C. F. Eck
Monsanto Researcn Corporation Mound Laboraton/' Miamisburg, Ohio 45342
I
I I
Stable Isotopes of the Atmosphere ".
Stable isotopes are becoming of increasing importance in the present technological world. Since 1946, when the electromagnetic separators a t Oak Ridge were applied to the separation of stable isotopes and later when thermal diffusion columns were used, the number of stable isotope shipments made annually to Federal users, educational institutions, and to private companies has been growing steadily (1-3). Some of these stable isotopes are obtained from air. Stable isotopes of air are used for low-temperature refrigerants, gas lasers, atomic projectiles, wavelength standards, tracers in medical and biological studies, and raw materials for radioisotope preparation, as well as for the fundamental investigation of isotope effects by the determination of each isotope's own properties. This article briefly presents the composition of air, the discovery of isotopes, their concentration in air, and r e views their current enrichment status. Elemental Composition of Air
Knowledge of the constituents of air and the existence of isotopes is relatively new, having been established in the present generation. While air was thought to be one of the elements in the days of the alchemist, by early 1894 it was shown to have the composition given in Table 1. . . .. ., ., . .' *. , , . .... ,. .., ,< .,' . , , . ." .,., " Known Composition of Air Early in 1894 Table 1.
.
.s
Gas
Concentration (vol 701
Nitrogen Oxygen Carbon dioxide
79 21 0.04
I n the same year, an unknown fraction that remained with nitrogen was finally identified as a new element, which was named argon, a "lazy" element. This new element was the first of a series found to be present in air. It helped to establish the periodic table. I n the short time of the next four years to 1898 the other elements helium, neon, krypton, and xenon were also discovered in air (4). Helium was actually first discovered in the sun's atmosphere 100 years ago. Further work established a more accurate composition of the atmosphere. The' nonvariable components of atmospheric air as reported by Mirtov (5) and based primarily on the studies of Glueckauf (6) are shown in Table 2. 'Mound Laboratory is operated by Monsanto Research Corporation for the U S . Atomic Energy Commission under Contract No. AT-33-1-GEN-53.
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Table 2. Nonvoriable Composition of Dry Ail sa-
-
Symbol
Name
Ha He Ne Na
Hydrogen Helium Neon Nitmgen Oxygen Argon Carbon dioxide Krypton Xenon
0 9
Ar COa Kr Xe
Concentration (ppm by volume) 0.5 5.24 18.21 780,840 209,460 9,340 300 1.14 0.087
Table 3. Variable Components in Air Gas Symbol
..
Name
-
-
CH. .
Mathane . . .... ...
CO SO, N20
Carbon monoxide Sulfur diolude Nltrous omde Ozone Nitrogen dioxide Radon Iodine Nitric oxide Ammonia
0 3
NO, Rn
L
NO NHJ
Concentration (ppm by volume) 1 5-2 0.06-1 0-1 0 5 0.01-0.1 0.0005-0.02 6 X lo-'' Trace Trace Trace
Nine elements are present in air according to this composition. Trace materials shown in Table 3 that vary for different locations are not included in this review. Isotopes
The discovery of isotopes was more recent than the discovery of the elemental composition of air. Matter had been thought to be composed of specific molecules of elements, each of uniform mass and with their own chemical and physical properties. With the discovery of radioactivity in 1895 and the subsequent wealth of investigative work on radioactive materials, their atomic weights, and their place in the periodic table, it was noted that lead from three radioactive sources had slightly different atomic weights. Soddy (7) presented the idea that elements existed that had the same chemical properties but differed in radioactive properties and atomic weight. The name for these materials, suggested by Soddy, was isotopes (equal place) since for a given element they occupied the same place in the periodic table. I n 1912 Sir J. J. Thomson noticed that the positive gaseous ions of a single element would produce several curves when deflected by electric and magnetic fields, instead of producing a single curve as expected. I n the study of neon, further
investigation revealed no one curve that corresponded to the accepted chemical atomic weight of 20.2. The two curves that were found corresponded to masses of 20 and 22. This was confirmed by other workers. Later neon-21 was also discovered. These data indicated that an element did not consist of only one kind of atom. These substances, neon-20, neon-21, and neon-22, are the stable isotopes of the element neon. The number signifies their weight in atomic mass units. Properties that are based on or influenced by mass vary accordingly. Each of these forms of neon has the same number of protons, 10, which is the atomic number, and the same number of electrons, 10; hence, these three substances have the same chemical properties. The mass diierence is caused by a differing number of neutrons. After this initial discovery, other elements were investigated to determine their isotopes. Of the 80 naturally occurring stable elements, 55 of them are polyisotopic (i.e., they have more than one stable isotope for a total of 242 stable isotopes). One third of these isotopic elements have two isotopes. Tin has the largest number of isotopes, 10 in all. Isotopic Composition of Air
