to 0.07 weight per cent sulfur after only 3 days of air-oxidation at room temperature. After 58 days of exposure, the sulfoxide sulfur increased to 0.20x. Additional exposure of the oil to air at 85°C for 10 days resulted in a sharp increase of the bases to 0.42 weight per cent sulfur before leveling off. The results show that these weakly basic materials were not present in the virgin crude but were easily formed under mild oxidizing conditions. Weak bases do not form in all crude oils, as evidenced by a similar air-oxidation test with crude oil VI. Prolonged exposure of this oil to air resulted in no increase in weakly basic material, indicating the absence of compounds which form or promote the formation of sulfoxides.
RECEIVED for review March 27, 1967. Accepted May 18, 1967. Division of Chemistry Petroleum, 150th National Meeting, ACS, Miami, Fla., April, 1967. This investigation was performed as a part of the work of American Petroleum Institute Research Project 60 on the Characterization of the Heavy Ends of Petroleum, carried out by the Bureau of Mines at Laramie, Wyo. Work was done under cooperative agreements between the Bureau of Mines, U S . Department of the Interior; the American Petroleum Institute; and the University of Wyoming. Reference to specific commercial materials or models of equipment in this report is made to facilitate understanding and does not imply endorsement by the Bureau of Mines.
p-Type Analysis by tisn Mass pectromet E. J. Gallegos, J. W. Green, L. P. Lindeman, R. L. LeTourneau, and R . M. Teeter Chewon Research Co., Richmond, Calif. We describe the first reported multicomponent grouptype analysis utilizing high resolution mass spectrometry for analyzing high boiling petroleum stocks without requiring fractionation via silica gel chromatography or its equivalent. The application of mass spectrometer group-type analysis to high boiling fractions has been restricted because it is necessary to employ costly and lengthy separations to reduce the number of spectral interferences caused by the great variety of components. It is well known that high resolution mass spectrometry eliminates many of these interferences. We have found that the data from the MS-9 high resolution spectrometer are sufficiently reproducible to take advantage of the small mass differences and thus eliminate the physical separation. This group-type procedure determines 19 components; seven saturated hydrocarbons with zero to six rings, nine aromatic hydrocarbons with one to four rings, and three aromatic sulfur compound types.
BECAUSE OF THE ABILITY of the early commercial mass spectrometers to give information on the carbon-hydrogen ratio and molecular sizes of hydrocarbon mixtures, mass spectrometry became an important analytical tool for assaying petroleum fractions. R . A. Brown (1) in 1951 was the first to use this spectral information for analyzing for compound types rather than for individual compounds. His procedure applied to gasoline received immediate and nearly universal acceptance in the petroleum industry. Increasing demand for processing of high boiling petroleum stocks caused interest in extending group-type analysis to these materials. O’Neal and Wier ( 2 ) in 1951 were the first to employ the necessary instrumentation and techniques for recording mass spectra of the heavier materials. Most heavy petroleum fractions contain such a large variety of components that a single spectrum cannot be interpreted in terms of compound types as can be done with gasolines. By 1956, however, several mass Spectrometer group-type methods (1) R. A. Brown, ANAL.CHEM., 23, 430 (1951). ( 2 ) M. J. O’Neal, Jr., and T. P. Wier, Jr., Ibid.,p. 830.
