Temperature Profiles of Turbulent Hydrogen Diffusion Flames Used in Atomic Fluorescence Spectrometry R. Smith, C. M. Stafford, and J. D. Winefordner Department of Chemistry, University of Florida, Gainesuille, Flu. 32601
The temperatures of H2/Air, H2/Ar/entrained Air, H2/Nz0, and Hz/entrained Air flames supported on total-consumption burners have been measured with the aid of an iridium/6% i r i d i u m 4 5 rhodium thermocouple. The flames have been used in atomic fluorescence and atomic emission studies, and results are quoted for flame temperatures measured with and without the aspiration of water. A full discussion of the use of thermocouples in measuring flame temperatures is given together with complete temperature contours and also vertical height profiles for a variety of flames. The principal source of oxidant in all the flames examined was entrained air, the main purpose of the aspirant gas being to create sufficient turbulence. As a result, the enhancement of atomic fluorescence quantum efficiency, obtained by aspiration using argon, is small (ea. 1 0 - 2 5 ) . Generally, easily volatilized and atomized elements can be determined above the visible portion of the flame, whereas other elements must be determined within the flame. For the latter elements, the height at which maximum fluorescence occurs does not necessarily correspond to the most favorable signal-to-noise ratio.
THE STRUCTURE AND physical characteristics of turbulent, hydrogen flames used in flame spectrometry, has received little study in the past, partly because of the complexity and small dimensions of the flames involved. Turbulent flames have been used to a considerable extent in many of the recent studies on atomic fluorescence spectrometry, and it would also appear that low-background, turbulent flames such as the H2/ Ar/entrained-air and H2/entrained-air flames may have some applications in atomic emission spectrometry. Recent studies in this laboratory ( I ) , between premix and total-consumption burners, indicate the considerable value of the latter in atomic fluorescence analysis as a result of higher uptake rate and simplicity. The characteristics of high-temperature turbulent flames have been investigated by Winefordner et al. (2), and Gilbert (3) who studied the Hz/02 and GH2/02 flames supported on Beckman total-consumption burners. Other work on Hz/air, H2/Ar/entrained-air9 H2/Oz and GH2/02 turbulent flames by de Galan and Winefordner ( 4 ) confirmed the earlier results obtained (2) for the high-temperature flames but indicated considerable lack of thermal equilibrium in the cooler H2/air and Hz/Ar/entrained-air flames. Additional results by Kirkbright et al. (5) for the H2/02 flame have been in good agreement with the values reported previously. All of the workers referred to have used spectrometric methods for measuring the flame temperature. In the higher temperature (1) M. P. Bratzel, R. M. Dagnall, and J. D. Winefordner, ANAL. CHEM., 41,713 (1969). (2) J. D. Winefordner, C. T. Mansfield, and J. T . Vickers, ibid., 35, 1611 (1963). (3) P. T.Gilbert, A.S.T.M. Spec. Tech. Publ., 269, 73 (1959). (4) L. de Galan and J. D. Winefordner, J. Quant. Spect. Radial. Transfer, 7,703 (1967). ( 5 ) G. F. Kirkbright, M. K. Peters, M. Sargent, and T. S. West, Talonto, 15, 663 (1968). 946
0
ANALYTICAL CHEMISTRY
flames, the Ornstein iron two-line method has been used ( 2 , 4 , 5)) although this gave results which are very much in error With the lower temperature flames (4). The sodium, linereversal method has been used with turbulent flames (4, 6)) but the work of de Galan and Winefordner (4) indicates that even sodium may give slightly high results when used with the lower temperature, turbulent flames. In addition, to the problems of thermal equilibrium, spectrometric methods have the disadvantage that they measure an average temperature across the flame and not the temperature at a specific point in the flame. This could be overcome