For the Cd-Ag system the same ratio had a minimum of loll. This astronomical difference is difficult t o explain. Nonideality can explain the Cd-Zn system t o some extent. Scheller and Treadwell (20) have studied this system and find an a: exptl. of 9.5 for 25% (by weight) zinc alloy with a 72y0cadmium yield. This compares t o an cy exptl. of 1.2 in this paper. Phase transitions have an effect on this binary alloy. Nonideality and phase transitions could, however, hardly explain the differences observed for the Cd-Ag system. A suspected source of error is the carrying of the contaminant through splattering and the volatilization of polyatomic clusters.
The simplicity of the carrier-free separation by distillation makes the method very attractive to the nuclear scientist. In addition, i t approximates more than any other separation a truly carrier-free condition, since the vapor contacts only the collector during the separation. By contrast, stable element impurities in chemical reagents used for methods such as extraction and ion exchange contribute t o reducing t h e specific activity of radioisotopes separated by these processes. Thus the future of the carrierfree and trace distillation of metals as a radiochemical separation appears t o be a bright one.
SUMMARY
We express appreciation t o R. W. Shideler for designing and constructing the induction heater, Harold Nass for irradiating many of the metals in the Ford Nuclear Reactor, and to Elaine Sheperd for assisting in some of the experiments.
ACKNOWLEDGMENT
Although it has not resulted in the degree of separation hoped for, distillation separation compares favorably with other radiochemical separation techniques, particularly when distillation is combined with preliminary separations such as the autoreduction of mercury onto copper. The separation of cadmium was preceded by a n electrolysis step because this procedure results in many contaminants carrying with the cadmium and thus provides a good test for the distillation method. Thus for most contaminating elements the distillation technique gives a separation comparable t o the ion exchange and extraction separations for cadmium ( 5 ) .
LITERATURE CITED
(1) Alvarez, L., Helmholz, A., Xelson, E.. Phus. Rat. 57. 660 (1940). (2) Baker, P. S., Duncan, F. R., Green, H . B., Science 118, 778-80 (1953). (3) Relgaev, Yu I., Zaidel, A. I., J . Anal. Chem. U S S R (Eng. - trans.) 12, KO.1, 25 (1957). (~, 4 ) Chem. Bna. News 39. 50 (March 6. 1961). ( 5 ) DeT'oe, J. R., Meinke, W.IT., ASAL. CHEY.31, 1428-32 (1959). (6) Endebrock, R. ST., Engle, P. hI.,
1J. S. At. Energy Comm. Rept. AECD
4146 (1953).
(7) 1Frauenfelder, H., Helv. Phys. Acta 23, 347 (1950). (8) *lohnson. F. 8.. Can. J . Phus. 31. 1136 (1953). (9) Kidson, G. V., Elsdon, W. L., Intern. J . A p p l . Radiation Isotopes 3 , 252 f 19.58).
295-7 (
(11) Ibid,
(12) Kroll, W. J.,'Vacuum 1, 163 (1951). (13) Meinke, W. W., Nucleonics 17, Yo. 9. 86-9 -- (19.59). \___",. ~
(14j Merines, J., J . Phys. Rad. 17, 90. 3, 308 (1956). (15) Nesmeyanov, H., Intern. J . A p p l . &Eation Isotooes 4. 16-29 (1958). (16) Parker, W.;h'ud. Znstr. 5 , 142-7 (1959). (17) Parker, W.,DeCroes, &I., Sevier, K., Zbid., 7, 22-36 (1960). (18) Parker, W., Grunditz. Y., Ibid., 14, 71-5 (1961). (19) Rayleigh, Lord, Phil. Mag. 4, No. 23, 2 (1902). (20) Scheller, JT. von, Treadx-ell, W.D., Helv. Chim. Acta 35, No. 3, 745-53 (1952). (21) Sherwin, C. T., Rev. Sei. Instr. 22, 339-41 (1951). (22) Sunderman, D. N., Meinke, IT.W., Ax.4~.CHEM.29, 1578-89 (1957). (23) Ibid., 31, 40 (1959). (24) Thomas, J. F., Sanborn, E. K., hIukai. 11..Tebbens. B. D.. Zbid.. 30. 1954 (1958). (25) Weinstein, E. E., Pavlenko. L. I., Belyaev, Y. I., Intern. J . A p p l . Radiation Isotopes 2 , 196 (1957). RECEIVEDfor review September 20, 1962. Accepted October 30, 1962. Division of Analytical Chemistry, 136th Meeting, ACS, Atlantic City, K. J., September 1959. Work supported in part by the U. S. Atomic Energy Commission.
