V O L U M E 2 3 , NO. 1 2 , D E C E M B E R 1 9 5 1
1735
WAVE LENGTH IN MICRONS Figure 9.
Infrared Absorption Spectra
A . 1 2-Bis(2 3-butanedionedioximo-N,N’)nickel(II) B . 1,’2-Bis(2~3-butanedionedioximo-d-N,N’)niokel(II)
(4) Buswell, A. hl., Dieta, V., and Rodebush, W.H., J . Chem. P h ~ s . , 5 , 84 (1937). (5) Buswell, A. hf., Rodebush, IT. H., and Roy, M. F., J . A m . Chem. Soc., 60, 2444 (1938). (6) Cambi, L., and Saego, L., Ber.. 64, 2591 (1931). (7) Cavell, H. J., and Sugden, 9.. J . Chem. Soc., 1935, 621. ( 8 ) Davies, hI., J . Chem. P h y s . , 15, 739 (1947). (9) Diehl, H., ”Applications of the Dioximes to Analytical Chemistry,’’ p. 22, Columbus, Ohio, G. Frederick Smith Chemical Co., 1940. (10) Feigl, F., A N A L . CHEM.,21, 1298 (1949). (11) Ferguson, R. C., and Banks, C. V..I b i d . , 23, 448, 1480 (1951). (12) Ferguson, R. C., 1-oter, R. C., and Banks, C. \-., N i k r o c h e m i e p e r , Mikrochim. Acta, 36, 11 (1951). (13) Fernelius, \$-. C.. Larsen, E. M., XIarchi, L. E., and Rollinson, . 520 (1948). C. L., Chem. Eng. ~ l ’ e u ~ s26, (14) Godycki, 1,. E., Rundle, R. E., Voter, R. C., and Banks, C. V., J . Chem. Phgs., 19, 1205 (1951). (15) Iictelaar, J. A. A,, Rec. t r a c . chim., 60, 523 (1941). (16) Klemm, V. W., Jacobi, H., and Tilii, \\-., Z . anorg. u. allgem. Chem., 201, 1 (1931). ( l i ) Milone, XI.. A t t i X o congr. i n t e r n . chim., 2, 346 (1938). (18) IIiloiie, hI. and Tappi, G., Atti accad. sci. Torino, Classe sci. fis., mat. nnt., 75, 445 (1940). 119) Pauling. L.. “Nature of the Chemical Bond,” Ithaca, Pi. Y., Cornel1 University Press, 1945.
(20) Pfeiffer, P., Be?., 63, 1811 (1930). (21) Pfeiffer, P., and Richare, J., I b i d . , 61, 103 (1928). (22) Randall, H. M.,Fowler, R. G., Fuson, N., and Dangl, J. R., “Infrared Determinations of Organic Structures,” New York, D. Van Kostrand Co., 1949. (23) Rauh, E. G., Smith, G. F., Banks, C. V., and Diehl, H., J . Org. Chem., 10, 199 (1945). (24) Reed, 9. A , , and Banks, C. V., Proc. Iowa A c a d . Sci., 5 5 , 267 (1948). (25) Reed, S. A , , Banks, C. V.,and Diehl, H., J . Org. Chem., 12, 792 (1947). (26) Rundle, R. E., and Godycki, L. E., Department of Chemistry, Iowa State College, private communication. (27) Sugden, S.,J . Chem. Soc., 1932, 246. Ihid., 105, 2187 (1914). (25) Tschugaeff, L. .1., (29) T-ander Haar. R. T\-., Voter, R. C., and Banks, C. V., J . Ore. Chem., 14, 830 (1919). (30) Voter, R. C., and Ranks, C. V.,XSAL. CHEJI.,21, 1320 (1949). (31) Voter, R. C., Banks, C. T., and Diehl, H., I b i d . , 20, 458, 652 (1948). (32) Westrum, E. F.,Jr., and Piteer, K. S., J . A m . Chem. SOC.,71, 1940 (1949). (33) \\-illis, J. B., and Mellor, D. P., I b i d . , 69, 1237 (1947). RECEIVED M a y 28, 1951. No. X I 1 in a series on “Chemistry of the uic-Dioximes.” Contrihution 164 from the Institute for Atomic Research a n d Department of Chemistry, Iowa State College, Ames, Iowa. Work performed in the Anies Laboratory of the AEC.
