Determination of Microgram and Submicrogram Quantities of Uranium by Neutron Activation Analysis H. A. MAHLMAN and G. W. LEDDICOTTE O a k Ridge National Laboratory, O a k Ridge, Tenn.
from uianium-234 IS negligible A hen short-time irradiations are used because the half-life of uranium-235 is of the order of 9 X 108 years, and the percentage abundance of the uranium-234 isotope is very low. Smales ( 7 ) and Seyfang and Smales (6) have used Reaction 2 to determine total uranium in minerals and uranium-235 in uranium mixtures. The uranium content of the samples can be determined by measuring the amount of barium-140 radioactivity produced in a predetermined time of irradiation relative to a comparative standard. Barium-140 ( t l i 2 = 12.8 days) emits both 8 and y radiation. A radiochemical separation of the barium-140 is usually made from a solution of the irradiated material. Reaction 3 was used in the work reported herein. Either the uranium-239 activity] or the neptunium-239 daughter activity] or the plutonium-239 daughter activity produced in this reaction is proportional to the amount of uranium present in the sample. The short half-life of uranium-239 (23.5 minutes) necessitates a rapid radiochemical separation. However, rapidity is not always conducive to an efficient removal of the contaminant radioactivities produced from other elements that may be present in the sample. On the other hand, it would take considerable time to obtain sufficient plutonium-239 a activity from the decay of the plutonium-239 for good sensitivity. Thus, neptunium-239 (f1/2 = 2.33 days) was considered as the best radionuclide to use in a radiochemical separation procedure. Determination of Neptunium-239 Radioactivity. As neptunium-239 decays, it emits both 8 and y radiations. The radionuclide has a spectrum of 7 y ($), the energies of which are in the range of 0.057 to 0.50 mev. Radiations of such energy can be conveniently detected by means of a y scintillation counter having a sodium iodide crystal (thallium activated) (1). The ,3 radiations can be detected by means of a Geiger-Muller counter. Comparative Method of Analysis. The amount of iieptunium-239 measured by gamma counting is directly proportional t o the initial amount of uranium-238 in the sample. If the sample and a comparative standard [a known weight of uranium oxide ( V 3 0 , ) ] are irradiated siniulmneously, processed chemically, and counted under similar conditions, then the amount of uranium in a sample can be calculated as follows:
Microgram and submicrogram quantities of uranium have been determined in synthetic samples, ores, and soils by neutron radioactivation analysis. The principles of the activation analysis method used in this determination and the processing of irradiated samples are discussed. This method of analysis is a sensitive and specific method for determining uranium in concentrations as small as 0.1 y per gram with a probable relative standard error of 10%. Concentrations of uranium in quantities as small as 0.0001 y per gram can be determined by neutron activation analysis.
T
H E rapid development of nuclear science in recent years has brought into existence a new method of analysis for the assay of trace elements contained in many different materials. Known as radioactivation analysis, the method is based on the measurement of nuclear radiations, either a,8, or y, from radioactive isotopes that are induced artificially in the stable isotope(s) of an element by interactions with the nuclear particles (such as neutrons and deuterons) produced either in a chain-reacting pile or in a charged particle accelerator. Radioactivation analrsis is extremely sensitive for most elements. It is also specific, because each induced radionuclide has its own particular decay constant and type( s) of radiation, neither of which is exactly duplicated in any other radionuclide. Contamination, as experienced in most conventional methods of analysis, is negligible, because the contaminants must be present before nuclear irradiation in order t o undergo nuclear reaction. The fundamentals of radioactivation analysis have been discussed by Boyd ( 2 ) and by Taylor and Havens (8). More recently, Leddicotte and Remolds (3, 4)have reported on the use of the Oak Ridge Kational Laboratory (ORNL) graphite reactor to determine submicrogram and microgram amounts of many elements by neutron activation analysis. (.4n analytical service by radioactivation analvsis is nom a part of the Oak Ridge Xational Laboratory program). A t least 70 elements can be determined in a varietv of materials a i t h eensitivities of detection ranging from 0.00001 to 1 y by the neutron activation analysis method. Xeutron radioactivation analysis a t O R S L has been used to determine microgram and submicrogram concentrations of uranium in synthetic samples, ores, and soils.
Weight of uranium in sample = Corrected count of Np-239 corrected count of Sp-239 per gram of uranium in comparator
DETERMINATION OF URANIUM BY NEUTRON RADIOACTIVATION ANALYSIS
where the corrected counts include corrections for counter background and the decay of neptunium-239 radioactivity in Goth sample and comparative standard. Because the amount of neptunium-239 formed is dependent upon the concentration of uranium-238 present in the irradiated sample, it can be readily seen that any alteration in the natural ratio of the uranium isotopes will cause bias results. The bias is limited on uranium-235 depleted uranium but may be serious xhere the opposite situation is present. The necessity of monitoring the flux of neutrons, xhich is usually a difficult quantity to measure or control exactly, is eliminated by the use of the comparative sample. Neutron Source. The ORKL graphite reactor was used as the neutron source for all the work described herein. The necltrvn flux of this reactor is about 10'2 neutrons per sq. cm. per second.
