Anal. Chem. 1982, 5 4 , 765-768
765
Identification of Dicyclic and Tricyclic Hydrocarbons in the Saturate Fraction of a Crude Oil by Gas Chromatography/Mass Spectrometry J. Stuart Richardson” and Denis E. Mllller Phillips Petroleum Company, Phlllips Research Center, Bartlesvllle, Oklahoma 74004
A saturate fractlon of an oll whlch was generated prlmarlly from terrestrial organlc matter was analyzed by gas chromatography/mass spectrometry. We searched for speclflc organlc compounds that could be used as unamblguous lndlcators of terrestrlal Input. Two Isomeric ClS and two isomerlc CIB dlcycllc hydrocarbons were Identified as well as 11 trlcycllc hydrocarbons. Structural characterlzatlons were based on mass spectral data and structures of probable blochemical precursors. Skeletal structures of the trlcycllc compounds occur in four dlstlnct types. Only three of the four types may be used as lterrestrlal blomarkers.
Certain organic molecules in oils known as “biomarkers” or “chemical fossils” can provide much insight about the depositional environment and source of the organic matter. At present, gas chromatagraphy/mass spectrometry (GC/MS) is the definitive method for identifying these molecules. The evolution and current scope of the technique are well documented (1-9). The work describes the GC/MS analysis of the saturate fraction of a Tertiary crude oil generated from terrestrial organic matter. The oil contains an n-alkane distribution typical of higher plant waxes. The preponderance of 24ethylcholestane relative to cholestane also indicates a high terrestrial input to the ~3ourcekerogen (10). The structures of several dicyclic and tricyclic hydrocarbons are determined based partly upon extrapolation of known mass spectral fragmentation behavior of standard compounds and partly upon known structures of probable biological precursors. EXPERIMENTAL SECTION The saturate fraction of the crude oil was obtained by silica gel chromatography using n-pentane as the eluting solvent. One hundred milligrams of t h e crude oil was placed at the top of a column containing 50 g of‘activated silica gel (Grade H, 100-200 mesh, Davison Chemical Co.). The silica gel was activated at 325 “C overnight. The sample was fractionated by continuous elution with approximately 450 rr~Lof n-pentane (Burdick and Jackson, Distilled In Glass). The total dry weight of the saturates fraction was 66.6 mg. This fraction was dissolved in 6.7 mL of methanol/toluene solution (20:80 v/v) to which was added 2.2 mL of saturated urea/methanol solution. The nonadduct fraction was separated from the n-alkanelurea clathrate by decantation, and the solvents were evaporated under dry nitrogen. Excess urea was dissolved in water. The saturates were redissolved in nhexane, decanted from the water, and concentrated. For the GC/MS analynis, a 60 m X 0.25 mm i.d. fused silica capillary column coated with SE-54 (J and W Scientific) was used in a Perkin-Elmer Sigma 3 gas chromatograph directly coupled to a Kratos MS-25 mass spectrometer. One microliter of sample solution (in n-hexane) was injected onto the column at room temperature. The injector port was designed in this laboratory by G. J. Greenwood. The tiplitless injection yielded a neat sample weight of 112 fig. Temperature programming of the column was delayed until elution of the solvent to take advantage of the “solvent effect” (11). Then the column oven was temperature programmed from 40 to 300 “C at a rate of 4 “C/min. The mass 0003-2700/82/0354-0765$01.25/0
spectrometer, operated in the E1 mode, was magnetically scanned at a rate of 1 s/decade using an ionizing voltage of 35 eV. The GC helium carrier gas flow rate was 2 mL/min. Selected mass chromatograms and further data reduction including background corrected partial mass spectra were obtained by using a Kratos DS-55 data system.
