Excitation Processes in Flame Spectrometry JAMES
H. GIBSON, WILLIAM E. L. GROSSMAN,
and W. D. COOKE
Baker laboratory, Cornell University, Ithaca, N. Y.
b Experimental procedures have been devised through which it is possible to understand quantitatively some of the fundamental processes of flame spectrometry. The effect of various parameters involved in the enhancement of emission by organic solvents can b e evaluated quantitatively. In the case of sodium and calcium the enhancement is caused partially by more rapid evaporation of solvent and partially by an increase in flame temperature. In the case of tin, a further contribution of chemiluminescence is noted. By flame temperature measurements, population distributions, and ground state concentrations, it is possible to predict flame emissivity to a few per cent. The rate of solvent evaporation, oxide formation, and flame dilution effects are discussed in relation to their effect on emission. Some possible mechanisms for the excitation of tin b y collisions with free radicals are discussed.
I
of elements by the measurement of their characteristic emission from flames, it is well known that a large number of factors affect the intensity of the emission. These factors can be divided into several major categories which include fuel mixtures, burner design, methods of aspiration, solvent effects, and interferences. Many authors have discussed the influence of these variables on emission characteristics ( I , 7, 10). However, one of the major problems in flame spectrometry revolves around the disagreement among various workers in the field regarding experimental observations. Some workers find, for example, that the presence of sodium enhances potassium emission while others observe the opposite effect (Y, p. 276). The effect of anions on the emission of calcium is a well known phenomenon that has been studied by a large number of investigators. However, under certain circumstances the effect is not seen (3). One of the reasons for these discrepancies is the complexity of the over-all processes giving rise to excitation in flame spectrometry. Some of these steps are shown in the following scheme (1). Solution feed -+ atomization -., evaporation -., precipitation of salt N THE DETERMINATION
-
266
ANALYTICAL CHEMISTRY
decomposition of salt + formation of metal vapor 7excitation process + excited metal emi8sion (1)
-
The situation is further complicated by the fact that metallic salts decompose into a variety of species with competing equilibria such as
weighed against the reproducibility required in any particular situation. Even though the processes involved are complex, it should be realized that the basic equations regarding emission from a flame are relatively simple. If the metal is thermally excited, the Boltzmann distribution applies ; Line Intensity = .V*
Most of the above reactions are kinetically controlled since the material passes through the flame in a few milliseconds. Since the steps neither go to completion nor reach equilibrium, the observations will be greatly dependent on the particular eupsrimental conditions chosen. For example, if the emission near the top of a flame is monitored, the evaporation and decomposition steps are allowed more time to proceed, and the results obtained may well differ from those observed lower in the flame. The first step, the rate of solution feed, is easy to control and this should always be done in serious research in flame spectrometry. The dependence of solution feed on sample viscosity, gas flows (la),and burner encrustation gives rise to complications that are better avoided. In this work this parameter was controlled by force feed of sample solution using a motor-driven hypodermic syringe. In routine analysis, of course, this is an added experimental burden and its use has to be
3
n
L,
T2
L2
POWER SUPPLY AMPS
Figure 1.
(3)
This equation assumes the line shape does not change (or the qlit is wide enough to integrate the line) and self absorption is negligible. The two variables, teniperature a d ground state concentration, can be measured by line reversal techniques and atomic absorption, respectivelv. Since the variables involved can be measxired quantitatively, it was decided to apply such measurements to developing an understanding of the nature of flame spectrometry. One of the topics chosen for study \%asthe use of organic solvents instead of aqueous solutions to enhance emissivity of spectral lines and bands. This phenomenon has been studied by a number of investigators, particularly Dean (I). The actual mechanism of enhancement is not fully understood, and it was hoped that a quantitative approach would shed some light on the basic process. To determine quantitatively the role the solvent plays in the intensity of atomic emission, it is necessary to consider the physical and chemical ef-
LAMP
0-60
a
Line reversal apparatus
fects separately. The physical effect involves the rate of release of solid species into the flame and is dependent on droplet size, evaporation processes, crystallization of solids, etc. If the rate of evaporation of aqueous solutions is a slow process, the use of organic solvents might increase the amount of solid salts in the flame nith a resulting increase in ground state concentration and emission. The chemical effect of the organic solvent would show up as an increased temperature, which would also result in an enhanced emission. Measurement. of ground state concentrations and flame temperatures would aid in evaluating the relative effects of these tn-o factors.
