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Circular Slot Burner-Droplet Generator System for High-Temperature Reaction and Vapor Transport Studies B. M. Joshi and R. D. Sacks* Department of Chemistty, University of Michigan, Ann Arbor, Michigan 48 109
A premixed, laminar flow nitrous oxide-acetylene flame from a circular slot burner Is described. Sample solution in the form of equally spaced, uniform size droplets produced by a piezoelectrically driven pinhole droplet generator Is introduced along the flame axis through a hole in the burner body. This produces 4-8 pm diameter analyte particles and results in a radiation plume beginning at one point in the flame. Dropto-drop variation in the axial location of the plume origin is f l mm. A detailed flame temperature prollle is presented which was obtained by the Fe two-line methlod. The flame has a 7 cm long X 10 mm diameter regioin with fairly uniform temperature of 2850 f 50 K and constant rise velocity of 1160 f 17 cm/s. Radial vapor transport of Ca from CaCI, particles appears to be diffusion controlled in this region of the flame. The measured value of isotropic diffusion coefficient for Ca is within the range of expected values.
Elemental determinations with atomic spectroscopy using nebulized solution samples are becoming so frequent and important in many technologies that there is growing interest in the details of all phases of the droplet-plasma interaction. These include droplet and vapor transport and atom formation and loss as well as radiative processes. Thus, a n instrumentation system of relative simplicity, designed specifically for such detailed studies, would be worthwhile. Ideally, such a system should combine two hardware elements, a well characterized, large volume, constant temperature transport and reaction zone and a system for introducing, in a controlled fashion, test species to study in the plasma or probe species to characterize the plasma. Laminar flames may be better suited for these studies than arc plasmas because the former can be designed to have a large region of fairly uniform temperature. The importance of a point source of free atoms for the reliable measurement of vapor transport properties is discussed by Ashton and Hayhurst ( I ) . The piezoelectrically driven droplet generator developed by Hieftje and Malmstadt (2, 3 ) provides a n elegant means for the controlled introduction of test or probe material. This paper describes a simplified form of piezoelectric droplet generator and a circular slot burner designed specifically for use with the generator. T h e burner has been designed t o produce a laminar flame with a large volume zone of relatively uniform temperature and constant rise velocity. These conditions are necessary to obtain reliable vapor transport data as well as information on atom formation and loss processes. A circular slot burner, where uniform droplets can be injected along the flame axis, was chosen for its design simplicity as well as for reducing problems associated with the droplet stream trajectory. Nitrous oxide-acetylene was chosen as the combustion mixture for several reasons. First, its relatively high temperature may provide a wider range of potential applications as well as simplify the extrapolation of observed trends to higher temperature arc plasmas. Second, it is popular as a free atom generator and excitation source for analytical spectroscopy. Third, there are less available 0003-2700/79/035 1-1781$0 1.OO/O
transport and kinetic data for this flame relative to some lower temperature oxy- or air-hydrocarbon flames. Finally, premixing under laminar flow and burning conditions is straightforward. Premixing is useful for reducing thermal gradients in the flame reaction zone. In addition to hardware design, data are presented which provide information on flame temperature variation, rise velocity, and its reproducibility, and finally on the vapor transport properties of the combined droplet generator-slot burner system. A companion report presents a preliminary study of atom loss and transport processes for Cia and Mn (4).
