Improvements in Carbon-14 Assay of Organic Compounds by Dry Combustion Masayuki Harnada and Eiko Kawano, Radiation Center of Osaka Prefecture, Sakai, Osaka, Japan
in the combustion M procedures and counting system of carbon-14-labeled compounds used in ODIFICATIOSS
this laboratcry afford certain advantages over the apparatus which was originally designed by Anderson, Delabarre, and Bothner-By ( I ) and Christman et al. ( 2 ) . Improvements are described chiefly with regard to a dry Combustion system which allows reduction of nitrogen oxides in a stream of nitrogen and to a low background counting system. One of the important problems during combustion of organic compounds is the removal of nitrogen oxides. This problem is magnified when the product of the combustion is desired for isotopic assay. In the procedure for isotopic compounds which has been developed by Anderson, Christman et al. ( I - 5 ) , the nitrogen oxides produced during the dry combustion of nitrogen-containing organic compounds are successfully removed from the gas stream by manganese dioxide. However, in this procedure specially prepared manganese dioxide is necessary and, in general, the combustion of nitrogen-containing compounds is more troublesome than that of nonnitrogen-containing compounds. Rlitsui, Yoshikawa, and Furuki (7) have developed a new method in which a sample for microdetermination of carbon and hydrogen in organic compounds is burned in a stream of nitrogen gas by adding a suitable amount of osygen generated electrolytically. Our combustion system for C14-labeled organic compounds was set up based on the above principle. This system not only gives complete removal of nitrogen oxides without any extra component such as pure manganese dioxide, but makes the operation simpler than previous systems. To reduce the background counting rate, metal (Kovar) cathode counting tubes were designed for use in a lead and iron shielded anticoincidence counting system. By this procedure, the background counting rate was reduced to about 4 c.p.ni.
I
1
I
COPPER ABSORGAUZE BENT
CATALYST
Figure 1 .
I
REDUCED COPPER GAUZE
I
I
BOAT
Fillings of the combustion tube
sorbent for halogen and sulfur, granules of metallic silver are used ( 6 ) . As a reducing agent, commercial 40mesh copper gauze is used after reduction with methanol. The combustion tube is packed from the end with absorbent, catalyst, and finally the reduced copper gauze roll. A diagram of a packed combustion tube is shown in Figure 1. Apparatus. The assembled apparatus is shown in Figure 2. Electrolytic Cell. An electrolytic cell was designed for the generation of oxygen gas. It has platinum wire (0.4 mm. in diameter) electrodes and the solution is a mixture of 30% hydrogen peroxide and a small amount of sulfuric acid. The cell is followed by tubes of heated copper oxide granules and Anhydrone to remove any trace of impurity. Trap and Manometer System. Except for the elimination of the manganese dioxide trap, gas collection and measurement system is just the same as the system designed by Christman et al. ( 2 ) . Counting Tube. Metal counters of a design similar to the ordinarv glass counter - were used to reduck the background counting rate. The cathode metal is Kovar and the volume is about 100 ml. Procedure. After the apparatus is assembled, the trap section is evacuated and then nitrogen gas is
passed through the heated combustion tube a t the rate of about 6 ml. per minute for 3 t o 4 hours. During this period, the temperatures of the two furnaces are adjusted with the stationary furnace a t 700' t o 800' C. and the auxiliary furnace a t 500' to 600" C., respectively. After cooling, the boat containing the weighed sample (2 t o 8 mg.) is inserted into the combustion tube and pushed to the central part of the automatic combustion furnace; then the combustion is started. In this procedure the combustion of the sample is carried out in two steps. I n the initial stage, the time required is 10 minutes and the heating temperature of the sample is gradually raised to 300' C. In the second and final stage, another 10-minute period, the temperature is raised to 800' C. After the furnace has been switched off, nitrogen gas is allowed to flow for an additional 10 minutes to sweep the combustion products into the condensing trap. During the combustion period, nitrogen gas is passed through the combustion tube and trapping system a t the rate of about 6 ml. per minute together with a certain amount of electrolytically generated oxygen as described below. The electrolytic cell is connected parallel to the same terminals of the power supply as the automatic furnace. The amount of oxygen generated during the initial stage is very small. When the combustion reaches the final stage,
-
n
EXPERIMENTAL
Reagents and Filling of t h e Combustion Tube. To complete the combustion in a stream of nitrogen, the combustion catalyst must have not only good catalytic properties but also a high oxygen-releasing ability. The catalyst used by the present authors is a mixture of cupric oxide, manganese dioxide, and cobaltic oxide which was recommended by Mitsui, Yoshikawa, and Furuki ( 7 ) . As the ab-
4-
HIGH VAC.
Figure 2. A. B. C.
D. E.
Assembled apparatus
Purification furnace O x y g e n gas generator Combustion furnace Automatic combustion furnace Stationary furnace
F. G. H. 1. 1.
Auxiliary furnace W a t e r trap Carbon dioxide trap Two-liquid manometer Counting tube
VOL. 38, NO. 7, JUNE 1966
943
Table 1.
