1. Natural Abundance of N15

Since 1929, when N15 was discovered by Naude In the band spectra of NO [1(19)],' there have been many measurements of the abundance ratio NN15) of nitrogen isotopes. A summary of alues measured for the abundance ratio and of istings in tables of isotopes from 1930 through 1952 is given in table 1. The table was compiled from the bibliography, section 4.1. It shows the late when each value was reported and the reference.

In 1937 the Committee on Atoms of the Interational Union of Chemistry made its first report 1(2)]. N15 was listed as having an abundance of 38 percent. Each following report of this com

mittee gave the same value through the sixth. report in 1942 [1(3)], which listed only changes to be made in the previous report. This value was still quoted as the standard when Schaffer [1(29)] issued A New Table of Isotopes in 1949. However, the table by Hollander, et al. [1(11)], based on values recorded in the literature or by private communication through approximately December 1952, gives a new value of 0.365 percent as measured by Nier [1(21)] in 1950. This value is the one presently used by the authers.

Apparently plants and animals do not fractionate nitrogen isotopes during anabolism and catabolism as is the case with hydrogen in plants.2 3 This is the conclusion that Schoenheimer and

Comments

2 T. Titani and M. Harada, The concentration of heavy isotopes in carbohydrates, Bul. Chem. Soc. Japan 10, 205 (1935).

3 E. W. Washburn and E. R. Smith, The isotopic fractionation of water by physiological processes, Science 79, 188 (1934).

of equal distribution of nitrogen isotopes in air and organic compounds [1 (30)]. The only exception noted was an increase in N15 concentration by more than 50 percent as found by White and Yagoda in 1950 for very old radioactive minerals [1(42)]. They later suggested the explanation [1(45)] that nitrogen may have been a component of the minerals at the time of formation, and that the ratio of N15 to N14 increased as a result of more rapid diffusion of the lighter isotope. The possibility of such an increase also being true for chemically bound nitrogen in crude oil and coal deposits was investigated in 1951 by Smith and Hudson [1(32)], but no increase over atmospheric nitrogen isotopic content was found.

Isotopic separation takes place in the atmosphere due to settling in the earth's gravitational field, and has been detected above 40 kilometers. This possibility was pointed out by Lindemann and Aston [1(16)] in 1919, and was tested in 1948 and 1950 by McQueen [1(17)] when air samples were collected from the altitude range 40 to 60 kilometers and were analyzed for percent separation using a 60° Nier-type mass spectrometer. The method of analysis compared the ratio of the intensities of the NN14 molecular ion beam. (mass No. 28) to that of the N14N15 molecular ion beam (mass No. 29) in the sample to that in a standard sample collected at ground level.

2. Methods of Concentrating N15

The usual methods of separating and concentrating isotopes [2(4)] are chemical exchange, gaseous diffusion and mass diffusion, which are more frequently used for large scale production, and gas centrifuge and thermal diffusion, which are more frequently used for small scale production. Among other methods that have been used for separation of nitrogen isotopes are the electric discharge and electrolysis. Two reports giving general descriptions and comparisons of different methods of separating and concentrating isotopes were published in 1947 by Benedict [2(4)] and in 1948 by Vick [2(51)].

The method of obtaining the highest concentration of N15 noted in the literature is that of Clusius, who inserted an electric discharge in the gas stream of a thermal diffusion column, thereby dissociating the molecules into atomic nitrogen. In 1947 he and Becker used this method to isolate NN15 to 98.9 percent [2(9)] and in 1950 he used it to produce 99.8 percent pure N15N15 [2(7)].

3. Methods of Measurement Methods of measurement of N15 include the mass spectrometer, gas density, crystal suspension, microwave spectroscopy, and optical spectroscopy.

The bibliography listed in section 4.3 includes some articles that give little information about the method of measurement but do state the accuracy obtained or the size of sample required.

of measurement on basis of precision, reproduc bility, or accuracy obtained because of th numerous difficulties involved. Seldom are thes values given in units that allow comparison wit another method of measurement. Also the de scription of how the data were analyzed to obtai the value or an actual tabulation of the data i seldom complete enough for recalculation. Aver age deviation, for instance, has no absolut significance as does standard deviation of th mean or coefficient of variation. In genera however, the accuracy of an isotope measuremen depends on the isotope concentration of the sam ple being analyzed and the precision of the instrument. Nier in 1948 [3(46)] points out that absolute ratios are not as important in trace work as are changes in the ratio as compared t standards. Thus some sources of error are auto matically eliminated and a higher degree accuracy is obtainable.

