High Frequency Induction Furnace in the Determination of Radiocarbon A. M. GAUDIN and HORACIO E. BERGNA Massachusetts Institute o f Technology, Cambridge, Mass.
.4ssay of materials labeled with carbon-14 commonly involves their oxidation to carbon dioxide, which is then measured directly by a Geiger counter or in a carbon dioxide ionization chamber. .4lternatively, the material may be converted to a solid, such as barium carbonate, before radioassay. The use of high frequency induction heating in an oxygen atmosphere provides a fast, reliable method of oxidizing radioactive carbon to carbon dioxide, which can then be determined in the usual way. The method has been successfully applied to the combustion of radiocarbon in mixtures of labeled lauric acid and quartz and in radioactive solutions. The radioactive carbon dioxide has been assayed with internal Geiger counters to prove the reliability of the combustion method.
carry a labeled organic agent. To induce heat in a nonconducting mineral, electrolytic irop powder was mixed with the sample, as suggested by Jackson ( 4 ) for the determination of organic carbon in soil. +Actually a mixture of iron and tin is used to lower the melting point of the iron. The sample should fuse completely during oxidation. Smith and Hockenyos (6) remark t h a t failure to obtain fusion results in incomplete removal of carbon as carbon dioxide in the determination of carbon in highmelting alloys.
_ _ _ _1
B
OTH dry and wet combustion techniques have been used in tracer work to oxidize carbon to carbon dioxide. Dry combustion, as in the Liebig and sodium peroxide fusion methods, and n e t combustion, as in the persulfate and Van SlykeFolch methods, are described in the literature. The use of a high frequency induction furnace provides a fast and convenient method of dry oxidation, securing a reproducible and reliable combustion. Induction heating has been used for the determination of carbon in high-melting alloys (6) as well as in low-carbon iron and steel (8). A rapid method of determining minute quantities of carbon 111 metals using the high frequency induction furnace has been described ( 7 ) . Recently, electrolytic iron powder has been used to indaice heat in a nonconductor such as soil ( 4 ) . I n this way, rapid and accurate determinations of carbon in soils are being made. I n the present investigation t h e induction furnace has been applied to the combustion of radiocarbon. Mixtures of labeled lauric acid and quartz and a radioactive solution were chosen t o illustrate the method. Using the gas counting technique described by Miller and Bro1Y.n (1, 6 ) adopted in this laboratory ( S ) , the reproducibility of the oxidation procedure 1%-asaecertained. The carbon dioxide evolved x a s purified in a leakproof train, using chemical absorbents for water and sulfur dioxide. special procedure was used for humid and liquid samples from which much water vapor was evolved.
L_ _ _ _ - - - _ _ - - - -
.I
FISHER INDUCTION C A R B O N APPARATUS
ATMOSPHERE
mF=*
e
e
e
RECOVERY AND PURIFICATION SYSTEM
T O VACUUM MANIFOLD AND MANO M ETRlC SYSTEM
Figure 1. Apparatus for burning sample and isolating active carbon dioxide Solenoid valve Magnesium perchlorate Caroxite Combustion tube a n d coil Manganese dioxide 6. Flowmeter 7. Platinum oxidizer 8. Liquid nitrogen trap 9. Liquid nitrogen a n d isohexane mixture a t
1. 2. 3.
4:
- 140° C.
If the powder containing the sample, iron, and tin is placed on and then covered by a layer of electrolytic iron, satisfactory conditions are obtained with the use of the Fisher furnace, which is credited by the manufacturers with reaching temperatures of 1600" C. Samples containing quartz and 3, 4, and 5 mg. of radioactive lauric acid were prepared in the following way.
APPARATUS
A Fisher induction carbon apparatus, originally designed for the determination of carbon and sulfur in steels, was used as a high frequency induction furnace. It is commercially available with a leakproof train containing chemical absorbents for water and sulfur dioxide. T h e outlet of the instrument was sealed to a vacuum svstem which has five liquid nitrogen traps. Three of them suffice to recover the carbon dioxide when the tvater content of the sample is low. When water is present, the two extra traps are used to distill the carbon dioxide from the water
SAMPLE
An Alundum boat was one-half filled x i t h granular Alundum and approximately 0.5 gram of powdered electrolytic iron was then spread evenly over the Alundum. . One gram of quartz was thoroughly mixed with the powdered radioactive lauric acid and approximately 0.5 gram of a 50-50 mixture of granular tin and electrolytic iron powder. This mixture was placed on the bed of electrolytic iron powder in the boat and covered with additional electrolytic iron and granular Alundum.
Because the method is intended primarily for tracer use in mineral engineering, quartz was chosen as a suitable mineral t o
As adsorbed coatings of radiocarbon-marked organic agents on minerals have been measured by depending on low-temperature
Figure 1 is a schematic drawing of the apparatus
467
468
ANALYTICAL CHEMISTRY
combustion methods (g), it is reasonable to expect that even more reproducible results are obtained a t the high temperature obtained in the induction furnace. Accordingly, special tests were not made to determine whether adsorbed coatings could be measured as readily as mixtures. Radioactive solution samples were prepared in the following way.
