ACKNOWLEDGMENT
The authors thank R. M. Brumfield of Potter and Brumfield, Inc., for donab ing the short-time time-delay relay which was designed and built for this project. They also thank T. J. Oyster who helped in the construction of the flash apparatus.
( 7 ) Booman. G. L., Pence, D. T., Ibid., p. 1366. (8) Copeland, A. W., Black, 0. D., Garrett, A. B., Chem. Revs. 31, 177 (1942). \ - I
~
3)
Delahay, P., “New Instrumental thods in Electrochemistrv.” D. 726.
Kuwana, T., J . Phys. Chem. 66, 2456 (1962). (20) Pitts, J. N., Jr., Letsinger, R. L.,
Taylor, R. P., Patterson, J. M., Recktenwalk, G., Martin, R. B., J. Am. Chem. SOC.81, 1068 (1959). (21) Porter, G., Wilkinson, F., Trans. Faraday SOC.57, 10 (1961). (22) Schenck, G. O., Meder, W., Pape, M., Proc. Intent. Conf. Peaceful Uses At. Energy 29, 352 (19.59). (23) Schwarz, W. M., Shah, I., ANAL. CHEM.35, 1770 (1963).
LITERATURE CITED
( 1 ) Beckett, A., Porter, G., Trans. Faraday SOC.59,2038 (1963). (2) Berg, H., Collection Czech. Chem. Communs. 25, 3404 (1960). (3) Bern. H.. 2. Anal. Chem. 216, 165 (19667: ‘ (4) Berg, H., Schweiss, H., Nature 191, 1270 (1961). (5) Berg, H., Schweiss, H., Tresselt, D., Exverimentelle Technik der Phwsik 12, i i 6 (1964). ( 6 ) Booman, G. L., Holbrook, W. B., ANAL.CHEM.37, 795 (1965).
S. P. PERONE J. R. BIRK Department of Chemistry Purdue University Lafayette, Ind.
\ - ,
\___-,-
19) Pitts, J. N., Jr., Johnson, H. W., Jr.,
PRESENTED at the Great Lakes Regional ACS Meeting, Chicago, Ill., June 1966. Work supported by Public Health Service Grant No. CA-07773-02 from the National Cancer Institute. Acknowledgment is made by J. R. B. for a Fellowship granted by Standard Oil of Ohio.
Measurement of Dispersion Efficiencies and Rates in Total-Consumption Aspirator-Burners SIR: One of the major obstacles in quantitatively characterizing the processes occurring in all types of flame spectrometry, in which total-consump tion aspirator-burners are used, is that of describing the manner in which the aspirated solution is dispersed in the flame gases. Unlike chamber-type aspirator-burners with their pre-mixed laminar flames (3, 6), total-consump tion aspirator-burners produce turbulent flames which are very difficult to evaluate experimentally. T o complicate the situation further, there are few analytical techniques available for observation of the dispersion of solution droplets in a burning flame. Therefore, it is the purpose of this paper to elucidate some of the characteristics of droplet dispersion in flames produced by total-consumption aspirator-burners. This is done by measurement of the intensity of radiation reflected from water droplets in the flame gases. THEORY
For the purposes of this discussion, the dispersion efficiency, d,, will be defined as follows: the ratio of the amount of solution dispersed into droplets of less than 1.1 microns by means of nebulization and evaporation to the total amount of solution entering the burner under a given set of experimental conditions. The dispersion efficiency can be determined by measuring the ratio, R , of the intensity of radiation reflected from water droplets at 0.55 micron with the flame burning to the intensity reflected with the no flame-
Le., only the aspirating gas flowingand then subtracting this ratio, R, from unity-i.e., d e = 1 - R. This method of measurement is based on experimental evidence which shows that the fraction of radiation reflected as a function of wavelength is a constant in flame gases studied for all wavelengths greater than 0.45 micron; this indicates that there is only a negligible signal due to light scattering from droplets having a diameter of the same order of magnitude or smaller, than the wavelength of observation (2). Several medium-bore, a small-bore, and a large-bore capillary total-consumption aspirator-burners were studied, and all of them showed this same behavior. Therefore, the results of this study should be generally applicable to total-consumption aspiratorburners with similar dispersion efficiencies. At wavelengths less than 0.45 micron, the Rayleigh type scattering curve is observed to be superimposed on the reflected signal. The wavelength of 0.55 micron is arbitrarily chosen, and reflection of incident radiation results primarily from droplets of about 1.1 microns and larger-Le., approximately two times the wavelength of observation
(4).