were published for determining separately the compositions of saturate and aromatic fractions. The disadvantage of this scheme is the time-consuming silica gel separation step which doubles the number of samples that must be analyzed, increases the opportunities for error, and limits the overall accuracy by the precision of the split. In this work, we made use of high resolution mass spectrometry, which has only recently become commercially available, to eliminate most of the interferences between saturated and aromatic hydrocarbons and, further, to eliminate the interferences of many hydrocarbons with a number of sulfur compounds. This is the first published report of a quantitative group-type procedure which takes advantage of high resolution mass spectrometry for the analysis of high boiling petroleum fractions without a preliminary silica gel separation. This analysis is useful as a rapid and inexpensive procedure for supplying the petroleum chemist with a quantitative estimate of the major components in the saturate and aromatic portions of crude oils and refinery stocks. No attempt has been made to include all molecular types present in petroleum products. This deficiency becomes more pronounced with the higher boiling fractions. A number of additional components could be added to the present scheme with additional effort. However, the law of diminishing returns eventually forces one to abandon many of the minor components with a sacrifice of some completeness and accuracy. Alternatively, one is forced to use a more elaborate scheme involving separation techniques with consequent increase in cost of analysis. As presently constituted, the method is considered useful for distillate fractions in the 500 ’950” F range. EXPERIMENTAL
Instrumentation. The high resolution instrument used is an MS-9 made by Associated Electrical Industries, Ltd. A 2-pl sample is introduced into an all-glass heated inlet (3). The instrument is adjusted to give 5000 resolution at mass 250. This ensures optimum resolution for the portion of ~
~~
(3) R. M. Teeter and W. R. Doty, Rev. Sei. Innstr., 37,792 (1966). VQL. 39, NO. 14, DECEMBER 1967
* 1833
21 2 Monocycloparaffins
1 Paraffins
CnH, n+2 CH,,
3 Dicyc lopa raff ins 4 Tricycloparaffins
CnHz
Tetracycloparaffins 5IAiicylbenzenes Pentacycioparaffins
CnH, n-z
"..,
8 Alkyl Naphthalenes
9 Acenaphthenes 10 Fluorenes
-
a
11 Phenanthrenes 12'.Pyrenes
Ct14 n-s
13 Chrysenes 14 Benzothiophenes 15 Dibenzothiophenes 16 Naphthobenzothiophenes
Figure 1. Segment from a high-resolution mass spectrum of the 800"-950° F fraction of an Arabian crude
the spectrum used in computation and reproducibility of results from sample to sample. We have found that results are quite temperature dependent, especially for the aromaticsaturate breakdown. A control sample is therefore generally used to ensure correct operating temperature. The instrument is equipped with a temperature controller to maintain the source at 240" C. The heated inlet is held at 300" C. Scan time for the high resolution spectra in the work reported here was 45 minutes using an early model of the MS-9 and oscillograph recording. This has been speeded up to 5 minutes with fast scan amplifiers and galvanometers recently installed. Group-Type Calibration. The features of a high resolution mass spectrum which made an extension of group-type analysis possible are shown in Figure 1. The upper portion of the figure gives part of a high resolution (5000) mass spectrum for an 800"-950" F fraction of an Arabian crude. In the lower portion of the figure are listed the 19 compound types considered. These include seven saturated hydrocarbon types with zero to six rings, nine aromatic hydrocarbon types with one to four rings, and three aromatic sulfur compound types with two to four rings. No oxygen or nitrogen compounds are included in the calculation. Fortunately, these compounds are not present to any large extent in distillate fractions of petroleum. A low intensity ion which corresponds to dibenzofuran (m/e 192) has been noted in some crude fractions, but this and others like it are presently ignored in the calculations. A number of other polynuclear aromatic hydrocarbons and sulfur compounds have also been ignored even though their presence is suggested in the mass spectra as is apparent from a careful study of Figure 1. A considerable amount of additional research would be necessary to identify and calibrate for these components. At each nominal mass in the range m/e 268-283, there is a multiplet of three to four 1834
@
ANALYTICAL CHEMISTRY
individual peaks. It is these separate peaks in the multiplet which are the essence of a high resolution spectrum and which are used in the computation, as will be explained. In the portion of the spectrum shown, the saturated hydrocarbons with masses 270-82 contain 20 carbons. Those immediately below 270 have 19 carbons and those above 282,21 carbons. The conventional means of classifying hydrocarbons in mass spectrometry is by number of hydrogen atoms in the empirical formula, CnHz,+,. For the seven saturated hydrocarbons under consideration, z runs from +2 for the paraffins to - 10 for the hexacycloparaffins. As indicated in Figure 1, the parent peaks for the saturates are those components of the multiplets with the heaviest mass as expected from the high hydrogen-to-carbon ratio. The compounds with z = - 12 (i.e., naphthalenes) have the same nominal molecular weights as the compound with one less carbon atom and z = + 2 (Le., paraffins). However, because of the reduced hydrogen-to-carbon ratio, the actual mass for a naphthalene is 0.094 mass unit less than that for a paraffin and, hence, at high resolution appears as a second peak in that multiplet. In general, the hydrogen-deficient compounds with z = - 12 to z = -24 appear as the second components in the multiplets and those with z = -26 to z = -38 appear as the third components. The sulfur atom has a distinctive mass defect relative lo hydrogen and carbon; and, hence, compounds containing sulfur will usually appear as separate peaks in the high resolution mass spectrum. However, it is possible for a sulfur compound and a hydrogen-deficient hydrocarbon to have sufficiently close exact masses to cause overlap at the resolution used in this work. Identification of a characteristic peak for each of the 19 compound types is given in the spectrum of Figure 1. Of these 19 components, there are only three seis of two types each which have the same characteristic peaks. The interfering components are connected by brackets in Figure 1.