The mass spectrometer is the instrument mos widely used for analyzing N15/N14 ratios. It re quires that the sample be in gaseous form, usuall N2. This is not the form in which the isotope introduced into most experiments, nor the for in which it is usually reclaimed. The chemica procedure for changing the sample to gaseou form was outlined by Sprinson and Rittenber [3 (40)] in 1949.

Winter [3(47)] published in 1948 a descriptio of the operation and history of the mass spectrom eter. He states that for an abundance ratio c 1 percent or greater, an accuracy of 1 percent ca be obtained with practice and, using extreme car and expert personnel, an accuracy of 0.3 percen can be obtained. The time of measurement i about 45 min per sample, which could be reduce somewhat by using an automatic recorder. Brow in 1951 [3(4)], reporting on his work with adenine states that the size of N2 sample preferred wa about a milligram. Rittenberg (3(30)], 1942 found that the mass spectrometer he used coul detect 1 part in 10,000 of 50 atom percent N diluted with normal urea, and it was felt tha results with the mass spectrometer were affected by the purity of the sample. Later i was realized, however, that CO and NO presen either in the sample or in the decomposition prod ucts of the ion beam can cause errors difficult to detect [3(14)].

no

Roberts [3(33)] published in 1948 å report o the gas density, liquid density, and thermal con ductivity methods of isotope analysis. He point out that the mass spectrometer is expensive an requires a high degree of operational skill and tha the alternate methods of analysis that he describes although they may be incapable of the accurac of the mass spectrometer and may demand ex ceptional care in the chemical purification o samples, are nevertheless relatively cheap t initiate and involve techniques well within th

4 W. J. Youden, Statistical methods for chemists (John Wiley & Sons, Ine New York, N. Y., 1951).

tope ratios by atomic weights is time-consuming, requires large samples, and is not sensitive to small variations in concentration.

Research on the development of the microwave spectrograph for analysis of nitrogen 15 in ammonia was reported in 1950 by Southern, et al. 3(39)]. N15 could be determined to approximately 3 percent of its concentration in the range 0.38 percent to 4.5 percent. The minimum sample size was 0.00015 mole of gas, which was mostly recoverable. A standard curve was used for alibration. Each total analysis required 2.5 hr, including 20 to 30 minutes for the analysis itself and 2 hr between samples.

The optical spectroscopic method of N15 analysis is the one in which this laboratory is primarily interested. Following is a brief résumé of the work done on it as noted in the literature.

In the discovery of N15 and measurement of its abundance ratio, Naude [3 (23,24)] and Urey and Murphy [3(44)] in 1929 and 1931 used the continuous spectrum from a hydrogen lamp, passed the light through NO in a variable pressure tube, and photographed the isotopic separation with a Hilger El quartz spectrograph. In 1939 Kruger 3(19)], in order to follow the progress of his work on concentrating N15, excited N bands with an electrodeless discharge and used the isotope separation of band heads as a guide. Only rough accuracy was needed. In 1947 Clusius and Becker 3(7)], also following the progress of isotope concentration, excited N2 in a hollow cathode tube and observed the (2,0) band of the 2d positive system of N2 at 2977.5 A. In 1948 Holmes [3(15)], in measuring atomic nitrogen isotope shifts, obtained high intensity and eliminated most backTound N2 bands by exciting a mixture of N2 in He with an electrodeless discharge of about 30 me. In order to get the lines sharp enough to resolve the isotope structure, the cross section of the tube. was made a thin rectangle and the discharge was mmersed in liquid nitrogen. The lines were then observed with a Hilger E478 spectrograph in series with a Fabry-Perot interferometer. In 1950 Hoch and Weisser [3(14)] excited N2 gas in fan electrodeless discharge tube of pyrex at 3 to 5 mm pressure. They were developing a method insensitive to most impurities, particularly those undesirable for the mass spectrometer, as well as one that works with a much smaller sample. They observed the (1,0 and 0,1) violet emission bands of the 2d positive system of nitrogen with a Hilger spectrograph. From the plates obtained, intensities were measured with a Moll microphotometer and the relative accuracy obtained was 2 o 3 percent. In 1952 Dieke [3(8)] obtained a patent for a method in which the nitrogen is ncorporated into cesium uranyl nitrate to enhance the spectral line separation and a fluorescence pectrum is produced with an arc light. This is observed with a light-sensitive tube to obtain an electric current as indication of the relative amount of N15 present.