Mass Spectrometric Determination of Molecular Weights of Components of a Mixture MURRAY EDEN, BERN4RD E. BURR, AND ARNOLD W. PR.ATT National Cancer Znstitute, AVationalInstitutes of Health, Bethesda, Md.
P
RESENT methods of qualitative analysis using t h e mass spectrometer are dependent on t h e availability of a catalog
of spectra for all compounds t h a t are t o be identified (‘7). T h e rapidly growing literature dealing with mass spectrometer analysis affords a reasonably extensive compilation for such comparative studies, b u t in certain fields of application such d a t a are decidedly incomplete; thus there remains some difficulty associated with identification of unknown compounds or a mixture of unknown compounds. T h e desirability of procedures t h a t would supplement t h e usual methods is apparent. This paper describes a procedure for determining t h e molecular weight of unidentified components by the use of mass spectrometiic data. It has long been known t h a t t h e rate of effusion of a gas is in-
versely proportional t o t h e square root of t h e molecular weighta relationship first accurately formulated b y Graham (2). T h e hydrodynamic properties of t h e molecular beam in t h e mass spectrometer inlet system have been described in detail b y I-Ionig ( 3 ) and recently this diffusion property has been used b v Friedel and Sharkey ( 1 ) t o determine t h e concentration of D + in t h e spectrum of HI) containing a n appreciable concentration of hydrogen. I n fact, it is standard practice t o use t h e diffusion rate of n-butane as a means of calibrating the mass spectrometer (8). T h e inlet s j s t e m of t h e mass spectrometer a t the authors’ disposal is equipped mith a gold foil containing txvo holes approximately 0.043 mm. in diameter. Generally, the piessure on t h e high pressure side of the orifice is initially maintained at 40 microns. With this magnitude of pressure and considering the
ANALYTICAL CHEMISTRY
1736
from the data iserr fourid t o agree with the accepted \alrirb to hettcr than 17~. The cninpiitcd molecular weights of a niirticre of organic ~ 1 1 1 pounds were within ty'of the accepted %slues. \Iea*urement of the effusion rates of gaseous enmponcitts should extend the rffrrtibcnrqs of ma49 spectrometry in so far as it contrihutec to qualitative ana1)sis. It may also he of taliie in stridier of molecirlar a3sociation in thr tapor phasr and, more geiirrall>, in studies of t a p o r phaw reart inn rquilihria.
The application of mass spectrometrj to the analysis of mixtures has been limited by the inability to identify compounds without prior knowledge of the mass spectra of each component. The present study was undertaken to see whether gaseous effusion rates measured by the mass spectrometer might be used to calculate the molecular weights of the components in a volatile mixture. The method was applied to a mixture of permanent gases and the molecular weights of the components computed
peaks representing ionic. fr:ipiiwnl P ilc~rivcdfrom t he szinic 11701rcular species will h a w identical rffusion rate ronst,arits.. T h u s i f the effusion rates of an idrntifiabli~romponetit and ) i n unkiio\r.n component are m r a s u r d in t h r saiiic' mixture, t lit- ino1ccul:ir weight of thc unkiiown raii hr 1~a1c~ul;itrd utilizing C;ixh:iiii's expression
ratio between the foil orifice diameter and the molecular mean free path (under these conditions the mean free path of nitrogen is about 1.4 mm.), one expects that gases flowing through the leak will obey Graham's law closely ( 4 ) . The mass spectrometer is readily adaptable as a microeffusiometer. It supplies a measure of the partial prcssure of each gaseous component of the sample. The peak height corresponding t o a n y ratio of mass t o charge is a function of the partial pressures of the molecular species in the sample. This relationship can be represented by the equations
Hzk
= Szk
X PE
k,(ni.
H?k
..
