Spectrometric Properties and Analytical Applications of Premixed Oxygen-Hydrogen Flames N. V. Mossholder,' V. A. Fassel,' and R. N. Kniseley Ames Laboratory-USAEC and the Department of Chemistry, lowa State University, Ames, lowa 50070
Observations on 14 different elements indicate that, in spite of its high maximum temperature (2950 K ) , the premixed oxygen-hydrogen flame offers few advantages as an atomization cell for flame atomic absorption spectrometry. Powers of detection are inferior to those determined in the fuel-rich nitrous oxide-acetylene flame, and serious solute vaporization interferences are observed in the relatively cool, very fuel-rich flame required for maximal dissociation of even moderately stable metal monoxides. The equivalency of emission detection limits for several elements in the oxygen-hydrogen and nitrous oxide-acetylene flames is attributable to the low spectral noise level of the former flame in most regions of the spectrum. Degrees of metal monoxide formation, calculated for Na, Fe, Be, and Ti using a thermodynamic flame model, explain the experimental results and iilustrate that even when solute vaporization is complete, elements that form very stable monoxides cannot be appreciably atomized in the premixed oxygen-hydrogen flame.
Several physical properties of the premixed oxygenhydrogen flame (1-173, which are summarized in Table I, make it appear attractive as an atomization cell or excitation source for analytical spectrometry. The emission and absorption spectra are strikingly simple and its reported temperature is only -150 K lower than that of the premixed nitrous oxide-acetylene flame (18). Thus, signifi1Present address, Celanese Fibers Company, P. 0. Box 1414, Charlotte, N.C. 28201. 2 To whom requests for reprints should be sent. H. H. Lurie and G . W. Sherman, Ind. Eng. Chem., 25,404 (1933). H. P. Broidaand K. E. Shuler, J. Chem. Phys., 27, 933 (1957). J. D. Winefordner, C. T. Mansfieid, and T. J. Vickers. Anal. Chem., 35, 1611 (1963). L. De Gaien and J. D. Winefordner, J. Ouant. Spectrosc. Radiat. Transfer, 7, 703 (1967). J. H . Gibson, W. E. Grossman, and W. D. Cooke, Anal. Chem., 35, 266 (1963). L. Simon, Optik (Stuttgart), 19, 621 (1962). R. Edseand L. R. Lawrence,Jr., Combust. Flame, 13,479 (1969). R. N. Kniseley, in "Flame Emission and Atomic Absorption Spectrometry," Vol. 1, J . A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, N. Y., 1969, Chapter 6. G. H. Dieke and H. M . Crosswhite, Johns Hopkins University Bumblebee Report 87, 1948. J . A. Fiorino, Ph.D. thesis, lowa State University of Science and Technology,Ames, Iowa, 1970. A . G . Gaydon, "The Spectroscopy of Flames," Chapman and Hall, London, 1957. H. G. Wolfhard and W. G . Parker, Proc. Phys. SOC.,London, Sect. A, 62, 722 (1949). A. G . Gaydon and H. G. Wolfhard, Proc. Phys. SOC.,London, Sect. A , 65, 2 (1952). P. J. Padley, Trans. FaradaySoc., 56, 449 (1960). R. W. Pearse and A. G. Gaydon, "The Identification of Molecular Spectra." 3rd ed, Wiley. New York, N. Y., 1963. R. D. Hudson and V. L. Carter, J. Opt. SOC.Amer., 58, 1621 (1968). S. D. Rains, Pittsburg Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1968. J. B. Willis, J. 0. Rasrnuson, R. N . Kniseiey. and V. A. Fassel, Spectrochim. Acta, Part 6, 23, 725 (1968).
