measure of the error in determining the added n-parafiins. Table VI compares data obtained from a detailed mass spectrometric analysis of a C g cut n-ith t h a t obtained by the mass spectrometer hydrocarbon-type analysis, follon-ed by calculation of the n-paraffin and isoparaffin values. Mass spectrometer data in Tables IV, V, and VI were obtained using paraffin parent peak sensith-ity ratios shown in Table I. DISCUSSION
The major value of this method lies in its ability to show directly changes in the isoparaffin-n-paraffin ratio in full range gasolines as a result of various processes. This is shown clearly in Table IV, where reformer feed and reformer product data are listed. The feed and product are from the same operation. The difference between the isoparaffin and n-paraffin ratios is marked. The availability of such data is of prime importance in many refining and pilot plant operations. The accuracy of this method is limited primarily by the validity of the assumed isomer distribution at each carbon number, and to a lesser extent by the determination of the paraffin carbon number distribution. The data of Table I sholy rather wide variations in the monoisotopic parent peak sensitivities of the isoparaffins a t each carbon number. Hence it is not feasible to assume an equal-part distribution of the isomers at each carbon number in calculating the weighted average isoparaffin sensitivities. The isomers must be weighted to achieve maximum accuracy of analysis. The thermodynamic equilibrium data of Prosen, Pitzer, and Rossini ( 7 ) are here used to approximate the isomer distribution in each carbon group C6through CS. The CP through CI1 isoparaffin sensitivity data were obtained from a
single reformer feed gasoline. Although choice of these data is arbitrary, their use is based on the successful application of the method to the three nonolefinic gasolines shown in Table IV. The virgin naphtha is a Kuw-ait naphtha. The reformer feed is a gasoline produced by hydrogenation of the material obtained from tliernial and catillytic crackingof a blend of four California crude oils. The reformate is the same gasoline, catalytically reiomitd. The satisfactory determinatio:i of the nparaftin content of the three gasolines demonstrates the accuracy and versatility of the method, when the data of Table I are used. Obviously, the empirical nature of such data may preclude t,hcir use in certain operations-eg., isomeriz%tion--. where the process may introduce a marked change in isomer equilibrium. In these cases a different weighting may be required. I n all cases, isomer weightings should be based on best available information, t o produce maximum accuracy of analysis. For example, isoparaffin sensitivity data based on the CC-C7 isoparaffin distribution d a t a from seven crude oils ( 8 ) may be used to supplement the isoparaffin sensii5ivit.y data here presented. -4program is being undertaken in this laboratory to establish less empirical isoparaffin sensitivity data at each carbon number, Cg through Cll, from the widest possible variety of gasolines. These data will be presented in a later communication. Until these data are available, the present empirical values will be used. Although best accuracy may not always be obtained, the method is providing d a t a for operating units that would not otherwise be available. ACKNOWLEDGMENT
The authors express their thanks to
the inanagcmcnt of the Union Oil L o . of California for permission to publish this paper. They appreciate the assistance of R. N. Fleck and C. G. Kight, who performed the Molecular Sieve extractions of the n-paraffins, and thank J. K. Fog0 for many helpful consultations and the preparation of the heavy gasoline cuts which serve as part of the calibration data. LITERATURE CITED
(1) Am. SOC.Testing Materials, ASTXI
Committee D-2 Report, Appendix IV, Sec. O(C), 1956. ( 2 ) Brown, R. A . . Consolidated Encrineering Corp., Mass Spectrometuer Group Rept. 71 (November 1949,.
