V O L U M E 2 2 , NO. 4, A P R I L 1 9 5 0 aiuperometric inctlinds in the case of fresh citrus fruits is 0 to 1%. The storage orange shows a somewhat greater difference, which may be due to impurity which produces a wave a t the ascorbic acid potential but does not react with the dye. The lower polarographic value for the canned orange juice might be due to impurity which reacts with the dye but is not indicated by the polarograph. The indication of the presence of glutathione is most striking in the cases of the potato and guava. The polarographic and amperometric ascorbic acid results in the case of the potato are in fairly close agreement. In the case of the guava it was not possible to compare results by the two methods, because it xas impractical to measure the height of the ascorbic acid due to the presence of interfering material. This might be chloride if its concentration is about M or greater. I t could not be cysteine, because its first wave diffusion current should be 1 pa. or more before the second wave near the potential of the ascorbic acid begins to form. The authors' rcqults and a studv of the results of others indicate
529 that thv amount of glutathione in citrus fruits is usually low. The fact that polarogranis of fresh citrus fruit juices show such well developed ascorbic acid waves is due to a low content of glutathione and interfering materials. The polarograph, therefore, should prove a 'useful instrument in rapid routine determinations of ascorbic acid in fresh citrus fruits. LITERATURE CITED '1) Cozzi, D., Ann. chim. applicatn, 29, 434 (1939). :2) Gillam, W. S., IND.ENG. CHEM., ANAL.ED.,17, 217 (19453. (3) Kirk, M. M.,Ibid., 13, 625 (1941). (4) Kodicek, E., and Wenig, K., Nature, 1 4 2 , 3 5 (1938).
(5) Kolthoff, I. M., and Barnum, C.,
J. Am.
Chem. Sac., 63, 3061
(1940).
(6) Okada, Ikonosuke, J . A g r . Chem. SOC.Japan, 19, 749-58 (1943). (7) Osterud, Th., Tek. Ukeblad, 86, 216 (1939). IS) Ramsey, J. B., and Colichman, E. L., IND. ENG.CHEM., ANAL. ED.,14, 319 (1942). ,R) Schwarts, K., Z. anal. Chem., 115, 164 (1939). RECEIVEDAugust 22, 1949.
Measurement of Radiocarbon as Carbon Dioxide inside Geiger-Muller Counters M A X W E L L LEIGII EIDINOFF Queens College, Flushing, K. Y . , and Sloan-Kettering Institute for Cancer Research, New York, N . Y . The measurement of radiocarbon activity as carbon dioxide admixed with carbon disul6de vapor has been studied as a quantitative procedure over the pressure range 2 to 175 cm. of mercury. The counting rate was found to be directly proportional to the quantity of active gas sample. I n the region 1 to 4 cm. of carbon dioxide pressure the average deviation from the straight line drawn through the origin for eleven measurements was less than 1.29%. At a total pressure of about 50 cm. the average deviation of seven measurements from the straight line drawn for
M
ILLER and Brown (1, 6) have recently reported satisfac-
tory counting characteristics in the Geiger-Muller region a t carbon dioxide pressures from 10 to 50 em. of mercury admixed with 2-em. pressure of carbon disulfide vapor. Skipper, Bryan, White, and Hutchison ( 7 ) used a voltage supply permitting counting of samples a t pressures up to 35 em. of carbon dioxide. Inasmuch as this counting method is very efficient even for relatively large samples, it was decided to study this procedure as a quantitative method of analysis for radiocarbon over as wide a pressure range as can be conveniently handled. Measurements were made in the range 2 to 10 cm. of carbon dioxide pressure. In this pressure interval, the threshold voltage range is about 1450 to 2200 volts (8). An advantage of operation in this range is that routinely available voltage supplies can be utilized. Measurements were also made a t pressures extending up to 175 em. At this latter pressure the threshold voltage is close to 7500 volts. At pressures between roughly 0.5 and 2 atmospheres, the sensitivity factor with respect to even the windowless counting of barium carbonate becomes significant. Gas trains and auxiliary apparatus provide for the routine conversion of a barium carbonate sample to carbon dioxide and quantitative transfer of the precisely measured gas to the evacuated Geiger-Muller counter tube. Memory effects, reproducibility,
counting rate versus moles of standard active sample was 0.4%. Plateau lengths over 200 volts with slopes usually less than 2% per 100 volts are obtained over the threshold counting range of 1500 to about 8000 volts. The gas trains and apparatus for conversion of barium carbonate to carbon dioxide showed negligible memory effects and permitted precise measurement of the quantity of carbon dioxide inside the counter tube. The construction of GeigerMuller counter tubes and auxiliary equipment is descrihed.