An Alundum boat was one-half filled with granular Alundum and approximately 0.5 gram of powdered electrolytic iron was spread evenly over the Alundum as in the preparation of solid samples. Next, 1 ml. of radioactive solution was evenly added dropwise to the bed of electrolytic iron powder in the boat, which was left overnight in a desiccator with anhydrous magnesium erchlorate. When the concentration of the solution 80 require$ enough solid inactive lauric acid was added to the boat to carry the isotopic carbon. A layer of granular Alundum was then addcd to the boat and the sample was ready for determination (see Figure 2).
carbon dioxide, provided it is not radioactive carbon dioxide, but the technique of measuring radioactivity with an internal Geiger counter includes a blank to determine the background. RESULTS
Results obtained determining carbon-I4 in mixtures of quartz and radioactive lauric acid are shown in Table I, which presents the data of 14 different determinations. For each of the three amounts of lauric acid, the standard deviation of a single obseivation is under 3.5%. This value is considered satisfactory for the kind of counters used.
Table I.
Results Obtained with Mixtures of Quartz and 3, 4, and 5 Mg. of Radioactive Lauric Acid Counting Rate, Counts per Minute per RIg. 3-Ma. 4-1hfe. 5-ME. aamde sampte sample 254 255 250 272 260 265 247 262 271 259 243 270
EXPERIMENTAL PROCEDURE
The boat and sleeve were pushed into position in the induction furnace. The apparatus was fired and the effluent gases passed through three traps cooled by liquid nitrogen and an extra safety trap cooled by liquid nitrogen and open to the air. The complete cycle lasted 2 minutes, after which the three traps where gases and water were frozen were isolated from the induction apparatus and the atmosphere by closing appropriate taps in the vacuum system. The oxygen was then pumped out. When the chemical absorbents were sufficient to prevent water contamination of the carbon dioxide, the latter was allowed to evaporate from the liquid nitrogen traps, and was then ready for measurement.
M I X T U R E OF O U A R T Z , POWDERED R A D I O A C T I V E L A U R I C A C I D AND 5 0 - 5 0 M I X T U R E OF G R A N U L A R T I N AND ELECTROLYTIC IRON POWDER
GRANULAR A L U N D U M
I
IRON
FOR S O L I D SAMPLE R A D I O A C T I V E SOLUTION SAMPLE ( A N D S O L I D GRANULAR ALUNDUM
I POWDERED ELECTROLYTIC IRON
FOR L l O U l D
Figure 2.
Average
250 257
255
259
An experiment in duplicate was also made using a dilute aqueous lauric acid solution instead of powdered solid agent. The solution contained 20 mg. per liter and 1.080 =k 0.003 ml. of this solution was used in each test. The total net activity of each sample was 494 =t10 counts per minute in both cases. The agreement is excellent. APPLICATIONS
P ~ W D E R E D ELECTROLYTIC
INACTIVE LAURIC ACID WHEN N E C E S S A R Y )
7.54
-I"
SAMPLE
Preparation of combustion boat
When water was not completely absorbed by the chemicals &B in the case of humid or liquid samples, it remained in part frozen with the carbon dioxide in the three traps. I n this case the contents of these traps were allov-ed to warm up and diffuse through a glass spiral a t - 140' & 2" C. (boiling solution of isohexane in liquid nitrogen) to another trap cooled with liquid nitrogen. These two traps were otherwise not used. The operation was performed under vacuum with the stopcocks to the pumps closed. Fifteen minutes sufficed for the operating volume of carbon dioxide to diffuse and be recovered entirely free of water in the end liquid nitrogen trap. Assay of the radioactive carbon dioxide has been made with internal Geiger counters using a technique already described (5). It is not important if all the ingredients in the boat give off
The high frequency induction furnace may be used for the oxidation of carbon-14 to carbon dioxide in the determination of labeled collectors for tracer work in mineral engineering. Minerals with the radioactive substance adsorbed, as well as humid samples or radioactive solutions, may be conveniently oxidized using this combustion method. An internal carbon dioxide counter or a carbon dioxide ionization chamber can be used for the final counting of radioactive carbon dioxide. Alternatively the carbon dioxide may be converted to a solid, such as barium carbonate, before radioassay. Samples other than those encountered in mineral engineering research work can probably be treated in the same way. It is believed that radioactive sulfur may be also determined using induction heating to convert the sulfur to sulfur dioxide, but experiments to support this opinion have yet to be made. ACKNOWLEDGMEXT
The authors wish to express their appreciation to the Atomic Energy Commission and to Armour & Go. for. providing the funds that made this research possible. LITERATURE CITED
(1) Brown, S. C., and Miller, W.W., Rev. Sci. Instr., 18, 496-600 (1947). (2) Gaudin, A. M., and de Bruyn, P. L., Trans. Can. Inst. Minin(r Met. Engrs. (Can. Mining Met. Bull.), 52, 148-54 (1949). (3) Gaudin, A. M.. de Bruyn, P. L.,Bloeoher, F. W., and Chang, C . S., Mining & Met., 29, 432-5 (1948). (4) Jackson, M. L., Soil.Sei. Proc., 16,370 (1952). (5) Miller, W. W., Science, 105, 123 (1947). (6) Smith, G. F., and Hockenyos, G. L., IND.ENG.CHEM.,ANAL. ED.,2, 38€(1930). (7) Stanley, J. K., and Yensen, T. D., Ibid., 17, 699-702 (1945). (8) Wooten, L. A . , and Guldner, W. G.. Ibid., 14, 835-8 (1942). RECEIVED for review May 28, 1954. Accepted November 1, 1954.