Dean and Carnes (1) have measured the droplet size distribution of solutions aspirated by total-consumption aspirator-burners without the flame and obtained a near-Gaussian distribution having a mean droplet diameter of 19.6 microns when aspirating water. Under these conditions, essentially all of the droplets from a total-consumption as-
pirator-burner with no flame will result in a reflection signal. Therefore, with only aspirating gas flowing, the intensity of the reflected signal is proportional to the total number of droplets and to their total surface area, and so
where IR’ is the intensity of reflected radiation with only aspirating gas (no flame), S, is the surface area of a particular droplet (i),ST the total surface area, ni is the number of droplets, n T is the total number of droplets, k is a constant which depends on the geometry of the droplets, and the summation is over all droplets greater than 1.1 microns (when the flame is o f f , essentially all droplets are greater than 1.1microns). With the flame on, the intensity of reflected radiation, I B ,which is also indicative of droplets larger than 1.1 microns (when the reflection is measured at 0.55 micron) is given by
IR
=
Sini
k a
It should be pointed out that many droplets are less than 1.1 microns when the flame is on and these droplets will not contribute significantly to the reflected signal. Therefore, the dispersion efficiency, de, is given by
It is interesting to note that the dispersion efficiency, de, is directly related to the atomization efficiency, e, as deVOL. 38, NO. l l , OCTOBER 1966
e
1593
fined by Winefordner and coworkers (8) by e =
k'd,
(4)
where k' is a constant which should be close to unity and depends upon the efficiency of producing salt particles from droplets smaller than about 1.1 microns. All measurements made in these studies were obtained using distilled water rather than salt solutions, in order to measure only the dispersion efficiency. Upon dehydration of salt solution droplets, salt particles would have resulted which could cause a significant scattering error. The rate of dispersion, Rd, is defined as the rate of decrease in the fraction of droplets larger than 1.1 micron with respect to time. The rate constant for dispersion, kd, can be calculated by plotting some function of R os. time that the droplets spend in the flame gases. These plots are obtained by measuring R as a function of height of observation (of reflection) in the flame. The height of measurement of reflection in the flame can be related to the time the droplets have spent in the flame gases (3). EXPERIMENTAL
Apparatus. A 150-watt xenon arc (No. 416-992, American Instrument Co., Silver Spring, Md.) was focused by means of a quartz lens onto the flame under consideration at a 90' angle to the optical axis composed of the entrance slit and collimator of a Czerny-Turner grating monchromator (No. 4-8400, American Instrument Co.). The arrangement of the experimental components is identical to that used for atomic fluorescence flame spectrometry by Veillon et al. (6). A parallel beam of about 5 em. in diameter was passed through the flame in order to ensure that all the droplets were within the solid angle of the incident radiation. A laboratory - constructed light trap box (6) was used to keep out reflected light from other sources than water droplets, and a shutter was placed between the xenon arc and flame in order to measure the
background signal which was subtracted from each measured signal. The light signals were detected by an RCA 1P28 multiplier phototube and an electrometer (No. 26-770, Jarrell-Ash Go., Waltham, Mass.) and recorded on a strip-chart recorder (Servo/riter 11, No. PSolWGA, Texas Instrument Co., Houston, Texas). Four Beckman total - consumption aspirator-burners were investigatedthree different No. 4020 (medium bore) and one No. 4050 (small bore) burners. The gas flow rates to the burners were regulated by two-stage regulators and a Beckman step-down pressure regulator, and the flow rates were measured by means of rotameters (No. 4-15-2, Ace Glass Corp., Vineland, N. J.). Other experimental parameters were as follows: phototube voltage, 900 volts; monochromator entrance and exit slit widths, 0.1 mm.; wavelength of observation, 5500 A; output of electrometer and full-scale voltage range of recorder, 1.0 mv. hocedure. The burner being studied was positioned a t the desired height with respect to the monochromator entrance slit, the aspirating gas flow was adjusted, and distilled deionized water was aspirated into the burner. The reflected light signal, I R ' , was measured over the background signal (the signal remaining when the shutter was placed between the xenon arc and the flame). This background signal was usually only slightly larger than the dark current at the voltage impressed on the multiplier phototube. After the fuel gas was turned on and the flame ignited, the reflected light signal, IR, was measured over the flame background signal. No reflection signal was observed from the flame without water being aspirated. RESULTS AND DISCUSSION