(3) Fleck, R. N., Wight, C. G., private
communication(4) Leith, JV., ANAL. CHIXI. 23, 49.1 (1951). (5) Lumpkin, €1. E., Thomas, E. \I-., Elliott, Annelle, Zbid., 24, 1398 (1952). (6) Nelson, K. H., Grimes, M. D., Heinrich, B. J., “Determination of nParaffins and n-Olefins in Petroleum Distillates,” Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, PIT. J., September, 1956. (7) Prosen, E. J., Pitzer, IC. S., Rossini, F. D., J. Research h‘utl. Bur. StunduTds 34, 403 (1945). (8) Rossini, F. D., Mair, B. J., Streiff, A. J., “Hydrocarbons from Petroleum,” Chap. 24, Reinhold, ?Sew York, 1953. (9) Schwartz, R. D., Brasseaux, D. J., “Determination of Normal Paraffins in Olefin-Free Petroleum Distillates by Molecular Sieve Sorption and Refractometry,” Division of Analytical Chemistry, 131st Meeting, ACS, Miami, Fla., April, 1957. (10) Sobcov, H., ANAL.CHEM.24, 1908 (1952). (11) Truter, E. V., Chem. PTOC. Eng. 35, No. 3, 75 (March 1954). RECEIVEDfor review February : , 1957. Accepted November 18, 1957.
Mass Spectrometer-Type Analysis for Olefins in Gaso me LOUIS MIKKELSEN,’ R. L. HOPKINS, and D. Y. YEE2 Petroleum Experiment Station, Bureau o f Mines, ,The inability of the mass spectrometer to distinguish between olefins and monocycloparaffins has hindered its use in analyzing hydrocarbon mixtures containing both compound types. This paper presents a method of making this distinction on the basis of differential mass spectra. Benzenesulfenyl chloride will react quantitatively with olefins in a hydrocarbon mixture to form a high boiling addition product. The vapor pressure of this product is low enough to contribute negligibly to the mass spectrum of the nonolefinic components in the mixture. By com-
U. S.
Department o f the Interior, Barflesville, Okla.
parison of mass spectra before and after treatment with this reagent, the olefin, monocycloparaffins, coda (cyclomono-olefins, diolefins, and acetylenes), and dicycloparaffin mass peaks may b e determined separately. The composition of the sample can then b e calculated by a slight modification of the commonly used hydrocarbontype analyses.
ples containing appreciable concentrations of olefins or coda (cyclo-olefins, diolefins, and acetylenes) compounds. A widely used method (4) requires a n auxiliary olefin analysis such as bromine number (3) or fluorescent indicator adsorption (a). It does not take dicycloparaffins into account. Another method ( 7 ) requires removal of olefins by bromination-steam distillation be-
M
’ Present address, National Research Corp., Cambridge, Mass. Present address, Kew York University, Instrumentation Laboratory, New York 53, N. Y.
for hydrocarbon-type analyses of gasolines by mass spectrometry previously reported have several limitations when applied t o samETHODS
VOL. 30,
NO. 3, MARCH 1956
317
fore analysis. It includes dicyclopara h s but not coda compounds. The method presented here permits the analysis of samples containing paraffins, monocycloparaffins, olefins, aromatics (benzene and alkylbenzenes), dicycloparaffins, and coda compounds by a combination of a simple, rapid chemical treatment and the use of differential mass spectra. The sets of mass peaks characteristic of olefinic compounds (olefins and coda compounds) are determined separately from those of nonolefinic compounds (paraffins, monocycloparaffins, dicycloparaffins, and alkylbenzenes). Concentrations of each of these six compound types may then be calculated by a modification of the Brown method (4). APPARATUS A N D REAGENTS
A CEC Model 21-103-C (modified) mass spectrometer having a room temperature inlet system was used in this work. Samples were introduced into the mass spectrometer by means of a mercury orifice system and a 0.001-ml. capillary dipper. Sample pressures (microns of mercury) were measured with the CEC Model 23-105 micromanometer (Consolidated Engineering Corp.). * Benzenesulfenyl chloride, the reagent used for the analysis, was not commercially available when this method was developed (now available from Cyclo Chemical Corp., 1930 East 64th St., Los Angeles 1, Calif.). The method of synthesis is similar to that described by Lecher and Holschneider (6). The compound was prepared by chlorinating benzenethiol a t -25" C. in carbon tetrachloride. Chlorine was allowed to flow slowly into 200 ml. of carbon tetrachloride a t -25" C. Twenty-eight grams of benzenethiol was added slowly while the mixture mas being stirred. A solid yellow complex, probably phenyldichlorosulfonium chloride, C&SC&, separated immediately and sometimes interfered with stirring. When interference became serious, the temperature of the mixture was allowed to rise to above -10" C., a t which point the complex dissociates into benzenesulfenyl chloride and chlorine. The remainder of the benxenethiol, usually only a small amount, was added a t this temperature. After addition of the benzenethiol, the carbon tetrachloride and excess chlorine were removed by using a water aspirator. The product was transferred to an all-glass still and distilled a t reduced pressure. The reagent distilled a t 70" C. a t 4 mm. The yield was approximately 85%. The reagent must be prepared only in small quantities-about a one-month supply-because it has a tendency to decompose, evolving hydrogen chloride upon prolonged storage. It should not be stored in a sealed ampoule or a sealed screw-cap bottle, as sufficiently high pressures to explode the container may develop. 3 18