All of the nine elements in air have isotopes for a total of 32. Asimov (8) showed the isotopes present. On the basis of the natural abundance data from Walton and Cameron (9) for neon and from Lederer, Hollander, and Perlman (10) for other isotopes and the assumption of random distribution of isotopes in a compound mole cule, the isotopic composition of air is given in Table4. Quantities of Isotopes in Air
Large quantities of atmospheric isotopes are available (11). To obtain a better idea of these quantities, a 350 ton per day (3.17 X lo5 kg/day) oxygen plant, used to supply oxygen for industrial purposes, would handle but not necessarily separate the approximate quantities of the relatively scarce isotopes shown in Table 5. Some of the oxygen plants also separate the stable noble gases, except for helium. These gases furnish the feed stock for separation of their isotopes a t the AEC's Mound Laboratory in Miamisburg, Ohio. Enrichment of Air Isotopes
Hydrogen has a high natural abundance, is manufactured by a number of industrial processes, and is readily available commercially. Deuterium, because of its low abundance in air, has been enriched using other 'feed materials. Early work on this separation was done by distillation (18) and electrolysis (IS). I n 1933, Taylor, Eyring, and Frost (14) separated 420 ml of 99% DsO by electrolysis. Since that time largescale separation of aeuterium has been performed. Even though the heliumisotopes exist a t an extremely low concentration in air, some isotope separation work has been done using the superfluid properties of helium4 (16), distillation (16), and thermal diffusion (17). I n the latter work, Bowring used three small columns to separate helium-3 enriched to 10% at a rate of 2 ml per week. Later, helium-3 available from nuclear technology activities of the AEC was enriched above 99% by thermal diffusion (18). A future source may d e Volume
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velop from the storage of commercial helium, which is recovered from selected natural gas streams by the Bureau of Mines in its conservation program. I n this process helium-bearing natural gas is cooled to separate the helium by condensing the hydrocarbons. The cooled helium could serve as feed for a superfluid-distillation separation. This helium has a high natural abundance of helium-4 and is the regular commercial product, separated from natural gas. Neon is separated from a crude rare gas fraction obtained in the production of oxygen by the air liquefaction-distillation process (19). It is interesting that about the time that neon isotopes were discovered, Chapman (20) predicted from theory the thermal diffusion effect in gases. He proposed @ I ) in 1919 that neon isotopes could be separated by thermal diffusion, which later turned out to be a simple and effectiveway to separate isotopes. Initial isotope separation attempts were made by distillation (22). Separations were also made using a diffusion method depending on the diffusion velocity difference through a porous wall, using Hertz mercury pump units (23). Clusius and Dickel, using thermal diffusion, developed a thermogravitational column which multiplied the separation effect many-fold. With this method ($4) in 1940 they separated neon-20 and neon-22 of 99.8 and 99.7% concentration, respectively. Clusius enriched a small quantity of neon-21 to 99.G70 (25) in 1956. The end isotopes were separated directly by thermal diffusion. For the middle isotope neon-21, an auxiliary gas (deuterated methane) was used with the neon feed in the thermal diffusion process to isolate the neon-21 more readily. Neon-21 has been enriched to 62% (26) directly by thermal diffusion. Nitrogen is separated from air in conjunction with oxygen separation by the air liquefaction-distillation process. The regular commercial cylinder nitrogen has a high nitrogen-14 isotopic concentration of over 99%. Early consideration for the separation of the heavy stable isotope of nitrogen of mass 15 was given to two processes: distillation of ammonia (27), and diffusion of gaseous nitrogen (28). Chemical exchange (29) has been used extensively in experimental work for separating this isotope. I n 1950 Clusius (30) used thermal diffusion to enrich nitrogen-15. Because of the equilibrium among I4Nz, I5N2,and 14N15Nhe , iuserted an electrical discharge to equilibrate the isotopic molecules. He separated lSN2 enriched to 99.8%. For larger scale separations, Clusius, et al. ($I), and McInteer and Potter (92) demonstrated that nitrogen15 could be enriched in conjunction with oxygen-17 and oxygen-18 as NO by low-temperature distillation. Oxygen has a high natural abundance of greater than 99.7% oxygen-16 and is available readily as regular commercial oxygen. For a higher concentration, water with a high oxygen-16 content was made in 1953 by distillation (33). Distillation (34) and electrolysis (35) of water have enriched the other two isotopes, oxygen-17 and oxygen-18. Distillation of oxygen (36) itself has also been studied. Oxygen-17, the middle isotope of this ternary system, has been more difficult to separate than the middle isotope of neon. Water distillation and gaseous thermal diffusion (97) are being used to enrich oxygen-l7to 607,. Oak Ridge National Laboratory has a combination 708