4.1. Natural Abundance and Physical
Properties

(1) F. W. Aston, Mass spectra and isotopes, p. 130 (Longmans, Green & Co., New York, N. Y., 1942). (2) F. W. Aston, N. Bohr, O. Hahn, W. D. Harkins, and G. Urbain, First report of the Committee on Atoms of the International Union of Chemistry, J. Chem. Soc., (London) 1910 (1937).

(3) F. W. Aston, International table of stable isotopes. Nature 150, 515 (1942).

(4) L. Aujeszky, Isotopes in the atmosphere, Időjárás (in Hungarian) 53, 289 (1949) [also Nuclear Sci. Abstracts 4, 382 (1950)].

(5) K. T. Bainbridge and E. B. Jordan, Atomic masses of hydrogen, helium, carbon and nitrogen isotopes, Phys. Rev. 51, 384 (1937).

(6) H. Bethe, Masses of light atoms from transmutation data, Phys. Rev. 47, 633 (1935).

(7) J. Bigeleisen and L. Friedman, The infra-red spectra of N15N14016 and N14N15016. Some thermodynamic properties of the isotopic N2O molecules, J. Chem. Phys. 18, 1656 (1950).

(8) R. T. Birge and D. H. Menzel, The relative abundance of the oxygen isotopes and the basis of the atomic weight system, Phys. Rev. 37, 1669 (1931).

(9) R. W. Cole, Exchange magnetic moments of nuclei with almost closed shells of nucleons, Phys. Rev. 89, 883 (1953).

(10) G. Herzberg, The nitrogen isotope of mass 15, Z. Physik. Chem. (in German) B9, 43 (1930). (11) J. M. Hollander, I. Perlman and G. T. Seaborg, Table of isotopes, Revs. Mod. Phys. 25, 469 (1953). (12) E. Ingerson, Nonradiogenic isotopes in geology: a review, Bul. Geol. Soc. Am. 64, 301 (1953). (13) I. Kirshenbaum, The vapor pressure and heat of vaporization of N15, J. Chem. Phys. 9, 660 (1941). (14) H. Krüger, Enrichment of N15 isotope and spectroscopic study of N15, Naturwissenschaften (in German) 26, 445 (1938).

(15) H. Krüger, The enriching of the N15 isotope and some spectroscopic investigations on N15, Z. Physik 111, 467 (1939).

(16) F. A. Lindemann and F. W. Aston, The possibility of separating isotopes, Phil. Mag. 37, 523 (1919). (17) J. H. McQueen, Isotopic separation due to settling in the atmosphere, Phys. Rev. 80, 100 (1950). (18) G. M. Murphy and H. C. Urey, The relative abundance of the nitrogen and oxygen isotopes, Phys. Rev. 41, 141 (1932).

(19) S. M. Naudé, An isotope of nitrogen, mass 15, Phys. Rev. 34, 1498 (1929).

(20) S. M. Naudé, The isotopes of nitrogen, mass 15, and oxygen, mass 18 and 17, and their abundances, Phys. Rev. 36, 333 (1930).

(21) A. O. Nier, A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium, Phys. Rev. 77, 789 (1950). (22) K. Ogata and H. Matsuda, Masses of light atoms, Phys. Rev. 89, 27 (1953).

(23) K. Ogata and H. Matsuda, Substandards of atomic mass, Natl. Bur. Standards Circ. 522, 59 (1953). (24) G. H. Palmer, Isotope abundance measurements, Mass Spectrometry, 108 (1952).

(25) W. G. Proctor, F. C. Yu, On the nuclear magnetic moments of several stable isotopes, Phys. Rev. 81, 20 (1951).

