Cyanogen-Oxygen Flame New Source for Quantitative Determination of Microgram Amounts of Metals BERT L. VALLEE and ANTHONY F. BARTHOLOMAY Biophysics Research Laboratory, Department of Medicine, Harvard Medical School and Peter Bent Brigham Hospital, Boston, Mass.
studies, appeared to be potential sources for elements requiring higher excitation energies. Both of these flames have been shown to approach temperatures of 5000" K., considerably higher than temperatures currently obtainable with other flames. Such considerations led to an investigation of the usefulness of the cyanogen-oxygen flame as a spectroscopic source ( I , i4, 15).
The c j anogen-ox>gen flame has been successfully employed as a spectrochemical source for the quantitative determination of aluminum, barium, calcium, cobalt, chromium, copper, iron, lead, magnesium, manganese, nickel, and strontium. Absolute amounts varying from 0.36 to 36.0 y of metals may be determined at the present time. When adequate quantities of the gas can be produced, the cyanogen-oxygen flame holds great promise as a refinement and simplification of trace metal analysis. The toxicity of cyanogen has not presented any problems during the experimental work.
T
HE flame, compared to the arc and spark, is a stable source. The intensity of radiation emitted is constant with time, and the spectra excited are relatively simple. For some elements, only two or three lines are observed, thus making it easy to isolate emission lines or band heads for measurement from the continuum. However, high resolution increases the line-background ratio, improving both the sensitivity and precision and minimizing interference from other radiating species in the flame (7-9,16).
-Luminous Outer Cone
-Intensely
Table I.
Bright Inner Cone
Temperatures of Some Flames Temperature,
Fuel-Oxidant Mixture Hydrogen-air Hydrogen-oxygen Acetylene-air Acetylene-oxygen Methane-air Methane-oxygen Ethylene-air Propane-air Propane-oxygen JIanufactured gas-air Manufactured gas-oxygen C yanogen-oxygen
From Gaydon (4) 2100 2810 2250
..
2737
..
2776
..
4580
C. _ _
-Burner
From Mavrodineau and Boiteux (IO) 2115
2690 2050 3110
Figure 1. Schematic diagram of cyanogen-oxygen flame
1955
2720 1895 1925
lii0 2800
..
The present paper presents the first quantitative spectrochemical data obtained with this source. MATERIALS
The temperature of the flame is controlled largely by the energy of the reaction between fuel and oxidant, but it is markedly influenced by the further decomposition of the products of combustion and the heat capacity of the resultant fragments. Therefore, the temperature of the flame does not reflect all of the energy made available by the reaction. A portion of the initial energy is utilized to heat the fragments to the final flame temperature, which is thus a function of the fuel, oxidant, and sample employed ( 4 ) . Table I lists the temperatures in degrees centigrade for various fuel-oxidant mixtures. The temperature of most flames conventionally used in analysis range from 2000' to 3100" K. (4). They are adequate only to excite elements of low excitation potential, such as the alkali metals and alkaline earths. These elements may be excited sufficiently so that high analytical sensitivity results, even though the flame is relatively cool. The cyanogen-oxygen (IS) and hydrogenfluorine flames ( I I ) , which have been the recent subject of physical