to some extent by the use of narrow light beams and subsequent mathematical treatment (Abel Inversion) of the results, however, in this instance it appears that the use of an alternative method of temperature measurement might prove advantageous. The results given here, describe the use of a thermocouple probe for temperature measurement in flames. The method is susceptible to many errors and is not simple to use although results may be obtained rapidly and easily once the thermocouple has been calibrated. All the flames used were supported on Hetco or Zeiss burners as these burners give better sensitivities in atomic fluorescence (7) than do Beckman burners; this is the first report on temperature measurements carried out in conjunction with these burners. Finally, the recent use of the premixed flame produced with a Hetco burner, reported by Mossotti and Duggan (8),has led to an investigation of such flames for atomic fluorescence (9). For the sake of completeness the temperature profiles of premixed Hz/air and premixed H2/N20 flames supported on a total-consumption burner (9) have been included in the results of this publication, even though these flames have, as yet, no uses in atomic spectrometry. EXPERIMENTAL
Apparatus. BURNERS.The total-consumption burners used were Jarrell-Ash 82-341 HETCO (Jarrell-Ash, Co., Waltham, Mass.) and the Zeiss (Carl Zeiss, Inc., New York, N. Y.) burners. The two burners were almost identical, and many parts were interchangeable. There appears to be no difference in comparable flames burned on each of these burners, although differences in solution uptake rate occurred. For premixed flames, on total-consumption burners, the Tescom hand-welder (Tescom 11-1101, Tescom Corp. Minneapolis, Minn.) premixing chambers were used as recommended (8). Gas flow rates were measured on rotameters, calibrated with the respective gases used by means of a wet test meter. THERMOCOUPLE. Thermocouples were made from 0.005inch diameter iridium and iridium-rhodium ( 6 0 ~ 4 0 ~ ) wire (Sigmund Cohn, Co., Mount Vernon, N. Y.) which (6) J. H. Gibson, W. E. L. Grossman, and W. D. Cooke, ANAL. CHEM., 35,266 (1963). (7) M. P. Bratzel and J. D. Winefordner, Anal. Letters, 1, 43 (1967). (8) V. G. Mossotti and M. Duggan, Appl. Optics., 7, 1325 (1968). (9) M. P. Bratzel, R. M. Dagnall, and J. D. Winefordner, unpublished work, University of Florida, Gainesville, Florida, 1969.
+
+
1.0 \
-
I
1.0
0.5
0
05
t
L
1.0
Figure 1. Full temperature profile of Hz/Ar/entrainedair flame (B), all dimensions in cm, expanded abscissa was welded in a natural-gas/Oa flame to give a bead junction of cu. 0.01-inch diameter. The joined wires were mounted in two-bore, ceramic insulators of high-purity recrystallized alumina, having an outer diameter of 0.03 in. (Omegatite 350, TRA 005132, Omega Engineering, Inc., Stamford, Conn.), and the insulators were protected along two thirds of their length by inconel metal sheaths (INC 1165-6, Omega Engineering). The welded junctions and exposed leads were thinly coated with zirconia cement (ESP1-MR32, Electronic Space Products Inc., Los Angeles, Calif.), to prevent catalytic effects and the assembly was mounted in a two-pin connector having copper terminals (MP-UNCOMF, Omega Engineering). Thermocouples were mounted on a manually-operated horizontal/vertical traverse mechanism having the horizontal traverse capable of positional precision of b0.04 mm and the vertical traverse capable of *0.2 mm. Measurements of thermoelectric e.m.f. were by means of a digital millivoltmeter (Digitec, United Systems Corp., Dayton, Ohio) which was calibrated in units of 0.02 mV and which operated on a null-balance principle, having effectively infinite impedance at balance. LINE REVERSAL.A premix Hz/air burner with facility for a flowing nitrogen sheath and designed by Zeegers (10) was used for line-reversal calibrations of the thermocouple. (10) P. J. T. Zeegers, Ph.D. Thesis, Utrecht, Netherlands (1966).
