Anal. Chem. 1980,
The determination of high concentrations of analytes using the emission of the sample in the ICAP with the help of a flame resonance spectrometer eliminates the requirement for dilution or the search for and the disadvantages of a less sensitive fluorescing line. As stated before ( 2 ) ,the great value of the ICAP-excited AFS technique lies in the excellent spectral resolution of the atomic fluorescence measurement. Therefore, specific analytical problems can be solved without the need for an expensive, high-resolution monochromator.
LITERATURE CITED (1) N. Ornenetto, S, Nikdel, J. D. Bradshaw, M. S.Epstein, R . D. Reeves, and J. D. Winefordner, Anal. Chem., 51, 1521 (1979). (2) M. S. Epstein. S. Nikdel, N. Ornenetto, R. Reeves, J. D. Bradshaw, and J. D. Winefordner, Anal. Chem., 51,2071 (1979). (3) N. Omenetto and J. D. Winefordner, Prog. Anal. Atom. Spectrosc., 2, 1 (1979).
52, 287-290
207
(4) J. P. S. Haarsma, J. Vlogtman, and J. Agterdenbos, Spectrochim. Acta.. Part E, 31, 129 (1976). (5) R. C. Fry and M. B. Denton, Anal. Chem., 49, 1413 (1977). (6) A. Walsh, Analyst(London), 100, 764 (1975). (7) T. Hollander, B. J. Jansen, J. J. Plaat, and C. Th. J. Alkemade, J . Quant. Spectrosc. Radiat. Transfer, IO, 1301 (1970). (8) H. G. C. Human and R. H. Scott, Spectrochim. Acta, Part E,31,459 (1976). (9) J. M. Ondov, W. H. Zoller, I. Olmez, N. K. Aras, G. E. Gordon, L. A. Rancitelli, K. H. Abel, R. H. Filby, K. R. Shah, and R. C. Ragaini, Anal. Chem., 47, 1102 (1975). (10) M. S. Epstein, T. C. Rains, and 0. Menis, Can. J . Spectrosc., 20, 22 (1 975).
RECEIVED for review July 19,1979. Accepted October 22,1979. N. Omenetto would like to thank the Committee of Internal Exchange of Scholars for the granting of a Fulbright travel fellowship*Research by AF-AF0SR-44620-76-C005 and by WPAFB Contract number F33615-78-C-2036.
Aerosol Monitoring System for the Size Characterization of Droplet Sprays Produced by Pneumatic Nebulizers John W. Novak, Jr. and Richard F. Browner" School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
An aerosol monitoring system has been developed which gives size distributions of spray droplets produced by common nebulizers used for analytical atomic spectrometry. The system provides a means for systematic characterization of droplet sizes of low volatility liquid sprays in the range 0.1-10 bm. Droplet distributions have been obtained using a cascade impactor operated In series with an electrical aerosol analyzer. These measurements are necessary to provide a sound practical and theoretical bask for the development of more effective means of solution transfer to flames and plasmas.
The nebulization of liquid samples is a critically important process in analytical atomic spectrometry, in that sample solutions are normally introduced into flames and plasmas as finely dispersed mists generated by pneumatic nebulizers. Willis ( I ) has made a detailed study of the operation of certain pneumatic nebulizers and has related a number of atomization problems experienced in analytical atomic spectrometry directly to nebulizer characteristics. Willis has suggested that for flame atomic absorption spectrometry, both atomization efficiencies and chemical interferences are a function of the droplet size distribution produced by the nebulizer. The droplet size distribution of the analyte solution spray determines: (i) the transport efficiency of the sample from solution to flame or plasma (2); (ii) the rate of desolvation of the wet droplets (3); (iii) the rate of vaporization of the dried salt particles remaining after desolvation (3) and finally, (iv) the magnitude of many interference effects (4,most commonly solute vaporization interferences. Consequently, both the magnitude of the analytical signal and its degree of freedom from some common interference effects are critically related to the nature of the spray entering the flame or plasma. The spray characteristics themselves are determined by the design of the nebulizer/spray chamber combination. O003-2700/80/O352-0287$0 1 O O / O
In order to understand better the operation of atomic absorption spectrometry ( U S ) and inductively coupled plasma (ICP) nebulizers in common use, it is essential first to accurately determine the droplet size distributions of the sprays they produce. Only with this information can design improvements be made which are based upon the actual data of relevance, rather than on a combination of indirect data such as signal magnitude, signal stability, interference effects, etc. T o date, the information available on droplet size distributions produced by various pneumatic nebulizers is of little direct relevance to any of the parameters of importance noted above. This is a direct consequence of the techniques used for collection of the data ( I , 3, 5 , 6 ) which are incapable of reliably measuring droplets smaller than approximately 10 pm in diameter. As will be shown in this and later studies: (1)a significant fraction of the mass of a typical droplet spray is contained in particles below 