Cyanogen was obtained from the Cyanamide Co. of America, New Products Division, New York, N. Y. A burner constructed in the authors' laboratory allows the combustion of cyanogen and oxygen and the introduction of the sample solution as a fine fog suspended in the oxygen stream. The oxygen is first passed through a nebulizer-an atomizer plus a trap-which allows for the return of large droplets. The sample fog and oxygen are premixed with equimolar amounts of cyanogen and flow out of the burner through a small hole, 1.1 mm. in diameter, a t the top of the burner, where the flame is formed. The flame, diagrammatically shown in Figure 1, is narrow with a very bright, bluishwhite inner cone and a blue outer cone. The flow rate of both cyanogen and oxygen is about 22 ml. per second. The known toxicity of cyanogen occasioned precautions. Though they have not proved neceesary thus far, such measures continue to be observed and are given here as background information. Cyanogen has a characteristic, pungent odor, distinctly noticeable a t concentrations far below its toxic levels in air ( 6 ) , which is one of the best indications of leakage of the gas into the surrounding air space. When equimolar quantities of oxygen and
1753
1754
ANALYTICAL CHEMISTRY Figure 2 shows working curves for the alkaline earth metals magnesium, calcium, strontium, and barium. Amounts of 3 p.p.m. of calcium, barium, and strontium and 10 p.p.m. of magnesium could be measured conveniently. The slopes of the curves are equal. Figure 3 similarly represents the parallel working curves obtained with manganese lines 4034.490, 4033.073, and 4030.755, and Figure 4 shows working curves for the chromium lines 4234.05 and 4274.803 and the aluminum line 3961.52. The limits of detection are apparent from the figures. The working curves for cobalt, iron, and nickel were similar to those sholyn in Figures 2 to 4. All working curves are linear over a wide range and have slopes close to 45". The sensitivity for molybdenum, tin, zinc, and cadmium was much poorer than that of the other elements analyzed. Unfortunately, the ultimate lines (raies ultimes) of these elements could not be measured because they lie outside of the range of the spectrograph which was employed. DISCUSSION
1
I
3
IO
30
100
CONCENTRATION IN P.P.M.
0.i6
I.'2
3.6
(2
-
AMOUNT OF METAL GOING THROUGH BURNER IN 4 MINUTES Figure 2. Working curves for strontium, calcium, barium, and magnesium
cyanogen interact, cyanogen cannot be detected among the resulting products, carbon monoxide and nitrogen. Because carbon monoxide, however, is odorless as well as toxic, it must be removed by exhaustion and adequate ventilation (1000 cubic feet per hour). All rubber tubing was secured with copper wire, and the connections were wrapped with filter paper impregnated with a 3% ethyl alcohol solution of gum guaiac and a 0.1% solution of copper sulfate. When these reagents come in contact with cyanogen, the Schoenbein-Pagenstecher (S) reaction takes place instantly, resulting in an intense blue color which ultimately turns brownish red. I n this manner slight leaks were detected instantly. Though never employed, a gas mask containing a canister for hydrogen cyanide gas (type GMK canister, Mine Safety Appliances Co., Pittsburgh, Pa.) rras kept in readiness a t all times. The intense emission of ultraviolet light by this flame required the use of goggles to avoid burns. Stock solutions (made with water as solvent) containing aluminum, barium, bismuth, cadmium, cobalt, chromium, copper, iron, lead, magnesium, manganese, molybdenum, nickel, tin, strontium, and zinc were made from Spec Pure metals or salts (Johnson Matthey Co., London, England) in concentrations of 10,000 p.p.m. Sample solutions of 0.33, 1.0, 3.3, 10, 30, 100, and 300 p.p.m. were prepared by serial transfers.