ANALYTICAL CHEMISTRY
PROCEDURE
The sample treatment may be summarized in the following manner : A mass spectrum of the original sample is obtained. About 0.5 ml. of the sample is placed in a small test tube. The sample is cooled t o 0' C., and the reagent is added, one drop at a time, until an excess is present. The excess reagent is removed by shaking with mercury, and the precipitate formed is separated by centrifuging. A mass spectrum of the treated sample (supernatant liquid) is obtained.
Because of the possibility of evaporation losses, the treatment was applied only to depentanized gasolines. Benzenesulfenyl chloride reacts with olefins to form p-chlorosulfide addition products (6). The reagent itself is dark red; the addition products are virtually colorless. Materials are present in some thermally cracked stocks that react to give colored products. These stocks can be analyzed, but the calculation procedure described here must be modified in order to do so. As the reagent is added to the sample one drop a t a time, the color of each drop is neutralized until no unreacted olefins remain in the sample. The sample is cooled to 0" C. while the reagent is added. This is done because the reaction is exothermic, and if the olefin content is high a considerable amount of heat is generated; and because it reduces the possibility of the reagents reacting with compounds other than olefins. Reagent is added a t the rate of one drop each 2 minutes until an excess is noted, as indicated by the color. The excess reagent may be removed by adding mercury and shaking. The mer-
Table 1. Peak Heights from 0.001 MI. of a Commercial Gasoline and Treated Olefin Blend m/e
41 43 55 57 67
Reaction Product from Olefin Blende 12.6 7.1 5.9 17.0 1.8
3.1
85
1.1 1.2 0.4 1.8
91
92
95
105 106 112 241 243 277 0
b
P P
0.1 0.0
4.4 23.1 26.3
8.0
1 . 9 microns. = 33.2 microns. =
Commercial Gasoline* 2430
2604 1416 1590 145 587 650 64 61 62 508 351 301 102 21
85 85 96 4941 5195 725
cury forms a solid precipitate with the excess reagent, and this is separated from the liquid by centrifuging. I n practice the supernatant liquid has a very light yellow color. The supernatant liquid is a mixture of the nonolefin hydrocarbons present in the original sample and the p-chlorosulfide reaction products of the reagent with the olefin compounds. The vapor pressure of the reaction products formed is low enough so that their contribution to the mass spectrum of the remaining hydrocarbons in the supernatant liquid is negligible. To obtain an indication of the magnitude of this contribution, a synthetic blend containing approximately 15 pure olefins having an average molecular weight of 98 was prepared. A portion of this blend was made to react completely with benzenesulfenyl chloride, and 0.001 ml. of the reaction product was introduced into the mass spectrometer inlet system. A pressure of 1.9 microns was obtained, compared with about 35 microns obtained from an equal volume of an average depentanized gasoline. Table I shows the principal peak heights resulting from the olefin reaction product, compared with those obtained from an equal volume (0.001 ml.) of a commercial unleaded gasoline. Summation peaks commonly used in hydrocarbon-type analyses are also shown for the gasoline and the reaction product. It has been concluded that the contribution of the reaction product to the mass spectrum of the supernatant liquid can be neglected. CALCULATIONS
When the mass spectra indicated above have been obtained, the summation peak heights indicated in Table I1 are measured and calculated on both records. The summation groups in Table I1 are slight modifications of those introduced by Brown (4) and Lumpkin ( 7 ) . The ratio 2 7 7 B / 2 7 7 A (where 2778 is the summed peak height for the original sample and z i 7 A is the value after treatment) is obtained. As alkylbenzenes do not react nith benzenesulfenyl chloride, and olefins do not have appreciable peaks a t these masses, 2 7 i B / 277n is the ratio of the number of moles of alkylbenzenes present in the mass spectrometer inlet system after introduction of the original sample to the number of moles present after introduction of the treated sample. The mass peaks contributed by other nonolefin compounds should vary by the same ratio. Therefore, multiplying each of the summation peaks for the treated sample by 2 7 7 B / 277n has the effect of calculating the part of these peaks in the original sample that is contributed by nonolehic compounds. Subtraction of these values
r
'35
Table 11.
