V O L U M E 2 2 , NO. 2, F E B R U A R Y 1 9 5 0 LITERATURE CITED
.Ilexander, L., Murray, T. M . , and dshley, S.E. Q., ISD. Ex+. CHEM., h . i L . ED., 19, 4 1 i - 2 2 (1947). (2) Chipman, J., and Fontana, M . G., Ibid., 7, 391-5 (1935). ( 3 ) Derge, G., Am. Inst. Mech. Engrs., Tech. Pub. 1544 (1943). (4) Derge. G., Piefer, IT.,and Richards, J., Ibid.. 2362 ( 1 9 4 8 ) . (5) Guldiier, W.G,, and Beach, A. L., ANAL.CHEY.,22, 366 (1950). (6) Kroll, IT. J., ailti S r h l e r h t o ~ iA, . IT,, Trans. Elecfrochem. Soc., 93,
(I
)
24i-58 (1948).
303 ( 7 ) Nauehtoil. ,J. .J.. and 15, T50-1 (1943).
Uhlie. H . H . . 1 ~ 1 )l . : ~ ( ;( .' H E M . , Asar . l : ~ . , I
(8) Phragmen. G., and Treje. 11.. .Ifi)ikor,tc.rets -Ann., 124, 511-:31 (1940).
(9) Taylor, Lyiilan, "Metals Handbook." g g . 408-11, ('levelantl, .Imerican Society for Metals, 1948. RECEIVED September 26, 1949. Presented before tlir Divhion of Industrial a n d Engineering Chemistry, Titaniiim Symposium, a t the 116th l l w t i n r of the AJIERICAB CHEXICAL SOCIETY, Atlantic City. S. J. Other papri. in the symposium were published in the Fehriiary I ~ M Iissue of Indusir.i'a1 and Engineering Chemistry.
Quantitative Techniques with Carbon 14 F ILLIAM H. B E I M E K
A N D GEORGE J . ATCHISON The Dow Chemicul Company, Midland, ,l.lic-h.
Experimental methods are described w-hich ha\e been deFeloped for use w i t h small amounts of carbon 14 labeled compounds of high specific acti\it?. Techniques are presented for establishing the relationship of weight to radioacti\ it? using 3 to 15 micrograms of the laheled compound, preparing carbonate samples for counting, and using infrared spectrophotometr? for the deterniination of chemical puritj of carhon 14 compounds.
M
UCH of the ~ ~ w w ~kvith c l iradioisotopcs as trarers has had as its aim qualitativrs information concerning t h e fate of labeled atoms in chemiral 01' biological systems. T h r extrenw sensitivity of thc method of measurement makes it possit)lcb to utilize radioisotopes also as a quantitative analytical tool. ("art)on 1-4 is being used tiy a nunitwr of lahoi~atoric~s as a mt~thotl 01' :tnalysis for very small amounts of latirlrd conipouritt~iii the products of complicatd rtlactioiis. I n ordcx~,to carry out quantitative measurements it is necessai'~~ to estahlisli the relationship twtiveen weight of the labeled material arid its radioactivity. The Iwst method of tlrtc,rmining this relationship depends upon the proceduw for counting and the compound t o be assayed. Because of thr, self-absorption 01' the lo\v-energy t)rta-ra>, of carbon 11 it has h w n the practice to count all samples as a rc.producii)lc solid, such as barium carhonate, or in the gas phase as carhon dioxide. If the labeled material to be assayed is a uniform solid, a small amount can be weighed to the desired accuracJ- and conve1,ted to the counting cornpound and its radioactivity can be measured in the same way in which the product materials will tie assayed. Greater tiifficulties are encountered when the lahelrd compound is of high specific activity, when evrn a few milligrams can wpresent 10 to 100 microcuries. It is not advisahle to handle samples of this activity in the same apparatus that will be usc~llater for analytical samples containing activity on the order of background. Gaseous or volatile ronipounds are difficult to sample tiy direct weighing. APPARATUS FOR ALIOUOTING 3lICROGR.AM SAMPLES O F VAPOR
An apparatus is described here foi, sampling gaseous or volatile compounds of high specific activity, using a closed glass system which facilitates handling without exposure of the material to the atmosphere.
By use' of a vacuum manifold a kiiovm small volume of the gas or vapor is isolated a t low pressure, distilled into a glass ampoule, and sealrd Lvith a fiamc.. The ampoule can then be introduced into a combustion tube and the gas burned to carbon dioxide. By using an all-glass vacuum system, which can he flamed bet ween exposures t o the radioactive matei,ial, arid a small volume (this was 0.1187 nil.) it is possihle to measure out amounts of the compound on the ordrr 3 to 15 micrograms with an accuracy of *lto2C,. EXPERI31ENTAL
The apparatus is diagramed in Figure 1.
