oxalate, citrate, phosphate, phthalate and bicarbonate buffers have been determined. Attempts to determine the composition of the complex in acetate buffers gave anomalous re-
sults. Oxalate, citrate and phosphate destroyed the complex. Tests in phthalate and bicarbonate buffers indicated a 1: 1complex, up to pH 7.75. ACSTIN 12, TEXAS
[CONTRIBUTION FROM THE SHELL
RECEIVED APRIL14, 1950
DEVELOPMENT COMPANY]
Mass Spectrometric Analysis of Low-molecular-weight Monodeutero-paraffis BY I>.P. STEVENSON AND C. I). WAGNER
The analysis of monodeutero-paraffns prepared in this Laboratory’ (all eight of the monodeutero-alkanes, C 4 4 ) presents certain problems which have been solved successfully by the use of mass spectrometric techniques. The general applicability of these techniques has made it desirable to treat these aspects in a paper separate from that dealing with the preparation and purification of the compounds. The mass spectrometer provides a sensitive means of detecting impurities in volatile substances, provided the impurities have molecular weights greater than the molecular weight of the substance under investigation or, more generally, are of such a nature that they produce ions not coincident in masscharge ratio with those of the substance under investigation. In general for impurities of molecular weight less than the substance under investigation to be determinable by the ordinary mass spectrometric analytical methods, a “pure” or standard sample must be available for comparison purposes. Such standard samples of monodeuteroparaffins are not available and ordinary methods of standardization are not applicable. For the determination of olefin and ordinary (undeuterated) alkane in the deutero-hydrocarbons, special methods were devised. These methods and the results obtained are described below. No description of the detection of impurities of higher molecular weight by the mass spectrometric technique is given here since these are adequately described in the literature. Dete&&on of Olefins.-As has been discussed in another recent paper,’ a characteristic side reaction in the synthesis of a paraffin by hydrolysis of a Grignard reagent leads to the formation of equal amounts of olefin and undeuterated para&. Since certain reactions of paraffins are very sensitive to trace olefin impurities,3 sensitive analytical methods are necessary to check the efficacy of chemical treatments employed to remove the olefins. In addition to having high sensitivity, the methods must consume a (1) C. D. Wagner and D. P. Stevenson. T m s JOURNAL, 79, 5784 (1850). The deuterium oxide used in the syntheses was supplied by Stuart Orpgen Company 011sllocation from the Isotopes Division, U. S. Atomic Energy Commission (2) H. W. Washburn, H. P. Wiley and S M Rock, I n d . Ens Chcm., Anal. Ed., I S , 541 (1943). (3) R C. Wackhcr and 13, Pines, Tma JOURNAL. 68, 1842, 2518
(lore).
very small sample, or be non-destructive, since the deuteroalkanes are laboratory preparations prepared on small scale. In principle, infrared absorption is applicable to this problem. Rasmussen and Brattain4 have observed that monoolefins, in general, have very strong characteristic absorption bands in the region 10-12.5 p where the corresponding paraffins have relatively very weak absorption. To the extent that monodeutero-paraffins do not differ significantly in their absorption in this region from that of the ordinary paraffins, infrared absorption measurements should afford considerable sensitivity to detection of olefins. However, for even the simple monodeutero-paraffins considered in this paper, i t is found that characteristic absorption bands do appear in this region (101 2 . 5 ~which ) render the method inapplicable in the absence of authentic standards for compari~on.