(1)
(2)
Hni
=
rr(ni. w t . ) z ' / 2
(6)
Peaks t h a t reprcsent the suin of ionic bontributiona f r o i i i iiiorc than one componcnt ran readily be distinguished from p ( ~ i k s arising from a single njolerular spwies. Expressions of t hr form of Equat,ion 5 niay be m i t t c n for rarh component taken hy itwlf, but it is obviouF from Equation 2it that, In H ; / H k plottrd ag:iinPt time will not be a straight lint. i f t \ r o or moR components i'oiitribute to this peak lwight.
for a single component, and
Hk = Hin
\r.t.),1/2
01'
I'ROCKDtJRK
Liquid samples of ahout 0.0005 nil. were introduce11 i i i t t i thc mass spectronietrr through a sint,ered disk (6). Gas samples were introduced by connwting t,he gas holder tn tmheinlct systcni by means of a standard-taper joint. The portion of the ~ a n i p l c in the leak manifold (ahout 300 nil.) w a isolated ~ from the r r s t of the system. A Puitahle pnrt,ion of t,he mam speririiiii \ive(imrs i n an
1737
V O L U M E 2 3 , NO. 1 2 , D E C E M B E R 1 9 5 1 elapieJ time of 30 to 45 niinutes. T h e v:iIues of log H,O/Hk \yere plotted against elapsed time (Figure 1 ) . ;\II instrumental constant. ci(ni. wt.);i/*, may be computed froni the effusion d a t a of any substance of known molecular wc’iglit, In practice, however, accuracy is gained by introducing :i conipound of known molecular weight as a n internal standard iii t w h determination. This niininiizes the vffect of ,changes in ti~niperature and small variations in ion potential, as well as second-order deviations from Graham’s law. T h e authors have chosrn the internal s t a n d a d so thiit its inolecular weight is not gwuter than about twice the p~~oliablr nioleculnr n-eight of the lightest component nor less than one Iiali’ the molecular weight of t hi. heaviest component. This critt.i,ioii was itssigned arbitrarily, but a n idea of its significance may be gaiiitd from the following e m mple.
l‘lie n~olecularweight of hydrogen detei,riiiiied with oxygen TVNS found to be 1.82. This ran hardly I i r iiiterpreted t o be anything other than hydrogen, but it does represent a n error of about 10%. T h e relative error clecreztses :I:: the ratio (m. wt.)k/(m. wt.)L approaches 1.
83 is different from the mean molecular weight of molecules contributing t o mass number 85. T h e ion of mass number 83 must contain two atoms of Cl“, while the ion of mass number 85 will contain one atom each of (21% and ( 2 1 3 7 . I t is a simple statistical operation t o calculate the average gram molecular weights of molecules that can ionize t o HCCI$’+ and HCC1WI37+, respectively. T h e relative amounts of CIS and C137have been taken t o be 0.746 and 0.254, respectively ( 5 ) . T h e mean molecular weights of s-tetrachloroethane for these t u o conditions are 167.41 and 169 03, respectively
Table 11. Condensable Vapors (‘oncentration
\lass No.
97.00
39 52 78 41 55
d prcip. H i , napne
5%
:w the internal sbandard
For permanent gases oxygen liuv been used as the internal stuadard, assuming a molecular weight of 32 and evaluating the eft’usion c o w t a n t determined froni the attenuation of the peak :it r r i e 32. For organic components in the range of molecular sytxiglits 30 t o 168, the benzene p w k ut 78 is genrriilly wtisfactory.
Cycloprntanonr
1.00
Chemiral Rlolecular Wt. 78.11
Molecular 13-t. Calcd. 77.2 78.5
84 11
84
Acetone
I 00
43 58
38 08
s-Tetrarlilorovtlia ne
1.00
83 85
187.415
169.03
(7Wa 84.1 84.7 87 5 58.1 58.4
178.3 177.0
Rcnzrnr ion current a t mass-charge ratio 78 was eniployed as internal atiindartl and assigned molecular weight of 78. h Srr t m t for discussion of mean molecular \veiglits of s-tetrachloroethane. u
RESULTS
Tiit. v:ilidity of this method WH checked by running several Iiiixtuwj of known components. Krsults obtained for the principal components in a synthetic* air nii?iture itre presented in Talilti I. T h e extent of agrwnir~ntc21n Lie wen b y comparing ( ~ 0 1 u i i i 13 1 ~s n d 4.