.?.
Group 241 243 267 267'
?ZCi-
, .~
x , e c ->
-
L
z '
4cQ
Summed Peak Heights for Hydrocarbon-Type Analysis
Peaks Summed 41 + 5 5 + 6 9 4-83 43 + 5 7 + 7 1 +85 67 81 95 67 68 81 82 95 96 77 4-78 + 7 9 $91 92 105 106
+ + ++ + + + +
277
-
w z
,", , 0 0 ;-
60
+ +
Hydrocarbon-Type Represented Olefins and monocycloparaffins Paraffins Dicycloparaffins Coda Alkylbensenes
+
r
-i
I
Table 111. 8 '0
23
1 A3
SE.~S,T,, T Y
,
1
,
,
I
Summation Peak Heights for a Commercial Gasoline
#
Olefin-Free Peaks (Treated Sample) ( X 1.262) 2232 3250 153 282 4959
I
'00
6C :O OF r - a L - C v E
823 AT NLSS 4 3
160
Original Sample 2924 3489 410 611 4959
Group 24 1 243 267 267' 277
Figure 1. Sensitivity of 243 in mass spectra of aliphatic paraffins 300
Treated Sample 1769 2575 121 224 3930
Olefin Peaks 692 230
..
329i
..
I
Table IV. ~
Dicycloparaffins Paraffins Monocycloparaffins Alkylbenzenes 17 '0 20
'
I
I
d0 60 80 S E N S I T I V I T Y OF " - B U T A N E
,
/ 100
' 120
0.032
0.110
...
... ... ...
... ...
*..
...
267 243 0:003
.. . ...
...
__ 267
__ 267
241
277
...
...
...
0:043
...
0:002
Sensitivity 267 = 98 divisions per micron. n-Butane sensitivity at mle 43 = 70.
1~
'
Calibration Coefficients
243 277 267 267
241 267 0.700
140
A T M A S S 43
Figure 2. Sensitivity of 241 in mass spectra of cycloparaffins 300
Table iic
II-
I I
L
LL
E
100
60
P O0
20
40
SE"iITIV1TY
from the peak heights of the original sample gives the contribution of the olefinic compounds. The molar volume of the p-chlorosulfide reaction products, which are introduced with the treated sample, is appreciably greater than that of the original olefins. Therefore, alkylbenzenes are less concentrated on a volume basis after treatment, and 277B/ 277~ is always greater than 1 when olefins are present. A typical set of such peaks is shown in Table 111. The average paraffin-cycloparaffin
Synthetic Blend
Paraffin-monocycloparaffina
Benzene Toluene Ethylbenzene 1-Hexene 5-Hexene 4-Methyl-2-pentene 2-Heptene 1-Octene 2-Octene
180
140
Figure 3. Sensitivity of 2 4 1 in mass spectra of mono-olefins
V.
Component
60
83
I00
,
'
20
$0
I
1
2,4,4-Trimethyl-l-pentene
1-Decene
Volume 50.0 5.0 35.0
5.0
l.O\
2.7'
\) l . 0 Ii 0.3J
5.0 100.0
OF n - e U T A N E AT V A S 5 4 3
Figure 4. Sensitivity of spectra of aromatics
277 in mass
Table VI.