1614
cant atomization for many elements and a minimum of spectral interferences are to be expected. Furthermore, the exceptionally wide limits of flammability for this flame, and the absence of the soot formation normally encountered in fuel-rich carbon-fuel flames, should allow a wide range of chemical environments to be produced. The analytical potential of this flame has, however, not been examined, apparently because burner systems which would allow both the introduction of sample aerosols and the safe combustion of the very high burning-velocity mixtures of oxygen and hydrogen have not been available. A burner especially designed (19) for the safe combustion of high burning-velocity gas mixtures has, however, provided the opportunity of experimentally evaluating the analytical utility of the premixed oxygen-hydrogen flame. This evaluation was the primary objective of this investigation. EXPERIMENTAL FACILITIES AND PROCEDURES Optical and Electronic Instrumentation. The characteristics and operating conditions of the spectrometer and electronics used in this investigation have been described (19). The importance of spatial resolution in the study of flame temperatures and atomic and molecular emission and absorption cannot be overstated. Large temperature gradients and nonuniform spatial distributions of species in the flame necessitate the examination of small, homogeneous flame volumes, if the results are to be meaningfully interpreted. By limiting both the aperture of the lens nearest the monochromator and the slit height to 2 mm, the angle of acceptance of the spectrometer and the divergence of the observed light leaving the flame were maintained a t small values. Thus, light that entered the monochromator (the solid angle of acceptance) originated in a section of the flame having dimensions a t the flame center of 2 mm in height and a width equal to the slit width. Since the light is not strictly parallel, however, these dimensions are slightly larger at the ends of the flame. Burner and Nebulizer. The burner used in this study was first described (19) and later modified (10) by Fiorino. A quiet, stable flame can easily be maintained on this burner, if a 7.62- by 0.025-cm slot is used. The burner head need not be cooled, even when the flame is operated for long periods of time. Flashbacks of oxygen-hydrogen mixtures seldom occur when this burner is utilized. If the flame does flashback, the detonation is mild because the overall combustion reaction results in a reduction in the total moles of gas,
2H2
+
0 2
-
2H2O
The O-ring sealed, blowout plug is ejected, but no damage to the burner results. The continuously variable nebulizer employed in this study (Varian Techtron, Walnut Creek, Calif.) allows independent variation of the solution uptake rate ( - 3 ml/min) and gas flow rates and produces an aerosol with a droplet size distribution ranging down to a few microns in diameter. Gas Supply and Flow Metering System. Gas flows were measured with a metering system similar to the one described by Fiorino et al. (19). Unfortunately, the system employed was being used concurrently for several other studies, so that internal modifications could not be made. Instead, accessories were added to adapt it for use with oxygen and hydrogen. The flow metering system, with accessories, is diagramed in Figure 1. Fiorino, R. N . Kniseley, and V. A. Fassel, Specfrochim. Acta, Part 6, 23, 41 3 (1968).
(19) J . A.
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
k
Table I. Physical Properties of the Premixed OxygenHydrogen Flame Property Values Ref Temperature Burning velocity Limits of flammability Background spectrum Emission
2630-3325 K 9 15 c m / s e c 9 4-91 6% HZ (for a flame containing 3% N2)
OH, 2607-3500
A
6000 A OH, 2607-3500
A
1900-2500 A Continuurn, 19002800 A 0 2
K L
(1-6) (7)
(8)
(9-73) (74)
Continuum (very w e a k ) , 2000Absorption
J
(9-73) (71-13, 15, 1 6 ) (70, 1 7 )
The production of fuel-rich flames required the use of very low oxygen flow rates. The samples were therefore nebulized by hydrogen rather than oxygen. The hydrogen flow was split into two streams; one entering the nebulizer and the other entering the burner directly. A needle valve (Mathison No. 100R, El in Figure 1) was inserted into the auxiliary flow stream to control the total amount of hydrogen entering the burner and to maintain a relatively constant flow through the nebulizer. Minor changes in the solution uptake rate could then be corrected by adjusting the nebulizer. Since valve El controlled the total hydrogen flow, the needle valve immediately preceding the rotameter (used when the fuel enters the burner directly) was opened fully, effectively removing it from the flow stream. In the oxygen flow system, the au ary oxidant line in the original system was closed, and a needle valve (Ez) was inserted immediately preceding the burner. As the oxygen flow rate was changed by varying E$, needle valve E2 was adjusted to maintain a constant pressure within the rotameter, since the rotameters must be operated at the same constant pressure a t which they were calibrated (20). The oxygen flow system was calibrated with a precision wettest meter (Precision Scientific Company, Chicago, Ill.). Since the hydrogen flow rates were beyond the capacity of the wet-test meter, the hydrogen