Compound Benzoic acid
Sample, mg.
Specimen Analyses
Theor.
Found
68.84 4.775 5.188 5.510 4.036 4.605 5.315 6.001 5.482
Acetanilide
c, %
69.35 68.35 68.90 69.28 69.12 68.92 68.73 68.85 71.09
4.285 4.235 4.135 3.985 4.610 4.235 4.440
71.36 71.38 71.50 71.04 71.39 70.75 71.38
Theor.
H, %
Found
4.95 4.61 4.78 4.49 4.90 4.84 4.42 4.76 4.91 6.71
0
6.69 6.97 7.12 6.50 6.71 6.38 6.60
2
4
6
B
Figure 3. Temperature gradient of the automatic furnace and rate of generation of oxygen from electrolytic cell 1.
II.
oxygen is generated at the rate of 0.7 ml. per minute and this amount is sufficient for the combustion (7'). Figure 3 shows both the temperature gradient of the automatic furnace and the rate of generation of the oxygen. During this operation, water produced is collected in the first trap (G), cooled with a dry ice-acetone mixture, and carbon dioxide is collected in the second trap ( H ) , cooled with liquid nitrogen. At the end of the combustion, stopcock (8) is closed, and the trap and measurement system is evacuated to a high vacuum; then the amounts of carbon dioxide and
water are measured by the two-liquid manometer described by Anderson et al. (I,@. Isotopic Assay. T h e total amount, or any desired portion, of carbon dioxide in the trap is introduced into a counting tube directly, and then P-10 gas (a proportional filling gas, 90% argon-10% methane) is added through the P-10 measuring bottle so as t o make the inner pressure of the tube 1 atmosphere ( 2 ) . Then the counting tube is set aside for a few hours to complete mixing of the gases. I n our counting system, the counting tube is surrounded by 24 G.-M. tubes
Table II.
Sample Benzoic acid
Calcd., d.p.m./mg. 7070" 1416b
Benzamide
Phenanthrene &Ethyl-2-propylacetanilide
2-Hexen-1-01-5-C'~ p-Aminobenzoic acid p-Bromophenyl benzoate-l-C14
inside a 5-cm. thick lead and a 5-cm. thick iron shield, and anticoincidence counting circuits are employed. RESULTS AND DISCUSSION
A list of analyses of carbon and hydrogen of specimens with this combustion method is shown in Table I. The oxidized zone of the copper gauze extends gradually toward the exit end after repeated combustion, but a reduced copper gauze still remains
c.p.m. 26595 27 169 6670 7766 7209 8357 6486 7054 7584 7264 2035 2294 44957 43813 48735 69178 57035 78966 7358 4026 364.3 456.6 14090 11817
Found d.p.m. 3 1642 32325 7936 9240 9690 7640 8577 9958 7727 8404 9035 8654 2421 2727 53488 52127 58059 82306 67947 93951 8765 4797 433.4 544.0 16786 14059
The value of the standard benzoic acid purchased from New England Nuclear Assay Corp.
* The calculated value of benzoic acid which was obtained by the dilution of above standard. 944
ANALYTICAL CHEMISTRY
Initial stage Final stage
Assay of C14-LabeledCompounds
Sample, mg. 4.629 4.775 5.768 6.624 7.146 5.510 5.298 6.354 4.895 5.254 5.636 5.430 6.220 6.972 2,914 2.932 3.302 2,432 1.975 2.804 3.902 2.082 4.170 5.197 6.064 4.996
IO 12 14 16 18 20
TIME (min.)
d.p.m./mg. 6836 6780 1370 1395 1356 1387 1619 1567 1579 1600 1603 1594 389 391 18358 17785 17583 33815 34404 33506 2246 2303 103.9 104.7 2768 2814
Average 6808
1379
1590 390 17909 33908 2275 104.3 2791
usable after 20 combustions or more. When it is exhausted, it can be reactivated by reducing with methanol. The results of the assay of various C14-labeled compounds are shown in Table 11. By our counting system the background counting rate can be reduced to ahout 4 c.p.m. With a normal sample which contains no more than 5% carbon dioxide in the counting tube, the plateau runs from 2000 to 2300 volts and there is no appreciable slope to the plateau or shifting of the plateau to higher voltages. Table I1 shows that nitrogen-containing compounds yield satisfactory results by the same operation as non-
nitrogen-containing compounds. The results with halogenated or liquid compounds are also satisfactory. After the measurement of relatively highly active comPoUndS, the background rate of the counting tube is as low as 5 c.p.m. after evacuation for a few hours. But it is desirable to heat to 120’ C. during evacuation after such measurements. ACKNOWLEDGMENT
The authors thank David R. Christman, Brookhaven National Laboratory, and Tetsuo llitsui, Kyoto University, for their kind help and valuable advice. The authors are
indebted to Tadashi Kawano for constructing glassware. LITERATURE CITED
( 1 ) Anderson, R. C., Delabarre, Yvette,
Bothner-By, A. A., ANAL.CHEM.24,
1298 (1952). (2) Christm% D. R., Day, N. E., Hansell, P. R., Anderson, R. C., Zbid., 27, 1935 (1955). (3) Christman, D. R., Paul, C. M., Zbid., 32, 131 (1960). (4) Christman, D. R., Stuber, J. L., Bothner-By, A. A., Ibid., 28, 1345 (1956). (5) Christman, D. R., Wolf, A. P., Zbid., 27, 1939 (1955). (6) Alitsui, T., Yamamoto, 0.7 Yoshikawa, K., Mikrochim. A c t a 1962, p. 521. (7) Mitsui, T., Yoshikawa, K., Furuki, C., Zbid., p. 385.