~ Since the infrared method of olefin detection was found to be inapplicable, i t was deemed desirable to develop a potentially absolute mass spectrometric method for olefin estimation in both paraffins and monodeutero-paraffins. This method is described in the following paragraphs. Owing to the nature of the ionization process, the appearance potential of the ion-fragment, CnHzn+, in the mass spectrum of the paraffin, CnH2n+ttZ, or CnHZn+lD,is necessarily 1.1 to 1.4 ev. greater than the appearance potential of the ion, CnHzn+,in the mass spectrum of the olefin, CnH2,. This follows from the fact that the appearance potential of CnHZn+in the mass spectrum of C,H2n+2or CMH2,,+1Dis greater than or equal to the ionization potential of the olefin, CnHan, plus the heat of dehydrogenation of the paraffin, CnH2nfi or CnH2&1D, while the appearance potential of CnHzn+in the mass spectrum of the olefin C,Hz, is approximately equal to the ionization potential of the olefin, CnH2,. The heats of dehydrogenation of the G-C, paraffin lie between f 2 7 and +33 kcal./mole or 1.2-1.4 ev.6 Thus, it should be possible to measure the (4) R S Rasrnussen and R. R Brattain, J . Chcm Phrr., 15, 120 (1940). (5) The authors are indebted to various colleagues, particularly Dr. F. S. Mortimer for infrared absorption measurements on the v d o u s G-0monodeutero-paraffins A detailed account of the infrared absorption of these substances wi11 be pubtished at a later date. (8) Kisttakowsky, s l el., Txrs Jouan&, 67, 85. 876 (19a6)
Dec., 1950
r
100.0
MASS SPECTkOMETkIC
-
-
-
V
ANALYSIS OF hf ONODEUTERO-PARAFFINS
i
100.0
30.0
8
.* $ .* w .* P
30.0
d
10.0
.* a,
.e
8
3.0 1.00
vj s ._ E
h
U
1.00
+I
c
a,
2 4 38
0.30
.e
0
!5 in
3.00
t Lc.
. I
2
10.0
.-++I
ICI
. 3
5613
0.30
.e
0.10
0.03
0.10
0.03 0.01
12 14 18 Ionizing electron energy, V- in volts (uncor.). Fig. 1.-Ionization efficiency curves of the methane ions, CHI+, CHa+ and CH2+. The specific intensities a t V- = 75 volts are CH4+ = 348, CHI+ = 261 and CH2+ = 25.8. TOcorrect voltage scale, V-, to an absolute basis add 4.1 e. v. in this figure and Figs. 2-5. (A) CHd --r CH4' e-, (B) CHI+ CHd" H E-, (C) CHI + CH,+ H2 E - .
4
6
+ +
8
10
+ + +
mass spectrum of the paraffin preparation a t such an electron energy that the only source of ions CxHzx.+ in the mass spectrum will be C,H2, impunty. In Figs. 2 through 5 there are shown the variation of specific intensity (sensitivity) of the ions CnHzx+z+, CnHzn+l+ and C,Hzn+ in the mass spectra of the paraffins, CnH2?z+2, 2 < n < 4 and CnHz,,+ in the mass spectra of the olefins C,HZn, 2 < n < 4 , with ionizing electron energy in the vicinity of the appearance potentials of these ions. In these figures, the specific intensities are in arbitary units such that 0.01 unit is the minimum detectable signal. The specific intensity units are the same for all substances. The ionizing electron energy scale is in volts, uncorrected for contact potentials, field penetration or contributions from the positive ion drawing out field characteristic of the Westinghouse Type LV mass spectrometer. From the appropriate curves in these figures, it follows that the absolute limits of detection of the various olefins in the corresponding paraffins are: 0.25y0 CzH4 in C2H6 measured a t 6.5 volts, O . O S ~ o C3H6in C3Hs measured a t 6.0 volts and O.OlyoC4Hg in C4H1m measured a t 6.0 volts. If the standard paraffins contain any olefin, then the above limits would be correspondingly reduced.
4 8 8 10 12 14 16 Ionizing electron energy, V- in volts (uncor.). Fig. 2.-Ionization efficiency curves of the ethane ions, GHjf and C2H4+, and the ethylene ion, CgH,+. The specific intensities at V- = 75 volts are: ethane, C Z H ~= + 250, C2Hs+ = 167, and GH,+= 784; ethylene, C?H,+ = 572. The specific intensity of C2H5+ from ethanc has been corrected for contribution from the ion C1zC1aI-I~+ due to the natural C13 content of the ethanc (see legend t o Fig. 1). (A) CJI4 + C2Hdf E-, (B) GH6+ CZHI+ He e-, (C) C1H6 + CzHsf f E-, (D) G H 6 -t CZHS+ H e-.