EXPERIMENTAL Figure 1shows a diagram of the entire system. The piezoelectric droplet generator is located directly under the burner so that the droplet stream passes vertically through the hole in the burner body. Tank air and a ballast chamber are used to force test solution through the generator pinhole orifice. Mirrors MI and M2 and beam splitter BS are used to image radiation from a selected part of the flame onto monochromators A and B. Two monochromators are used for flame temperature measurements by the two-line method. The outputs from the detectors are recorded simultaneously on a digital storage oscilloscope, which has an analog X-T output. Droplet Generator. A section view of the droplet generator is shown in Figure 2b. It is similar to a design described by Berglund and Liu ( 5 ) . A 4-cm o.d., 2.5-cm i.d., 0.6.cm thick ring shaped piezoelectric bimorph transducer (Gulton Industries) is used to establish a periodic deformation of a stainless steel plate containing a pinhole orifice through which the test solution is forced. This breaks up the liquid stream into a series of solution droplets. With the proper combination of transducer excitation frequency, pinhole diameter, and solution flow rate, droplets with very uniform size and spacing are obtained. The transducer is attached with conductive adhesive to a 4-cm diameter stainless steel flange. The flange is threaded onto a stainless steel support so as to seal the pinhole between two Teflon O-rings. Pinholes (10-20 pm diameter) formed in 9.5-mm o.d., 0.25-mm thick stainless steel disks (Optimation, lnc., Concord, Mass.) were used. A stainless steel feed tube is provided to supply solution to the small cavity below the pinhole. A drain tube also is provided. The drain tube is sealed during operation. Both surfaces of the transducer are tinned, and electrical contact is made through the stainless steel flange on the upper surface and through a spring-loaded contact on the under surface. The transducer is excited directly with a 10-V peak-to-peak signal from a function generator (Model F-34, Interstate Electronics Corp., Anaheim, Calif.). No ancillary electronics are used, and no provision is made for droplet charging or sorting. Thus, all droplets produced by the generator are introduced into the flame. Occasionally, owing to erosion of the pinhole, the solution jet comes out at some angle with respect to the pinhole axis and hence would not move along the flame axis. This problem was reduced by mounting the generator on a circular stand which contains three leveling screws. A 6.3-cm id., 8-cm long brass chamber pressurized by tank air is used to supply solution to the droplet generator. The sample delivery tube is placed in a beaker of solution which is sealed in the chamber. A membrane filter between the pressure chamber and generator is necessary since the pinhole is easily clogged by suspended particulate material. Solution flow rate is determined by inserting a Y-shaped stainless steel tube into the solution flow line. A screw-type syringe 1979 American Chemical Society
BIMOFIP*
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TEFLON O-RINGS
/SPRING CONTACT
\LIQUID
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Figure 2. Section drawings of (a)circular slot burner and (b) piezoelectric droplet generator
is used to inject a small air bubble into the side arm of the tube. The bubble enters the solution flow, and the time required for it to traverse a 50-mm long, 1.0-mm i.d. segment of precision-bore stainless steel tubing then is used for flow rate measurements. Droplet size is computed from the solution flow rate and the frequency of the excitation signal. This frequency is measured on a Nicolet Model 1090-A digital storage oscilloscope. For the range of solution flow rates used in this study, the uncertainty in droplet volume ranges from 1 to 6% using this procedure. Measurements of droplet diameter from photomicrographs confirmed that only one droplet is formed per oscillatory cycle of the piezoelectric transducer. Circular Slot Burner. A section view of the nitrous oxide-acetylene circular slot burner is shown in Figure 2a. The burner is machined in two parts from stainless steel blocks. When the two pieces are connected, a 0.40-mm wide slot is formed, which has a mean circumference of 29.9 mm. The height of the slot, which is important in maintaining laminar flow, is 32 mm. Solution droplets pass through the 6.25-mm diameter hole in the burner body and enter the combustion zone above the burner head. A water cooling chamber is machined to fit close to the burner head and is silver soldered to the burner body. With the fuel and oxidant flow rates used in these studies, the Reynolds number does not exceed 2000. Fuel and oxidant gases pass through tank regulators and rotameter-type flow meters equipped with needle valves. The gases are premixed in a 2.5-cm i.d., 10-cm long stainless steel chamber filled with glass wool. A hole in the side of the chamber is sealed with an A1 foil diaphragm, which serves as a safety pressure release in case of flashback. All connecting lines are 0.62-cm 0.d. copper tubing.