flow system was calibrated with a positive displacement gas meter (Sprague Meter Company, Bridgeport, Conn.). In all the experiments described below, a constant total flow rate ( 0 2 + Hz)of 50 l./min was maintained. For each value of 02/Hz studied, the proper 0 2 and Hz flow rates were calculated, and the rotameters were adjusted accordingly. Experimental results demonstrating the need for strict maintenance of a constant total flow rate will be discussed below. Solutions. Aqueous chloride solutions were used for all elements except Ag and Pb. AgN03 and P b ( N 0 3 ) ~as well as NaCl were dissolved directly in water and diluted to the final concentration. For the other elements, the metal or metal carbonate was dissolved in HC1. The excess acid was then removed by evaporating the solution to near dryness followed by diluting to the proper concentration. Temperature Measurements. The spectral line-reversal technique was used to measure temperatures in the oxygen-hydrogen flame. The principles of this method, possible errors, and precautions which must be taken to assure reliable results have already been discussed (21-26). The experimental technique used was that of Willis et al. (18). Data for the emissivity of tungsten were taken from de Vos (27). (20) C . Veillon and J. Y . Park, A n a / . Chem.. 42, 684 (1970). (21) A. G . Gaydon and H. G . Wolfhard, "Flames, Their Structure, Radiation and Temperature," 2nd ed, Chapman and Hall, London, 1960. (22) W. Snelleman, P h . D . thesis, State University of Utrecht, "BronderOffset." Rotterdam, the Netherlands, 1965. (23) W. Snelleman, Combust. Flame, 11, 453 (1967). (24) D . L. Thomas, Combust. Flame, 12, 541 (1968). (25) W. Snellernan, in "Flame Emission and Atomic Absorption Spectrometry," Vol. 1, J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, N . Y . , 1969, Chapter 7. (26) I . Reif, Ph.D. thesis, Iowa State University of Science and Technology, Ames, iowa, 1971. (27) J. C. de Vos, Physica (Utrecht!, 20, 690 (1954).
Figure 1. Gas flow regulation and metering system /A) oxygen, .(E) hydrogen, (C)precision diaphragm regulator, (D) pressure gauge, (E) precision needle valve, (F) rotameter, ( G ) float, (H) tapered glass tube, ( I ) toggle valve, (J) to oxidant port, (K) to nebulizer port, (L) to fuel port Measured flame temperatures should ideally reflect only the homogeneous temperature of the central axial flame channel, but the edges and ends of the flame may bias the measurements unless their effects are carefully excluded. Edge effects can be reduced by decreasing the acceptance angle of the spectrometer, and contributions from both the edges and ends of the flame can be reduced by selecting a thermometric species that exists as free atoms only in the flame cknter. Metals, such as Ca, which form stable monoxides and hydroxides in the cool outer edges of the flame, are particularly suitable. Additional experimental advantages obtained by using an alkaline earth element for the test element have been discussed by Willis et a2. (18). Optimization of Experiniental Conditions and D a t a Display-The "Response Surkace" Technique. In most experimental flame studies, only one independent and one dependent variable are studied a t a time. This technique; however, neglects any interactions which might exist between two or more dependent variables. Several statistical methods have been developed to study dependent variable interactions (28-31) but most of these involve considerable calculation. It has been shown (30, 31) that in the study of flames only a few parameters (fuel and oxidant flow rates, oxidant pressure, and position in the flame) have a significant effect on atomic line emission and absorption. Moreover, the observed dependence of emission intensity and absorbance upon the flow rate and oxidant pressure resulted, in part, from the dependence of the nebulization systems used upon these parameters. In this study, the effects of gas pressure and flow rate were minimized by using a nebulization system which was relatively independent of these variables. Even when nebulization effects were eliminated, however, the total flow rate determined the final flame volume and thus the amount of sample dilution. As discussed below, the lowest total flow rate commensurate with safe operation of the burner produced the maximal sample concentration in the flame. Thus, by maintaining the total gas flow rate a t this level, only two experimental parameters were important in the experiments described below-the oxidant-tofuel ratio and the flame site a t which the measurement was made. When the dependence of the desired response can be limited to two variables, it is usually simpler to determine the response surfaces solely by experiment, thereby avoiding the somewhat involved statistical calculations. Two types of response surface display have been employed in the study of flames. Malakoff et al. (32) used oblique projections of experimentally determined re(28) G. E. P. Boxand K. E. Wilson, J. Roy. Stat. SOC.,813, 1 (1951). (29) G. E. Box, Biometrics, I O , 16 (1954). (30) R. K. Skogerboe, Ph.D. thesis, Montana State College, Bozeman, Montana, 1963. (31) K. M . Ceilier and H. C. T. Stace, Appl. Spectrosc., 20, 26 (1966). (32) J. L. Malakoff, J. Ramirez-Munoz. and W. Z. Scott, Anal. Chirn. Acta, 42, 515 (1968).
