Isotope Distribution in the Unterzuuch AnuIysis of Labeled Oxygen Compounds

WILMER G. MILLER and LAURENS ANDERSON Department of Chemistry, College of Leffers and Sciences, and Deparfment of Biochemisfry, College of Agriculture, University of Wisconsin, Madison 6, Wis.

The internal distribuiion of oxygen isotopes has been determined in car- bon dioxide obtained from enriched water by the Unterzaucher method. The finding that distribution is non- random i s of significance in the calcu- ;ation of the oxygen-18 content of samples which trave been converted fo carbon dioxide by the Unterzaucher method preparatory to mass analysis. A simple equaiion for making these calculations is presented.

N THE USE of oxygen-i8 as a tracer, 1 the C'nterzaucher method (7, 8) is frequently employed to convert the oxygen of organic compounds to carbon dioxide for mass spectrometric analysis. I n this method, the sample is pyrolyzed to carbon monoxide over hot carbon [sometimes platinized (S)], and the carbon monoxide is oxidized to carbon dioxide with iodine pentoxide. The procedure is rather laborious, but gives reliable results with a wide variety of oxygen-containing compounds (3, 8). Doering and Dorfman (5 ) , in adapting the procedure for use in isotope analysis, showed that the oxygen-18-enriched oxides of carbon did not exchange iso- tope with either the quartz pyrolysis tube or the iodine pentoxide. Until the present investigation, the internal distribution of isotope in the carbon dioxide had not been determined. This communication presents data on this distribution and discusses its signifi- cance.

I n the mass spectrometric analysis of cnrbon dioxide, the atom fraction 0 ' 8

is c:iiciilated from the ratio of mass 46 to mass 44, as determined from the corresponding voltages or peak heights. I n devising equations for this calcula- tion. it is necessary to consider whether the iwtopes of oxygen are randomly (iistributcd in the carbon dioxide. As w i : half the oxygen of carbon dioxide o'nt:iined by the Unterzaucher process :s &:rived from the enriched sample, . ~ n d the other half from iodine pentoxide of norm:d isotopic abundance, one might ixpwt the distribution to be nonrandom. illat is. m e would expect only one of the two oxygen atoms of the carbon dioxide to be enriched in oxygen-18. On the other hand, one could suppose

r , , .

tha t the mechanism of the oxidat,ion of carbon monoxide by iodine pentoxide is such t h a t the oxygen-18 becomes equally divided between the two posi- tions (random distribution). Both as- sumptions have been made by investi- gators using the Unterzaucher method for the ,analysis of oxygen-Weontaining compounds; the equations of Doering and Dorfman (5) and Bender and Kemp ( I ) are based on nonrandom distribution, while that of Denney and Greeiibaum (4) is based on random distribution. So long as carbon dioxide containing no more than 1 to 2 atom yo 0 ' 8 is involved, the distinction is not im- portant, because assuming the wrong distribution will cause less than 1% relative error in calculating the isotope content of the sample. However, the error becomes significant with the greater enrichments which will be en- countered now tha t water containing 10 t o 90 atom Yo Ols is commercially available (2).

T o determine the isotope distribu- tion, water known to contaiii 8 to 10 atom yo 0 ' 8 was converted to carbon dioxide in an Unterzaueher-type apparatus. The carbon dioxide was collected in aliquid air trap and analyzed for masses 44 through 48 on a Consoli- dated Model 21-620 recording mass apectrornater. The remainder of the sample was then introduced into a 200- ml. borosilicate glass bulb, which was equipped with a sealed-in platinum fila- ment, and was equilibrated with the filament a t bright red heat until the ratios of the peak heights for the various masses became constant. The proce- dure of conversion of !\.ate7 to carbon dioxide, analysis, and equilibration was repeated several times \\-ith closely simi- lar results. Data from a typical run are presented in Table I. Thc values recorded were obtained by scanning the mass 44- t3 48-region 4 t o 6 times at each ana!)-sis; in the initial analysis the manifold was pumped out after the first set of scans, more sample n-as ad- mitted, and the scanning was repeated.