The burner and droplet generator are mounted as a single unit on a stand which is connected by an 8-thread per cm lead screw to a reversible, ac, 100 rpm motor. This permits the flame to be translated normal to both the flame axis and the optical axis a t a rate of 0.208 cm/s. Since the maximum diameter of the flame is about 2.5 cm, the flame image is scanned across the monochromator entrance slit at a rate of about 1 2 s/scan. The stand also has two vertical, screw-type support rods, which permit manual vertical positioning of the system so that radial scans of the flame can be made at different axial positions. Optical Monitoring. Monochromator A in Figure 1 is a GCA McPherson Model 218, 0.3-m instrument, and B is a GCA McPherson Model EU-700, 0.3-m unit. Both have first-order, linear reciprocal dispersion of about 2.5 nm/mm and use 100-pm wide, 1.25-mm long entrance slits. The mirrors, MI and M2 in Figure 1, are 7.5 cm in diameter and 37.0 cm in focal length and are positioned to image the flame axis on the entrance slits with unit magnification. Since the monochromators have different optical speeds, the mirrors are stopped down to 5.0-cm diameter when both monochromators are used for flame temperature measurements. This results in the mirror system being the limiting aperture for both monochromators and thus results in both instruments viewing the same plasma volume. A quartz, tungsten-iodine lamp (General Electric, Model 6.6A/T4/1CL) operated at 6.5 A as in Stair et al. ( 6 ) and positioned a t the normal location of the flame was used to obtain corrections for differences in monochromator efficiency and detector response as well as the beam splitter transmission at all wavelengths of interest. The 1P28 photomultiplier tubes were biased at -1000 V, and their outputs were measured across their 10-kQ load resistors, which were connected in parallel with 1.0-pF capacitors. This results in a 0.01-s time constant and significantly reduces noise from flame flicker. The detector signals are displayed simultaneously on a Nicolet Model 1090 A digital storage oscilloscope operated with a sampling interval of 0.1 s/point. Droplets were examined and photographed with an optical microscope by directing the droplet stream normal to the microscope optical axis and illuminating the stream with repetitive flashes from a strobe lamp. The lamp is triggered from the signal used to excite the bimorph transducer and has a combined pulse width and time jitter of about 1 ps. Data Reduction. Test solution concentrations were sufficiently high to mitigate the need for background corrections. Measured intensity profiles from scans across the flame were used directly for radial diffusion studies and deconvoluted for temperature measurements. A 50-radial zone model and a matrix approximation to the Abel integral equation are basic to the deconvolution procedure. Since the flame has an axis of symmetry, scans across the entire flame result in nearly symmetric intensity profiles. Thus, two intensity values are recorded for each of the 0.208-mm wide ring-shaped zones. These pairs are averaged to obtain the 50 values required with the deconvolution procedure. Flame Temperature and Rise Velocity Measurements. Flame temperature was measured using the two-line method with Fe as a probe element. A 1.0% solution of ferrous chloride (containing a trace of acetic acid to prevent hydrolysis) is introduced into the flame using a conventional pneumatic nebulizer. A rather concentrated solution is required because of the small transition probabilities of some of the most useful transitions. In addition, higher concentration results in useful radiation occurring lower in the flame. This increases the length of flame plasma for which temperature measurements are possible. Lines were selected using the criteria discussed by Kirkbright et al. (7). Statistical weights and transition probabilities were taken from the tables of Corliss and Bozman (8). The flame rise velocity is measured by a method similar to the one described by Snelleman (9). Small particles of aluminum are introduced into the flame by dispersing them in an air stream with a magnetic stirrer placed below the burner. The particles are photographed using their own radiation as they burn in the flame. The camera shutter speed is calibrated with a fast photodiode (EG and G, 040A) in place of the photographic emulsion. A flashlight is used as a source, and the output across the 1-kQ photodiode load is recorded with the digital storage oscilloscope using the desired camera shutter speed. A shutter
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open time of 1 ms was used for the rise velocity measurements. The length of the aluminum particle tracks on the emulsion was measured with a calibrated microscope. Reagents. Deionized, distilled water was used in all sample solutions. All reagent-grade salts were used. Solutions were filtered twice through 1-pm pore diameter membrane filters. No acids were added to the solutions since parts of the droplet generator were easily corroded.