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
1615
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STOICHIOMETRY l%/H*1
Figure 3. Response surface of temperature (K) in t h e premixe O2.H2flame
op+m Figure 2. Photographs. taken slightly off the optical axis of the spectrometer. of the stoichiometric (left. O p / H z = 0.50) and very fuel-rich (right, 02/H2 = 0.10) premixed oxygen-hydrogen flames
-
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sponse surfaces to illustrate the interaction 01 BDsOrDBnCe, meral concentration, and a third variable in atomic absorption measurements, and Skogerhoe et al. (33) projected the response 8urface onto the independent variable plane to display the dependence of vanadium emission intensity nn the oxidant-to-fuelratio and observation site in a turbulent oxygen-hydrogen flame. The planar projection format of Skogerhoe et 01. is employed throughout the discbssion that follows to illustrate and interpret the flame temperature and the emission and absorption of species in the premixed oxygen-hydrogen flame. The response of temperature (K), emission intensity, or absorbance is plotted against both 02/Hz, the molar oxidant-to-fuel flow ratio (abscissa), and Ht, the vertical position in the flame (ordinate), and points of equal response are connected with lines. The paint of maximum response is represented by +. Emission intensities 81Id absorhances are normalized with respect to the maximum va lue to facilitate the comparison of one element with anothei., The response surfaces were constructed by measuri ng the hack-. . . wmpera~urc ground corrected emission intensity, absorbance, or ~.-. at 44 evenly spaced values of 02/Hz and Ht. The data were processed with a digital computer and the corresponding response at each point was then calculated. For the emission Hnd absorbance response surfaces, the resulting points were plotted by the computer in seven groups, each with a different symbol. The paints in each group comprised a given range of relative intensities or ahsorhances, i.e., 1-10. 10-25, 25-40, 40-55, 55-70, 70-85, and 85100% of the maximum value. Lines were then drawn hy hand to separate these groups and the lines were marked with the relative intensity of the junction (1, 10, 25%. etc.). No experimental aids such as a motor driven burner assembly were used in this work. The intensities and absorbance data were recorded directly from the integrator readout onto keypunch forms. The time required to construct one plot was approximately 45 min. The reproducibility of the hand drawn lines was g o d ; variation in the position of a line was at most +3% of the maximum value. 1
.
.....
RESULTS AND DISCUSSION Visual Appearance. The premixed oxygen-hydrogen flame burning on a laminadlow slot burner is acoustically quiet and very stable. The visual appearance of the flame is highly dependent upon the oxidant-to-fuel ratio (331 R. K.
Skogerbae. A.
38, 1821 (1968).