Determination of the Total Oxygen Content of Organic Materials by Fast Neutron Activation R. A. STALLWOOD, W. E. MOTT, and D. T. FANALE Gulf Research & Development Co., Pittsburgh 30, Pa.
b A rapid nondestructive method for the determination of oxygen in organic materials, which requires no chemical processing and is relatively free of interference, is based on the irradiation of approximately 6 grams of sample in a 14-m.e.v. neutron flux of -108 neutrons per sq. cm. per second, and the scintillation counting of the radiation from the 7.4-second N16 produced b y the O'6(n,p)N*5 reaction. Oxygen concentrations greater than 100 p.p.m. can b e routinely measured to within 10% in less than 10 minutes b y this method; a t 30 p.p.m. the relative standard deviation on a result is estimated to b e about 25%.
6
0
ANALYTICAL CHEMISTRY
A
LTHOUGH oxygen in elementary
or
combined form is one of the most commonly occurring constituents of organic materials and its direct determination has been the subject of extensive investigation, the development and application of rapid instrumental methods have not kept pace with existing requirements. Prior t o 1939, methods for direct determination of oxygen in organic compounds were based on either complete oxidation of the compound with measurement of the oxygen consumed or catalytic hydrogenation (6) t o form water. Both techniques were cumbersome. required complex apparatus, and were excessively tedious and time-
consuming. Neither could be considered amenable to routine application. In 1939, Schutze (7) proposed a semimicromethod in which the sample is thermally decomposed in a stream of nitrogen and the cracked products are passed over carbon a t about 1000° C . The resulting carbon monoxide is then oxidized at room temperature with iodine pentoxide, yielding carbon dioxide and iodine, either of which may be determined and used as a measure of oxygen content. Unterzaucher (9) adapted the method t o the microchemical scale by making various improvements in the apparatus. Modifications permitting the use of larger sample sizes mere made by Dinerstein and
8 88 MEV
7 I2 6 92 6 14
This paper describes the techniques developed a t our laboratory for routinely determining oxygen in petroleum products and related materials by the activation method. These techniques lead to much more accurate analyses than those reported recently by Veal and Cook (IO). Also, considerably more emphasis has been placed on the routine analysis of volatile liquids containing from 50 p.p.m. to 0.1% oxygen.
0 016
Figure 1 .
Decay scheme for
EXPERIMENTAL N16
Instrumentation. A diagram of the sample transfer system with detail sketches of a sample container in the irradiation and counting positions is shown in Figure 2. The system is operated with dry air a t 15 p.s.i. and is equipped with three timers and four solenoid valves that automatically time and control the irradiation, transfer, and counting sequence. Samples are transferred through the 12-foot long polyethylene tube in less than 0.3 second. A residual pressure is maintained in the system during the irradiation and counting periods to assure reproducible positioning of the sample bottles. The neutrons are produced by bombardment of a water-cooled tritiated titanium target with deuterons from a 130-kv. accelerator. LL'ith a new target the 14-m.e.v. neutron flux on the deuteron beam axis 0.55 inch from the target (center of sample) is approximately 2 X 108 neutrons per sq. cm. per second with a 250-pa. magnetically analyzed (20' deflection) beam of monoatomic deuterons. A NaI(T1) scintillation counter 5 inches in diameter and 5 inches thick having a well 1 inch in diameter and 3 l / 4 inches long is used to detect the radiation from the K16 produced in the sample. The output of the counter is fed through an amplifier to either a
discriminator-scaler or a multichannel analyzer, the latter being needed only to determine the optimum discriminator setting for a given matrix material, as is discussed more fully in the section on interferences. The neutron output of the accelerator is monitored with a BF, counter located in the shield wall. The monitor count is used to normalize the data to a fixed neutron flux, thereby compensating for fluctuations due to changing beam and target conditions. Sample Preparation. The samples, which to date have been predominantly rather volatile liquid petroleum products, are poured into l/'r-ounce polyethylene bottles (weighing about 1.5 grams) fitted with extended dropper tips for sealing and machined polyethylene driving caps (see Figure 2). The tips are purchased with the bottles from the Ern0 Products Co., I'hiladelphia, Pa. Sample bottles are weighed before and after filling and the n-eight of the sample (-6.5 grams) is determined to 0.01 gram. For liquid samples containing less than 0.17, osygen and for all solid samples the filling and sealing operations are carried out in an atmosphere of nitrogen or helium. Under normal operating conditions, no attempt is made to remove the dissolvrd oxygen present in liquid samples a t the time of receipt. Solid materials containing more than 0.5% oxygen are packaged in the manner shown in Figure 3, the void spaces in the container being filled with helium. Irradiation and Counting. Samples are p u t in the irradiation position by inserting them into a sample loader located near the counter end of the transfer tube and by pressing a button which opens solenoid valves 1 and 2 (Figure 2 ) . The irradiation, delay, and counting sequence are initiated by a manual switch which simultaneoucly starts the irradiation timer and t h e neutron monitor and directs the deuteron beam onto the tritium target by energizing a beam deflector located in