Table I. Fragment Peaks Used for High Resolution 19-Component Mass Spectrometer Group-Type Analysis Compounds Formula Fragment peaks Paraffins CnHzn+z 71, 85, 99, 113 Monocycloparaffins CHz, 69, 83, 97, 111, 125, 139 Dicycloparaffins C$Hzn-2 109, 123, 137, 151, 165,
Middle Eastern Crude
a Silica Gel
9 Aromatics
Lumpkin's Matrix
Hood and O'Neal's Matrix
Figure 2. Preparation and analyses of calibrating samples
They have in each casq a common empirical formula; at z = - 6 alkylbenaenes interfere with tetracycloparaffins, at z = - 8 benzocycloparaffins interfere with pentacycloparaffins, and at z = -10 benzodicycloparaffins interfere with hexacycloparaffins. The groups of peaks with odd masses are the fragments from heavier components. Also present in the spectrum are isotope contributions from 13C and s4S, The spectra are not corrected for these isotopes. Their contribution is incorporated in the matrix calculation. The calculation procedure adopted for this work is essentially the mating of the matrix method of Hood and O'Neal ( 4 ) for the analysis of zero- to six-ring saturated hydrocarbons plus alkylbenzenes with the matrix method of Hastings, Johnson, and Lumpkin (5) for the analysis of the aromatic and sulfur compounds. Both methods employ fragment peaks, and Table I lists the masses of the peaks used for each component. The identities of most of the calculation peaks were confirmed by exact mass measurements using the peak matching technique. Consideration had to be given to two items to complete the formulation. First, the relative sensitivities for the components from the two mated methods had to be adjusted to ensure an accurate aromatic-saturate split; and second, some off-diagonal matrix elements had to be added to the new, larger matrix to compensate for the interactions of the interfering saturated and aromatic types. This part of the work was implemented by preparing some narrow fractions from an 8O0"-90Oo F distillation cut of a Middle Eastern crude as outlined in Figure 2. The starting material was separated on silica gel into aromatic and saturated fractions. The saturated fraction was dewaxed and then subjected to thermal diffusion. The thermal diffusion cuts from the calibrating materials were analyzed on a con-
(4) A. Hood and M. J. O'Neal, Jr., Proc. of Joint Conference Organized by Hydrocarbon Research Group, Inst. of Petroleum and ASTM Committee E14, Univ. of London, September 24-26, 1958. (5) S. H. Hastings, B. H. Johnson, and H. E. Lumpkin, ANAL. CHEM., 28, 1243 (1956).