Component Paraffins Monocycloparaffin Dicycloparaffinsc Alkylbeneenes Olefins Codac
a
Silica gel concentrate from a gasoline.
Analysis of a Synthetic Blend
Blended 50 0 45.0 5.0 0
Volume yo M.S. analysis0 41.8 8.9)50.7
Standard Deviation*
44.6 4.7
o:ii 0.19 ..
0
0
0.14
0.25
Typical analysis. Based on 10 analyses. * Value zero for all analyses. 5
b
VOL. 30, NO. 3, MARCH 1958
319
~ _ _ _ _ _ _ _ ~ _ _ _ _ _ _ _ _ _ ~
~
Table VII.
Compounds Used in Stock Blends
Paraffins and Monocycloparaffins 2-Methylpentane" 3-Methylpentane" 2,3-Dimethylbutane* n-Heptane" n-Octaneb Iso-octanec Cyclopentanec CyclohexaneB hIethylcyclohexaneb Cyclomono-olefins 1-Methylcyclopentenee 1-Methylcyclohexenee 1-Ethylcyclohexenee
Olefins 2-Pentenee 1-Hexenea 2-Methyl-2-pentened 4-Methyl-2-pentenea 2-Heptene" 3-Heptened I-Deceneu Diolefins 1,2-Pentadienef l-cis-3-Pentadknee 1-truns-3-Pentadienee 1,4-Pentadienee 2,3-Pentadiene6 1,S-Hexadiene' 2,CHexadienef 2,5-Dimethyl-2hexadienef
1,3-Dimethylcyclohexenef 3,5,5-Trimethylcyclohexene 3,3,5-Trimethylcyclohexene' Alk ylbenzenes Benzeneb Toluene* Ethylbenzene"
Phillips Phillips Phillips Phillips
Table VIII.
standard sample. '' APT API-45 Ohio State University. Filtered through silica gel.
pure grade. research grade. Spectro rade. technicaf grade.
Typical Analysis of Synthetic Blends Containing Diolefins and/or Cyclomono-olefins
Component Paraffins Xonocycloparaffins Die ycloparaffins hromatics Olefins Diolefins Paraffins Monocycloparaffins Dicycloparaffins .iromatics Olefins Cyclomono-olefins Paraffins Monocycloparaffins Dicycloparaffins -4romatics Olefins Cyclomono-olefins and diolefins
Table IX.
37
Paraffins Monocycloparaffins Dicycloparaffins Aromatics Olefins Coda Triob Paraffins Monocycloparaffins Dicycloparaffins Aromatics Olefins Coda
AIS
oil" ~$U 1
0
35.0
- j
l.5i
37.4 11.2
1; 1
+
49 9
3-4 9
36.2
0 3,5. 0 10 3
39.0
Paraffins, P, = P P' Monocvclouaraffine, CP, = CP CP' Olefins; 0, A O - CP' Dicycloparaffins, D, = as originally calculated Coda = as originally calculated Aromatics, A , = as originally calculated
Analysis of Four Gasolines
FI.4
Br S o .
Catalytic Reformate I E( 26.97 10.2137.3 36.4 . ., . 0.2 60.5 62.2 ... 2 . 22 1.4 2.1 ... ... ... Commercial Gasoline 2B 35.7) 16.3 53.5 51.4 ...
A6:;) :;)
, I
0 34 8 10 2'
0
Br S o
51.4
12
Volume % Component
T-olume 76 11s FIA
Blended
T'olume __ .~ C,; 1185 FII Br S o . Ttiernlallj. C r d x d Gasoline i 26 4)
60.6
...
12.2
20.6
...
12." 12.8127.5
18.8
32.3
30 30.9160.3 3 0
2.4) L +) Straight-Run S a p h t h : ~5 33.5' 5 9 . 5 ~ 9 3 . 0 92.7 ...