+
+
+
+
In the case of the deutero-paraffins, the limits of detection of C,Hz, should be lower than those quoted for the ordinary paraffins since the formation of CnHz,f from CnH2,+1D requires the removal of HD from the molecule-ion, a process of lower intrinsic probability than that of removal ?, and thus the specific intensity of HZ from CnH21r+ TABLE I ANALYSES OF MONODEUTERO-PARAFFINS Mole % C~LH?~
hf.S. CHdi .. CzHs-di 0.1 C ~ H ~ - I - ~. n~R CaHs-2-di .Os n-C4Hla-i-dl .ni n-C4Hl0-2-dl .01 i-C4HlO-l-dl .01 i-C~Kio-Z-di .02
M.Sn 4.0 h 0 . 5
..... , . , , . .. . . , .. . . . . ... .. . . .. .... . . .. . . . . . . . .. . ,
Mole % CnHtni? M.S.b h1.S.C 4.2 in.; ,...... 2 . 3 * 1.0 . . . . , . . . . . . 2 . 1 i I .n . . . . . , . 1.0 t 1 . O
. ..
0.5
2 .o 2 .o 0.5
A
0.4
* 0.4 * 1 .o
*0.2
.. . .... ... .... . .. . . . . . . . . .. .
I.Rd
... .,. ... C 2 ~ -
Fig .i--Ioiiiratiori cfiiclenc) cui v e i 01 fhipropaxle IOIIS, C?llsc, C € 1 7 - d i d C1H6- aiid thc propylene ion Cd3b' T h e specific intensities .it 1.- = 7,i volt5 ax. propane, CIHs+ = 310, C.HiT = 'L4O.ind C-IIb = 41.4;propylene, ligilld to Flg 11. (A I C9IL + -+
4
f
-
Dec.. 1950
5615
hIASS SPECTROMETRIC ANALYSIS O F htONODEUTER0-PARAFFINS
preparation of C,Hz,+1D is to convert all of the hydrogen atoms of a portion of the sample to a mixture of Hz, HD and Dz, and then determine the relative concentration of the isotopic hydrogens and thus the atomic ratio of H to D in the preparation. A simple calculation permits the estimation of the CnH2,+2, provided this is the only hydrogen-containing impurity of the preparation. If a is the mole fraction C,HZ~+Z and 1- a the mole fraction of C,Hz,,+lD, then
100.0
30.0
g
.*
.".*$
2
10.0
Y
3.00
v.
'6 1.00
or Q!
= [1
+ (2n f l ) ( D / H ) I / D + (D/H)I
The most convenient method for completely converting a hydrocarbon to hydrogen is to expose the hydrocarbon to metallic chromium a t temperatures between 800 to 1000°. The large free energy of formation of the chromium carbides, Cr5Cz and Cr3Cz7assures the essential completeness of the reactions
+
+ +
+ 2)Hz or + 2)Hz
5nCr 2CnHzn+2+nCr6Ca (2% 3nCr 2CnH2,,+z+nCraCs f (2n
This method of conversion of hydrocarbon to hydrogen is preferable to combustion to water from which the hydrogen may be generated for analysis because it avoids the danger of exchange of D for H between water and strongly adsorbed ordinary water on vessel walls and other surfaces. The mass spectrometer gives a reasonably accurate analysis of the resultant Hz, HD and Dz mixture and the mass spectrum permits a sensitive measure of the completeness of the conversion of the hydrocarbon to hydrogen as well. The above described method was applied to the analysis of the methane-d sample and gave 4.0 * 0.5% CH4 in the CHBD. In the case of higher monodeutero-paraffins, the method, and any method depending upon determination of the gross D,IH ratio, suffers from the high dilution of 1 hydrogens present the deuterium by the 2n and thus tends to require an increasingly accurate measurement of the composition of the Hz, HD and Dz mixtures. A second method in the case of the monodeutero-paraffins of determining the C,HZ,+Z content of CnH2,+1D samples is one that uses the same principles as that of the mass spectrometric method of determining olefin impurity. The dppearance potential of the ion CnH2,+1+ in the inass spectrum of CnHZll+2and hence the appearance potential of CnHznDf in the mass spectrum of CnHza+1D is greater than the appearance potential of C%HZ,+Z+ in the mass spectrum of CnHzn+2 by 1 to 2 volts, i. e., by the difference between the dissociation energy of the first C-H bond of CnHzn+z+and the difference between the vertical and adiabatic ionization potential
+
(7) 11
K. K.Kelly, Bureau of Mines Bulletin 407, 1937, pp. 10 and
j,
Y .*
8
Y .e
0.30
v
'F.