0.5
0
0
.5
1.0
Figure 2. Full temperature profile of Hz/Air flame (E), all dimensions in cm, expanded abscissa This burner was capable of supporting a range of Hz/Ot/N* flames having uniform temperature over the flame crosssection and height. A tungsten strip lamp (Eppley Laboratory, Inc., Newport, R. I.), was calibrated in terms of absoluteo energy output by the makers between 2500 A and 7500 A at 35 A using National Bureau of Standards reference sources (EU-238 and EU-275E) and was used as a background light source. The schematic arrangement of the linereversal apparatus was similar to that described by Gibson, et ul. (6) and Snelleman (11); a 0.5-m Ebert grating monochromator (Model 8200, Jarrell-Ash Co.) having a 20-p by 2-mm slit, was used with an RCA 1P-28 photomultiplier operated at 850V. A solid state nanoammeter (11) and galvanometric recorder were used for amplification and readout. Procedure. THERMOCOUPLE CALIBRATION. Blackburn and Caldwell (13) have provided calibration tables for the variation of the thermal emf of iridiumliridium-rhodium thermocouples. At temperatures above 1200 O K , radiational heat losses from the thermocouple become appreciable, and a correction must be applied to compensate for the apparent reduction in the measured temperature. The radiational correction was obtained by measuring the temperature of well-defined, uniform, laminar flames by the sodium linereversal method and with the uncalibrated thermocouple. (11) W. Snelleman, Comb. and Flume, 11,453 (1967). (12) T.C. OHaver and J. D. Winefordner,J. Chem. Educ., in press. (13) G. E;. Blackburn and F. R. Caldwell, J. Res. NES., 68C, 41 (1964). VOL. 41,NO. 7, JUNE 1969
947
2
4
I
0
10
12
Heighticml
Figure 4. Temperature variation along central, vertical axis of Hz/Ar/entrained-airflames A, B, C
OK
2000
1000
1.0
0.5
0
0.5
1.0
Figure 3. Full temperature profile of reversed Hz/ entrained-air flame (G)
The difference between the thermocouple and reversal temperatures, AT, was found to fit Equation 1 to lt25 "K for the range 1760 "K-2350 OK:
where: AT = temperature reduction due to radiational heat lossLe., difference between uncorrected thermal
couple temperature and line-reversal temperature. k = constant depending on thermocouple. To = thermocouple temperature, uncorrected for radiation losses. The general procedure ,for the sodium line-reversal method has been discussed in detail (4, 6, IO, II) and will not be repeated here. Reversal measurements were carried out using 1000 ppm, 500 ppm, 100 ppm, 50 ppm, and where possible 10 ppm concentrations of sodium solution introduced into the flame via an aspiration chamber. The reversal temperature was found by extrapolation of these results to low sodium concentrations-Le., 10-100 ppm. The brightness temperature of the tungsten light source at 5890 A was found from the makers absolute energy calibration and 948
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ANALYTICAL CHEMISTRY
2
4
6
10
12
Height krnl
Figure 5. Temperature variation along central, vertical axis of Hz/Airflames D, E, F
radiation tables after correcting for an intensity reduction of 8% (II) for the quartz lens used to focus the radiation on the flame. HZ/O~/NZ premixed flames were used together with the nitrogen sheath; nitrogen was used as the aspirant gas. Reversal temperatures for hydrogen flames using the premix burner could be reproduced to within 20 "K of the values published by Zeegers (IO) for the same gas flow rates and burner. TEMPERATURE PROFILES. Complete profiles of selected flames (Figures 1-3) were obtained by scanning the whole width of the flame at height intervals of 0.5 cm or less. The variation in temperature with height, for the flame center only, was measured for more flames (Figures 4-6) with and without the aspiration of water. For all the flames used, the vertical temperature profiles could be reproduced to f10 "K at any point on the central axis of the flame, and the maximum measured temperature of any flame could also be reproduced to =!=lo OK. The experimental gas flow rates, maximum measured temperatures, and other details are given in Table I. Thermocouple temperatures were corrected for radiational heat losses and for small errors involved in the use of uncompensated iridium/copper and copper/iridium-rhodium connections as the reference junction (13).
~
~~
~~
~
~~
Table I. Flame Types and Characteristics Flame A B C D
E F G H I J
Type Hz/Ar/Ent.-air 6‘
HZ/Air
Fuel (I./min)
Aspirant (l./min)
HzO uptake (ml/min)
T max
9.3 14.0 17.0 9.3 14.0 17.0
5.9 5.9 5.9 5.9 5.9 5.9 16.0 4.75 20 psib 20 psib
2.07 2.07 2.07 1.08 1.08 1 .08 0.92 2.93 2.36 2.46
1894 2040 2122 2287 2256 2208 2182 2525 2630 2250
(OK)
Hz/Ent.-air ... HdND 15.0 premix H2/NZ0 16 psib premix Hz/Air 16 psib a Calculated as the minimum amount required for complete combustion of hydrogen. * See text for explanation.