RESULTS AND DISCUSSION The Dicyclic Hydrocarbons. On the basis of previous experience, it was anticipated that dicyclic hydrocarbons, if present, would most likely arise from products produced by the biological cyclization of terpenes. For example, geranylgeraniol (as its pyrophosphate) is regarded as the primary precursor of a large class of natural Cz0diterpenes found in higher terrestrial plants. It is known to undergo biological alterations leading to a wide variety of naturally occurring dicyclic terpenoid acids and alcohols (12). Subsequent dehydration and reduction of these, as would occur during petroleum genesis, would result in dicyclic terpane “biomarkers”. Another possible source of dicyclic hydrocarbons is the thermal alteration of higher terpanes formed originally by enzymatic processes. Squalene, a naturally occurring triterpene, will cyclize to give a number of tri-, tetra-, and pentacyclic terpenoids (12). Their geochemical alteration that leaves any two adjacent rings intact could give dicyclic hydrocarbons. Bendoraitis has suggested that a plausible source of dicyclic terpanes could be the degradation of tetra- and pentacyclic terpanes (13). The m / z 123 mass chromatogram, shown in Figure 1, has been used to detect the possible presence of dicyclic terpanes. The structure associated with this fragment is 123
Mass spectra of two dicyclic terpane isomers of molecular weight 208 (compounds 1 and 2) are shown in Figure 2. Spectra of isomers of molecular weight 222 (compounds 3 and 4) are also shown in Figure 2. The mass spectrum of one of the isomers (compound 1) is very similar to one published by Bendoraitis (13). He detected two CI5 dicycloalkanes in a Loma Novia crude oil and he postulated two possible pentamethyldecalin structures
I
II
Either of these structures is consistent with the mass spectrum of compound 1. Loss of a methyl group would account for the (M - 15)’ fragment (m/z 193) while other fragments could be rationalized by the fragmentations
0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
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TIME M l h 21 02
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'Igure 1. Mass chromatograms showing relative positlons of diycioaikanes and distribution of tricyclic terpenoids having prominent alr 163 fragments. Compound 2 is an isomer of compound 1. The m / z 123 fragment is the base peak in the mass spectrum of 2 as opposed to the m / z 193 base peak of 1. The difference is probably due to configurational effects. It has been shown, for example, that the fragment from the loss of the equatorial methyl of trans-syn-2-methyldecalin is larger than the (M 15)+ fragment of trans-anti-2-methyldecalin in which the methyl is axial (14). On the basis of the structures of probable precursors and the mass spectral data of the 2-methyldecalins and the unknowns, we believe that compound 1 may be the pentamethyldecalin shown as structure I, in which the configuration at C-8 is R (equatorial methyl) and at C-9 is S (equatorial methyl). Compound 2 may be structure I with an S configuration at C-8 (axial methyl). Compound 3 appears to be a homologue of compound 1. A structure consistent with the mass spectrum and in accord with probable precursors is 123
c,
The expected fragmentations are indicated. Such a structure would account for the m/z 165 fragment in the mass spectrum (Figure 2) not observed in the spectra of compounds 1 and 2. The (M - 29)+ fragment (m/z 193) is indicative of loss of an ethyl substituent. This fragment is not observed in the mass spectrum of compound 4 (Figure 2). To assure that these spectra, as well as subsequent ones presented for tricyclic hydrocarbons, are indicative of compounds free of double bonds, we performed IR and NMR analyses on the sample. From these analyses, no significant amount of unsaturation was evident. Mass spectra of compounds 1-4 were compared to that of authentic decahydro-1,4-dimethyl-7-(l-methylethyl)naphthalene, a related compound of molecular weight 208. On the basis of this comparison, compounds 1-4 are estimated to be greater than 90% pure. It is possible that more than one compound may be contributing to a given mass spectrum.