(26) W. G. Proctor and F. C. Yu, The magnetic moments of Mn55, Co59, C137, N15 and N, Phys. Rev. 77, 716 (1950).

(27) D. Rittenberg, A. S. Keston, F. Rosebury, and R. Schoenheimer, Studies in protein metabolism. II. The determination of nitrogen isotopes in organic compounds, J. Biol. Chem. 127, 291 (1939).

other inert gases, in the fumaroles of hot boraxbearing Tuscan springs. Geochemical aspects of the composition of the fumaroles, Mem. Accad. Italia, Classe Sci. Fis., Mat. Nat. 8, 533 (1937). (29) J. J. Schaffer, A new table of isotopes, Bol. Fac. Ing. Montevideo (in Spanish) 3, 355 (1949). (30) R. Schoenheimer and D. Rittenberg, Studies in protein metabolism. I. General considerations in the application of isotopes to the study of protein metabolism. The normal abundance of nitrogen isotopes in amino acids, J. Biol. Chem. 127, 285 (1939).

(31) G. T. Seaborg, Table of isotopes, Revs. Mod. Phys. 16, 1 (1944).

(32) P. V. Smith, Jr. and B. E. Hudson, Jr., Abundance of N15 in the nitrogen present in crude oil and coal, Science 113, 577 (1951).

(33) H. G. Thode, The vapor pressures, heats of vaporization and melting points of N14 and N15 ammonias, J. Am. Chem. Soc. 62, 581 (1940).

(34) H. G. Thode, Variations in abundances of isotopes in nature, Research (London) 2, 154 (1949). (35) H. von Ubisch, The mass spectrometer and its use II, Fra Fysik Verden No. 4, 229 (1949).

(36) H. C. Urey, The thermodynamic properties of isotopic substances, J. Chem. Soc. (London), 562 (1947). (37) H. C. Urey and L. J. Greiff, Isotopic exchange equilibria, J. Am. Chem. Soc. 57, 321 (1935).

(38) H. C. Urey and G. M. Murphy, The relative abundance of N1 and N15, Phys. Rev. 38, 575 (1931). (39) A. L. Vaughan, J. H. Williams, and J. T. Tate, Isotopic abundance ratios of C, N, A, Ne, and He, Phys. Rev. 46, 327 (1934).

(40) M. H. Wahl, J. F. Huffman, and J. A. Hipple, Jr., An attempted concentration of the heavy nitrogen isotope, J. Chem. Phys. 3, 434 (1935). (41) K. Way, L. Fano, M. R. Scott, and K. Thew,

Nuclear data. A collection of experimental values of half lives, radiation energies, relative isotopic abundances, nuclear moments and cross sections, Natl. Bur. Standards Circ. 499, 12 (1950). (42) W. C. White and H. Yagoda, Abundance of N15 in in the N occluded in radioactive minerals, Science 111, 307 (1950).

(43) D. W. Wilson, A. O. C. Nier, and S. P. Reimann, Preparation and measurement of isotopic tracers (J. W. Edwards, Ann Arbor, Michigan, 1948). (44) R. W. Wood and G. H. Dieke, The nuclear spin of N15, J. Chem. Phys. 6, 908 (1938).

(45) H. Yagoda and W. C. White, Ratio of N15/N14 in gases occluded in radioactive minerals, Phys. Rev. 78, 330 (1950).

4.2. Methods of Concentrating

(1) J. W. Beams and F. B. Haynes, The separation of isotopes by centrifuging, Phys. Rev. 50, 491 (1936). (2) E. W. Becker and H. Baumgärtel, Concentration of N15 by means of the chemical exchange method, Angew. Chem. (in German) [A] 59, 88 (1947). (3) E. W. Becker and H. Baumgärtel, The enrichment of N15 with a two-fold exchange arrangement, Z. Naturforsch. (in German) 1, 514 (1946). (4) M. Benedict, Multistage separation processes, Trans. Am. Inst. Chem. Engrs. 43, 41 (1947).

(5) H. Brown, Thermal separation ratios calculated from viscosity data, Phys. Rev. 57, 242 (1940).