179, 193 149, 163, 177, 191, 205, 219, 233, 247 Tetracyclouaraffins CnH2n-e 189, 203, 217, 231. 245. 259, 273, 287, 30i Pentacycloparaffins CnHzn-8 229, 243, 257, 271, 285, 299, 313, 327,. 341,. 355 Hexacycloparaffins 269, 283, 297, 311, 325, 339, 353, 367, 381, 395, 409 Akylbenzenes 91, 105,119,133 Benzocycloparaffins 117, 131, 145, 159 Benzodicycloparaffins 129, 143, 157 Naphthalenes 141," 1 5 5 , ~1690 Acenaphthenes 167,' 181,e 195,a 2Oga Fluorenes 1 ~ 5 5 1, ~7 9 , ~193,a 207a Phenanthrenes 191,a 205,a 219O Pyrenes 215,a 229,a 2435 Chrysenes 241,a 255,a 2 6 9 Benzothiophenes 147.a 161.a 175Q Dibenzothiophenes CnHnn-l& 197,* 211,b 225b Naphthobenzothiophenes C,,HZ~-ZZS247,3 261,b275b a Second component in multiplet (Le., 0.094 mass unit less than
Tricycloparaffins
C,Hzn-4
saturated component). Third component in multiplet (Le., 0.18 mass unit less than saturated component).
ventional CEC 21-103C mass spectrometer in addition to the high resolution MS-9 mass spectrometer using Hood and O'Neal's calibration. Results from the low resolution instrument were assumed to be correct, as a similar instrument was used in their calibration. Slight differences in results from the two instruments were found, and the starting matrix was adjusted to bring the results from the high resolution instrument into agreement with those from the low resolution instrument. No modifications were made to the Hastings, Johnson, and Lumpkin matrix for calculating the aromatic and sulfur compounds. Sensitivities were then adjusted to fit the analysis of blends of the saturated and aromatic fractions which were considered as known. The off-diagonal matrix elements connecting components with z = -6, z = - 8, and z = - 10 required trial-and-error fitting to give accurate results for the alkylbenzenes, benzocycloparaffins, benzodicycloparaffins; and tetra-, penta-, and hexacycloparaffins. Fortunately, the most abundant fragment peaks for the interfering components occur in different mass ranges. The saturated hydrocarbons with z = - 6 , z = -8, and z = -10 involve four, five, and six rings, mostly fused, while the corresponding aromatics involve one, two, and three rings. As Table I shows, the fragments used in the analysis reflect these differences. The final inverse matrix developed from this procedure is given in Table 11. Tables IIIA and IIIB illustrate some of the results on mixtures made from the calibrating samples. The final 19component matrix was used to compute the results shown. The analyses for the saturated and aromatic portions of the sample are given in the first two columns. The results for the saturated fraction in Table IIIA show essentially 100% nonaromatic hydrocarbons, and those for the aromatic fraction show approximately 13 saturated hydrocarbons. These results are consistent with separations expected on silica gel and are evidence that the nondiagonal terms in the matrix connecting these components are in good order. Column 3 gives the composition expected for a 50/50 blend VOL. 39, NO. 14, DECEMBER 1967
* 1835
001n
0
0
0
0
0
0
0
0
0
O
36
@
ANALYTKAL CHEMISTRY
O
O
O
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Q