RESULTS AND DISCUSSION General Performance Characteristics. The droplet generator produced uniform droplets in the frequency range from 2.8 to 15 kHz and for solution flow rates ranging from 0.048 to 0.26 mL/min. This results in droplets in the size range from 30 to 80 km in diameter. Over most of this frequency range, an audible tone occurred, which became louder a t several resonance frequencies. I t was found that the generator would start more easily if the cavity under the pinhole contained air rather than solution when the ballast chamber was pressurized. When acidic solutions were used, rapid erosion of the pinhole occurred. This caused the solution stream to exit the pinhole a t some angle with respect to the pinhole axis. Square wave or pulse signals applied to the transducer produced more uniform droplets and over a larger frequency range than sinusoidal signals. This rather surprising observation may result from the more rapid or impulse-like deformation of the pinhole during the rising and falling portions of the excitation signal. This may be more effective in breaking up the solution stream than the more gradual deformation occurring with sinusoidal excitation. When the droplet stream was observed with a microscope a t about 5 cm from the pinhole, repetitive flashes of the strobe lamp showed that spatial jitter or uncertainty in the droplet location usually was less than about 0.1-droplet diameter or 3 to 8 pm. The burner operated without flashback with gas flow rates of 4.85 to 9.54 and 2.31 to 4.63 L/min for nitrous oxide and acetylene, respectively. These flow rates resulted in Reynolds numbers ranging from 560 to 1940. The flame was quite large with a maximum diameter of about 2.5 cm and a length of about 50 cm. Very little flicker or wavering of the flame was observed from the burner head up to about 25 cm from the head. Extensive flicker was observed in the upper half of the flame. With a slightly fuel rich (reducing) flame, a well defined red interconal region was observed about 8 cm above the burner head. The base of the flame below the interconal region is hollow because of the sample introduction port in the burner body. T h e flame has very low luminosity and is essentially transparent in this region. The stream of solution droplets from the generator was clearly visible in this region. Figure 3 shows a composite of photomicrographs of uniform and nonuniform droplets together with photographs for both types of droplets in the nitrous oxide-acetylene flame. With the droplet generator frequency and shutter speed used, these photographs correspond to the integrated effect of several hundred droplets in the flame. With uniform droplets, the analyte emission appears to originate from one point in the flame. This is because droplet desolvation and the vaporization of the resulting micro crystal require just about the same time for each droplet if they are of uniform size and have the same trajectory in the flame. As the analyte vapor is convectively carried through the flame at the characteristic rise velocity, radial diffusion results in a comet-shaped plume of analyte emission. T h e plume appears most intense just above its origin. As radial diffusion increases the plume diameter, decreasing analyte number densities result in a decrease in peak intensity. T h e jitter in the point of origin of analyte emission typically ranges from f0.25 to f l . O mm
:a! Figure 3. Photomicrographs of droplets produced by the piezoelectric generator and photographs of the nitrous oxide-acetylene flame for (a) uniform and (b) nonuniform droplets
with the larger values occurring higher in the flame where flicker is more severe. This is comparable to the 1.25-mm axial resolution of the optical system as determined by the length of the monochromator entrance slits. Thus, for measurements made sufficiently high in the vapor cloud that the particle vaporization time can be neglected, the droplet generatorslot burner system should approximate a point source of free atoms with an axial location uncertainty of about f l mm. The presence of nonuniform droplets in the flame is very apparent since the point of origin of analyte emission wanders capriciously along the flame axis. When satellite droplets (small droplets associated with larger ones) occur, two emission origins are observed, a lower intensity one lower in the flame and a higher intensity one higher in the flame. Empirical fine frequency tuning to establish a sufficiently stsble emission plume is straightforward. I t should be noted t h a t single-droplet processes are not observed in these studies but rather those associated with a steady-state approximation to the single-droplet situation. Since a t normal operating frequencies of about 10 kHz, the vapor from 100 droplets passes through the observation zone in a single RC time constant of the measurement circuit, an averaged response for several hundred nearly identical droplets is obtained. Flame Rise Velocity. A reliable measurernent of flame rise velocity is essential if the system is to be used for vapor transport studies. From the length of the A1 particle tracks on 1.0-ms photographs of the flame, the rise velocity is found to be relatively constant a t 1160 cm/s in the region from 8 to 20 cm above the burner head. The rise velocity is somewhat lower near the base of the flame. Several separate measurements of rise velocity in the 8- to 20-cm region resulted in a relative standard deviation of 1.5%. Flame Temperature. T o obtain reliable temperature measurements, it is necessary to obtain intensity values from different regions of the flame. This requires deconvolution of the measured radial intensity profiles. For a plasma with an axis of symmetry, the intensity I H a t a distance R for the plasma axis can be related to the measured intensity IX obtained a t location X on a scan across the flame profile through the inverted form of the Abel integral equation ( I O ) .