RESULTS AND DISCUSSION
Thermocouple Errors. Operating characteristics of iridium/ iridium-rhodium (60%-40%) thermocouples have been given by Blackburn and Caldwell (13), and sections of the volume edited by Herzfeld (14) deal exhaustively with these thermocouples and associated equipment. The thermocouple materials used were chosen because of their ability to withstand typical flame conditions without failure or appreciable change in thermoelectric characteristics. Alumina insulators were used despite their high conductivity at temperatures above 2000 OK ( 1 4 ) ; the insulator used had a resistance in excess of 5 K ohms when immersed in a flame at 2100 OK for a depth of about one inch. This test was carried out using a thermocouple with an unwelded junction. Catalytic effects were noted with uncoated thermocouple junctions, and therefore a coating of zirconia ceramic was applied to prevent this. The use of silica coatings as recommended by Fristrom and Westenberg (15) was not suitable in this instance as the silica rapidly vaporized and at high temperatures may react with iridium or rhodium (14). Temperature differences of ca. 300 OK were noted in some turbulent hydrogen flames between coated and uncoated thermocouples, whereas differences of less than 10 OK were observed when the same thermocouples were inserted in premixed natural gas/air flames. In one instance, an apparent maximum temperature of over 2650 OK was noted for a hydrogen/air turbulent flame when measured with an uncoated thermocouple; when a zirconia coated thermocouple was used the maximum temperature noted was 2287 OK. The radiation heat loss was determined by measuring the temperature of uniform flame gases using the thermocouples and line-reversal method. This represents the largest correction applied and also the greatest source of error. A variation in the gas velocity of the turbulent and calibration flames will affect the heat transfer to the thermocouple and result in an error in the radiation correction. However, Kaskan (16) has shown that the heat transfer depends on the fourth root of the reciprocal of gas velocity and as a result the expected error, arising from differences in velocity between
(14) C. M. Herzfeld, “Temperature,” 3 (Part 2), .Reinhold, New York, 1962. (15) R. M. Fristrom and A. A. Westenberg, “Flame Structure,” McGraw-Hill, New York, 1965. (16) W. E. Kaskan, “Sixth Symp. (Int.) Comb.”, Reinhold, New York, 1957, p 134.
I
1
I
I
I
Entrained-air requiredm (I./min)
T max with HzO (OK) 1560 1812 21 10 2110 21 35 2133 2104 2405 2530 2058
I
23.3 35.0 42.5 17.4 29.1 36.6 40.0 25.7
I
I
I
I
I
I
I
2
4
6
8
IO
12
I
I H e i g h t (cml
Figure 6. Temperature variation along central, vertical axis of reversed Hz/entrained air flame G,H ~ / N z Oflame H , premixed H2/Nz0flame I and premixed Hz/Air flame J turbulent flame and calibration flame, will be less than 32 OK for a turbulent flame temperature of ca. 2400 OK. This estimate is based on maximum gas rise velocities measured by the authors for turbulent flames and also on published (10) rise velocities for the type of flames used in calibration. Smaller uncertainties will arise from the differences in, thermal conductivity, viscosity, and isothermal gas densities between the two types of flame. These considerations will also apply to different regions of the same turbulent flame. A total error for the radiation correction of about 8%--i.e., 32 OK for a flame at 2400 OK with a thermocouple having a radiation correction of 400 OK is to be expected. The error involved in using uncompensated iridium/copper and copper/iridium-rhodium connections as the reference junction was eliminated by the application of the tabulated (13) emf values for these couples at room temperature. This compensation is of the order of 10-20 OK. Conduction errors were minimized by the thermocouple design which ensured the junction leads were in contact with flame gases over a distance of about 1 cm. Errors due to the effect of the thermocouple acting as a heat sink on the flame will be negligible because of the small dimensions of the thermocouple and the high mass burning rates of the flape gases. VOL. 41,NO. 7, JUNE 1969
949
Heating effects due to the dissipation of kinetic energy of the flame gases on striking the thermocouple are of minor importance in the turbulent flames investigated here and have been included to some extent in the compensation for the radiation error. For a gas rise velocity of ca. 80 m sec-’, this error would beless than 2 OK (17). The millivoltmeter used for measuring thermoelectric emf had a large impedance at balance, and no corrections were necessary for the thermoelectric current required to drive the meter. The thermoelectric emf tables (13) have an expected maximum error of ca. 10 OK at the upper temperature limits of the thermocouples resulting from variations between lots of wire. The wire used in this study was obtained from the same source as that of Blackburn and Caldwell (13), and a temperature error of =t0.4% has been assumed for possible differences between tabulated and experimental thermal emf. Systematic errors due to the effect of the thermocouple on the flame have been minimized as far as possible by thermocouple design but will not have been completely eliminated. Such errors arise