For comparison of the observed values with those predicted by the two types of isotope distribution, the isotope con- tent of the original water was calculated from the mass ratios 45/44 and 46/44, assuming nonrandom distribution. The

values obtained were 0.45 atom yo OI7

and 8.55 atom % 0 I 8 . Theoretical peak heights for masses 47 and 48 (nonrandom distribution) and masses 45 t o 48 (random distribution) were then calculated from these figures for oxygen-17 and -18 content. The cal- culated peak heights are tabulated in Table I.

The calculations were based on simple equations in which the abundance of carbon dioxide of a given niass is set equal to the sum of the probabilit.ies of occurrence of each of the isotopic spe- cirs having tha t mass. Two types of carbon dioxide molecules have to be considered: those with oxygcn atoms of the same mass--e.g., C1*O1601-and those with oxygen atoms of different masses-eg., Cl2O1601*. If the dis- tribution is random, the probabilities of occurrence are acaoZ for the species of the first type, 2acaoao~ for species of the second type, with ac: cyo, and a0* denoting the atom fractions of C of mass c, 0 of mass 0, and 0 of mass of in the carbon and oxygen, respectively, of the carbon dioxide sample (9). (These expressions are generally appli- cable; in the present case the quantities a. and CY"' are the means of the normal abundances of Oo and 0"' and tho abun- dances of 0" and 0"' in the isotopically enriched compound pyrolyzed.) If the distrihui inn is nonrandom, the ::orre- sponding expressions are a,q,c,)a,(,) and a c a o ( n ) a o ' ( e ) + acao(e)aor(n), where aorn) and ao*(n) denote the normal abundances of Oo and O"', and and eo'(+ the abundances of Oo and 0"' in the isotopically enriched compound.

The nornial abundance valucs usrd for the carbon and oxygeii isotopes were determined by direct anaiyses of the iodine pentoxide and carbon which served as reagents. The figures were: GI3, 1.05 atom 7 0 ; 0 ' 7 , 0.037 atom %; and OI8, 0.204 atom yo. Isotopic sub- stitution was considered to have no effect on the fragmentation of carbon dioxide in the mass spectrometer. 'This assumption is not strictly true (ti), but, the error invclved is negligible. The isotopic species which were taken into account were: mass 44. C'*016016; mass 45, CW'6017 and C13016016. mass 40. C1?O16O1*; mass 47, C1701701; and C13016018; and mass 4% Cl?O1sO~s. Calculations showed that the contribu- tions of the remaining specips could be ignored. Thus, although C13016017 ac- counts for about 0.270 of the mass 46

1668 ANALYTICAL CHEMISTRY

peak in normal carbon dioxide, samples enriched in oxygen-18 are relativdy leas enriched in oxygen-17 (8). and under thew circumstances the contribution of C1JO’QoL7 is nut significant.

Table I shows that, before equilibra- tion, the mass 47 peak was five times greater than the mass 48 peak, and that the mass ratio 48/44 increased tenfold during equilibration. These facts aione are sufficient evidence that the distri- bution of oxygen isotopes in the carbon dioxide formed in the Unterzaucher procedure is nonrandom. The agree- ment between the peak heights ob- served before and after equilibration and the values calculated for nonrandom and random distribution, respectively, is in all cases within experimental error, and thus lends fu!l support to this con- clusion.

The authors have found the following simple equation, derived from the prob- ability expressions described above, useful in calculating the results of mass spectrometric analyses of Unterzaucher sampies. z = atom fraction 0 1 8 in the com-

R = ohsrlvrd rat-io, mass 46/mam 44

then

pouna

R‘ = R - 0.00204

R‘ l + R ’

z = -

This equation takes into account the oxygen introduced by the iodine pent-

Table 1. Mass Distribution in Carbon Dioxide from the Unterzaucher Procedure

Relative Peak Heights & Standard Deviations Mass Obsd., before Calcd., non- Obsd., after Calcd., random NO. equil. random dist. equi!. dis: 44 1OO.OOO 45 1 . 5 9 B f 0 . 0 0 8