-
Sacks and Walters ( 1 1 ) developed a matrix solution to this equation based on a model where the plasma is divided into 50 concentric zones of equal width. T h e intensity I R is assumed to be constant in a given zone. With the small zone
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
88 8888
Table I. Data for Fe Lines Used in Flame Temperature Measurements line no.
wavelength, nm
3.86 x 4.60 X 1.78 x 6.40 x 9.64 x 1.17 x 3.25 X 1.60 x 2.10 x
370.5 372.2 373.5 374.8 375.8 382.0 385.6 389.6 390.0 spectral line pair A B 6 6
g x A, s-'
8 9
3
1
6 6 6 5
7 2 4 2
10' 10' 109 10' lo8 109
10" 10' 10'
excitation energy, eV 3.396 3.416 4.177 3.416 4.255 4.102 3.265 3.291 3.265
I:
temperature range, K 1970-2140 2130-2330 2340-2530 251 0-273 0 2590-2840 2800-3100 3130-3460
IIi/
width (0.208 mm) used here, this assumption is reasonable. T h e iron two-line method (12) was used for temperature measurements because no ancillary radiation source is required and reliable measurements are possible over a wide range of temperatures. If the flame is in local thermodynamic equilibrium, the temperature is given by
lOmm Smm Omm 5 m m lomrn Figure 4. Isotherm contour map of the slightly fuel-rich nitrous ox-
ide-acetylene flame Subscripts 1 and 2 refer to two lines in the iron spectrum, and E , A , g, A, and h are the excitation energy, transition probability, statistical weight, wavelength, and Boltzmann constant, respectively. The intensities II and 1, are radial values (ZR) obtained from the deconvolution procedure. Table I lists the line pairs used here along with their corresponding temperature ranges. Preliminary studies with the droplet generator-slot burner system indicated that with the attainable droplet sizes, the region from 9 to 20 cm above the burner head is the most useful for vapor transport and analyte emission profile studies. Figure 4 shows the temperature profile for a slightly fuel-rich nitrous oxide-acetylene flame. The temperature is indicated for a 7-mm radius, 11-cm long cylindrical region. Owing to poor intensity measurement precision near the periphery of the flame, the temperature could not be determined reliably beyond 7 mm from the flame axis. This poor precision resulted from low line intensities in the cooler periphery of the flame together with greater noise, which may result from air entrainment. The maximum flame temperature was 3120 K, and this occurred 8.5 cm above the burner head. This is just above the red, interconal (red feather) region. T h e flame has two nodes of high temperature a t 15 and 19 cm above the burner head, respectively. These may result from movement of the flame gases toward the flame axis after they emerge from the burner slot together with the effects of air entrained in the base of the flame from the sample introduction port. I t can be seen from Figure 4 t h a t the temperature is relatively constant in a 5-mm radius, 7-cm long cylindrical region beginning about 13 cm above the burner head. In the region near the flame axis, the uncertainty in intensity measurements ranged from 0.5 to 1.0%. This results in a temperature uncertainty of 10-18 K. Near the periphery of the region studied, uncertainty in measured intensity ranged from 2 to 3% and corresponds to a temperature uncertainty
of 30-60 K. Uncertainties in the values of transition probabilities and statistical weights used in Equation 2 are difficult to evaluate and are not considered here. The temperature values reported here are in reasonable agreement with values reported by other workers. Willis et al. (13) reported a maximum temperature of 3150 K in a stoichiometric nitrous oxide-acetylene flame with an oxidant-to-fuel ratio of about 3. They also reported an average temperature of about 2900 K up to 5 cm above the burner head for a slightly fuel rich flame with an oxidant to fuel ratio of 1.6. Kirkbright et al. (14) reported an average temperature of about 2800 K a t 12 mm above the primary reaction zone of a nitrous oxide-acetylene flame. Transport Processes. Free atom formation and loss as well as radiative processes are most conveniently studied when the vapor transport properties of the excitation source are well characterized. If radial vapor transport is diffusion controlled, analyte particle number densities from a point source of free atoms can be estimated if the species' diffusion coefficient and flame rise (convective) velocity are known or measurable. Ashton and Hayhurst (1)discussed the experimental and theoretical considerations for diffusion coefficient measurements in flames. A 0.5-mm i.d. tube carrying - l + m diameter solution droplets generated by an ultrasonic nebulizer was used to approximate a point source of free atoms. The method is based on a comparison of radial vapor flux to axial flux under conditions where axial diffusion can be neglected relative to convection as the principal mechanism for axial vapor transport. This condition is met when VZ >> D, where Vis the convective velocity of the flame, Z the height in the flame relative to the point source of free atoms, and D the diffusion coefficient. Thus, if measurements are made sufficiently far from the point source that 2 >> D / V , axial diffusion can be neglected. Three conditions in the source are necessary for reliable transport studies. First, the source must contain a transport