from the mixing action of the thermocouple on the unburned gases issuing from the burner, the effect of the turbulent wake above the thermocouple, and the averaging effect due to the size of the thermojunction. Most of these errors are of unknown magnitude and are inherent in all thermocouple measurements in flames; the mixing effect is mainly important at low height above the burner in regions of steep temperature gradient and of little analytical importance. Random errors resulting from positioning precision could be reduced to less than 10 OK even for flame regions where the temperature gradients were large but the temperature steady. The above discussion of the main possible sources of error leads to a maximum uncertainty of about A50 OK for flame temperatures of 2400 OK and about *40 OK at 2000 OK. Although the estimated error is larger than might be expected using the line-reversal method, better resolution of temperature contours is possible using thermocouples. Simon (18) has measured the sodium reversal temperatures of a turbulent Hz/Oz diffusion flame and found a maximum difference of 150 O K between the average temperature and the tem perature in the center of the flame, resulting from radial inhomogeneity of the flame temperature. Flames Examined (see Table I). H2/Arflames B and C and H2/air flames E and F were representative of the flames used by previous workers (7, 19) in atomic fluorescence. Flames A and D were examined to extend the range of flow rates so that virtually all the flames likely to be used for analysis would be covered. To facilitate comparison, the argon or air flow rates were maintained at 5.9 1. rnin-’ for flames A through F. Hz/entrained-air flames have been used by Ellis and Derners (20) for atomic fluorescence and appear to have some applicability in atomic emission spectrometry. Hydrogen was supplied to the flame through the burner port usually used for the aspirant gas, and the flow rates were limited by the hydrogen flow required for aspiration and lift-off of the flame from the burner. It is quite possible, however, that lifted Hz/entrained-air flames could be used for analysis. (17) E. F. Fiock, L. 0. Olsen, and P. D. Freeze, “Third Symp. on Comb. Flame and Explosion Phenomena,” Williams & Wilkins, Baltimore, 1949, p 655. (18) L. Simon, Oprik, 19,621 (1962). (19) K. E. Zacha, M. P. Bratzel, J. D. Winefordner, and J. M. Mansfield, ANAL.CHEM., 40, 1733 (1968). (20) D.W. Ellis and D. R. Demers, ibid., 38, 1945 (1966). 950
ANALYTICAL CHEMISTRY
-...........
YY.Y.....
OK
2200
...........2320 -=.------
‘K
.
............. 2:.::.:“ .
2000
....
. ,
1800 5
10
15
20
Hydrogen Flow [I.rnin‘’)
Figure 7. Maximum temperatures of Hz/Ar flames A, B, C, and Hz/Air flames D, E, F, having the same flow of aspirant gas (5.9 1. min-l) but different fuel flow rates The 2320 O K asymptote represents the calculated adiabatic temperature of the H,/Air flame The premixed H2/air, premixed Hz/NzO,and unmixed Hz/ NzO flames have recently been examined in this laboratory (9) and show considerable advantages over the flames, commonly used in atomic fluorescence spectrometry. Because of the complex mixing arrangements used for the premixed flames, the gas characteristics given in Table I, are gas pressures. However, a detailed description of how to reproduce the premixed flames will be given in another report currently being prepared for publication. Flames H , I, and J have the same gas flow rates and compositions as those examined in the fluorescence studies (9). An unmixed Hz/NzOflame, having gas flow rates corresponding to flames B and E-i.e., HZ = 14.0 1. min-l and NzO = 5.9 1. min-’ caused failure of the thermocouple and unfortunately could not be included in the results. Similarly, H2/02 turbulent flames could not be included in this study because of their high temperature; however, measurements of the temperatures of these flames have been made by several authors (2-6), and there is general agreement of the published values. Premixed Hz/Ar turbulent flames were found (9) to give no advantages in analysis over the unmixed Hz/Ar flames, as the aspirant did not act as an oxidant and the degree of turbulence was not reduced appreciably by premixing. Consequently, these flames were not included in this study. Temperatures of Flames. Temperature profiles of H,/Ar flame B, Hz/air flame E and Hz/entrained-air flame G are given in Figures 1, 2, and 3, respectively. The variations of temperature along the vertical axes of the flames listed in Table I are given in Figures 4, 5, and 6. The maximum temperatures found along the vertical axes of these flames are listed in Table I. The maximum adiabatic temperature for a H2/airflame has been calculated by Lewis and von Elbe (21) as 2320 O K , and it appears that this temperature is approached as the flow of hydrogen relative to air is decreased to the stoichiometric ratio (Figure 7 ) . However, such a flame would be far too small for analytical purposes, and most useful flames are apparently very fuel rich. As the ratio of hydrogen-to-air is increased, the flame becomes larger, and the maximum temperature decreases until a limiting value of about 2180 O K is reached. (21) B. Lewis and G. von Elbe, “Combustion, Flames and Explosions of Gases,” 2nd Ed., Academic Press, New York, 1961.