and constancy of measured counting rate per unit specific activity have also been studied. EXPERIMEIYTAL
The gas lines and apparatus shown in Figure 1provide for preparation and storage of purified inactive carbon dioxide, preparation of diluted active gas reservoir bulbs, conversion of a barium carbonate sample taken for analysis to carbon dioxide and measurement of the quantity of gas obtained, and quantitative transfer of the evolved carbon dioxide to a Geiger-Muller counter tube containing a suitable pressure of carbon disulfide. Preparation and Storage of Inactive Carbon Dioxide. Sodium bicarbonate in the glass container, U ,is heated to about 350" C. hy the Nichrome heating element, A , after that section ot the line has been evacuated. The evolved carbon dioxide is stripped of water vapor in traps E and E' surrounded by 5 dry iceCellosolve (ethylene glycol monoethyl ether) mixture. After rejection of the initial portion of evolved gas, the carbon dioxide is allowed to fill the %liter gas bulb, R. Conversion of Barium Carbonate to Carbon Dioxide. With an empty container, K , in place, the conversion system is flushed with a carbon dioxidefree nitrogen stream from tank C (99.9% nitrogen), through the flowmeter capillary, B,the Ascarite-filled tube, G, and traps E, E', and F , and is vented at the mercury bubbler, D. Traps E and E' are immersed in a dry ice slurry
530
ANALYTICAL CHEMISTRY
B"
Figure 1. Gas Lines for C1402Counting D. G.
J.
M.
Mercury bubblers Ascarite-containing tubes Splash traps Small vacuum end manometer
M'. Long vacuum end manometer
S. Cold finger for vapor condensation using liquid nitrogen V. To high vacuum system (not shown) Y. Grease seal standard-taper joint I
while both traps labeled F are immersed in liquid nitrogen. After the small container or centrifuge tube holding the barium carbonate sample ( u s u a h 1.5 t'o 40 mg.) is inserted a t K , several milliliters of 20% perchrorlc acid solution contained in H are slowlv added to the sample holder. Perchloric acid solution was chosen because the barium salt is soluble. The evolved carbon dioxide is swept along by the nitrogen stream and condensed in F , while water vapor is removed a t E and E'. In order to condense residual carbon dioxide, stopcock 1 is closed and the gas stream is slon71y pumped out through stopcock 5 until the pressure is about 3 cm. Stopcock 4 is then closed and the nitrogen is completely removed from traps F (as checked by McLeod gage reading) by pumping for several minutes, The quantity of carbon dioxide is measured in the constantvolume mercury manometer, .$f'', after the carbon dioxide in F is transferred to the cold finger, S, next to stopcock 9. The transfer is accomplished in less than a minute by opening stopcocks 6 and 9 after S is surrounded by liquid nitrogen. In order t o minimize the transfer of traces of water vapor, trap F adjacent to stopcock 6 is immersed in a dry ice bath during the latter operation. The small volume (with stopcock 10 closed) is used when approximately 10 to 30 mg. of barium carbonate are converted. Tube W is made from 2-mm. capillary tubing. For samples up t o several hundred milligrams of barium carbonate the larger volume includes calibrated bulb T . The gas sample is then quantitatively transferred to the Geiger-Muller counter tube, X, by condensation inside S. Carbon disulfide vapor inside the bulb between stopcocks 7 and 8 is similarly transferred into X by condensatlon. The carbon disulfide liquid reservoir is container N . Two-liter bulbs P and Q contain active gas samples needed for study of gas counting as a quantitative procedure. The samples were prepared by conversion of active barium carbonate (as described above) and dilution with suitable quantity of inactive gas from R. The stopcocks, V , lead to the high vacuum pumping svsten~ consisting of a Welch 2-stage Duoseal pump, mercury