5-pm diameter, and (2) the great majority of droplets greater than approximately 20 pm do not reach the flame or plasma in any case and so are largely irrelevant to analytical spectrometry. There are very real difficulties associated with the determination of droplet size distributions from pneumatic nebulizers operating under realistic conditions. The high particle number densities of the sprays (approximately lo6 particles ~ m - and ~ ) wide particle size distributions can lead to significant measurement errors, even when sophisticated instrumentation is used for their determination (7-9). Recent reviews of available particle sizing techniques ( I O , 1I ) aided in the selection for this study of a cascade impactor for large droplets (12-14) (20.4 pm) and an electrical aerosol analyzer for droplets in the range 0.032-1.0 pm diameter ( 1 5 - l a . However, these systems are generally used at much lower particle number densities than those found in pneumatic nebulizer sprays (12, 16, 18, 29). Instrument requirements led to the need for extremely high dilution of the aerosol 1580 American Chemical Society
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stream, without disturbing the particle size distribution. In addition, the efficient removal of droplets > 1.0-pm diameter from the aerosol stream prior t o its entry into the electrical aerosol analyzer was essential, as these droplets can give rise t o erroneous signals. The development and testing of a system which meets these requirements and which is suitable for characterization of droplet sprays produced by pneumatic nebulizers is described in the following text. EXPERIMENTAL Aerosol Monitoring Apparatus. An Anderson cascade impactor (Anderson 2000 Inc., Model 21-000) was used for the sizing of droplets in the range 0.4-11.0 pm in diameter. In addition, a back-up filter stage for absolute collection of submicron particles was added for some studies. In almost all experiments a preimpactor head and modified first stages (14, 20) were used. The necessity of using the modified impactor when sizing aerosols containing a large fraction of particles over 10 pm in diameter has been well justified (14). The Anderson sampler was operated a t 28.3 L min-' air flow rate using a Doerr 1/3 horsepower vacuum pump (Doerr Electric Corp., Cedarburg, Wis.). The calibration of the cascade impactor was checked by collecting 4.6 f 0.2 pm diameter dioctylphthalate droplets, generated with a laboratory constructed monodisperse aerosol generator of the Berglund and Liu type (21),through the experimental system. The calibration was found to be accurate to within the manufacturers' quoted limits (2~10%).This was felt to be sufficiently accurate for the present study. Particles I 1pm in diameter were sized by an Electrical Aerosol analyzer (Thermo-Systems, Inc., Model 3030). This instrument gives a size distribution in the range 0.032-1 Mm every 1.5 min. The Model 3030 is an advanced version of the prototype developed by Liu, Whitby, and Pui (16). The instrument's operation is based on unit charging of the droplets by passing them through a corona discharge, then measuring the electrical mobility of the charged aerosol. Although the ouput of the electrical aerosol analyzer (EAA) is an electrical mobility spectrum, this can be easily converted to a particle size distribution from standard calibration curves provided with the instrument. The 50 L min-' air flow required through the EAA was supplied by a Welch vacuum pump (Model 1402, Sargent-Welch Scientific Co., Skokie, Ill.). All air flows through the system were accurately monitored by a Matheson Linear Mass Flowmeter (Model 8116, Matheson Gas Products, Rutherford, N.J.). The system's air supplies were dried with silica gel, then filtered through two 0.02-j~mfilters arranged in series. Aerosol Generation and Sampling. Perkin-Elmer AAS nebulizers, with adjustable sample capillary needles, were used in these studies and dioctylphthalate (DOP) was used to generate the test aerosols. A syringe pump (Model 341, Orion Research, Inc., Cambridge, Mass.) was used when a controlled DOP flow to the nebulizer was required. Sample probes were made from Pyrex glass tubing. The aerosol was isokinetically sampled at all points in order to avoid biasing the size distribution (22, 23). RESULTS AND DISCUSSION Development of the Aerosol Monitoring System. A schematic representation of the monitoring system is shown in Figure 1. The main novelty of this system is that it utilizes a cascade impactor operated in series with an electrical aerosol analyzer. The cascade impactor provides an integrated size distribution for a n entire test, while the EAA gives a size distribution profile every 1.5 min during a test. Table I lists the size ranges measured and shows how each instrument fits into the measurement scheme. I t should be noted that there is overlap of measurement intervals between the cascade impactor and the EAA, which serves to check that each instrument is operating properly. This check is accomplished by placing the absolute filter in the cascade impactor assembly and collecting all submicron particles that would normally have gone t o the EAA. The mass collected on the absolute