It was thought, at first, that incomplete combustion of hydrogen occurred especially as the amount of entrained-air required for complete combustion appeared excessively large. However, analysis of the flame gases, by gas chromatogaphy after extracting a sample using a stainless steel probe, indicated that hydrogen combustion was complete at least to (the limit of measurement). In these experiments, helium was used as a reference tracer and was mixed with hydrogen before combustion. It appears, therefore, that the decrease in temperature is dependent on the amount of entrained-air (and hence turbulence) required for complete combustion. The higher the degree of air-entrainment, the greater the exchange of heat with the surroundings and the greater the possibility of entrainment of excess air from the atmosphere. These generalizations appear to be borne out in the case of the H2/Ar/entrained-air flames A , B, and C (see Figure 7). When the ratio of hydrogen-to-argon is small, a cooling effect due to the presence of argon will be expected, as well as a possible increase in the degree of turbulence. At higher flow rates of hydrogen, the cooling effect of argon should be reduced, and eventually the H2/Ar flame should exhibit a temperature approaching that of a Hz/air flame for which the ratio of hydrogen to aspirant-air is also large. The results in Figure 7 indicate that this expectation is fulfilled at least within the range of gas compositions examined in this study. The premixed Hz/air flame used in this study had a temperature which was representative of the unmixed flames. However, the flame was not stoichiometric, and some degree of air entrainment was necessary to ensure combustion. Similarly, the two H2/N20flames could not be directly compared, as their compositions were based on their analytical utility. However, the reduction in turbulence obtained by premixing has been demonstrated photographically by schlieren (8) and shadow (9) techniques, and it is expected that premixing would give higher flame temperatures because of this. Flame Structure. The unmixed Hz/Ar, H2/air, and Hz/ N20 flames are characterized by the same shape (see Figures 1 and 2). Immediately above the burner, the flame widens in a similar manner to a hydrogen diffusion flame. In this region (0-8 mm above the burner), mixing of the aspirant and fuel gases is incomplete, and turbulence is not developed. About 2 cm above the burner, the flame shows a pronounced pinching effect, and it is likely that air entrainment becomes important in this region, above which the flame widens to give a transparent, almost invisible and poorly defined flame which is typical of the extended or fragmented reaction zones of turbulent flames at high Reynolds numbers. The inner flame structure of the unmixed flames is difficult to observe because of their transparency. An unburned gas zone, in the center of the flame, extending 2 cm or so above the the burner, is usually obvious, but little structure can be observed visually. In the H2/N20 flame, however, a yellow reaction zone is easily observed and appears as a cone extending from a little above the “pinched” region at the edges of the flame (ea. 2 cm above the burner) to between 3.5 and 4.5 cm in the center. This yellow cone is about 1 cm wide in the unmixed flame and about 3 mm wide in the premixed flame. The temperature profiles for the central, vertical axes of the unmixed flames (Figures 4, 5, and 6) show quite pronounced inflexions, between the burner tip and the region of maximum temperature. The inflexions appear as decreases in temperature above an otherwise gradually increasing profile. In the Hz/air and Hz/N20 flames, they occur just above the regions where the aspirant gases react with hydrogen (this can
be observed visually with the H2/N20 flame), and with the H2/Ar flame, the inflexions occurs at a height where the “pinching” effect is greatest. In the unmixed flames, the inflexions of the temperature profiles are most apparent as the flow of aspirant relative to fuel is increased. The same inflexions were noticed with premixed flames Z and J (Figure 6 ) but are less obvious. Aspiration of Water. The flames considered, hitherto, have been ‘dry’ flames and have not had water aspirated through them, as is the usual practice in analysis. The measurement of flame temperature is even more complicated by the fact that the evaporation of water droplets in the flame is only partially complete and at heights lower than 3 cm above the burner, water droplets will actually cool the thermocouple considerably but will reduce the temperature of the flame to a much lesser