Anal. Chem. 1983, 55, 123-126
123
Determination of Petroleum Sterane Distributions by Mass Spectrometry with Selective Metastable Ion Monitoring Geoff A. Warburton Kratos Limited, Barton Dock Road, Urmston, Manchester M3 12LD,United Kingdom
John E. Zumberge" Cities Service Research, Box 3908, Tulsa, Oklahoma 74 102
Conventional GC/MS rinaiysis of crude oils and Sedimentary rock extracts for steranes, in which the m / z 217 fragment ion is monitored, often reveals a complex and incompletely resolved structural and stereoisomeric mlxture of C2,, Czarand CZ9 steranes. Increased speclficlty can be achieved by monitoring the spontaneous (unlmolecular) fragmentatlon of sterane parent ions occurring in the first field-free region of a double focusing mass spectrometer. The sterane metastable parent ion transiitions, corresponding to 372' 217', 386' 217', and 400' 217+, can be separately observed during a single GC/MS run by using a programmable power supply to vary the accelerating voltage while holding the magnetic and electrostatic fields at approprlate constant values.
-
-
-
el a
1000
100
-
0
,
!
zc-.*.-*-*
,
,
,
,
,
I
200
1000
600
"
'
I
1400
1800
1400
18W
SCAN UUMBER
100
~
1
4
- 94472
I
372'-217*
0 -
3 ----Steroidal hydrocarbons are common constituents of crude oils and ancient sedimentary rocks. Sterane distributions can be used as indicators of petroleum source rock depositional environments because the carbon atom skeleton of steranes is a remnant of the biochemical precursor steroidal structure (1). For example, a relatively greater abundance of CZ9 steranes (e.g., stereoisomers of 24-ethylcholestane) over C27 steranes could suggeEit that the precursor organic matter contained more land-derived biochemical compounds rather than those derived from marine organisms since sterol distributions dominated by C29components are characteristic of vascular land plants (I). Steranes are also useful parameters in petroleum exploration as correlation and thermal maturity indicators (2-4). With increasing temperature, the biologically derived 20R isomer of 5~~(11),14~~(11),17~~(11)-steranes is isomerized to the 20s configuration, which is not found in biological systems (3-5). Oils and sedimentary rocks which have experienced different degrees of thermal maturation can, therefore, have correspondingly different 2OS/2OR sterane ratios. In oil-oil and oil-source rock correlation studies, sterane distributions are used to identify oils which share a common source and thermal history (4). Routine capillary ga3 chromatographic/mass spectrometric (GC/MS) analysis of aliphatic hydrocarbon fractions of crude oils or sedimentary rock extracts allows the monitoring of the electron impact (EI) mc/z 217 fragment ion which is the base peak in 14a(H)-sterane E1 mass spectra (6). The resulting mass fragmentograms frequently reveal complex and incompletely resolved structural and stereoisomeric mixtures of C2,, C28, and CZ9steranes. Most problematic is the coelution of rearranged CZgsteranes (also known as diasteranes) with CZs and CZ7normal steranes ( 3 , 4 ) . Thus, chromatographic coelution of sterane homologues places constraints on the accurate measurement of geochernically significant sterane components.
SUM OF METASTABLE TRANSITIONS
- 109903
I."
+-"I..-
200
600
1000
!R 386*-
217+
I
ZW
6W
1OW
li
I400
Figure 1. (a) Sterane dlstributlons in a crude 011 from the Wllllston Basin, Montana, using selective metastable ion monitoring (SMIM); part b represents the C27H48steranes while parts c and d show the C28H50 and C29H52steranes, respectively.
In order to increase the specificity of petroleum sterane determinations, we used selective metastable ion monitoring (SMIM) in the GC/MS analyses of a number of crude oil aliphatic hydrocarbon fractions. Monitoring the spontaneous (unimolecular) fragmentation of sterane parent ions occurring in the first field-free region of a double focusing mass spectrometer (in which the electrostatic field precedes the magnetic sector) allows for the discrimination of steranes with disparate molecular weights. The most common steranes (both regular and rearranged) in petroleum have molecular weights of 372 amu (C27H48), 386 amu (Cz8Hs0),and 400 amu (C29H52). The
0003-2700/83/0355-0123$01.50/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
124
SUM OF METASTABLE TRANSITlOh'S 1439
RETENTION TIME (MINUTESSECONDS) 1558 1919 2139 2358
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2W
2858
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I
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2618
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SCAh KUMBER
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'
200
600
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I'
1400
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"
2nn
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600
-
-'
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1000
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loon
1400
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, ,
,
i8nn
i
SUM OF METASTABLE TRANSITIONS 372'-
600
1000
18W
200
2W
1wO
Steranes in a sedlment sample (Coupvray)extract from the Paris Basin (3).
-
500
18W
1SW
14W
Figure 2.