nebulizer
I
I
Figure 1. Measurement system for aerosol particle sizing (a marks air inlets). I, initial aerosol dilution chamber; 11, secondary concentric dilution chamber; 111, isokinetic sampling probe; IV, cascade impactor; V, isokinetic sampling probe; VI, final aerosol dilution chamber; EAA, electrical aerosol analyzer
Table I. Measurement Ranges of Cascade Impactor and Electrical Aerosol Analyzer" particle size ranges, ym
> 10 10.0-9.0 9.0-5.8 5.8-4.7
\
I
1.1-0.7
0.7-0.3 2 0.32-0.18 0.18-0.10 0.10-0.06 0.06-0.03
I
EAA
I
In this study the final range 0.06-0.03 pm was not used. a
filter should correspond to the mass calculated from the EAA measurements with the filter removed. In most instances, the total mass of sample 50.4 pm in diameter calculated from the EAA measurement was about 10% higher than the mass expected from the filter measurement. A likely reason for this discrepancy is partial sample loss from the absolute filter by evaporation or by reentrainment of the small aerosol droplets into the air stream of the cascade. Nevertheless, this discrepancy was not large enough t o suggest any major source of error. Dilution System. The initial data collection was performed using very low flow rates of DOP t~ the nebulizer from a syringe pump. These data were used later as a reference to ensure that the dilutor system did not upset the droplet distribution. At higher (>0.1 mL min-') flows of DOP, the droplet concentration (droplet mass ~ m - was ~ ) too high to be measured effectively without further dilution of the initial spray produced in Chamber I of the system. High particle concentrations have been a problem for many aerosol research projects and suitable dilutor systems have been designed (24, 25) to meet the individual needs of specific projects. However, these were primarily used for dry aerosols with lower number densities than aerosols produced by nebulizer sprays and both were found experimentally t o be unsuitable for this study. Consequently, a new system was developed for this study which served t o dilute the aerosol without altering the size distribution. This is shown diagrammatically as parts I and I1 in Figure 1. The inner tube of the first dilution unit essentially replaces the burner chamber. This allows auxiliary air to be added, simulating actual AAS operation, before the aerosol moves into the dilutor tubes. The auxiliary air flow
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
0.1
0.2
0.5
1
10
5
2
289
Droplet diameter, pm Flgure 2. Reproducibility of droplet size distributions. iii New Perkin-Elmer nebulizer.
was controlled to give the original aerosol stream a threefold dilution. Using typical AAS gas flow, Le., approximately 20 L min-' (nebulizer plus auxiliary gas), the new design dilutor is capable of a thirty-fold dilution of the original nebulizer spray. In addition, the dilutor serves to increase the velocity of the aerosol, thereby allowing isokinetic sampling of only a small portion of diluted aerosol a t the dilutor's end. The isokinetic sample probe is shown as Part I11 in Figure 1. In this study a tube about 1/13 the cross-sectional area of the dilutor served as the sample probe for the cascade impactor. By using this combination a 400-fold dilution was achieved, putting the droplet concentration well within the working range of the cascade impactor (Part IV in Figure 1). T h e EAA measures aerosols in the concentration range between 1 and 1000 r g m-3. This necessitated an additional dilution of the aerosol stream leaving the cascade impactor before it entered the EAA. The dilution system for the EAA is shown as parts V and VI in Figure 1. Its operation is as follows: (1)the aerosol leaving the cascade impactor is allowed to enter the expansion chamber, from which a small portion is isokinetically sampled; (2) the sampled portion is then further diluted by make-up gas added through a dilutor before entering the E M . Table I1 summarizes the details of dilutions made on the original aerosol stream and the total dilution achieved. The final aerosol concentration is within the optimum working range of the EAA. T e s t i n g of the Aerosol Monitoring System. In evaluating the performance of the monitoring system, it is important to consider possible aerosol distribution changes which might occur before and during sampling. Evaporation of the DOP droplets in an air stream has been shown to be negligible (26). Aerosol sampling was carried out isokinetically in order not to upset the droplet distribution at sampling points. One very important factor to consider in evaluating the quality
1111 Two-year-old Perkin-Elmer nebulizer
Table 11. Dilution Factors for Components of Systema dilution factors stage I (chamber) stage I1 (dilutor) stage I11 (sampler 1) stage IV (cascade) stage V (sampler 2) stage VI (make-up gas)
3x 10 x 13x 1x
24x 4x
37 000 total a
See Figure 1 for explanation of stage numbers.