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
(test) zone of uniform temperature. Second, the convective velocity V must be relatively constant in the transport zone, and third, the zone must be large enough to prevent loss of test vapor by radial diffusion. An additional requirement is t h a t measurements be made sufficiently high in the vapor cloud so that the finite time required for particle vaporization can be neglected. Under such conditions, the intensity I measured in a narrow region at location 2 along the flame axis and the intensity Io integrated across the entire vapor cloud a t the same axial position is given by
(3) where AX is the width of the region in which I is measured. With the optical system used here, AX is just the width of the monochromator entrance slit. From Equation 3, a plot of (AXZo/I)' vs. 2 should be a straight line if radial vapor transport is diffusion controlled. A linear plot was obtained for the region from 1.5 to 3.0 cm above the analyte emission origin for -4-pm diameter CaC12 particles in the slightly fuel-rich nitrous oxideacetylene flame. The particle size, assuming spherical geometry, was estimated from the droplet diameter and the solution concentration. The Ca free atom concentrations in the measurement zone from these very small particles should be sufficiently low to neglect self-absorption effects. The good linear fit suggests that radial vapor transport is under diffusion control or a t least can be described in terms of a simple, isotropic diffusion coefficient in the region from 1.5 to 3.0 cm above the origin of analyte emission. In addition, it appears that atomization of the micro crystals is sufficiently localized to approximate a point source of free atoms for measurements made a t least 1.5 cm downstream from the source. I t should be noted t h a t even if compound formation is appreciable, a plot of ( A X I o / I ) 2vs. 2 should be a straight line if radial vapor transport is diffusion controlled. However, here D is a n effective diffusion coefficient for the metal in both chemical forms and is given by ( I ) , (4)
1785
Here, D , and D,, are the diffusion coefficients for the metal and metal compound, respectively, and f is the ratio of metal compound to free metal concentration. If the droplet generator-burner system is a nearly ideal point source, the ( A X I O / O 2vs. 2 plot should have a zero intercept. The intercept occurs -0.5 mm above the origin of analyte emission. I t should be noted that the axial spatial resolution is 1.25 mm. Thus, this experimental arrangement can be defined as a virtual point source of free ;itoms located -0.5 mm above the origin of analyte emission for measurements made sufficiently downstream from the point of solute particle vaporization. From the slope of the ( A X I o / I ) 2vs. 2 plot and the 1160 cm/s flame rise velocity, the diffusion coefficient for Ca is found to be 7.4 cmz/s. The intensity measurements used here were obtained 10 to 12 cm above the burner head. The flame temperature in this region is 3030 to 3070 K. The value of D found here is within the range of values predicted from Snelleman's (9) work assuming that D increases with the square of temperature (15).
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
(14) (15)
A. F. Ashton and A. N. Hayhurst, Trans. Faraday Soc , 66, 824 (1970). G. M. Hieftje and H. V. Malmstadt, Anal. Chem., 40, 1860 (1968). G. M. Hieftje and H. V. Malmstadt, Anal. Chem., 41, 1735 (1969). B. M. Joshi and R. D. Sacks, Anal. Chem., following paper in this issue. R. N. Berglund and B. Y. H. Liu, Environ. Sci. Techno/., 7, 147 (1973). R. Stair, W. E. Schneider, and J. K. Jackson, Appl. Opt., 2, 1151 (1963). G.F. Kirkbright, M. Sargent, and S. Vetter, Spectrochm. Acta, Part 6, 25, 465 (1970). C. H. Corliss and W. R. Bozman, "Experimental Transition Probabilities for Spectral Lines of Several Elements", hbfl. Bur. Stand(U.S.),Monogr. 5 3 (1962). W. Snelleman, FhD. Thesis, University of Utrecht, U b e c M , The Netherlands. 1965. C. J. Cremers and C. R. Birkebak, Appl. Opf.,5 , 1057 (1966). R. D. Sacks and J. P. Walters, Anal. Chem., 42, 61 (1970). I. Reif, V. A. Fassel. and R. N. Kniseley. . S.m 3 r o c h i n . Acta, Part B. 28, 105 (1973). J. B. Willis, J. 0. Rasmuson, R. N. Kniseley. and V. A. Fassei. Spect-ochim. Acta, Part B , 23, 725 (1968). G. F. Kirkbright, M. K. Peters, M. Sargent, and T. S.West, Talanta, 15, 663 (1968). R. Papoular, "Electrical Phenomena in Gases", American Elsevier. New York, 1965, Chap. 7.
RECEIVED for review February 23,1979. Accepted June 8,1979