Indium Capsules for Microdetermination of Carbon and Hydrogen G. E. Secor and L. M. White, Western Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture, Albany, Calif. 94710
have been used to Iliquids contain weighed samples of volatile for analysis by mass specNDIUM CAPSULES
trometry ( 1 ) and gas chromatography ( 2 , 3). Indium encapsulation should also have many advantages over existing techniques for handling volatile, unstable, and hygroscopic samples for carbon and hydrogen analysis. T o test the suitability of indium capsules for use in this determination, a large variety of solid and liquid samples encapsulated in indium were analyzed for carbon and hydrogen by a conventionzl Pregl-type micro method. The results were as good as those with platinum boats or glass capillary tubes. I n some determinations, a voluminous residue formed when solid samples containing sulfur or halogens were combusted in the capsules. This did not occur with volatile liquid samples. EXPERIMENTAL
The capsules were made from 2-mm. i.d. X 3-mm. 0.d. (0.080-inch i.d. X 0.120-inch o.d.), 99.9901, pure indium tubing (The Indium Corp. of America, Utica, Y. Y.), as received, cut into 15-mm. lengths. The ends of the tubes were sealed by cold-welding with a swaging tool (Wilkens Instrument and Research, Inc., Walnut Creek, Calif.) or with smooth, flat-tipped forceps. Because indium is a soft metal, care was taken to make sturdy welds without frangible ends; and the capsules were handled carefully between weighings to avoid loss of weighable amounts of indium through contact kvith other objects. When solid samples or additives were added to the capsules, precautions were taken t o leave the weld area clean if it was essential to have an air-tight seal. When a n
additive was needed for the combustion, it was added t o the sample inside the capsule, and the capsule was shaken vigorously after sealing. The capsule, containing 3 to 7 mg. of sample, was inserted into the combustion tube in a Coors No. 110, size 4/0 porcelain combustion boat. The combustion train and general procedure for the microdetermination of carbon and hydrogen with indium capsules were the same as those used routinely in this laboratory with platinum boats or glass capillaries. After the combustion was completed, the indium was removed partially from the porcelain boat by “peeling” and final traces were removed by boiling in 1: 1 nitric acid. Molten indium wets glass and adheres to it on cooling. If indium Table 1.
should contaminate the walls of the combustion tube, the metal can be oxidized by burning an organic sulfur compound in an open boat in the combustion tube and letting the vapor pass over the hot indium. The residue formed is removed with a moist cotton swab. RESULTS AND DISCUSSION
Indium resists corrosion, has a low melting point (156.4’ C.), and is available commercially in the form of highly pure (99.99yo) tubing in a wide range of sizes. It self-welds at room temperature under moderate pressure. The capsules used in this study cost about 8 cents each and weigh ap-
Analytical Results for Indium Encapsulated Compounds
Carbon, % Theoretical Founda Acetone Methanol 2-Chloroethanol Ethyl acetate Di-n-propyl sulfite Benzene Bromobenzene Iodobenzene Toluene Pvridine Bknzaldehy de Vanillin Urea
2,PDinitrophenylhydrazine
62,04 37.49 29.84 54.53 43.35 92.26 45.90 35.32 z1.25 45.92 79.23 63.15 20.00 36,37 47.40 32.76 41.82 60.01 47,05 20.88 66.26
S-benzylthiuronium chloride 2-Iodophenol PBromobenzoic acid 4Fluorobenzoic acid Potassium acid phthalate* Sodium tartrate .2HzO* Triphenyl phosphate* Av. of ttr-o or three replicates. * Combusted with WOa inside of capsule.
61.75 37.63 29.99 54,66 43.47 92.14 46.23 35,39 91.37 75.87 79.04 63.31 20.12 36.17 47.47 33.02 42.11 60.02 47.12 21.04 66.25
Hydrogen, % Theoretical Found0 10.41 12.58 6.26 9.15 8.49 7.74 3.21 2.47 8.75 6.37 5.70 5.30 6.71 3.05 5... 4-.7
2.29 2.51 3.60 2.47 3.51 4.63
10.47 12.54 6.35 9.26 8.40 7.81 3.41 2.66 8.79 6.50 5.89 5.37 6.72 3.01 5.45 2.46 2.58 3.67 2.56 3.51 4.69
VOL. 38, NO. 7, JUNE 1966
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