diffusion pump, and trap surrounded by a dry ice mixture. '4ctive samples are disposed. of by condensation in a cold finger attached t o a flask contalnlng Ascarite (not shown in Figure 1). The inactive carbon dioxide used in these studies REAGENTS. was made by heating sodiuy bicarbonate (Eimer and A4mend, tested purity reagent) a t 350 c. High specific activity carbon dioxide nras prepared bp addition of 20% perchloric acid ( c . P . , Eimer and hmend, tested purity reagent) t o barium carbonatr containing carbon 14. Malllnckrodt carbon disulfide (analvtical reagent, 46-47' C . boiling point) was used without further purification. Design of Geiger-Muller Counter Tubes. The principal type of tube used in the carbon dioxide counting studies is shown in Figllre 2. The borosilicate glass tube has a "cold finger" exten-
sion, A , at the lower end to provide for condensation of tube contents when immersed in liquid nitrogen. The chemicallv plated silver cathode surface, C-D, is coated with a thin lajer ( 1 ) of colloidal graphite (Aquadag). The cathode lead enters a t F . The 4-mil tungsten anode wire, E , is surrounded by glass shields, B, which extend into the volume defined by the cathode. These shields eliminate spurious pulses a t the N-elded joint leading to the thick tungsten wire sealed through the glass and serve to define the counting volume. The tube is fitted by attachment at the ground-joint grease seal, H , and the precision-ground Stopcock, G
Figure 2. Geiger-Miiller Counter Tube for Gas Counting
illthough tubes constructed in this manner and having diameters of 10, 15.5, 32, and 55 mm. were used in some experiments the results presented in thi. paper were obtained using tubes that were approximatelv 15 5 nim in inside diameter and about 15 to 16 cm. in cathode length Two such tubes are referred to in this paper as tube 1 and tube 2 and have an effective volume of 29 and 32 ml., respectivelv. I n a lead housing of &cm. thickness, the background count is approximatelv 35 counts per minute. Electronic Equipment. The counting studies at Ion pressures were carried out using a scale of 64 circuit and a 2500-volt stabilized voltage supply. A modified Neher-Harper circuit containing a quenching tube and cathode follower was used. This circuit contained two 6AG5 tubes. Satisfactory quenching action and plateau lengths were obtained for all the counting tubes used in these studies by using a 6-megohm grid resistor and a grid bias voltage in the range 6 to 9 volts (extinguishing circuit 1). The counting studies a t carbon dioxide pressures over 10 cm. were carried out using a scale of 64 circuit. .4n electronicallv stabilized voltage supplv (Model 1090, Nuclear Instrument and Chemical Companv, Chicago, Ill.) v a s used in the range 2000 to 5000 volts. The high poqitive voltage was applied to the central wire anode. For roltages greater than 5000 and up to 8000, a suitable number of 67-volt dry cells in series vere used to augment the electronic qupplv Neher-Harper Extinguishing Circuit for Higher-Voltage Counting. The Seher-Harper circuit used for counting at voltages above 2000 volts differs from the conventional KeherHarper circuit in the specific vacuum tube used. Ordinarv radiotype tubes commonly used, such as 6AG5 or 57, are not designed for large plate-filament voltage differences. It was found that an 81 1 transmitter type tube gave very satisfactorv rewlts even
531
V O L U M E 22, NO. 4, A P R I L 1 9 5 0 hen plate-filament voltages up to 5000 were used. -I 150volt cathode bias supply was obtained from the scaling circuit. .I resistance bleeder was used to supply a negative grid bias for the 811 tube between t,he limits of approximately 15 and 60 volts. I t was observed that a grid resistance of 10 megohms aiid a grid bias close to 37 volts gave satisfactory counting characteristics for the 15.5-mm. diameter tubes over a wide rang