The reactions of cyanogen with air and oxygen and their kinetics have been studied for many years. I n 1914 Reis ( l a ) analyzed the products and the spectrum of this flame. On the basis of theoretical considerations, he suggested the flame temperature to be 4740' C. Following renewed interest in this source, Thomas, Gaydon, and Bren er ( I S ) measured the flame temperature experimentally by a determination of the vibrational inteneity distribution of the cyanogen bands. They found it to be 4800' =k 200' K. in stoichiometric cyanogen-oxygen flames. Conivay, Grosse, and Wilson i d ) measured the temperature of the cyanogen flame directly by means of the line reversal method, using the sun as a compaiison radiator, and found the temperature to be 4640" f 50' K. when cyanogen and oxygen were equimolar. Their calculated value, 4835' zk 50' K. is in good agreement with that of Thomas, Gaydon, and Brewer. .4n increase in pressure to 10 atm. raised the flame temperature to 5050" K. Calculation of the theoretical temperature was based upon the assumption that the reaction betnreen cyanogen and oxygen is
CPNz
+ 02
+
2CO
+
N 2
+E
INSTRUMENTATION
A 1.5-meter Wadsworth spectrograph (Jarrell-Ash Co., Boston, Mass.), with a grating of 15,000 lines per inch, and Eastman Kodak type 103-0 film were employed for this work. The slit width was 65 microns. The reciprocal linear dispersion was 10.8 A. per mm. in the first order. The arrangements of the camera, in conjunction with the sensitivity of the film, confined selection of lines for measurement to the region between 2800 and 5200 A. T h e exposure time was 4 minutes. The amount of sample consumed d u r h g this period was measured carefully to allow the calculation of the total amount of metal introduced into the burner per total exposure time. I n this manner the absolute sensitivity of the method could be determined: A total of 0.12 cc. of solution passed through the burner during the 4 minutes' exposure time. The film was developed for 5 minutes in D19 developer and fixed in a hypo bath for 20 minutes. No internal standard was employed. Densitometry was performed with an A plied Research Laboratories microdensitometer, the slit width o r which was 10 microns.
'-t-&n-
CONCENTRATION IN P.P.M.
I
1.2
I
3.6
I
c
12
AMOUNT OF METAL GOING THROUGH BURNER IN 4 MINUTES Figure 3.
Working curves for three manganese lines
V O L U M E 28, NO. 11, N O V E M B E R 1 9 5 6 A
10.0-
5.0-
-t m + ii 2.0z w
2 1.0-
3 w
p:
0.5-
CONCENTRATION IN P.P.M.
1.2
3.6
12
AMOUNT OF METAL GOING THROUGH BURNER IN 4 MINUTES Figure 4.
Working curves for two chromium lines and aluminum line
when equimolar quantities of both reactants are present. The high temperature of the flame is apparently due to the highly exothermic nature of the reaction, coupled with the fact that only a small fraction of its products dissociate. Theoretically, therefore, this flame should provide adequate energies to excite elements other than the alkali metals and the alkaline earths. The first data to be obtained on the excitation of spectra of elements in the cyanogen-oxygen flame have shown it to be a good spectroscopic source for the excitation of aluminum, barium, calcium, cobalt, chromium, copper, iron, magnesium, manganese, lead, and strontium (1, 14, 15). Interference in the spectra from the presence of cyanogen band heads, initially thought to be a major handicap, has been found to be of no consequence. The outer cone hardly emits cyanogen band spectra, and clear spectra devoid of cyanogen interference are readily observed. This is easily accomplished by proper alignment of the optics and the source in relation to the camera. There are no data on the relative sensitivities to be expected for different lines of the same element in the cyanogen flame, of course. Figure 3 is particularly pertinent in this regard. In the ‘WIT Wavelength Tables,” the manganese lines 4030.755, 4033.073, and 4034.490 are assigned relative arc intensities which compare as 5 to 4 to 2.5 ( 5 ) . The working curves obtained for these lines intercept the log intensity axis at 1.8, 1.3, and 1.0 (Figure 3). The intensity ratios of these lines in the cyanogenoxygen flame are 5 to 3.6 to 2.8, comparable to the intensities observed in the arc within the limits of experiment. Similarly, the “ M I T Wavelength Tables” list arc intensity ratios for the three chromium lines, 4254.346, 4274.803, and 4289.721, as 5 to 4 to 3; in the cyanogen flame analogous results were obtained for these lines. The energy of excitation available in the cyanogen flame presumably causes electronic excitation phenomena closely akin to those occurring in the direct current arc. The present data demonstrate the feasibility of quantitative spectrochemical analysis with this source. The data show good reproducibility on triplicate determinations, which is significant because an internal standard was not employed. The unit slope of the working curves is similar to that obtained for other sources under ideal conditions.