extent. However, this represents a region of the flame which has no analytical utility. Analytical measurements are usually made at, or above, the hottest part of the flame. Over this region, the cooling effect of water on all the flames examined represented a temperature decrease of 5-1 5%, depending on the flame type and solution uptake rate (see Table I) but independent of flame height. Turbulent Flames in Atomic Fluorescence. Although several reports have appeared from this laboratory (7, 19) concerning the use of turbulent hydrogen flames in atomic fluorescence, it is not generally appreciated that many elements are determined most satisfactorily above the visible portion of the flame. This is particularly true for volatile elements which are easily atomized such as Cd, Cu, Hg, Ag, Zn, etc., while the more difficultly atomized elements such as Fe, Mg, Ni, etc., are best determined within the visible portion of the flame itself. The low background of the region above the flame is mainly responsible for some of the extremely low limits of detection which have been obtained using turbulent flames. The authors have re-examined the atomic fluorescence of several metals in solutions containing no matrix elements, and found the fluorescence above the flame to be quite genuine. For example, when a continuum excitation source is used, the atomic fluorescence spectral line can be observed when the monochromator is scanned over a narrow wavelength region. Elements such as magnesium and iron, which are normally determined within the flame, appear to give their maximum signals at the region of maximum flame temperature. These observations have since been confirmed as part of this study. In this study, the atomic fluorescence from flame segments about 5 mm high was observed at different heights above the burner. Although the fluorescence emission from metals such as magnesium and iron was greatest in the high-temperature region of the flame, this was not necessarily the best region to use for analytical purposes. With magnesium, the background, due to the OH emission around 2852 A, is high at the region of maximum temperature. Consequently, it is necessary to optimize the height of measurement on the basis of signal-to-noise considerations, rather than on the basis of only flourescence intensity. In the case of iron, however, the atomic resonance line (2483 A) occurs at a wavelength where flame background is fairly low, and as a result the atomic fluorescence determination of iron can be carried out at the region of maximum temperature, whereas the determination of magnesium is carried out at the tip of the visible portion of the flame where the background noise is lower. The results of Bratzel and Winefordner (7) have been optimized with regard to signal-to-noise, and it is instructive to compare their VOL. 41, NO. 7 , JUNE 1969
951
heights of measurement for various elements, with the temperature profiles of Figures 4 and 5. The relationships between heights of observation and fluorescence emission for the premixed and unmixed Hz/air and Hz/NzO flames will be the subject of a series of comparative papers (9)and will not be discussed here. With laminar flames, the use of Ar/Oz mixtures in place of air generally results in increased fluorescence quantum efficiencies of the order of two-to-five-fold. Because of the
high degree of air-entrainment in the turbulent flame, this enhancement was found to be small and of the order of about 10% in the most favorable cases after allowance for different solution uptake rates had been made. Received for review February 10, 1969. Accepted March 21, 1969. This work was supported by AFOSR (SRC)-OAR, U.S.A.F. Grant No. AF-AFOSR-69-1685.
Determination of Arsenic by Atomic Absorption Spectrometry with an Electrodeless Discharge Lamp as a Source of Radiation Oscar Menis and
T. C. Rains
Analytical Chemistry Division, National Bureau of Stanabrds, Washington, D.C. 20234
The determination of arsenic in cast iron and highpurity selenium metal by atomic absor tion spectrometry was facilitated by the extraction orthe arsenic with diethylammonium diethyldithiocarbamate (DDDC) followed by a stripping process from the organic phase by displacement reaction. In addition, the high background absorption of arsenic radiation encountered in various oxidant-fuel systems was overcome with an argon (entrained air)-hydro en flame. The electrodeless discharge lamp was Vound to be an excellent high intensity source of radiation for use in atomic absorption in the far ultraviolet region. The detection limits for arsenic in an aqueous medium free of interfering cations was 0.1 pg/ml.