Table I.
Prrriiauent Cases
Concentration. Species
%
O X .\le (‘On .i
22 76 00 65
\[ass
so,
32 “8
0 45
44
0.90
40
.\lolerular 11-t Calcd.
The reproducibility of effusion constant for any given constituent is about 0.5yo. I n the case of the permanent gases the determination of molecular weight is in error h y 1%. The condensable vapois have a n equally good reproducibility for the rates of effusion, but in a few cases--e.g., s-tetrachloroethanethe molecular weight is in error by 4%. It is possible that this error m a l he decreased by choosing a n internal standard with a molecular weight more nearly equal to the molecular weight of s-tetrachloroethane. DISCUSSION
;):;(
44 3
39 9
,’ O Y ?8i.n uwd as internal standard u u d awignzd niole.cular weight 32.
The results of a four-component organic. mixture are presented i n Table 11. Figure 1 is a Ieprrsentatiori of the curves from \vhicli the diffusion constants were obtainrd. Only one curve for each component has been plotted, the other curves for each component being virtually suprriniposable. I t can be seen t h a t tlirre of the plots of log e / H k against time for the mass numbers are straight a n d pass through the origin. In the case of mass uumber 85, a curve drawn through all the e q w i m e n t a l points drviatev slightly from linearity. Reference to scans of the pure substancw shows t h a t componrnts of both s-tetrachloroethane and cyclopentanone contributed to the peaks a t mass numbers S3 and ai. Even though the coiitribution of the cyclopentanone K:IY about 5% of the total a t m s s number 85 a n d 3.5% a t mass number 83, t h a t may have been sufficient to cause the deviations froni linearity. I n view of these considerations, the slopes of the plots of mass numbers 85 and 83 were determined from t h e last s e ~ e i points, i so t h a t the lines extended t o t o do not pass through the origin. If the lines had Lern drawn through the origin, tlir calrubt+ values of molecular weights would have been 152.6 i n the case of mass nunilwr 85 and 171.0 in t h e case of mass IluInlIer 53. Interpretation of the brhavior of‘ s-1,1,2,2-tetrachloroethane iii this mixture is complicated by another special consideration. Iri the first place, the mean niolecular weight of molecules of s-tptrac’hloroethane t h a t can contributr ions of mass number
T h e range of applicability of effusion rates to the determination of molecular weights cannot be assigned on the basis of the results obtained so far. Several limitations may be anticipated. Any molecular species that undergoes association or a n y components that react under the conditions of t h e experiment will produce spurious results. Similarly, compounds t h a t are strongly absorbed on the walls of the apparatus may not be suitable for such a computation. Of t h e several dozen compounds studied thus far, only a a t e r fails t o produce a reasonable result. There is evidence of a second-order deviation from the behavior predicted by Graham’s law, but it may be minimized by choosing a suitable internal standard. These second-order effects will be discussed in a later publication. LITERATURE CITED
(1) Friedel, R. A . , and Sharkey, A. G., Jr., J . Chem. P h y s . , 17, 584 (1949). (2) Graham, T., Phil. Trans., 136, 573 (1846). (3) Honig, R. E., J . A p p l i e d Phys., 16, 646 (1945). (4) Knudsen, &I.,Ann. P h y s i k , 28, 75 (1909). O., and Hanson, E. O., P h y s . Rev., 50,722 (1936). (5) Piier, (6) Taylor, R. C., and Young, W. S., IND.ENG.CHEM.,ASAL. ED., 17, 811 (1945). ( 7 ) W-ashburn. H. TV., “Mass Spectrometry,” pp. 587-639, in ”Phys-
ical Methods in Chemical Analysis,” W. G. Berl, ed., Piew York, Academic Press, 1950. (8) TTiley. H. F., “Operation and Maintenance of the Consolidated Engineering Corp. hlass Spectrometer,” Vol. I, pp. 66-7, Consolidated Engineering Corp., Pasadena 4, Calif., 1946.
RECEIVED April 6 1951