O
O
O
0
O
O
O
O
O
O
Q
O
0
O
O
O
O
O
O
Table IIIB. Check Analysis to Test Sensitivities and Nondiagonal Matrix Elements Used in Calculations Liauid volume 7 1:l Blend of aromatics and saturates, columns 1 and 2
Table IIIA. Check Analysis to Test Sensitivities and Nondiagonal Matrix Elements Used in Calculations Liquid volume 1:1 Blend of aromatics and saturates, columns 1 and 2
'I2
'12
column 1 plus Type Paraffins Monocycloparaffins Dicycloparaffins Tricycloparaffins Tetracycloparaffins Pentacycloparaffins Hexacycloparaffins Total saturates Alkylbenzenes Benzocycloparaffins Benzodicycloparaffins Naphthalenes Acenaphthenes Fluorenes Phenanthrenes Pyrenes Chrysenes Total aromatics Benzothiophenes Dibenzothiophenes Naphthobenzothiophenes Total sulfur compounds
MS
Saturated Aromatic '12 fraction fraction column 2
matrix results
0 3 4 4 0.2 0.8 2 13 7 7 7
18 14 12 7 3 1 1 56 3.7 3.5 3.5
17 12 10 6 3 1 4 53 2.8 3.0 2.7
0 0.1 0.1
3 7 10 19 15 4 78 2 5 3
1.5 3.5 5.0 9.7 7.5 2.1 39 1.o 2.5 1.5
1.4 3.1 4.8 9.0 12.5 2.9 42 0.8 2.2 2.1
0.2
10
5
5
36 25 20 9 6 2 0.9 99 0.5 0.1 0 0 0 0 0.4 0 0.3 1.4
column 1 plus Saturated Aromatic '12 fraction fraction column 2
Type Paraffins Monocycloparaffins Dicycloparaffins Tricycloparaffins Tetracycloparaffins Pentacycloparaffins Hexacycloparaffins Total saturates Alkylbenzenes Benzocycloparaffins Benzodicycloparaffins Naphthalenes Acenaphthenes Fluorenes Phenanthrenes Pyrenes Chrysenes Total aromatics Benzothiophenes Dibenzothiophenes Naphthobenzothiophenes Total sulfur compounds
0.9 2.8 10.7 20.6 30.4 19.4 11.3 96.5 0.2 0.7 0.1
1.6 3.3 1.3 1.5 1.1 2.6 0.5 12.0 4.1 2.3 2.8
1.3 3.0 6.0 11.0 16.0 11.o 6.0 54.0 2.0 1.5 1.5
1.6 2.5 6.3 11.0 18.8 11.6 6.4 58.3 0.8 1.3 1.3
0.3 0.4 0.3 0.7 0.8 0.0 3.5 0 0
1.2 1.3 5.8 18.9 31.0 10.3 77.8 0.8 2.6
0.8 0.9 3.0 9.8 15.9 5.1 40.6 0.4 1.3
0.7 0.9 2.9 9.5 15.6 4.1 37.2 0.4 1.1
0 0
6.7 10.2
3.4 5.1
3.1 4.6
Table IV. High Resolution Mass Spectrometer Group-Type Results Liauid volume X , Arabia California 500 O650" F
Saturates Paraffins Monocycloparaffins Dicycloparaffins Tricycloparaffins Tetracycloparaffins Pentacycloparaffins Hexacycloparaffins Total saturates Aromatics Alkylbenzenes Benzocycloparaffins Benzodicycloparaffins Naphthalenes Acenaphthenes Fluorenes Phenanthrenes Pyrenes Chrysenes Total aromatics Sulfur compounds Benzothiophenes Dibenzothiophenes Naphthobenzothiophenes Total sulfur compounds
650 800' F O-
800"950" F
500 650" F O-
650 n800" F
MS
matrix results
800O950" F
Louisiana 500 O650" F
650 '-
800" F
38 7 5 3 3 2 0 58
21 7 4 2 3 4 0 41
15 7 7 5 3 3 5 45
2 18 15 11 6 4 0 56
2 9 9 8 10 4 4 46
3 5 8 14 15 6 5 56
7 21 15 11 9 3 0 66
4 11 11 9 11 8 3 57
13 4 3 4 1 1 2
10 4 2 0.7 1 0.4 6 11 3 38
7 9 10 5 6 5 0
7 7 5 2 5 6 12 6 0 50
3 4 4 2 4 4 7 9 3 40
7 6 9 4 3 4 0 0 0 33
5 4 4 2 2
28
8 2 2 0.6 1 4 10 5 2 35
9 5 0 14
19 0 24
0 0
5
7 6 4 17
0 0
42 2.4 0.1 0 2
2 2 0 4
2 1 0.7 4
5
13 4 0 39
0.8 0.6 0 1
VOL. 39, NO. 14, DECEMBER 1967
0.6 3 0 4
e
1837
Comparison of Sulfur Contents Determined by MS Group-Type and X-Ray Fluorescence MS data Average Mole molecX-ray, sulfur ular Weight Z weight Z Sample compounds weight sulfur sulfur
Table V.