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Figure 2. Mass spectra of compounds 1-4. If so, they must be closely related configurational isomers not affecting the gross structures presented. The same is true of mass spectra of tricyclic hydrocarbons presented later. The Tricyclic Hydrocarbons. Tricyclic terpanes may be derived from diterpenoids such as abietic or pimaric acids (15). They may also arise as an intermediate from the Markownikov cyclization of squalene (12) to yield
I
ILL
After geochemical degradation involving the alkyl group, R may contain 1 to 11 carbon atoms. Abietic and pimaric acids are formed by cyclization of geranylgeraniol (as its pyrophosphate) in higher plants (12). Squalene is a common metabolic intermediate produced by the condensation of two farnesyl pyrophosphate units and is generally considered as the precursor of most important types of pentacyclic terpanes (12). The significance of tricyclic terpanes as potential biomarkers is enhanced in that they may
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
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60 40 20 0
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Figure 3. Mass chromatograms showing distribution of tricyclic terpenoids having proinent m l z 191 and m l z 177 fragments.
be more resistant to biodegradation than their tetra- or pentacyclic counterpairts (7,16, 17). It has been suggested (1,2,16) that tricyclic terpanes having the skeletal structure I1[1will have a prominent m/z 191 mass spectral fragment cont,aining rings A and B. The reasoning is based, in part, on mass spectral fragmentation pattetns of pentacyclic terpanes having the same A,B ring moiety. The fragmentation trends can be extended to other tricyclic structures such as fichtelite according to
220
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,
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100
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200
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350
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135
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Fichtelite's mass spectrum has been published (18). Extrapolating these results to tricyclic terpanes having the basic skeletal structures IV, V, and VI, the following prominent fragments would be expected for the indicated ring scissions:
163
~
m
m
41
A reasonable fragmentation mechanism for structure IV leading to the m / z 16,3 fragment which can readily be extended to structures 114 V, and VI is 95
!
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'm _- 163
I
Figure 1gives an m / z 163 computer-generated mass chromatogram showing the distribution of tricyclic terpanes having prominent m/z 163 fragments (compounds 5-9). Similar
1
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247
I
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150
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Figure 4. Mass spectra of compounds 5-10.
300
350
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
Table I. Base Skeletal Structures of Tricyclic Terpanes Skeletal Type
MOLECULAR
Y
WEIGHT
'
290
*
~
*
m
*
m
~*~
z ALTERNATE POSSIBILITIES
chromatograms of m / z 177 and m / z 191 fragments (Figure 3) have been used to obtain distributions of other tricyclic terpanes (compounds 10-15). Mass spectra of tricyclic terpanes along with proposed structures for compounds 5 through 10 are given in Figure 4. Structural assignments are based on probable precursor structures and mass spectral data. Compounds 6 and 7 are apparent stereoisomers and each may be represented by two possible structures. The mass spectrum alone cannot unambiguously differentiate the two. Subtle differences of their mass spectra include m/z 137 > m / z 135 for 6, while m/z 135 > m/z 137 for 7. Similarly, m/z 149 is greater than m / z 151 for 6 while the reverse is true for 7. Compounds 8 and 9 may have type V structures. An aromatic analogue of the type V structure, simonellite, is known to occur in sediments from the North Atlantic Ocean (15). An ethyl substitution is indicated by the (M - 29)' fragment ( m / z 233) in the mass spectrum of compound 10. Spectra of tricyclic terpanes in the lower mass region ( m / z 50-200) consist essentially of the same fragments differing only in relative intensity. Spectra presented were compared to that of authentic fichtelite. On the basis of this comparison, the spectra were estimated to be greater than 90% pure. If the general structures we have proposed for the tricyclic terpanes are arranged according to molecular weight and skeletal structure (types 111, IV,V, and VI), the result is Table I. Compounds 11-15 are included. A definite clustering of structural types is seen. The tricyclic terpanes a t or below molecular weight 276 consist primarily of structural types IV, V, and VI, while those at or above molecular weight 304 have only the type I11 structure. There is mass spectral evidence for additional tricyclic terpanes having structural types IV,
V, and VI a t or below molecular weight 276 which are not included in Table I, primarily because their mass spectra contain peaks from fragments of overlapping components. There is no evidence for terpane types IV, V, and VI having molecular weights greater than 276.