1755 Because supplies of cyanogen were severely curtailed and were not sufficient for extended experimentation, the investigation was confined to examination of the most critical features of the flame. These circumstances precluded extensive investigation, modifications, and improvements in burner design, further adjustments of the flow rate of the gases, the optimal characteristics of the sample, and its optimal rate of introduction into the flame. Similarly, extensive studies of reproducibility and precision could not be undertaken. The choice of an internal standard was contingent upon detailed knowledge of the spectra of any elements to be employed for this purpose and the volatilization of their salts in this flame. The amounts of cyanogen available were insufficient to carry out such examination. The data here presented show the analytical sensitivity obtainable with this flame in the quantitative identification of elements not generally subject to excitation with ordinary flame-. The use of the hydrogen-oxygen flame under otherwise identical conditions did not result in any detectable spectra for the transition elements. Not only is the cyanogen-oxygen flame highly sensitive, but the quantitative measurements, exemplified by the working curves, are excellent in view of the fact that no internal standards could be employed. The limitations imposed by the presently restricted availability of the gas will constitute but a temporary delay in the implementation of the desirable features of this analytical procedure. Further improvement should result from the use of photomultiplier detectors in place of the photographic plates. The cyanogen-oxygen flame is a valuable addition to the sources presently available for spectrochemistry. Extension of this work should develop a useful technique in the analysis of many metals in the microgram and millimicrogram range. ACKNOWLEDGMENT
The authors take pleasure in acknowledging the assistance and advice of Ralph E. Thiers. LITERATURE C I T E D
Baker, M. R., Vallee, B. L., J . Opt. SOC.Anier. 45, 773 (1955). Conway, J. B., Wilson, R. H., Jr., Grosse, A. V., J . Am. Chem. SOC.76, 499 (1953). Dennis, L. M., “Gas Analysis,” llacmillan, New York, 1913. Gaydon, A. G., Wolfhard, H. G., “Flames, Their Structure, Radiation and Temperature,” Chapman & Hall, London, 1953. Harrison, G. R., “hlIT Wavelength Tables,” Wiley, Yew York, 1952. Henderson, Y., Haggard, H. W.,“Noxious Gases and the Principles of Respiration Influencing Their Action,” Reinhold, Xew York, 1927. hIargoshes, M., Vallee, B. L., A N ~ LCHEM. . 28, 180 (1956). hlargoshes, XI., Vallee, B. L., “Direct Reading Flame Spectrometry. Principles and Instrumentation,” U. S. Department of Commerce, O5ce of Technical Services, P B 111743, 1956. hIargoshes, M., Vallee, B. L., “Flame Photometry and Spectrometry. Principles and Applications,” in “Methods of Biochemical Analysis,” vol. 111, David Glick, ed., Interscience, Kew York, 1966. hlavrodineau, R., Boiteux, H., “L’Analyse Spectrale Quantitative par la Flamme,” Masson, Paris, 1954. “Energy Transfer in Hot Gases,” p. 111, Natl. Bur. Standards (V. S.),Circ. 523, 1954. Reis, A, 2. Physik. 88, 515 (1914). Thomas, X., Gaydon, A. G., Brewer, L., J . Chem. Phys. 20, 369 (1952).
Vallee, B. L., “A Synopsis in the Instrumentation and Principles of Flame Spectrometry,” in “Methods of Trace Analysis,” John H. Yoe, ed., Wiley, Kew York, 1956. Vallee, B. L., Baker, h l . R., AKAL.CHEY.27, 320 (1955) (abstract). Vallee, B. L., hIargoshes, M., Ibid., 28, 175 (1956). RECEIVED for review April 9, 1956. Accepted June 25, 1956. This work has been supported b y a contract between the Office of Naval Research, Department of the Navy, and Harvard University, Contract NR 119-277, and by grants from the Howard Hughes Medical Institute and the Research Corp., New York, N. Y.
