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Preface

This literature survey, concerning the nitrogen isotope N15, was made as
an aid to a study to extend the optical spectroscopic method of isotope anal-
ysis to the measurement of N15/N14 ratios. It is not intended to be a complete
bibliography of this extensive field. For convenience, the bibliography is
grouped according to subjects. Therefore, some references appear in more
than one group.

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Bibliography on Nitrogen 15*

M. W. Chapman and H. P. Broida

References to the literature on nitrogen 15 that has appeared from 1919 to 1952, inclusive, with a few later references, are given. The citations relate to the abundance of N18 naturally occurring, its physical properties, methods of concentrating it, methods of N15/N14 measurement, and the synthesis and uses of N15 compounds.

1. Natural Abundance of N15

Since 1929, when N15 was discovered by Naude the band spectra of NO [1(19)],1 there have been many measurements of the abundance ratio N/N15) of nitrogen isotopes. A summary of alues measured for the abundance ratio and of stings in tables of isotopes from 1930 through 152 is given in table 1. The table was compiled om the bibliography, section 4.1. It shows the Ete when each value was reported and the ference.

In 1937 the Committee on Atoms of the Interational Union of Chemistry made its first report 12)]. 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

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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 bas 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 N14 N14 molecular ion beam (mass No. 28) to that of the N14 N15 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 N14 N15 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 th instrument. Nier in 1948 [3(46)] points out tha 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 fort in which it is usually reclaimed. The chemics 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 no 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)].

Roberts [3(33)] published in 1948 a 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

W. J. Youden, Statistical methods for chemists (John Wiley & Sons, Ine

New York, N. Y., 1951).

range of the chemist. The measurement of isotope 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.

2

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 background N2 bands by exciting a mixture of N2 in He with an electrodeless discharge of about 30 mc. 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 an 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 microphotoneter and the relative accuracy obtained was 2 to 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 he 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. Bibliography

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 N15 N14016 and N14 N15016. 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). Kirshen

(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 N14, 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).

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