of the data produced by this system is the loss of droplets due to settling or impaction on the p r i m w dilution chamber walls. Although this represents a significant percentage of the total mass of sample nebulized, the larger droplets settle or impact in a somewhat analogous manner during an actual AAS or ICP analysis using conventional equipment, and so actually contribute little to the analytical signal. Considerable work has been done evaluating sources of error in data acquired from cascade impactors (19,27) and the E M (28). It is apparent that once these instruments are properly calibrated they have high reliability. System Evaluation. Some preliminary data were collected to determine the precision of the measurement system. During one week, several runs were made using identical operating conditions with both new and old Perkin-Elmer nebulizers. Figure 2 shows the variation in measurements. The monitoring system appears to measure quite consistently. However, an interesting observation is the spread of data using a twoyear-old nebulizer. This type of fluctuation in the droplet distribution may ultimately affect the precision of real analyses by altering droplet transport efficiencies, atomization rates, etc.
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This method of plotting is chosen in preference to a histogram plot because it allows plotting of data taken over unequal particle size ranges. This is not possible with the histogram method. Both the cascade impactor and the EAA produce unequal size range data. When the log of the droplet diameters is plotted, the area under the curve is approximately equal to the aerosol mass in the droplet range of interest. This also facilitates a visual approximation of the mass median diameter. Actual droplet number distributions have also been calculated and their plots are somewhat bell-shaped.
5
LITERATURE CITED (1) (2) (3) (4)
[
Droplet dlarnefer
prr
Flgure 3. Droplet size distributions for DOP and silicone oil aerosols produced under identical conditions. Nebulizer: Perkin-Elmer concentric design. Nebulization air flow: 5.6 L min-'. Air pressure: 1.17 X lo2 kPa above atmosphere (17 psig). Liquid flow (DOP and silicone oil): 0.012 mL min-'. 0 Dioctyl phthalate. A Silicone oil
It should be emphasized that all data on droplet size distributions presented herein refer exclusively to DOP aerosols. The difficulties of working with aqueous droplet sprays are considerable but work is currently underway to achieve this goal by a variety of approaches, including laser scattering experiments. Nevertheless, data obtained with DOP are directly applicable to studies with aqueous sprays. Essentially, the main anticipated difference with aqueous sprays is a shift to larger mean droplet diameters, and preliminary studies have conf i e d this practically. The particle size distributions obtained with DOP allow a valuable comparison to be made between different nebulizers and their operation with different solution and gas flow rates. T o the best of our knowledge, these data have not been obtained by any means in the critical size range < l o pm prior to this study. The ability of the system to properly measure distributions produced by different solvents was also investigated. The properties of silicone oil which determine the size distribution of droplets produced under the same nebulizer conditions (i.e., surface tension and viscosity) are similar to those of DOP. Figure 3 shows that silicone oil and DOP produce similar distributions. The slightly lower mass median diameter of droplets formed from silicone oil is predicted in general terms from the Nukiyama and Tanasawa equation (29). Therefore the system does appear to respond in a predictable manner to droplet size distribution changes and should be adaptable to other solvent systems. Data Presentation. The ordinates of Figures 2 and 3 represent the mass distribution function which normalizes droplet mass distributions to a uniform sizing interval Adp.