1616
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Flguie 4. Horizontal temperature profiles at 2.0 cm above the burner top
as illustrated in Figure 2. The primary reaction zone of the stoichiometric flame (with respect to the gases passing through the burner) shoani in the left half of this figure is hlue-white i n color, approximately 1 mm in height, and is formed approximately 0.5 mm from the top of the burner. The post reaction zone or “burnt gas ;egion” is pale blue and nearly transparent. This flame IS 7.6 cm long, approximately 12 cm high, and less than 1 cm wide at 2 cm above the top of the burner. As the flame is made increasingly fuel-rich, the luminosity of both zones decreases and the size of the flame increases. At oxidant-to-fuel ratios less than 0.10, the flame is invisible in normal light, hut in a darkened room the shape of the flame is that illustrated in the right half of Figure 2. When the composition of the grrses leaving the burner corresponds to Oz/hz 5 -0.10, the limit of flammahilitji is exceeded. Combustion is then supported primarily by oxygen entrained from the atmosphere and the long primary reaction zones at the edges of the flame result. The dark central portion of the flame in this figure represents the burnt gas region. This very fuel-rich flame (Oz/H2 = 0.10) is more than 20 cm high and approximately 4 cm wide at 2 cm above the burner top. Temperature. T h e variation of temperature in the premixed oxygen-hydrogen flame as a function of Oz/Hz and Ht (temperature response surface) is illustrated in Figure 3, and Figure 4 shows the variation in measured horizontal temperature profiles as a function of Oz/H2. These fig-
* ANALYTICAL CHEMISTRY, VOL. 45. NO. 9. AUGUST 1973
*
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4000
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Emission background spectrum of the stoichiometric,premixed oxygen-hydrogen flame
ures exhibit several interesting features. Of primary interest is the 2950 K maximum temperature in Figure 3, which was observed a t 02/Hz = 0.50 and Ht = 0.25 cm. At this stoichiometry, the primary reaction zone, because of its small size and proximity to the burner, is outside of the angle of acceptance of the spectrometer. Thus, the nonthermal temperature distributions commonly observed in the primary reaction zone (34) probably do not contribute to the measured value. The maximum temperature measured in the premixed oxygen-hydrogen flame is approximately 75 K higher than the maximum temperature of the nitrous oxide-hydrogen flame (35) and less than 125 K lower than the corresponding value in the nitrous oxide-acetylene flame (18). The value 2950 K is also higher than most others reported for this flame even though the flame examined in this study contained appreciable amounts of water. This value is in good agreement, however, with the theoretical maximum temperature of -3083 K (36) considering the effects of heat losses and nebulized water. Another interesting feature of the oxygen-hydrogen flame is the unusually large temperature difference ( - 1250 K ) between the stoichiometric (Oz/H2 = 0.50) and very fuel-rich (Oz/Hz = 0.05) flames. Figures 3 indicates that only 150 K of this difference occurs between O2/H2 = 0.50 and 0.30, whereas the difference is -600 K between Oz/H2 = 0.30 and 0.15 and -500 K between O 4 H 2 = 0.15 and 0.05. The horizontal temperature profiles shown in Figure 4, as well as the response surface in Figure 3 and visual observations of the flame. support the conclusion that at oxidant-to-fuel ratios less than -0.10, entrained atmospheric oxygen plays an important role in the combustion process. This deduction is most strongly supported by the interesting inversion of the horizontal temperature profiles. When primary oxidant (that which passes through the burner) supports the flame, the highest temperatures should be observed in the flame center, and the temperature should decrease near the edges of the flame as the entrained air dilutes and cools the flame gases. Figure 4 A. G. Gaydon and H. G . Wolfhard, "Flames, Their Structure, Radiation and Temperature," 3rd ed, Chapman and Hall, London, 1971. J. €3. Willis, V. A. Fassel, and J. A. Fiorino, Spectrochirn. Acta, Part B. 24, 157 (1969). E. V. L'vov, "Atomic Absorption Spectroscopy." Translated from Russian by Israel Program for Scientific Translations. Document AEC-tr-6979 available from Clearinghouse for Federal Scientific and Technical Information, Springfield, Va. 22151,
clearly shows this effect for O2/Hz = 0.30 and 0.40. As 02/H2 decreases from 0.20 to 0.05, there is an increasing tendency for the flame temperature to be lower in the flame center and higher near the edges of the flame. This behavior is in harmony with the existence of a diffusion flame supported primarily by atmospheric oxygen. The profiles in Figure 4 do not extend farther than 4 mm from the flame center because no Ca emission could be detected beyond this point. Background Spectrum. The spectral region in which the stoichiometric, premixed oxygen-hydrogen flame emits is illustrated in Figure 5. The bands shown in this figure are all emitted by the OH radical. No emission continuum is evident in Figure 5 because a relatively low sensitivity was required to keep the OH bandhead on scale. At higher amplifications, a weak emission continuum could be observed throughout most of the spectral region illustrated. The OH bands shown in Figure 5 are also observed in absorption, as are 0 2 bands and an absorption continuum which underlies the 0 2 band system (IO). Response surfaces of OH emission intensity and absorbance (at the 3064 A bandhead) are found in Figure 6. The absorption response surface in this figure does not indicate a relative change in the concentrations of the OH molecule because the partition functions of OH change significantly over the wide range of flame temperatures. In addition, the OH bandhead is comprised of several superimposed components whose relative intensities may vary with temperature. The maximum OH response (emission intensity or absorbance) on both surfaces is located 0.5 cm above the burner in a slightly fuel-lean flame (02/H2 = 0.55). Furthermore, both surfaces exhibit steadily diminishing response with decreasing Oz/H2. This behavior reflects the reduction of the concentrations of all oxygen-containing flame species as the flame is made more fuel-rich. The drop in OH emission with decreasing oxidant-to-fuel ratio is particularly rapid, illustrating the exponential dependence of emission intensity on temperature (compared with Figure 3). Emission and Absorption of Representative Elements. The response surfaces of atomic absorbance and emission described below illustrate the oxidant-to-fuel ratio and observation site that produce the maximum response in each mode. These plots supply the experimental information necessary for the determination of detection limits and, in addition, provide valuable information