(6) K. Clusius, Separation of isotopes by thermal diffusion, Review of Programme, IUPAC Plenary Lectures, Stockholm and Uppsala (in German), 53 (1953).

(7) K. Clusius, Separation tube. IX. Preparation of pure heavy nitrogen N15, Helv. Chim. Acta (in German) 33, 2134 (1950).

tion of N15-rich nitrogen by means of the gas density balance, Z. anorg. u. allgem. Chem. 251 92 (1943).

(9) K. Clusius and E. W. Becker, The separation tube VI. Isolation of the mixed molecule N14 N15, Z Naturforsch. (in German) [A] 2, 154 (1947).

(10) K. Clusius, E. Becker, and H. Lauckner, Enrichmen of N15 by exchange, Sitzber. Math. Naturw. Abt Bayer Akad. Wiss. Munchen 145 (1941).

(11) K. Clusius and G. Dickel, New Process for separation of gas mixtures and isotopes, Naturwissenschafter (in German) 26, 546 (1938).

(12) K. Clusius and G. Dickel, The separation tube. I Principles of a new method of gas and isotop separation by thermal diffusion, Z. physik. Chem [B] 44, 397 (1939).

(13) K. Clusius, G. Dickel, and E. Becker, Pure oxvger isotope O218 and nitrogen isotope N14N15, Natur wissenschaften (in German) 31, 210 (1943).

(14) A. N. Davenport and E. R. S. Winter, Diffusion properties of gases. V. Thermal diffusion of carbon monoxide, nitrogen and methane, Trans. Farada; Soc. 47, 1160 (1951).

(15) E. David, Separation of isotopes with continuously working separation tubes, Z. Physik (in German 134, 377 (1953).

(16) H. Eyring and A. Sherman, Theoretical considera tions concerning the separation of isotopes, J Chem. Phys. 1, 345 (1933).

(17) I. G. Farbenind. A.-G. (Alvin Krauss, inventor) Isotopes of oxygen and nitrogen, German Paten No. 632,071 (July 2, 1936).

(18) R. Fleischmann, Concentration of N15 by the separat ing-tube process of Clusius and Dickel, Physik. Z (in German) 41, 14 (1940).

(19) W. Groth and P. Harteck, Separation of isotopes b gaseous diffusion, Z. Physik. Chem. (in German 199, 114 (1952).

(20) G. Hertz, A method of separating isotope mixture by diffusion through streaming mercury vapor Z. Physik (in German) 91, 810 (1934)

(21) (22)

D. A. Hutchison, Efficiency of the electrolytic separa tion of N isotopes, Phys. Rev. 75, 1303 (1949) D. A. Hutchison, Efficiency of the electrolyti separation of potassium isotopes, J. Chem. Phys 14, 401 (1946).

(23) R. C. Jones and W. H. Furry, Calculation of th thermal diffusion constant from viscosity datɛ Phys. Rev. 57, 547 (1940).

(24) R. C. Jones and W. H. Furry, The separation of isc topes by thermal diffusion, Revs. Mod. Phys. 18 151 (1946).

(25) I. Kirshenbaum, J. S. Smith, T. Crowell, J. Graf and R. McKee, Separation of the nitrogen isotope by the exchange reaction between ammonia an solutions of ammonium nitrate, J. Chem. Phys 15, 440 (1947).

(26) H. Krüger, Enrichment of N15 isotope and spectro scopic study of N15, Naturwissenschaften (i German) 26, 445 (1938).

(27) H. Krüger, The enriching of the N15 isotope and som spectroscopic investigations on N15, Z. Physik 111 467 (1939)

(28) A. N. Murin, Thermodiffusion method for the separa tion of isotopes, Uspekhi Khim. 10, 671 (1941). (29) R. Nakane, Concentration of heavy nitrogen isotop (supplement), Repts. Sci. Research Inst. (Japan 28, 413 (1952)

(30) R. Nakane, Concentration of heavy nitroge isotope concentrating efficiency of the packe column and its liquid hold-up determined b radioactive cobalt, Repts. Sci. Research Inst (Japan) 28, 276 (1952).

(31) E. Ogawa, Mechanism of isotopic exchange reactions Bul. Chem. Soc. Japan 11, 425 (1936).