Mass-Spectrometer Study of the Rare Gases by Vernon H
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---1 I U. S. Department of Commerce Research Paper RP1799 National Bureau of Standards Volume 38, June 1947 Part of the Journal of Research of the National Bureau of Standards Mass-Spectrometer Study of the Rare Gases By Vernon H. Dibeler; Fred L. Mohler, and Robert M. Reese A study of the appearance potentia ls and isotope abundance of tbe rare gases has been made \yith a Consolidated Engin eering Corp. mass spectrometer as a check on the operation of the instrument. The res ults a re comparable to previously published data. In addition, some nell' experimental value of ionization poten tials have been obtained. Generally, good agreement of the isotope abundance measurements with previously published values obtained on other types of in struments and with chemica]]y determined atom ic weights, indicates a minimum of discri m ination effects. The relative yalues of the ion currents for the rare gases were obtai lled and p.lotted against th e atomic n um bel'. K 0 simple relation betwcen sensitidty and atomic Humber is evident. I. Introduction comparison of the resul ts obtained by different types of instrumen ts is useful. Mass spectrometer studies of all the rare gases have been published. Bleakney [1),1 and more II. Experimental Procedure recently Stevenson and Hipple [2] , have measured the appearance potentials of sin gly and multiply The Consolidated mass spectrometer used in the charged ions in helium, neon, and argon. Nier presen t work has been described elsewhere [8]. [3], Aldrich and Nier [4], and Vaughan, Williams, Briefly , it consists of a tungsten-filament source and Tate [5] have publish ed accurate valucs of of electrons, the energies of which can be con the isotope ratios of the rare gases. A survey of trolled between 0 and 100 v by a wire-wound all the rare gases wi th a Consolidated Engineering poten tiometer and measured to :r O.5 percent by Corporation mass spectrometer was primarily un . a calibrated voltmetcr. The positive ions formed dm'taken by the authors as a check on the per in the electron beam are dra\\Tn out by a fixed formance of the instrument. As the results sup potential of about 3 v for appearance poten tial plement published work in several respects, and measurements, whereas a variable potential (ap as a precision has been obtained that is com proximately 1 percent of the ion accelerating parable with the best pu blished work, it seemed voltage) is used in the isotope abundance measure worth while to publish the results. The relative men ts. The positive ions arc then accelerated by values of ion cunen ts obtained under similar con a variable electri c field and deflected into 180- ditions in a series of gases having similar elec degree arcs by a suitable magnetic fi eld. They tronic structm e but differing in atomic weigh t arc separately amplified by an electrometer tube are of interest. Some new valLl es of ionization and recorded by galvanometers. The recorder potentials have also been obtained. The ob contains four galvanometers in parallel with served valu es of the isotope ratios serve further sensitivities in the ratios of 1 : 3 : 10 : 30. Thus to corroborate accepted valu es of isotope ratios the 8-in. photographic record has an effective and compu ted n,tomic weights. Some recent wid th of 240 in. papers by Coggeshn,ll [6] and WashbUl'n and The appearance potentin,l measurements were B erry [7] concerning discrimination in the ion made in the usual manner and evaluated according source indicate thn,t this may appreciably influence to the m ethod of "initial breaks" described by observed isotope ratios. In this connection, a Stevenson and Hipple [9] . \ 'Vhenever possible, the isotope abundance 1 Figures in brackets indicate the li terature references at the end of this paper. measurements were mn.de at sfweral different pres- Mass-Spectrometer Study of Rare Gases 617 sures and values of electron energy and curren t. impact and by optical methods. The doubly Lack of discrimination in the apparatus was shown chargerl helium ion was not mcaRurcd because of by indepedence of the abundance ratios of these interfercnce by lIt ion. The hydrogen was ' pos values [3]. sibly released from the interior surfaces of the The rare gases used in this investigation were mass spectromet<?r when helium was admitted as portions of samples submitted to this l aborator~' background runs