7'5 B
D
E
C
F
EXPERIMENTAL
Flame Temperature Measurements. There are a variety of different procedures available for measuring flame temperatures (4). The sodium line reversal technique, although experimentally difficult-, is probably more widely used t'han any other. It assumes that the sodium atoms are in thermal equilibrium wit,h the flame gases and the choice of sodium arises from the fact that the pop(1.1ations of atomic levels of alkali metals seem most likely to assume a l of these reactive species would be higher in the H 2 : 0 , flame. There is a further difference in the two flames, however, that might offer some explanation for the larger concentration of tin atoms in the cooler flame. The H 2 : 0 2 flame is turbulent with no well defined reaction zone. I n the case of the Hz:air flame the reaction zone is clearly defined and it is possible that this well defined high energy region could give rise to the abnormally large concentration of tin vapor observed in this zone. Some further evidence arises from the fact that inert diluents such as argon and nitrogen decrease the burning velocity of hydrogen by a factor of four (4) so that the SnO molecules are in the reaction zone four times as long. The thickness of the zone is also increased by a factor of two or three so that the possibility of a species in the reaction zone reacting with the SnO molecules is eight to 12 times more Oz. There is no likely in H 2 :air as in Hz: direct evidence, homver, that this mechanism is actually an important factor in the increased tin concentration. Role of the Excited State in Tin Enhancement. =Is previously mentioned the excited state concentration of tin atonis is abnormally high as indicated by t h e high reversal temperatures. Obviously, a chemiluminescent process plays a n important, if not the dominant, role i n t h e enhancement of emission. The use of H2:air flames in conjunction with is+ propanol gives the strongest emission for all lines of tin. The effect of increasing the amount of isopropanol on the emission profile of the 2863 A. line is shown in Figure 18. The abnormally high excitation could result froin two different types of chemical reaction: SnO
+X
-+
+ XO
(7)
Sn* f YZ
(8)
Sn*
or Sn”
+ Y + 2 -+
energy to make the process energetically feasible. Even in this case, the available energy is minimal and the process becomes feasible only if it is presumed that the CO product is fornied in its lowest energy state. A more serious objection to this mechanism arises from our inability to detect the 2478 A. line of carbon vapor in the H2:air-isopropanol flames. This is an allowed transition and the emission should be sensitive to the presence of small amounts of this vapor which is detectable by the 2478 A. emission in flames using organic fuels such as acetylene (8). However, it is not believed that the evidence is strong enough to discard Gilbert’s proposed mechanism as a possible process. Reaction 8 in which the e x i t e d tin state is chemically produced from the ground state requires considerably less energy than reaction 7 since the bond energy of SnO (5.7 e.v.) must be included in 7 . The energy of the excitation process Sn + Sn* is variable since many lines are excited in the isopropanol-fed flames. The energies of the excited states for the observed lines range from 4.3 t o 6.4 e.v. above the ground state (6). The temperatures involved, however, are high enough t o populate the many low lying electronic states of tin so that the AE values necessary range from 4.3 to 5.8 e.v. Two possible reactions which could produce the requisite amount of energy are : Sn t C H 4 - O H - t Sn* Sn
+ CH + 0
ACKNOWLEDGMENT
The authors acknowledge the value of many stimulating discussions with Q Won Choi and Arthur Underwood in the development of this research. I n the early days of this program the complexity of the problem seemed insurmountable and without this stimulation, success might never have been achieved. The support of the Air Force Office of Scientific Research is gratefully acknowledged.
(9) LITERATURE CITED
-+
Sn*
+ HCO
(10)
Reaction 9 is thermodynaniically feasible and, in fact, has alinost 2.0 e.v. above the maximum required energy for the excitation process. Furthermore, both C H and OH radicals are present in relatively high concentrations in H a :air-isopropanol flames as evidenced by strong emission from both band systems. This excitation process also fills the requirement of involving a carbonaceous species since strong enhancement is only observed when organic solvents are used. The fact that a three-body collision i- necessary does not exclude the process from consideration since excitation is a rare event under any circumstance. For example the three-body collision : Pb
The first mechanism is the one proposed by Gilbert (6) in which “X” represents carbon vapor. If reaction 7 is the correct mechanism, carbon nionoside seems to be the only common species with sufficiently high bonding
+ CO + HP
to excitation, and concludes that the three-body excitation is perhaps the preferred mechanism. The question as t o whether the excitation of tin is a two- or three-body process could probably be answered by changing the operating pressure of the flame. However, the study of both high and low pressure flames is experimentally difficult. The process 10 s e e m less likely than 9 since the concentration of atomic oxygen is quite low in Hz:air flames ( 5 ) . The species HCO, however, is believed to exist in such flames ( 5 ) . These mechanisins which are proposed to account for the excitation of tin can only be considered speculative since there is no direct evidence that they are actually involved in the formation of the excited states. The elucidation of the actual mechanism must be postponed until better methods are developed for following such reactions. Research along these lines is presently being pursued.
+ H + OH
-+
HIO
+ Pb*
(11)
has been proposed to account for chemiluminescence of lead in flame reaction zones (9). Gaydon (4, p. 222) discusses the question of three-body us. two-body reactions as probable processes leading
(1) Dean, John A., “Flame Phfitometry,” McGraw-Hill, New York, 1960. (2) Dean, J. A , , Carnes, W. J., ASAL. CHEW34, 192 (1962). von, I.2 . PJEanzeneraehr. (3) Filcek, ?* Duong. Bodenk. 85, 115 (1959). (4) Gaydon, A. G., Wolfhard, H. .G., “Flames, Their ytructure, Radiation, and Temperature, Chapman and Hall, London, 1960. ( 5 ) Gaydon,,, A. G., “The Spectroscopy of Flames. ChaDman and Hall, London, 1957. (6) Gilbert, P. T., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1961. ( 7 ) Herrmann, R., Alkemade, C. T. J., “Flammenphotometrie,” Springer-verlag, Berlin, 1960. 18’1 Mavrodineanu. R.. Boiteux. H., “L’Analyse Spectrale ’Quantitative par la Flamme,” Masson, Paris, 1954. (9) Padley, P. J., Sugden, T. M., Proc. Rou. Soc., (London) Ser. A 248, 248 (1958). (10) Poluektov, N. S., Feigl Anniversary Symposium, Birmingham, England, 1962. (11) West, A. C., Ph.D. Thesis, Cornell University, Ithaca, N. Y., 1961. (12) Winefordner, J. D., Latz, H. W., ANAL.CHEM.33, 1727 (1961). \ - I
RECEIVEDfor review April 4, 1962. Accepted December 31, 1962. VOL. 35, NO. 3, MARCH 1963
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