The four burners which were investigated gave quite different dispersion efficiency characteristics. No correlation could be made for different burners between the solution flow rate or the pressure required for a particular aspiration gas flow rate and the dispersion efficiency of the burner. Therefore, the dispersion efficiency for a specific aspira-
Table I. Comparison of Flow and Dispersion Efficiency Characteristics for Four Beckman Total-Consumption Aspirator-Burners Using an Oxyhydrogen Flame Aspilating gas Aspirating gas flow rate Solution flow Dispersion. pressure (p,s.i.) (l./min.) rate (ml./min.) efficiency, d e Burner No. 0.67 0.65 2.0 5 1 0.78 1.3 2.5 11 0.84 2.6 3.0 20 2b 18 2.0 2.3 0.91 3 5 2.5 0.65 0.86 4c 11 2.5 0.66 0.84 a Dispersion efficiency measured at 2 cm. above burner tip using a stoichiometric
oxyhydrogen flame * Burner could not be run at an oxygen flow of 2.5 I./minute. c Small bore burner.
1594
e
ANALYTICAL CHEMISTRY
tor-burner and flame should always be measured-Le., the dispersion characteristics of two medium-bore, total-consumption aspirator-burners as well as the solution flow rates (7) can be quite different. I n Table I, the dispersion efficiencies for similar experimental conditions for the four burners are given. It was observed, however, that when the same burner was used, a higher aspirating gas flow rate increased the dispersion efficiency at any given height above the burner (see Figure 1) over the range of aspirating gas flow rates studied. Three different flames were investigated using burner No. 1-the oxyhydrogen, the air-hydrogen, and the newly reported (6) argon-hydrogenentrained air flames. The efficiencies were measured as a function of the height above the burner tip and fuel-toaspirating gas flow rate ratio. It can be seen from Figure 2 that the fuel-toaspirating gas flow rate ratio has no effect. This is evidence to support the observation by Winefordner and Latz (7) that fuel flow rate does not affect the solution flow of a burner: When using the same oxidant gas flow rate, the only variation from flame-type to flame-type is observed at small heights above the burner tip. Height refers to the portion of the flame viewed by the spectrometric system and is taken with respect to the burner tip. This difference is probably due to the variation in the size of the inner cone of the flame. The oxyhydrogen flame burns closest to the tip and has a higher temperature. The airhydrogen and argon-hydrogen-entrained air flames have about the same flame shape, but the air-hydrogen is somewhat hotter as is evidenced by a more intense flame background (continuum) emission. It should be emphasized that the dispersion efficiencies measured in this communication have absolute significance only for the aspirator-burners used in these studies. If the analyst wishes to know the efficiency of dispersion of his aspirator-burner, flame type, and solution flow, then he must measure his own value as described in this manuscript. There is a great deal of variation in d e values for different total consumption aspirator-burners of the same type (see Table I ) ; this indicates the critical nature of the design of such aspirator-burners. It is recommended that all analysts record the dispersion efficiency [or atomization efficiency (8)] and list its value in their papers. The dispersion efficiencies measured in this manuscript generally show the same trends but are slightly higher than the atomization efficiencies (see Equation 4) measured by Winefordner, Mansfield, and Vickers (8) for similar flame conditions and solution flow rates. This indicates that the constant k' in Equation 4 is slightly less than unity. The relative standard deviation of the dispersion
I .
e o.*o
U Y
> u 2
y 0
0.60
LL LL
w
2 0,TO 0
v)
a w
zP
0,so
0,so
-
-
0
I
I
I
I
1
I
2
s
4
8
HEIGHT A B O V E BURNER (cm.)