The lal~rledcompound is put on t h(L vacuuni manifold at it in :I glass ampoule with a break-seal and gwund joint. The system is pumped thoroughly and outgassed with a Bunsen flame. The mercury is allowed to rise in the cutoff, ,If,which is made of 1.25cm. (0.5-inch) diameter barometer tubing and stopcock D is closed. The seal is broken on the sample anipoule with the steel projectile arid the active compound is distilled into B with liquid nitrogen. The mercurj. in X is lowered arid the pressure is react on the thermocouple gage, 5°C. Any noncondensable gases can he pumped away while holding the sample a t liquid nitrogeii temperature iri R. The glass system to the right of IIf is agaiii flamed and pumped. Then with 1) closed the liquid nitrogen at B is replaced by a hath a t a temperature t o give a vapor pressurtl within the range of the manomrter, .lf. The mercury is raisrd iii Jf and the sample is again frozrri w i t h liquitl nitrogen. TO DIFFUSION PUMP
Figure 1. Diagram of .Apparatus Thc mercury is raised in tube E to a p o i l i t ,just a frttctioii of a millimeter below the opening of the known volume, V , The p1ecsure to the left of JI is the vapor pressure of the sample at liquitl nitrogen temperature and the difference in the height (11' th(. mercury on the two sides of Jl is measured tiy a catlietomcltc*i after temperature equilibrium is establishcd. The temperature, is measured with an external thermometer. The me)I'ciii'y is raised in E until the level is well above the opcaning of i7, Thi. isolates the volume of gas T' at the measwed temperatui,cy and pessure. Jf is opened and the gas in thr sampling frozen into B again, keeping J' isolated with the mercu ThP system is again flamed and pumped until the ther gage shows its ultimate pressure reading, indirating a pressure loiver than 0.5 micron. The mercury is again raised in J f and then communication between volume T 7 and the system to the right of .lf is made by lowering tho mewury column in E . The gas trom I- is distilled into one of the ampoules, F , n.ith liquid nitrogen and the system is flamed. Th(, thrrmocouplr gage indicates
304 T a h h I.
Aliqnot
ANALYTICAL CHEMISTRY S a m p l i n g High Activity C a r b o n T e t r a c h l o r i d e Pressure
Mm.
Cnled.
-
Temp.
c.
Wt. of Alrwot 7
24.4 12.34 CClr 1-1 10.15 1-2 10.47 25.5 12.69 1.3 10.57 26.7 12.80 CCli 2-1 9.01 25.2 10.93 2-2 9.01 26.7 10.91 2-3 9.15 25.7 11.08 CCI, 3-1 9.58 30.3 11.43 3-2 12.50 30.1 14.91 3-3 8.96 30.5 10.44 CCli 4-1 9.25 26.4 11.18 4-2 11.40 26.2 13.78 4-3 9.37 26.2 11.32 COz 4-1 11.63 25.0 4.03 4-2 12.10 25.0 4.20 4-3 10.30 25.0 3.57 " C ~ C O used J as counting d i d for CCI,
Wt. of
BaCOa
Gmm 0.3455 0.1532 0.1450 0.1493
Sample Carbon in 1 G. BhCOa Count*/
0,1528
0.1476
0.1597 0.1533 0.1486
0.10Q4° 0,1096 0.1152
0.1133 0.1142 0.1152
Deviation from Averaxe
mi%./-+
41, ..
521.8 530.6 514.9 341.0 336.0 340.2 538.2 529.6 546.6 333.1" 335.4 341.1 354.2 348.2 370.3
-0.1 +l.5 -1.5 +O.6 -0.9 +0.3 0.0 -1.6 +1.4 -1.0 -0.3 +1.4
-0.9 -2.6 +3.6
4 and COI 4
when all the gas has been driven into the ampoule, which is then sealed off from the system. Repetition of this procedure Jlows as as many samples to be taken as desired. The same technique is used to meter out samples of known weight for use in tracer experiments.
present and a. fractionation may occur during the oomhustion which will produce erroneous counting analyses. Several attempts have been made to obtain an accurate ratio of weight to counts per minute on Oak Ridge barium carbonate in which 5.9% of the carbon is radioactive. The results are not reproducible, presumnbly because of nonuniformity of the barium carbonate.
barium carbonate is 2 mg. per 100 ml. of mater a t 18 '. An ali