Nuclear Data. The reactions of the uranium isotopes, uranium-234, uranium-235, or uranium-238 with thermal neutionsLe., neutrons having an energy of 0.0252 ev.-ran be used in this method of analysis. The percentage ahundances of these isotopes are 0.006, 0.72, and 99.274%, respectively. The reactions of these isotopes with thermal neutrons are summarized &$ follows: 234
,,U (n,?)fission products
In Reaction 1, the amount of uranium-235 activity prodiiced
823
ANALYTICAL CHEMISTRY
824
Self-Shadowing. It has been recognized that a neutron flus depression occurs a t the center of the sample undergoing irradiat ion; this is called self-shadowing The self-shadowing effects :tic negligible if solid samples are irradiated in 4 m m . inside diameter quartz tubing. Liquids are irradiated in quartz ampoules and contain lot? solute concentrations, thus attenuation of the neutron flux is similai to that of water. The presence i r i the sample of other elements which may have high neutronabsorption cross sections will result in low results. For example. gadolinium would cause serious neutron flux attenuation in the sample and would necessitate a preirradiation separation of the uimium by some convenient quantitative method. RADIOACTIVATION ANALYSIS OF SAMPLES THAT CONTAIN URANIL-M
Nuclear Irradiztion of Sample. Weighed portions of the samples and the coniparat'ive standard are put into small quartz tulie;. The tubes are closed with cork stoppers that are wrapped i n aluminum. They are then irradiat,ed in the reactor. After irradiation, the samples are allowed t,o decay about 4 hours and are then chemically processed as described below. The synthetic samples used in this laboratory hsd been processed bv a filter paper partition chromatography technique. Aft,er the separation, the paper was conveniently irradiated in short pieces of quarta tubing whose openings were plugged by means of cork stoppers. Chemical Separation of Neptunium-239. In most neutron nctivation analyses, a chemical separation is made to isolate the radioactivit'v of the element from all other radioactive species i n the sample. Usually an "isotopic carrier"-a known amount of the natural inactive element-is added to the solutions of both the irradiated specimen and the comparison samples. The solutions are then processed chemically to isolate the carrier and desired radioelement from other elements and contaminant radioactivit,ies. Small amounts of other elementa are added as holdback or scavenging ca,rriers to assist in the decontamination procc5-enhance this sensitivity. Considering all of the factors given above. it has been calculated that a t least 0.0001 y per gram of nranium cmi be tletected. RESULTS
Reproducibility. The prerision of analysis for ui,;iiiium is within = t ~ l O 7 ~ .The results report,ed in Tables I through I11 show the relative standard deviation for each set of tleterminations. Determination of Uranium in Synthetics. This investigation of the method of radioactivation analysis for uranium was first applied t80the determination of uranium in a series of eynthrt,ic aamples. The handling of t,he samples before irradiation has l,rrii d r ~ r ~ i b eabove. d The samples were irradiated for almut
.4CKNOWLEDGhIENT
The authors wish t,o arknodedge the work of T. C. Rains. J. H. Oliver, J. J. Manning. L. 11.Frakes, N. B. Tuck, S . F. Sharp. and W. L. Bruce in analyzing the irradiated materials. LITERATURE CITED
Borkowski. C. ,J., -1s.4~.CHEM.,21, 34s (1949). Boyd, G . E., Ibid.,21, 335-47 (1949). (3) Leddicotte, G. W., and Reynolds, S. .I Sitcleonics. ., 8 , S o . 3,
(1) (2)
62-5 (1951).
(4) Leddicotte, G. W.,and Reynolds. S. A , , ORSL R e p t . , 1443
(Jan. 2, 1953); AECD-3489 (declassified Jan. 27, 1953). (5) Seaborg, G. T., and coworkers, Metallurgical Project R ~ p t . , CN-2689, 41 (Feb. 15, 1945) (classified). (6) Seyfang, il. P., and Smales, 1. _1., AERE Rept., C/R 980 (Oc>t. 3, 1952) (unclassified). (7) Smales, A. A., Ibid., C/R 930 (May 13, 1952) (unclassified). (8) Taylor, T. I., and Hayens, WT.T., Jr., Nucleonics, 6 , S e . 4 54-66 (1950). (9) Way, K., Fano. L., Scott. JI., and Thew, K., Natl. Bur. Standards U. s., Circ. 499 (1950). RECEIVED for review Sol-ember 19, 1954.
Accepted January 17, 1953.
Analysis of Automobile Exhaust Gases by Mass Spectrometry J. K. WALKER and C. L. O'HARA, Consolidated Engineering Corp., Pasadena 15, Calif. IIass spectrometer techniques for the anal>-sisof automobile exhaust gases, and a sampling procedure for laboratory gas analysis are presented. These data are supplemented by results from continuous monitoring of exhaust gas composition at various engine speeds. The hydrocarbon content of automobile exhaust Yaries with engine speed, approaching steady state at high speed. The oxides of nitrogen produced, as indicated hi- continuous analysis, show an increase with engine speed.
A
U.4LPSIS of eshaust gases from automobiles has long presented a problem in the study of air pollution. The mass spwtrometer offers esceptional promise as a satisfactory means for complete analysis and monitoring of all combudon products from internal comliustion engines. \lass spectrometric tech-
niques for the determinntioii of c,shaust gases h a w Iwen reportrtl previously (1, .?, 9). The lubricating oil products of exhaust gases can be determined similarly ( 8 ) , but are not considered as nxtjor contributors to air pollution 111- virtue of their lolt-er vapor pressure. Khile reporting results of rwent mass spectrometric a n a I , ~ ~ s of engine rshauet, the problems encountered in analysis and t h e sampling techniques ava.ilahle should be reviewed. The greatest, single problem met in automolile exhaust gas analysis is that of obtaining represeiit'ative sampling. ;ilthough %yoof the conibustion products are gaseous at atmospheric pressure and anibient temperature, the other 15% are liquids condensed or ~ ( 1 sorbed below the operating tcmperatures. Previous reports have indicated the coiidensnhlr and soluble gaseous oomponentc other than u-ater to 1~ most significant in the problems of :iir pollution (4,10).