Figure 2. Variation of dispersion efficiency with different flames, different fuel to aspirating gas flow rates, and height above the burner as measured on a single burner.
efficiencies measured in this communication was 1.3% for a series of 10 measurements for several different values of d.. The rate constant for dispersion of water droplets in the flame can be obtained by plotting some function of (1 - de) or R-Le., the fraction of the total solution remaining in droplet form greater than 1.1 microns us. the time the water droplets spend in the flame gases. The time the water droplets spend in the flame is related t o the height above the burner tip. The reciprocal of the rise velocity (in cm./second) of the flame gases is simply the time (seconds) required for any given particle to travel 1 cm. Thus, if the orifice area of the burner and the flow rate of the aspirating gas are known, the approximate time can be calculated by dividing the orifice area in cm.2 by the gas flow rate in cm.3/second. This assumes that the velocity throughout the width of the flame gases is constant. By these measurements and plots, two rate constants were calculated. The rate constant for the region 1 to 3 cm. above the burner tip was found to be 4 X l o 4second-’ and the rate constant for the region 4 to 6 cm. above the burner tip was found to be 2 x lo* second.-’ All rate constants were calculated for studies using burner
Curve A. Measurements made with an oxyhydrogen flame. The oxygen flow rate was 2.5 I./min., and the same curve was obtained with hydrogen flow rates of 5.0, 7.0, 8.0, and 9.0 I./min. Curve B. Measurements made with an air-hydrogen flame. The air flow rate wos 2.5 l./min., and the same curve was obtained with hydrogen flow rates of 5.0, 6.0, 8.0, and 9.0 I./min. Curve C. Measurements made with an argon-hydrogen-entrained air flame. The argon flow rate was 2.5 l./min,, and the same curve was obtained with hydrogen flow rates of 7.0, 8.0, and 9.0 I./min.
KO. 1 . The rate expression for the 1- to 3-cm. region followed second order kinetics-Le., a plot of l/R us. time resulted in a straight line-and the upper (4-to 6cm.) region followed first order kineticsLe., a plot of log R us. time resulted in a straight line. The actual rate constants for dispersion calculated above are only useful for the burner used in these studies. However, the order of the rate expressions should be valid for all totalconsumption aspirator-burners. Of course, if the rate constants are known for a given set of experimental parameters, approximate values of d. can be calculated for any given height in the flame.
tometry,” P. T. Gilbert, trans., Wiley, New York, 1963. ( 4 ) Jenkins, F. A., White, H. E., “Fundamentals of Optics,” McGraw-Hill, New York, 1957. (5) Mavrodineanu, R., Boiteux, H., “Flame Spectroscopy,” Wiley, New c,
1965.
illon, C., Mansfield, J. M., Parsons,
M. P. PAR SONS^ J. D. WINEFORDNER Department of Chemistry University of Florida Gainesville, Fla. 32601
LITERATURE CITED
(1) Dean, J. A., Carnes, W. J., ANAL. CHEM.34, 192 (1962). (2) Green, H. L., Lane, W. R., “Particu-
late Clouds: Dusts, Smokes and Mists,” Van Nostrand, Princeton, N. J.,
1957. ( 3 ) Herrmrtn, R., Alkemade, C. T. J.,
“Chemical Analysis by Flame Pho-
1 Present address, Phillips Petroleum Co., Research and Development, Bartlesville, Okla. 74004
RESEARCH sponsored by AFOSR(SRC) - O A R , U. S.A. F. Grant No. AFAFOSR1033-66 with supplementary funds from Edgewood Arsenal, Md.
VOL 38, NO. 11, OCTOBER 1 9 6 6
1595