320
ANALYTICAL CHEMISTRY
+
Sensitivities are chosen from the curves given in Figures 1to 4 for P,, CP,, 0,. and A,. The sensitivity value for D,is given in Table IV. The coda sensitivity is 0.20 X sensitivity 277 for alkyl benzenes in the case of diolefins and acetylenes, and 0.79 X sensitivity 277 for cyclo-olefins. If something is known about the distribution of these compound types, a weighted value of the sensitivity may be used. If not, an average value of 0.65 will suffice. The corrected peak heights are divided by the appropriate sensitivities to obtain partial pressure contributions. The six partial pressures thus obtained are multiplied by mole-to-volume factors, and the composition in volume per cent is calculated.
1 ) I
a Obtained by using treatment described, but with a niodified method of calculation. It is included for illustration only. A class of compounds including triolefins, cyclodiolefins, cyclomono-olefins with unsaturated side chains, terpenes, and others, found only in thermally cracked gasolines and not normally considered as a component.
'
and aromatic molecular weights are calculated from the mass spectrum (1). Calibration coefficients given by Brown (4) are selected for paraffins, monocycloparaffins, and alkylbenzenes. Coefficients used for dicycloparaffins are shown in Table IV. With the coefficients selected, four simultaneous equations are set up and solved for the summed peak height contributions of paraffins, P, cycloparaffins, CP, dicycloparaffins, D, and aromatics, A, using the olefin-free peak heights calculated according to the procedure outlined above. Paraffins and naphthenes react slightly with benzenesulfenyl chloride. This effect is small, and corrections can be made a t the same time the olefins are calculated. Using the difference, peaks obtained for 243, 241, and 267', with the appropriate calibration coefficients as given by Brown (C), three simultaneous equations are set up and solved for the summed peak height contributions of paraffins, P',olefins, 0, and coda compounds. P' is added to P to obtain the corrected paraffin peak height. The assumption is made that paraffin and monocycloparaffins react in equal amounts, and P' is multiplied by0.76, the sensitivity ratio of monocycloparaffins to paraffins, to give a value CP'. This value is added to CP and subtracted from 0. The corrected, summed peak heights arrived at in this manner are:
RESULTS AND DISCUSSION
To establish the repeatability of the method, a synthetic blend (Table V) FTas prepared and subjected to ten independent analyses by the procedure outlined above. The standard deviation based on the ten analyses for each of the compound types is shown in Table VI,
together with a typical analysis. Several blends containing diolefins and/or cyclomono-olefins were prepared to determine the applicability of the method for samples containing these compound types. Table VI1 presents the compounds used in preparing various stock blends, and Table VI11 presents the results obtained from the analyses. Four gasolines were selected from the ASTM Section F, R.D. IV, coordinated cooperative program and analyzed. The four gasolines selected were: catalytic reformate (Sample lB), commercial gasoline (Sample 2B), thermally cracked gasoline (Sample 4), and striaght-run naphtha (Sample 5). The analyses of these gasolines, obtained in this laboratory by this method. are presented in Table IX. All the blends and the four gasolines were subjected to fluorescent indicator adsorption analyses ( 2 ) (standard wall and maximum intensity method), and olefin analyses from bromine number ( 3 ) obtained by D 1158-55T. for comparison with mass spectrometer results.