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.water distillation and thermal diffusion plant that has enriched the oxygen-17 isotope to about 50%. I n 1943, Clusius (98) used thermal diffusion to enrich 250 rnl of the heaviest stable isotope, oxygen-18, to 99%. Liter quantities of oxygen-18 have become available by water distillation and gaseous thermal diffusion (37). Argon, like neon, is separated from air by distillation (19). Initial work on the argon ternary isotope system was done in 1936 using the Hertz diffusionmethod using flowing mercury vapor (39). Argon-36 was enriched 40-fold. Thermal diffusion (40, 41) was also used. In 1953 Clusius and Meyer (42) enriched argon-36 twofold by distillation. Clusius (43), using thermal diffusion, enriched argon-36 above 99% and then, using hydrogen chloride as an auxiliary gas, enriched the middle isotope argon-38 to greater than 99%. Argon40 is present in air at an isotopic concentration of 99.67,. For special purposes it has been enriched by thermal diffusion to 99.957, a t Mound Laboratory. The concentration of carbon dioxide in the atmosphere is too low to allow economic removal for carbon isotope source purposes. In the separation studies of carbon isotopes, the Hertzian type diffusion (44) and distillation (45) were used in 1936. I n 1939 de Hemptiune and Capron (46) enriched carbon-13 to 50% using methane in a battery of 51 mercury diffusion pumps. I n 1951 Johns and London (47) distilled carbon monoxide to reach 60-707' carbon-13. I n 1965 Rutherford and Keller (@) used a thermal diffusion cascade to enrich methane to greater than 90% carbon-13, starting with natural abundance methane. This cascade was later used to reach a concentration of 95% carbon-13. Krypton is also separated from air (19). With six stable isotopes, krypton is not as easily separated into its individual isotopes as are binary and ternary systems. Thermal diffusion has been the primary method used to separate these isotopes. Groth and Harteck (49) used it in 1940. Clusius and Dickel (50) in 1942 enriched krypton-86 to 99.5% and krypton-84 to 98.2%. Gas centrifugation (51) has also been tried for separating krypton isotopes. The lightest isotope, krypton-78, has been enriched to 44.9% by Blais and Watson (52) using thermal diffusion. They similarly enriched krypton-80 to 69.8%. Hydrogen bromides as auxiliary gases have been used by Clusius and Hostettler (53) with thermal diffusion to enrich krypton-78 660.5%. Like most of the noble gases, xenon is also separated from air (19). Among the noble gases it has the largest number of isotopes, a total of nine,from mass 124 to mass 136. I n 1938 an unsuccessful attempt was made to separate these isotopes in a direct-current glow discharge (54). In 1949 thermal diffusion (55) in an 89-ft (27.1-111) long system was used to enrich a heavy fraction from a mass average of 131.3 to 134.3. Also tried in that year was a gas centrifuge (51). I n 1953 Clusiub, el al. (JG), using a 158-ft (48.2-m) long thermal diffusion system, enriched the heavy isotope xenon-136 to 99%. Xenon-124 has been enriched by thermal diffusion to 157, (57) by means of an apparatus described by Rutherford, et al. (58). tjubsequently, with a modified column arrangement, xenon-124 vxs enriched to 65% (59)). At an intermediate stage in the enrichment of xenon-124 by thermal diffusion the isotope xenon-129 was enriched to 60-63%.
Scale
A fission byproduct of the nuclear technology of the AEC is xenon. This fission xenon contains primarily the four heavy stable isotopes of xenon: masses 131, 132, 134, and 136. By using this fission xenon as a source of feed for thermal diffusion, xenon-131 has been enriched to 40% (60) in liter quantities. Electromagnetic separation has been used to separate milliliter quantities of xenon-131 enriched to 99% (61). Current Enrichment Status
Table 6 summarizes the status of the various isotopes, regarding scale and degree of enrichment. The scale of enrichment is classified roughly as follows Industrial Preparative Experimental
cubic foot (STP) quantities liter (STP) quantities milliliter (STP) quantities
Determination of properties and unique uses await the availability of many of these isotopes. Literature Cited
(8) (7) (8)
40, 1 (1932). Lewm, G . N., J . Amcr. Cham. Soc., 55, 1297 (1933). TAYLOR, H. 9.. E Y A I NH., ~ , ANY FROBT, A. A,, J. Cham. Phye., 1, 823