showed no trace of H+ ion or H t for mass spectrometric analysis by the Bureau of ion. To simplify the figure, the doubly charged Mines helium plants and the Air R eduction Sales neon curve is not shown. lL has a very low in Co. All were of greater purity than 99.5 percent tensity with an attendant increase ill error of Lhe by volume, the helium containing less than 0.1 determination of the appearance potential. The percent by volume of total impurity. slope of the ionization efficiency curve for doubly . charged argon continues to increase for approxi III. Discussion of Results mately 9 v after the initial break, whereas that portion of the curve for singly charged argon is 1. Ioniza tion Potentials about 2.5 v. This agrees with observations re ported by Stevenson and Hipple [2]. The ob Figure 1 shows the initial portions of the ioniza served critical potentials are ill good agreement tion efficiency curves for the singly charged ions of with computed values in spite of the shape of helium and neon and thf' singly and doubly the curves. charged ions of argon. Their critical potentials are summarized in table 1 and compared with some TABLE l.- Appeamnce potent.ials of singly and ln1ditply previously reported data obtained both by electron chaTged ions of the l'are gases I'e/ative to A+ [Appearance potent.ial, volts] Optical 'rhis papcr a Others Ion sourcesb He+ 24.4,±0.2 24.586 ____ __ Ne+ 21.5,±0.1 21.58 _____ __ 21.5± O.1 [I]. N e++ 61.7 ±2.0 62.L _______ 63.0±O.5 [I]. (Ne+) -(A+) =5.6,±0.15 [3] . III A+ c 15.76 15.76 ____ ___ lli.HO.1 [I]. (A++)-(A+)=28.0±0.5 [3] . AH 43.6,±0.2 43.62 __ . __ __ 44.0±0.5 [1[ . Kr+ 13.9,±0.1 14.009 ___ . __ Kr++ 38.6 ±0.2 38.6 [12] -.--- Xe+ 12.1,±O.1 12.138 _____ _ X e++ 34.1 ±0.2 33.3 [13] --- ~I---+--+-- • Corrected scale, assuming I (A+) = 15.76 ev . ... >I---+---+- bBacher and Goudsmit, Atomic energy states (McGraw-Hili Book Co., ;:: New York, N. Y., 1932). Conversion factor 8,066 cm-' used to convcrt from in f,' wave numbers t.o electron volts. c Observed value is 13.4o±0.1 v. Figure 2 shows the initial portions of the ioniza tion efficiency curves for singly and doubly charged ions of krypton and xenon. The plots for Iv'++ ion and Xe++ ion are, like A++ ion, curved over a range of 8 to 10 v beyond the appearance potential. 12 2. Isotope Abundance Ratios 40 42 44 46 48 so 52 54 IONIZING VOLTAGE (tJNCOR.) The measurement of the ratio of He3 to He4 is FIGURE I.- Initial porti()ns of the ionization efficiency curves for the ions He+, Ne+, A+, and A++. beyond the range of the Consolidated mass spectro meter, but has been measured by Aldrich and Nier The ordinate is arbitrary and different for some ions. The middle scale of the abcissa applies to curves II and III. The lower scale applies to curve 1 V. [4] as 1.7 X 10- 6• They used a mass spectrometer 618 Journal of Research of sufficient resolving power to show that the mass 3 peak was not HD. T able 2 lists the neon isotopes and the percent age abundance as determin d in this laboratory and by Vaughan , Williams, and T ate [5]. The maximum deviation in Lhe tables in this paper is intended only as an indication of th e reproduci bility of the m easurements. The accuracy is believed to be of the order of 1 percent of the abundance for isotopic n,b undances greater than 10 percent. With the neon isotopic masses listE'd by Pollard [10] a nd converting from the n,tomic ~I----+----I- to the chemical scale, a chemical weight of 20.182 is obtained. The accepted value is 20.183 (Inter '"~I-- _-+-__ I iii national Atomic Weights, 1947). ~ T ABLE 2.- Abundance of isotopes of neon m/c 20 21 22 ---- Percent Percent Percent 'rhis paper ______________ ______ 90.51 0.28 9.21 M ax imum deviatioll __________ ± 0.15 ±.02 ±0. 18 30 Vaughan. Will iams, and T ate IONIZING VOLTAG E ( UNCQR.) [8J " . --- -------_______ _______ 90. 0 . 27 9.73 FIGURE 2. - I nitial portions of the ionization efficiency curves for the ions Kr+, K1'++, and Xe++. Table 3 lists the argon isotopes n,nd their respec 1' he ordinate is arbitrary and different rrom some ions. The lower scale tive percentage itbundance. The comparative or the abcissa applies to curves III and IV. data are again those of Vaughan, Williams, and Tate [5], and thE' weight calculated from their values give a weight of 83.81. The accepted values is 39.950, The accepted value is 39.944. value is 83.7. The chemical weigh t computed from the abund ance and the isotopic weigh ts given by Pollard [10] T ABLE 4.