SUMMARY
A hydrocarbon-type analysis for gasolines containing paraffins, monocycloparaffins and dicycloparaffins, alkylbenzenes, olefins, and coda compounds is possible with the use of a simple chemical treatment and differential mass spectra. The treatment requires about 2 man-hours. Accuracy for olefins is comparable to that of fluorescent indicator adsorption and bromine number analyses in samples low in olefins. The data presented indicate that aromatic and olefin concentrations obtained by this method may be more reliable than fluorescent indicator adsorption or bromine number analyses with samples containing appreciable concentrations of coda compounds. ACKNOWLEDGMENT
The authors wish to acknoa-ledge the n-ork of Loyetta Curnutte in obtaining the mass spectra used. Jack H. Hale in ohtaining the fluorescent indicator ad-
sorption and bromine number analyses, and R . A. Brown and the mass spwtronieter group of the Atlantic Refining Co. for permission to use the four figures presented. LITERATURE CITED
(1) Am. SOC.Testing Materials, “ASThX
Standards on Petroleum Products and Lubricants,” Appendix IV, p. 953, 1956. (2) Ibid., Method D 1319-56T, p. 751. 1956. (3) Ibid., Method D 875-53T, p. 355, 1956 14) . , Brown. R. A., ANAL. CHEJL 23, 430 (1951). (5) Lecher, IS., Holschneider, F., Ber. 57, 755 (1924). (6) Lecher, H., Stocklin, Ibid., 58, 414 (1925). ( 7 ) Lumpkin, H. E., Thomas, B. W., Elliott. Annelle. ANAL. CHEJI. 24. 1389 (1952). ‘ RECEIVED for review June 12, 1957. Accepted Sovember 22, 1957. Work partly financed by contractual arrangement between the Department of the Army, Office of Chief of Ordnance and the Bureau of Mines. ASTM Committee E-14 on 3Iasj Spectrometry, Cincinnati, Ohio, 1956
Low Voltage Techniques in High Molecular Weight Mass Spectrometry H. E. LUMPKIN Research and Development Division, HiJmb/e Oil and Refining Co., Bayfown, Tex.
,The mass spectra of double bondcontaining molecules can be greatly simplified b y lowering the ionizing voltage in a mass spectrometer so that the energy available i s sufficient to form the molecule ion, but too low to form fragment ions. Use o f the low voltage technique in the higher boiling ranges of petroleum simplifies interpretation of the spectra. Aromatic and olefin calibration d a t a and examples of their use are given.
L
ow
VOLTAGE techniques in mass spectrometry have been used in these laboratories for some time for the analysis of olefins and aromatics in the gasoline boiling range. Simplification of the spectra a t the low ionizing voltages at which they are obtained by this technique also leads t o simplified calculation methods. As the mass spectra of petroleum fractions become more complex with increase in boiling range and carbon number, it is natural that a n y technique which results in simplification of the spectra and their interpretation is eagerly sought. This paper describes application of low
voltage techniques to the higher boiling fractions of petroleum. The principle of the method and many of its advantages and difficulties have been adequately discussed by Field and Hastings ( 2 ) ; therefore. this phase is covered in a very cursory manner here. The mass spectral peaks, both parents and fragments, of monoolefins, cyclic olefins, dicyclic olefins, and aromatics are often obscured by fragment peaks from these compound types as well as by parent and fragment peaks due to paraffin and naph,thenes in normal spectra produced with 70-volt electrons. When the mass spectrometer is operated SO that the bombarding electrons have only sufficient energy to form the molecule ion and insufficient energy to cause carbonhydrogen or carbon-carbon bond cleavage, only parent ions are observed in the spectra. If the ionizing voltage is selected at a relatively low value, only the molecule ions of compounds having an ionization potential a t or below the selected voltage are formed. -4s most of the paraffin and naphthene hydrocarbons have ionization potentials in the 10- to 13-volt range (*:),
and the double bond-containing niolecules have ionization potentials belov about 10 volts, it is possible t o select a n ionizing voltage which will give a spectrum composed almost exclusivelj of the parent peaks of double bondcontaining molecules. This is the principle of the method. K i t h proper calibration data this permits determination, by molecular weight groups, of mono-olefins, cyclic olefins and/or diolefins, dicyclic olefins and/or cyclic diolefins, and aromatics, in the presence of each other and of paraffins and naphthenes. A major advantage of this technique in the analysis of heavy petroleum fractions is the fact that the absence of fragment peaks makes unnecessary the use of simultaneous equations. The occurrence and abundance of fragment peaks in mass spectra of high molecular weight materials are not easily predictable Thus, the values of interference coefficients used in characteristic fragment methods ( I , 4 6, 8) are subject to some uncertainty due to incomplete separation of calibrant concentrates and possible lack of specificity of the few pure compounds VOL. 30,
NO. 3, MARCH 1958
321