Klipp (3) in an effort t o minimize errors in the analysis of low oxygen content petroleum products. Oita (6) made further modifications in applying the technique to light hydrocarbons. The problems of sensitivity and volatility were overcome by using a magnetically controlled section of spiral quartz tubing as the sample container, permitting the use of as much as 5 grams of sample. Although methods based on thermal decomposition require somewhat simpler apparatus and are less subject t o interference than the complete oxidation and catalytic hydrogenation methods, time requirements of the order of 60 to 70 minutes per analysis make them equally unattractive for routine use. The answer t o the oxygen analysis problem now appears to be fast neutron activation analysis. In the determination of oxygen by activation with fast neutrons ( I , 2, 4 , 8 , I O ) , the sample t o be analyzed is irradiated with neutrons of sufficient energy to initiate the 0I6(n,p)Y6reaction(Q = -9.62 m.e.v.). The 7.4-second "6 activity induced in the sample is then measured and the oyygen content computed from the slope of a calibration curve prepared from a series of standards containing known amounts of oxygen. Samples and standards are prepared, irradiated, POLYETMYLENE DlOPPEll $.OZ m y ETHYLENE TRANSFER TUBE, and counted in exactly the same way; BOTTLE {'ID X 093'WL.\ )Ip all actiTities are normalized to a fixed nriitron flus and sample n eight. Fast neutrons for this work are most conveniently produced by bombarding a tritiated target with deuterons in a rr.latively low voltage accelerator, the yield of the H3(d,n)He4reaction being such that an adequate output ( ~ 1 0 ~ ~ TARGET neutrons per second) of 14-m.e.v. neuI 3 0 Kqv DEUTERO trons is obtained a t accelerating voltBEAM ages as low as 125 kv. Either ,%ray or y-ray counting techniques can be employed to measure the W6 activity (see Figure 1). Because of the short half life of X16,the irradiation time and the time a t which the irradiation stops, as well as the counting time and the titile a t lvhich the counting starts, need to be very carefully controlled. An automatic timing and sample transfer system is therefore necessary; if accurate, Figure 2. Irradiation, transfer, and reproducible results are to be obtained, determination
/
S'DIA X S'TK I ( o I I T I 1 CRYSTAL
f
STEEL SHIELD
IO'TK
TK EORAL PLATE
i
"
counting sysiem for
oxygen
VOL. 35, NO. 1, JANUARY 1963
7
the drift tube of the accelerator. After the preset irradiation time (20 seconds) , the beam is automatically deflected onto a water-cooled slit stopping the generation of neutrons, valves 1 and 2 are closed and 3 and 4 are opened, and the delay timer is started. The delay timer then turns on the counting equipment for a preset counting time (usually 18 seconds) 0.3 second after being actuated. Experience has shown that the heaviest samples, and consequently the slowest to transfer, reach the counting position within the delay period. After the counting data are recorded. the cycle is repeated until the desired total count is accumulated.
POLYETHYLENE TUBE
fir,
SbHPLE
Figure 3. Sample container for irradiation of solid materials
DISCUSSION
Sample Containers. One of the most important problems t o be solved before oxygen analysis of liquids and powders by neutron activation can be p u t on a routine baPis is t h a t of obtaining a moderately sized (-5- to 15-m1.), cheap, disposable sample container. Ideally, the container material should be relatively free of ouj-gen, fluorine, and other elements t h a t give rise t o reaction products with short half lives. Short of this, the concentration of contaminants in the material should not vary significantly from container to container. Following a rough surrey by the fast activation method of the oxygen and fluorine contents of a number of possible container materials. polyethylene was selected for further study. Samples were either machined in the form of solid cylinders jn-eighing about 6 grams)
Table I.
xion
or cut into small pieces and sealed in a bottle in a n atmosphere of helium. They were irradiated and counted as described above. Relative y-ray activities per gram measured above a discriminator level corresponding to 0.5 m.e.v. for several types of polyethylene are given in Table I; a polypropylene value is included for comparison purposes. These results led us to the use of the commercially available 1/4-ounce polyethylene bottle (costing $37 per thousand) for our standard sample container. Subsequent activation measurements have shown that the polyethylene in these bottles contains approximately 320 p.p.m. of oxygen; changes in oxygen content from bottle to bottle are too
Comparison of Activities Induced in Polyethylene and Polypropylene b y Fast Neutrons Relati1e
activity
?*raterial Conventional polyethj lene Linear polyethylene Marlex 5003 PvZedical grade polrethylene Polyethylene from coriiniercial 1 'p-o