aE 0.10 ' 0.03 0.01
4 6 8 10 12 14 16 Ionizing electron energy, V- in volts (uncor.). Fig. 5.-Ionization efficiency curves of the i-butane ions, C4H10 +, C4H9 and C& +,and the isobutylene ion C4Hs +. The specific intensities at V- = 75 volts are, isobutane CaHlo+ = 71.6, C4H9+ = 45.3, C4Hsf = 5.32; isobutylene, Ci"+ = 333 (see legend to Fig. 1). (A) i-C,Hs + '24Hs" e-, (E) Z'-CIHIO C4HlO+ e-, (C) i-C4Hio -.* ChHp" H e-, (D) i-CdHio C4Hs" Hz e-. +
+ + +
+
+
+
+ +
of C,HZ,+Z.~Thus, the mass spectrum of a monodeutero-paraffin sample may be measured a t such an ionizing electron energy that the only 2 (corresponding source of ion of m/q = 14n to CnHZn+2or CnHznD+)is the CnH2n+2content of the C,Hz%+lD. In Fig. 1 through 5 the course of the pertinent ionization efficiency curves for the five C1-C4 paraffins is shown. Examination of the figures referred to reveals that the limits of detection of C,HZ,+Z in C,HZ,+lD by the method outlined in the preceding paragraph are: ca. 0.370 CHI in CHBD with 9 volt electrons, cu. 1.0% GHs in C Z H ~ D with 7 volt electrons, ca. O.4y0 n-C4Hloin n-C4HloD with 7 volt electrons and ca. l.070 Z'-C4HIO in Z'-C4H9D with 6 volt electrons, if it is assumed that the course of the ionization efficiency curve of C,rH2,D+ in the C,Hz,+lD mass spectrum is essentially coincident with that of CnHZn+l+in the C,H Z % mass + ~ spectrum. The very near equality of the appearance potentials of C3H7+ and CaHe+ in the propane mass spectrum renders this method inapplicable to C3H7D samples. It may be noted in passing that the much larger difference between the appearance potential of cvHZ%-3+ and C,-
+
(8) See Koffel and Lad, J Chcm. Phrs., 16,420 (1948), for a summary of appearance potentials in the mass spectra of paraffins.
5616
D. P. STEVENSON AND C. D. WAGNER
Hzn-2+ (acetylenes) and C,H2,-1+ and CnHzn+ (olefins) than for CnHZn+l+ and CnHantz+ (paraffins)results in this method having much greater sensitivity for the determination of C*H,,-z in C,H,,-BD or CnH2,rin CnHzn--1D than is the case for CnHtn+z in C,zH2,L+lD. For instance, 0.05% C2H2 is readily detected in either CzHD or CzDzby this method. Through measurements of the mass spectra a t the above indicated ionizing electron energies the estimates of the CfiHzn+2 content of the CnHzn+lDJ methane-d, ethane-d, n-butane-1-d, n-butane-2-d, i-butane-1-d and i-butane-2-4 shown in column 4 of Table I, were obtained. I t will be noted that in the case of i-butane-2-d the sensitivity is greater than that predicted from the curves of Fig. 5. This results from the very low probability of dissociation of a primary hydrogen by an isobutdne ion.g As a result, the specific intensity of the ion of m,lq = 58 in the mass spectrum of isobutane-2-d is very small (compared to that of this ion in the mass spectrum of isobutane-1-d or the ion mlq = 37 of isobutane-do) and becomes less than the limit of measurement a t a higher ionizing electron energy than does 5& or 57 for isohutane-1-d or isobutane-do, respectively. A third method, which like the second method for determination of CnHen+*in CnHen+lDdepends upon measurements of the mass spectra a t low ionizing electron energies, is also available. It is well known that the specific intensities of most ions in the mass spectra of hydrocarbons are linear functions of the ionizing electron energy over a limited range of the ionizing energy. This range, for such ions as CnH~n+aC and CnHln+l+,starts 1.5 to 2.0 volts above the appearance potential of the ion and continues for 6-10 volts, or