positive voltage-current characteristics of pulse
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Figure 6. Electrical circuit for producing high precipitating voltage pulses VOL. 40, NO. 4, APRIL 1968
845
MOBILITY 21, MOBILITY Zt
Figure 7. Precipitation of charged particles of two different electrical mobilities showing how uniform aerosol deposit is obtained on a collecting surface of area A
mm PARTICLE DIAMETER. MICRONS I O
OOOi
Table I. Uniformity of Aerosol Deposit and Efficiency of the PCPP Electrostatic Aerosol Sampler. Cover slip number 1 2 3 4 5 6
Efficiency
Diameter of test aerosol in microns 0.016
0.028
0.049 0.103 0.260 1.85
06.3
1.21 0.79 0.67 0.54 0.53 0.46 0.54
1.06 0.76 0.68 0.64 0.64 0.61 0.64
0.92 0.74 0.70 0.70 0.68 0.62 0.69
1.37 0.88 0.83 0.81 0.80 0.72 0.81
1.32 1.13 0.84 0.75 0.74 0.71 0.75
0.96 0.74 0.70 0.68 0.66 0.65 0.67
1.27 0.87 0.76 0.74 0.73 0.73 0.74
0 The numbers expressing the uniformity of the aerosol deposit given in this table for each of the six cover slips and for each of seven aerosols are the values of the ratio, 7, computed according to Equation 1 (see text). The efficiency of the sampler is based upon the average of the ratio for cover slips numbers 4 and 5.
were placed in the precipitating section, as in Figure 1, and the sampler was operated under standard conditions and in the pulse precipitating mode for a definite number of cycles ranging from 3000 cycles for the 0.016-micron aerosol to 200 cycles for the 0.26-micron aerosol. Simultaneously, a n aerosol sample was taken with a membrane filter and the volume of aerosol sampled was measured using a wet-test meter. The experimental set-up is shown in Figure 8. At the end of each test, the cover slips and the membrane filter were taken from the sampler and individually rinsed in demineralized water. The uranine content of the rinse water
Figure 9. Efficiency of PCPP electrostatic aerosol sampler
was then measured with a fluorometer. The results of the tests are given in terms of the following ratio,
which is given in Table I for each of the six cover slips and for each of the seven test aerosols. In Equation 1 C, is the uranine concentration in the rinse for the particular cover slip, V , is the volume of demineralized water used in rinsing the cover slip, n is the number of precipitating cycles, A is the area of the cover slip, h the vertical distance between the upper surface of the cover slip and the top of the precipitating region, Cfis the uranine concentration in the rinse water for the membrane filter, V I is the volume of demineralized water used in rinsing the membrane filter, and Va is the volume of aerosol sampled by the membrane filter as measured by the wet-test meter, Within the region where the aerosol deposition is uniform, the ratio, 7 , as computed by Equation 1, represents the absolute sampling efficiency of the sampler. The data in Table I show that over the area occupied by the two cover slips, numbers 4 and 5, this ratio is essentially constant. The efficiency of the sampler, based upon the ratio, 7 , for the two cover slips 4 and 5 , is shown in Figure 9 as a function of particle size. The efficiency varies by approximately 10% over a two-decade range in particle size, and over the entire range of particle size tested the variation was from approximately 54 t o 81%.
PRECIPITATING VOLTAGE V * -1000 BIAS
I
2.6 LITER/MINUTE
1
I
FLOURESCENT AEROSOL (DRY AND ELECTRICALLY NEUTRAL)
ROTAMETER
FILTER SAMPLER
-
” VACUUM PUMP
T
WET TEST METER
Figure 8. Test set-up 846
ANALYTICAL CHEMISTRY
0 0
Although the aerosol sample taken with the PCPP electrostatic sampler is not entirely representative, as the sampling efficiency depends to some extent on the particle size, the true size distribution of the aerosol can be determined by making appropriate corrections using Figure 9. Further, because the absolute sampling efficiency of the sampler is known, the absolute concentration and size distribution of the
aerosol can be determined from the aerosol sample. This is generally impossible with the conventional electrostatic aerosolsampler. RECEIVED for review August 31, 1967. Accepted December 26,1967. Research supported under AEC contract AT(11-1)1248 and carries the publication number COO-1248-11 under this contract.
A Venting Apparatus and Cleaning Procedure for Electron Capture Detectors Robert R. Claeys and Tommy Farr Department of Agricultural Chemistry, Oregon State Uniuersity, Coruallis, Ore.
ONEof the problems encountered with a tritium source electron capture detector is cell contamination. The detector becomes noisy with time, with a concurring decrease in the linear response range and available standing current. Frequent cleaning of the detector is often necessary. Two developments have led to a large increase in the linear response range and to reduction in noise and base line drift. The first development is an improved cleaning procedure for tritium foils, and the second is the addition of a carrier gas venting apparatus. The carrier gas is shunted into the atmosphere instead of passing through the detector during idle periods or when the solvent and contaminating materials are eluted from the column. The new cleaning technique results in a cleaner foil, and the venting arrangement reduces foil contamination from column bleedoff or the elution of contaminants. EXPERIMENTAL Cleaning Procedure. The tritium foil is first removed from the cell and placed in a 25-ml boiling solution of 5 % KOH in 9 5 z ethanol. Disposable gloves and safety glasses must be worn. With the use of tweezers, the foil is scrubbed lightly with a pipe cleaner for 2 or 3 minutes. The foil is removed from the solution and rinsed with 10 ml of hot 95% ethanol using a disposable pipet. The foil is then cleaned with an aqueous, thiourea, jewelers' solution (Jeweluster manufactured by E-Z-EST Products Co., Inc., Oakland,
Calif.) by scrubbing with a cotton pipe cleaner for 2 to 3 minutes, and rinsed with 10 ml of hot ethanol. The detector cell and leads are also cleaned with this solution. All liquid wastes are combined for counting or disposal. The waste solution from cleaning a 250-mCi tritium foil has been found to contain 0.001 to 0.04 mCi of 3H. Apparatus. The venting apparatus, shown in Figures 1 and 2, was installed in a MicroTek 220 instrument equipped with a parallel plate detector and a Varian Aerograph 550 instrument equipped with a concentric tube detector. The toggle valve is mounted outside of the oven, preferably on the front panel. When the toggle valve is open the carrier gas is restricted from entering the detector by capillary tubing. Upon closing the toggle valve the carrier gas flows through the capillary tube to the detector. Arrangements must be made to pass auxilliary nitrogen gas directly into the detector. With instruments employing a parallel plate detector a purge gas provision may already be present. With instruments employing a common base for electron capture and flame ionization detectors the nitrogen gas can be admitted through the hydrogen inlet. The auxilliary nitrogen gas is adjusted t o 20 ml/minute with a Nupro 2M needle valve (Nuclear Products Company, Cleveland, Ohio). After cleaning the tritium foil and installing of the venting apparatus, the linearity and sensitivity of both the parallel plate and concentric tube detectors were measured, and the results compared with previous detector responses. Both detectors were operated at a fixed dc voltage with nitrogen carrier gas.
G
B
I
Figure 1. Cross-sectional view of venting apparatus A. B.
C.
D. E.
F. G. H.
Gas chromatographic column Stainless steel Swagelok cao 1- X '/le-inch OD, i.047-inch ID, S.S. hypodermic tubing '/&ch OD copper tubing "4X +inch OD S.S. tubing 1- X l/l~-inchOD, 0.007-inch, S.S. capillary tubing Swagelok nuts Solder welds
DETECTOR BASE
TOGGLE VALVE
VOL. 40, NO. 4, APRIL 1968
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