CONCLUSIONS The presence or absence of specific tricyclic and dicyclic terpanes leads to the following conclusions. Precursors of tricyclic terpanes possessing structural types IV, V, and VI contain no more than 20 carbon atoms. Tricyclic terpanes of structural type I11 do not readily lose ring methyl groups during geochemical degradation. Tricyclic terpanes with structural types IV, V, and VI may arise from higher plant components derived from labdadienylpyrophosphate such as abietic and/or pimaric acids. Tricyclic terpanes with structural type I11 may arise from the biochemical cyclization of squalene or squalene-like terpenoids common to most organisms. We infer that tricyclic terpanes with structural types IV, V, and VI and dicyclic terpanes such as compounds 1-4 may be generally used as terrigenous biomarkers. ACKNOWLEDGMENT Our thanks go to G. J. Greenwood for reviewing the paper, to J. A. Lytle for obtaining much of the mass spectral data, to M. P. Fuller for IR spectra, and to D. M. Cantor for NMR data. LITERATURE CITED (1) Anders, D. E.; Robinson, W. E. Geochim. Cosmocbim. Acta 1971, 35,661-678. (2) Gallegos, E. J. Anal. Chem. 1971, 4 7 , 1151-1160. (3) Anders, D. E.; Dolittle, F. G.; Robinson, W. E. Geochim. Cosmochim. Acta 1973, 3 7 , 1213-1228. (4) Leythaeuser, D.; Hollerbach, A.; Hagemann, H. W. Adv. Org. Geochem. 1975, 3-20. (5) Wardroper, A. M. K.; Brooks, P. W.; Humberston, M. J.; Maxwell, J. R. Geochim. Cosmochim. Acta 1977, 4 1 , 499-510. (6) Slefert, W. K.; Moldowan, J. M. Geochim. Cosmochim. Acta 1978, 42, 77-95. (7) Slefert, W. K.; Moldowan, J. M. Geochim. Cosmochim. Acta 1979,
43. 111-126. (6) Ekweozor, C. M.; Okogun, J. I.; Ekong, D. E. U.; Maxwell, J. R. Chem. Geoi. 1979. 27. 11-28. (9) Phllp, R. P.;'Gllbert, F. D. APEA J. 1980, 2 0 , 221-228. (10) Tlssot, B. P.; Welte, D. H. "Petroleum Formation And Occurrence"; Springer-Verlag: Berlin, Heidelberg, New York 1978;Part 11, Chapter 3. (11) Grob, K.; Grob, K., Jr. HRC CC J . High Resoiut. Chromotogr. Chromotogr. Commun. 1978, 1 (l),57-64. (12) Wsmann, T. A.; Crout, D. H. G. "Organic Chemistry of Secondary Plant Metabolism"; Freeman, Cooper: New York, 1969;Chapters 10
and 11.
(13) Bendoraltls, J. G. Adv. Org. Geochem. 1973, 209-224. (14) Meyerson, S.;Weltkamp, A. W. Org. Mass Spectrom. 1989, 2 , 603-609. (15) Slmonelt, 0.R. T. Geochim. Cosmocbim. Acta 1977, 4 1 , 463-476. (16) Reed, W. E. Geochim. Cosmochim. Acta 1977, 4 1 , 237-247. (17) Rubenstein, I; Strausz; 0. P.; Spyckerelle, C.; Crawford, R. J.; Westlake, D. W. S. Geochim. Cosmochim. Acta 1977, 4 1 , 1341-1353. (16) Douglas, A. G.; Grantham, P. J. A&. Org. Geocbem. 1973, 261-276.
RECEIVED for review September 21,1981. Accepted December 14,1981. Part of this work was presented at the 29th Annual Conference on Mass Spectrometry and Allied Topics, May 24-29,1981, Minneapolis, MN. Our appreciation goes to the management of Phillips Petroleum Co. for permission to publish.