IN RECENT YEARS atomic absorption spectrometry (AAS) has provided an excellent means for the determination of a large number of elements. However, very little work has been reported for those elements, such as arsenic, which have groundstate resonance lines in the far ultraviolet region of the spectrum. Allan ( I ) in an unpublished paper in 1963 presented working curves for arsenic. The major difficulties in the determination of this element include the high absorption losses due to air and flame gases and the correction for the scattering of light due to solid particles. Slavin, Sebens, and Sprague (2) reported the determination of arsenic in glass by AAS but obtained a higher value than the one reported by chemical analysis. The authors attributed the high value to an improper correction for the light scattering which was made at 199 nm. To overcome the absorption of air and flame gases, Massmann (3) used an electrically-heated graphite cell and double beam spectrometer in an argon atmosphere which did not completely eliminate the difficulties encountered. Since the oral presentation of this paper, Dagnall and West (4) described the sensitivities of the arsenic electrodeless discharge lamp and Dagnall, Thompson, and West the fluorescence and analytical characteristics of arsenic (5). To make the method applicable to the determination of trace quantities of arsenic in a large variety of materials re-
(1) J. E. Allan, Fourth Australian Spectroscopy Conference, Canberra, Australia, August 1963. (2) Walter Slavin, Carl Sebens, and Sabina Sprague, At. Absorption Newsletter 4, 341 (1965). ( 3 ) H. Massmann, 2.Anal. Chern., 225, 203 (1967). (4) R. M.Dagnall and T. S. West. Appl. Opt., 7, 1287 (1968). (5) R. M. Dagnall, K. C. Thompson, and T. S. West, Tuluntu, 15, 677 (1968). 952
ANALYTICAL CHEMISTRY
quired the application of high-intensity electrodeless discharge lamps, the use of a total consumption burner with an argon (entrained airkhydrogen flame, and appropriate chemical separations. All contributed in overcoming the difficulties due to absorption and light scattering in the far ultraviolet region. In our investigation of the determination of arsenic in National Bureau of Standards, Standard Reference Material (NBS-SRM) high purity selenium and cast iron, parameters such as the source of radiation, the resonance lines of arsenic, interfering ions, separations, nebulizers, and oxidant fuel systems were investigated. EXPERIMENTAL
Reagents. Arsenic, Standard Solution was prepared in
"0, from NBS-SRM 83c. Diethylammonium diethyldithiocarbamate (DDDC), 0.1 solution in chloroform. Prepare this reagent fresh daily. 2-Thenoyltrifluoroacetone (TTA), lM, was dissolved in carbon tetrachloride. Store in a dark bottle. Ammonium Acetate Buffer Solution, 5 M ammonium acetate was adjusted to pH 4.5 with 5Macetic acid. Other solutions are prepared from ACS reagent-grade chemicals. Apparatus. The instrument is a 0.5m Eberte mount monochromator with a grating blazed for 3000 A. The hollow-cathode power supply and tube holder are used unmodified, as received from the manufacturer. The electronic circuitry consists of three units and a chart recorder. The three units are the 0 to 2100 V multiplier phototube power supply, the selective amplifier and synchronous detector, and the DCR-2 digital readout. The electrodeless discharge lamp is powered by an R F generator from a medical-type diathermy unit which supplies 100 W at 2450 MHz. The resonant frequency of ' 1 4 wave cavity described by Fehsenfield, Evenson, and Broida (6) is adjusted for the minimum reflected power by a tuning stub and a coupling slider on the cavity. To observe minimum reflected power, an R F power meter is placed between the cavity and power generator. The lamps were prepared as described by Dagnall and West (4). Slit Width. Because the 0.5m monochromator was equipped with adjustable bilateral slits, the slits were adjusted to give the maximum absorption of arsenic radiation with the (6) F. C. Fehsenfield, K. M. Evenson, and H. 0. Broida, Rev. Sci. Instrum., 36, 294 (1965).