Arabia 50Oo-65O0 F 650"-800" F 800"-950" F
15 24 17
196 248 320
2.4 3.1 1.7
1.5 2.3 2.5
2.5 3.4 3.7
196 248 320
0.41 0.43 0.37
0.67 0.92 1.o
1.4 3.4
195 248
0.23 0.44
0.16 0.41
stocks the number of three- and four-membered polynuclear aromatics and sulfur ring compounds is readily apparent. One of the good features of this analysis is the ease with which some sulfur compounds are included. However, as the method is now set up, it does not account for all of the sulfur compounds that are present, especially in the 800"950" F fractions. This is seen by referring back to Figure 1 and noting that there are several peaks for which no accounting is made and which possibly contain sulfur. As an example, the fourth peak in the multiplet at mass 282 might be C3 H7
California 500"-650" F 650"-800" F 80O0-95O0 F
The third and fourth peaks in the multiplet at mass 280 could be
Louisiana 500"-650° F 65O0-8OO0 F
of the two fractions based on the analyses in Columns 1 and 2. This is compared with the actual analysis of such a blend in column 4. The excellent agreement for the total saturates and total aromatics shows that the relative sensitivities employed are adequate. Likewise, the mutually good agreement of the alkylbenzenes, benzocycloparaffins, and benzodicycloparaffins indicates that adequate corrections were made for interferences from the tetracycloparaffins, pentacycloparaffins, and hexacycloparaffins. Table IIIB gives similar analyses for a mixture made from equal parts of the bottom cut from a thermal diffusion column which is rich in four-, five-, and six-ring cycloparaffins and of a highly aromatic fraction from a silica gel separation. The agreement for the alkylbenzenes, benzocycloparaffins, and benzodicycloparaffins in Column 3 and Column 4 is still quite good for this mixture which contains more condensed cycloparaffins. The total aromatic-total saturate split is also in good agreement. RESULTS
To demonstrate the utility of the high resolution mass spectrometric group-type analysis, we have applied it to several distillation fractions from three crude sources. The results are given in Table IV. A survey of the data confirms many of the generalizations already known about these materials. For instance, the high paraffin, low cycloparaffin content of the Arabian crude us. the low paraffin, high cycloparaffin content of the California and Louisiana crudes is confirmed. Also, the increase in the degree of cyclization with increasing molecular weight is apparent for each of the different crudes. The low degree of cyclization of the Arabian crude is also manifested in the aromatics fraction, although in the heavier
1838
@
ANALYTICAL CHEMISTRY
respectively. The weight per cent of sulfur accounted for by mass spectrometry has been computed and is given in Column 3 of Table V. Column 1 shows the total mole per cent of sulfur compounds as found by the group-type analysis. The average molecular weight of the sulfur compounds used to convert to weight per cent was determined from the mass distribution for each sulfur type. These averages differed only slightly. The weight per cent sulfur by mass spectrometry is compared with that determined by x-ray fluorescence in Column 4. The agreement for the Louisiana crude is very good. For the California crude, the mass spectrometer results are generally low. For the Arabian crude, the mass spectrometer results are high for the 500"-650" F and 650"800" F fractions. This may reflect somewhat low sensitivities for the sulfur compounds in the matrix. The low sulfur results by mass spectrometer are easily explained by the fact that not all sulfur compounds have been included as discussed above. In addition there are possibly some sulfur compounds containing two sulfur atoms that may be masked by others with only one sulfur atom. For example, Reid (6) has reported C&&+ in Middle Eastern ozokerite. These compounds would not be resolved from dibenzothiophene with the instrument condition used in this work. ACKNOWLEDGMENT
The authors thank T. W. Hansen for assistance with the mass spectrometer and Miss C. Feudo for assistance with preparation of data. RECEIVEDfor review June 8, 1967. Accepted August 22 1967. (6) W. K. Reid, ANAL.CHEM.,38, 445 (1966).