1OO.ooc; 1.579 f 0.004 1.575

46 9 .59 f 0 . 0 2 9 19 f 0 02 9 10 47 0.109 f 0.003 0.106 0.119 f 0.003 0 121 48 0.02 f O . O 1 0.019 0.19 f O . 0 1 0.21!

oxide used in the Unterzaucher method, and is based on a nonrandom distribu- tion of the oxygen-18 in the carbon dioxide. It is assumed that the only species contributing to the ma= 46 peak is C1201Q18. If the mass spectrometer used does not give the expected reading for standard (tank) carbon dioxide, R may be normalized by multiplying by 0.00409/Retd. If not all of the oxygens are labeled in the compound analyzed, multiplication of (z - 0.00‘204) by the appropriate factor gives the atom frac- tion excess in the labeied positions. In cme the sample is enriched in oxygen- 17 to the point that the quantity a17($) becomes significant with respect to 1, the right side of the equation is multi- plied by (1 - a,,(,,).

A C K N O W L E D G M E N T

The authors are indebted to R. A. Al- berty for use of an Unterzaucher appara-

Tetrahydrofuran-Water Mixture as a Po I a rog ra p h ic So Ive n t

Determination of the Lower Polyphenyls

LOUIS SILVERMAN, WANDA G. BRADSHAW,’ and MARY E. SHIDELER Atomics International, A Division of North American Aviation, Inc., Canoga Park, Calif.

b Tetrahydrofuran-water mixture is a satisfactory medium for polarographic studies of certain organic compounds. it is particularly adaptable for routine analyses because it may be pur- chased in a relatively pure form and any small amounts of impurities present may be quickly and easily removed by passing it through a column of acti- vated alumina. Tetrahydrofuran has good solvent action for supporting electrolytes such as tetrabutylammo- nium iodide and for many organic compounds. The diffusion currents ob- tained for organic compounds in this medium are higher than those ob- tained in dioxane-water solvent. Bi- phenyl and the terphenyls can be

determined quantitatively using this medium and reduction waves also have been obtained for p-bromo- diphenyl, nitrobenzene, naphthalene, triphenylene, 2-bromonaphthalene, anthracene, and pyrene.

E; A search for polarographic solvents I for biphenyl and the terphenyls, tee rahydrofuran was found t o have cer- tain advantages as a solvent in polaro- graphic work. Abrahamson and Reyn- olds ( 1 ) tried tetrahydrofuran as a solvent for the organohalosilanes, but no reduction was obtained. No other use of tetrahydrofuran in the polarc- graphic field has been mentioned in the literature.

tus and to Irving Shain for use of the mass spectrometer.

LITERATURE CITED

(1) Bender, M. L., Kemp, K. C., J . Am. C h m . SOC. 79, 1 1 1 , 116 (1957j.

(2) Chem. Eng. Mews 36, KO. 21, 57; (1958).

(3) Clark, S. J., “Quantitative hfethods of Organic Microanalysis,” p. 154, Butterworths, London, l95C.

(4) Denney, D. H., Greenbnum, M. .%.) J. Am., Chem. SOC. 79,979 ( 1057 ;.

(5) Doering, W. von E., Dorfman, E.. Ibid. , 75, 5595 (1953).

(6) Schaeffcr, 0. A., Owen, H. R., J . Chem. Phys. 23, 1305 (1955).

(7) Schutze, M., 2. anal. Chem. 116, 245

(8) Unterzaucher, J., B w . 73, 391 (1940). (9) Urey, H. C., Grriff, L. J., 2 . AWL.

Chem. SOC. 57,321 (1935) and referencw there cited.

(1939-40).

RECEIVED for revirm October 2, 1958. Accepted June 1, 1950.

Of the several solvents studied i n this investigation tetrahydrofuran dissolves both a suitable supporting elcctrolyte and the organic biphenyl and terphenyis, and also permits the attainment of very negative potentials ( -2 t o -3 volts). When used with tetr3butS13mnioniurn iodide as the supporting electrolyte, it offers a polarographic medium for or- ganic compounds in Lvhich the samples are easily and quickly prepared.

EXPERIMENTAL

Apparatus. The current-voltage

1 Present address, Lockheed hircraft Corp., Sunnyvale, Calif.

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