Willis, J. B. Spectrochim. Acta, Part A , 1979, 23, 811. Skogerboe, R. K.; Olson, K. W. Appl. Spectrosc. 1978, 32, 181. Bastiaans, G. J.; Hieftje, G. M. Anal. Chem. 1974, 4 6 , 901. Alkemade, C. Th. J. "Flame Emission and Atomic Absorption Spectrometry", Vol. I, Dean, J. A.. Rains, T. C., Eds.; Marcel Dekker: New York, 1969; Chapter 4. (5) Koirtyohann, S . R.; Pickett, E. E. Anal. Chem. 1988, 38, 1087. (6) Stupar, J.; Dawson, J. B. Appl. Opt. 1988, 7 , 1351. (7) Fuchs, N. A. Atmos. Environ. 1975, 9 , 697. (8) Sengupta, M.; Jana, S. S.;Biswas, D. N. Prog. Colloid Pdym. Sci. 1978, 63, 195. (9) Mercer, T. T.; Goddard. R. F.; Flores, R. L. Ann. Allergy 1965, 314. (IO) "Instrumentation for Monitoring Air Quality", ASTM Spec. Tech. Pubi. 555;ASTM: Philadelphia, Pa. 1974. (11) "Aerosol Measurements", Nati. Bur. Stand. ( U . S . )Spec. Pub/.412; Cassat, W. A.; Maddock, R. S., Eds.; U S . Govt. Printing Office: Washington, D.C., 1974. (12) May, K. R. J . Sci. Instrum. 1945, 22, 187. (13) Andersen, A. A. J . Bacterial. 1958, 7 6 , 471. (14) Hu, J. Nan-Hai Environ. Sci. Technoi. 1971, 5, 251. (15) Liu, B. Y. H.; Whitby, K. T.; Pui, D. Y. H. "A Portable Electrical Aerosol Analyzer for Size Distribution Measurement of Submicron Aerosols". Abstracts of Papers, 66th Annual Meeting of the Air Pollution Control Association, Chicago, Ill. 1973. (16) Whitby, K. T.; Liu, B. Y. H.; Husar, K. B.; Barsic, N. J. J. Colloid Interface Sci. 1972, 39, 136. (17) Whitby, K. T.; Clark, W. E. Tellus, 1966, 18, 573. (18) Wedding, J. B.; McFarland. A. R.; Cermak, J. E. Environ. Sci. Technoi. 1977. 11. 387. (19) Zinn,'B. T:; Powell, E. A.; Cassanova, R. A,; Bankston, C. P. Fire Res. 1977, 1 , 23. (20) May, K. R. Appl. Microbiol. 1984, 12, 37. (21) Berglund, R. N.; Liu, B. Y. H. Environ. Sci. Techno/. 1973, 7 , 147. (22) Davies, C. N. Br. J . Appi. Phys. 1988, 2 , 921. (23) Watson, H. H. Am. Ind. Hygiene Assoc. 0.1954, 15, 21. (24) Hare, C. T.; Springer, K. J.; Bradow, R. L. "Fuel and Additive Effects on Diesel Particulate. Development and Demonstration of Methodology", Society of Automotive Engineers, Warrendale, Pa., 1976. Paper No. 760130. (25) Kittleson, D. B.; Dolan, D. F. "Dynamics of Sampling and Measurement of Diesel Engine Exhaust Aerosols". Abstracts of Papers, Conference on Carbonaceous Particles in the Atmosphere, Berkeley, Calif., 1978. (26) Bitron, M. D. Ind. Eng. Chern. 1955, 47, 23. (27) Dzubay, T. G.; Hines, L. E.; Stevens, R. K. Atmos. Environ. 1978, 10, 229. (28) Marple, V. A. "Development, Calibration and Application of Size Distribution Instruments at the University of Minnesota", Nati. Bur. Stand. (U.S.) Spec. Pubi. 412; Cassat, W. A,, Maddock, R. S.,Eds.; US. Govt. Printing Office: Washington, D.C., 1974. (29) Nukiyama, S.; Tanasawa, T. "Experiments on the Atomization of Liquids in an Air Stream"; Hope, E., transl.; Defense Research Board, Department of National Defense: Ottawa, Canada, 1950.
RECEIVED for review June 29, 1979. Accepted November 1, 1979. This work was supported by the National Science Foundation under Grant No. CHE77-07618.