A N A L Y T I C A L CHEMISTRY, VOL. 45. N O . 9, AUGUST 1973
1617
2.00
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a W
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2 1.00 W
B
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a
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0.50
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0.2
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1
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FLAME STOICHIOMETRY
(02/H2)
Figure 6. Response surfaces of OH absorbance (left) and emission (right) in the premixed oxygen-hydrogen flame
I
Figure 7. Zn absorbance response surfaces obtained using changing (left) and constant (right) total flow rates
about the free-atom formation processes which occur in the premixed oxygen-hydrogen flame. The concentration of metallic species (free atoms and compounds) in a flame can be defined in the following way no. of metallic species passing through t h e flame/unit t i m e concentration = volume of gas passing through t h e flameiunit t i m e In nearly all studies of premixed flames, the number of metallic species passing through the flame per unit time is constant, since the nebulization rate is held constant. The normal method of adjusting the oxidant-to-fuel ratio in flame spectrometric experiments is to maintain a constant flow rate for the nebulizing gas and vary the flow rate of the other gas to adjust the oxidant-to-fuel ratio. This method, however, results in a changing total flow rate and therefore i n changing concentrations in the flame. In hydrocarbon flames, the range of oxidant-to-fuel ratios is typically small and the results of measurements
in these flames are probably not significantly biased by small changes in the total flow rate. In hydrogen flames, on the other hand, an extremely wide range of oxidantto-fuel ratios can be examined, and the results can be highly distorted unless a constant total flow rate is maintained. The degree of distortion which may be generated is illustrated for Zn in Figure 7 . The left-hand portion of this figure shows the response surface resulting from the use of a changing total flow rate. The hydrogen flow rate was held constant a t 35 l./min (the minimum required to prevent flashback a t all Oz/Hz ratios) and the oxygen flow rate was varied from 0 to 19.2 l./min. The other side of this figure illustrates the corresponding surface obtained, as were all the response surfaces discussed below, when a constant total flow rate of 50 l./min was maintained. The error introduced by failure to maintain a constant total flow rate through the burner is evident without a detailed analysis of the response surfaces. The formation of free atoms in flames is usually discussed in terms of solvent evaporation, vaporization of salt particles, and dissociation of compounds. Three other pro-
1618 * A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
cesses tend to reduce the concentration of free atoms in flames. These are compound formation, ionization, and dilution of the flame gases by air entrainment (37). All of these processes may occur either sequentially or simultaneously so that it is often difficult to assess their relative importance. Of particular importance is the major role played by the formation of metal monoxides in the atomization process, a subject which has been extensively investigated and interpreted (38, 39). This appraisal of the analytical potential of the premixed oxygen-hydrogen flame therefore includes observations on a broad list of elements possessing a range of metal-monoxide (MO) stabilities, as well as other selected properties such as excitation potentials of their spectral lines (40). In Table 11, these elements are classified, according to the shapes of the response surfaces, into three groups containing elements whose MO dissociation energies are 1 4 . 1 eV (Group I), 4.2-5.4 eV (Group 11), and 1 6 . 4 eV (Group III). (41). The response surfaces of Fe, Ba, and A1 were exceptional, so that these elements could not be assigned to any of the above groups. The absorption and emission response surfaces of Ag shown in Figure 8 are representative of all the elements in Group I. Since the shapes of these and the other response surfaces discussed below reflect the sensitivities of freeatom emission and absorbance to temperature as well as Oz/Hz and Ht, an effective interpretation requires a mental convolution of the response surface of temperature (Figure 3) with those of atomic emission and absorbance. (It is recognized that the large range of temperatures encountered in this flame may bias the experimentally observed absorbances because of their dependence on absorption line Doppler half-widths. The fact that several elements with very different atomic weights (Mg us. P b for example) exhibited nearly identical response surfaces indicates that the bias is slight.) Referring first to the ab(37) (38) (39) (40)
(41)
C. Th. J. Alkemade, in "Flame Emission and Atomic Absorption Spectrometry," Vol. 1, J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York. N. Y . . 1969, Chapter 4. T. G. Cowley, V. A. Fassel. and R. N. Kniseley, Spectrochim. Acta, P a r t B , 23, 771 (1968). V. A. Fassel, J. 0. Rasmuson, T. G. Cowley, and R. N. Kniseley, Spectrochim. Acta, Part B, 25, 559 (1970). W. F. Meggers, C. H. Corliss, and 6.F. Scribner, Nat. Bur. Stand. (U.S.)Monogr., 32, Part 1 (1961). A. G. Gaydon, "Dissociation Energies and Spectra of Diatomic Molecules," 3rd ed, Chapman and Hall, London, 1968.