until the specific intensity has attained ca. 5Et-6Oo/G of the maximurn value characteristic of ionizing energies between 50 and 100 volts. It is further known that for similar processes, the intercept of the extrapolated linear portion of the ionization efficiency curve 011 the ionizing energy axis is a tneasure oi the Appearance potential.lU Thus, over the raiige of ionizing electron energies for which the specific intensities of both C,,H2n+LAarid C,Ht,,+: are linear functions of the ionizing energy, the ratio o f intensities, CnHtnD+/CnHan+lD in the mass spectrum of pure CnH2rr+113,will be equal to ;L constant, independent of the ionizing electron energy, times the the ratio of intensities CnH2,+L + , ' C , H ~+~in+ ~ tilass spectrum of pure CnHzn+2. This may be proved as follows: If the specific intensities qf CnHznD', Ct*Hzn+~D+jCnH2,+1+ and CnHzn+2+ (9) D. P. Stevenson and C. D. Wagner. J Chcm. Phys., in publication. (10) For discussions of the significance of ionization efficlency curves and their interpretation. we Stcvensou J Chrrn P h y s , 10, 291 (1942), Stevenson and Hipple. Phys Rcu, 62, 237 (19421 Mariner and Blenkney tbid 72, 507 r ? r l 4 7 ) , Tlought rbtd 71 93 (lq47'
1701. 72
areid-], idg, ih-land iho, respectively, for the pure compounds, a d - l Jado, ah-1 and $0 are the slopes of the linear portion of their ionization efficiency Ida, Ih-l and Iho the corresponding curves, and Id-l, intercepts with the ionizing electron energy ( V - ) axis, then, over the range of V- for which the i's are linear
v- v- - P-1) Po) = d o ( v- - P o )
i"l =
Id-1)
&-I(
(1)
= ado( A* = ah-l(V$10
iba
and [CnH?nD+/CnH2n+ I D + ] /[CnH2n+1+/CnHm+2+]=
v- - Po v- - F a
(2)
The intercepts, I , are measures of the ap earance and P-I, and Id0 and 0 , which potentials, Id.-] will only differ through differences in the zero point energies of the initial and final states, i. e., by the order of 0.02-0.05 e.v. The appearance potentials themselves are of the order of 10 to 12 e.\--; thus, V- - Id-1,'V-- - Ih-1 and V- Iho/V- - Ida, will each differ from unity by 0.5% a t most and probably in opposite sense. Thus
&
where r is a constant to better than *0.5y0 over the specified range of V-. As a consequence of the above, over the specified range of V-, the specific intensity i m of the 2, corresponding to CnH2n+z+ ion, m / q = 14n plus CnH,,D+ in the mass spectrum of the C,H2,$+ID preparation can be written i, = Sk[P - HI rP,n-lirnTl (4) where S l , [P - HI and pm-l are, respectively, the specific intensity of CnHzn+2+ characteristic of pure CnHzn+2,the mole fraction of C,H2n+2, and the characteristic ratio, CnH2n+l+/CnHzn+2+, and is the specific intensity of the ion, m/q = 14n 3, corresponding to CnIIzn+lD+. From measurements of the mass spectra of pure C,Hzn+Zand the sample of C,HZn+ID a t two ionizing electron energies, VI- and Vz-, in the specified range of V-, there are obtained a pair of simultaneous equations in the two unknowns, [P - HI and I?. In order for the pair of equations to be solvable, the available range of V - is subject to the further conditions that S m and Pm-1 have different rates of change with V-. In the case of propane, the ionization efficiency curves for CaHa+ and C3H,+ are both linear for 9.0 6 V - 6 16.0 e.v. (uncorrected scale corresponding to Figs. 1 through 5) and dSa/dVand dp43/dV- differ most in the range 9.06 V-c12.0 e.v. Thus measurements of iu and i d 5 for the propane-1-d and propane-2-d samples were made at 0.5 e.v. intervals over 9.0 'L
+
+
+
< -