Table II. Properties of the Elements and Spectral Lines Examined
Group I
Wavelength, Element
A
Excitation pot," eV
Monoxide dissociation energy,b eV
Na
5890.0 3280.7 2138.6 2288.0 2170.0 2852.1 3247.5 3719.9 4607.3 4226.7 4254.4 2246.0 3961.5 5535.5 4379.2 3642.7
2.1 1 3.78 5.79 5.42 5.71 4.34 3.82 3.33 2.69 2.93 2.91 5.52 3.14 2.24 3.13 3.42
2.0 2.8 1.00
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FLAME STOICHIOMETRY ( 0 2 / H 2 )
Figure 9. Experimental response surfaces of Ca (10 pg/ml) absorbance (left) and emission intensity (right) 2.00
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FLAME STOICHIOMETRY (02/H2)
Figure 10. Experimental response surfacesof AI (10,000 pg/ml) absorbance (left) and emission intensity (right) fraction a t the site of maximal absorbance is very close to one for the Group I elements. The response surface of Ag emission intensity illustrates the exponential dependence of intensity on temperature. The temperature and Ag emission plots (Figures 3 and 8, respectively) display nearly identical shapes and the points of maximum intensity and temperature correspond exactly. The Ca absorbance and emission intensity response surfaces shown in Figure 9 are typical of the elements in Table I11 which have MO dissociation energies between 4.2 and 5.4 eV (Group 11). In contrast to the behavior of Ag and the other elements in Group I, the highest absorbance and consequently the highest free-atom number density of Ca is found in the very fuel-rich flame (Oz/Hz = 0.05) and there is a precipitous drop in absorbance as Oz/Hz increases to 0.50. Comparison of the response surfaces of temperature (Figure 3), and Ag and Ca absorbances (Figures 8 and 9), indicates that the oxygen concentration in the flame and not temperature is of prime importance in the atomization of the elements in Group 11. The fact that an oxidant-to-fuel ratio of 0.05 produces maximal atomization of these elements even though the 1620
temperature of this flame is only about 1700 K [compared with -2500 K for the air-acetylene flame ( 2 2 ) ] suggests that the oxygen concentration a t this point must be very low indeed. Although the absorbance response surfaces for Ag and Ca are very different, the corresponding emission plots are quite similar, again reflecting the sensitivity of the observed intensity to the flame temperature. The main difference in the emission response surfaces of Ag and Ca is the shift of maximal Ca intensity to a slightly lower oxidant-to-fuel ratio (from 0.55 to 0.40). It is logical to assign this shift to the rapid increase in the Ca free-atom partial pressure, although the lower excitation potential of the Ca line (relative to the Ag line) may be a contributing factor. Titanium and vanadium, the elements assigned to Group III, both have MO dissociation energies greater than 6 eV. Neither of these elements could be observed in the premixed oxygen-hydrogen flame in either emission or absorption. Evidently a combination of flame composition and temperature suitable for the atomization of these elements does not exist in this flame. The shape of the Fe absorbance response surface is intermediate between those of the elements in Groups I and
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 9, AUGUST 1973
Table I l l . Detection Limits Element
Ag AI
0a Ca Cd
Monoxide dissociation energy. eV 2.0a 4.6 5.8 4.3