-
217*
-
sterane metastable parent ion transitions (372' 217+, 386+ 217+, and 400+ 217+) are known to occur readily in the first field-free region (7). Gallegos (7) used metastable ion methods to measure the abundances of C2,, CZ8,and CZs steranes in a Green River shale extract, although only direct insertion mass spectrometry (not GC/MS) was used. Separation of isomers was therefore not achieved. More recently, Gaskell and Millington (8) and Finlay and Gaskell (9) used selected metastable peak monitoring in quantitative GC/MS to detect and measure dihydrotestosterone and testosterone in human blood plasma. In the present study, we have combined the features of an abundant metastable sterane fragment ion with high-resolution capillary gas chromatography to greatly increase the specificity of petroleum sterane determinations. In addition, the three different sterane parent ion metastable transitions were monitored in a single GC/MS run by using a programmable power supply to vary the accelerating voltage, all under data system control.
EXPERIMENTAL SECTION
Figure 3. A comparlson of sterane distributlons derived from selected ion monitoring (SIM) of the m l z 217 ion at 3000 resolution and SMIM from another crude oil from the Williston Basin.
a Grob type split/splitless injector (at 270 "C) and a 20-m fused silica capillary column coated with OV-1 (methylsilicone). Analyses were performed in the split mode (251). The GC column oven temperature program was the following: 60 "C to 100 "C at 35 "C/min and then to 230 "C at 10 "C/min, subsequently ramping to 280 "C at 2 "C/min. The He flow rate through the GC column was about 1 mL/min made up to 30 mL/min prior to entering the GC/MS interface which consisted of a jet separator held at 250 "C. Mass spectrometer parameters used were the following: source temperature = 220 OC; electron beam current = 100 MA;and electron voltage = 60 eV. The three metastable parent ion transitions monitored were 217.1956+,386.3913' 217.1956+,and 400.4069' 372.3756' 217.1956' which correspond to C27H48,C28H6~, and C2&62 sterane isomers, respectively. By use of an authentic cholestane standard in the direct insertion probe, the m/z 217 ion was located by adjusting the magnet setting. The electrostatic voltage (E) and acceleration voltage (V) supplies were unlinked, and the E reference voltage was supplied by the internal reference of the mass spectrometer. The V reference was supplied by a binary programmable power supply driven by DS55 software. Increasing the accelerating voltage from 2 kV to 3.429 kV allowed the transition 372+ 217' occurring in the first field-freeregion to be located at the collector (IO). Similarly, increasing V to 3.558 217' and 400' kV and 3.687 kV allowed the transitions 386' 217' to be located, respectively. The data system, DS55, was programmed to switch to each of these voltages repetitively during the GC/MS run and record the signal obtained. The dwell time on each transition was 150 ms; a sweep of A50 ppm was applied to the V reference supply to ensure collection of peak top data. Data from the MS25 were acquired through a 200-kHz preprocessor interface operating at a sampling rate of 100 MS. The MS25
- -
-
-+
Aliphatic hydrocarbon fractions were obtained from pentane-deasphalted crude oil samples (subsequent to light end evaporation) which were subjected to combined alumina/silica column liquid chromatography. Also, a cyclic/branched aliphatic hydrocarbon fraction from a sediment extract of a sample (Coupvray)from the Paris Basin (3) was examined. The aliphatic hydrocarbon fractions were then analyzed with a Kratos MS25/DS55 GC/MS system equipped with metastable ion monitoring facilities. The GC was a Carlo Erba 4160 fitted with
-
-
125
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983 SUM OF METASTABLE TRANSITIONS
SUM OF iMETASTABLE TRANSITIONS loo
-- ions = 4 8 4 m
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-._..
i -
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600
1000
1400
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eon
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1000
SCAN NUMBER
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SCAN NUMBER
DIASTERANES
'1
1
600
200
1400
1000
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100 7 100%: 166136
386'-
217*
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200
600
1000
1400
iaw
Figure 4. Steranes in a nondegraded Colombian crMe oil using SMIM.
resolution was set to 600 (10% valley) for the SMIM experiments. Conventional single ion monitoring (SIM) experiments were also performed on the hydrocarbon fractions at 3000 resolution using standard DS55 SIM software in order to compare the resulting sterane distributions to those obtained during SMIM experiments. The rnlz 217.1956 ion was monitored along with two other masses in order to simulate the same conditions as in SMIM.
RESULTS AND DISCUSSION Figure 1shows the metastable ion sterane distributions in a crude oil aliphatic hydrocarbon fraction from the Williston Basin in Montana. In this figure, b represents the 372' 217' (C27steranes) transition while c and d show the 386' 217+ (C, steranes) and 400' 217+ (C, steranes) transitions, respectively. The top portion of each figure, a, is simply the computer summation of b, c, and d normalized to the most abundant component. Prior to SMIM, only the CZ9steranes (i.e., the stereoisomers of 24-ethylcholestane) could be used for thermal maturation determinations (e.g., 20S/20R) with any degree of confidence because the Cn and C, stereoisomers usually coelute with Cze diasteranes (3,4). Also, in a number of samples an unknown component, which is not a regular Cm sterane, coelutes on O'V-1 with the 20s isomer of 24-ethylcholestane (11). Hence, using SMIM, both Cn and C, sterane ratios can be more precisely determined, as well as Cze ratios. In addition, the CZ7isomers can be quantitatively compared to the Czsand Czehomologues by separately summing the total ion currents in b, c, and d, respectively. In this manner, the relative abundance of sterane homologues can be used for determining source rock depositional environments.
-
-+
-+
zw
6W
1400
Figure 5. Sterane distributions in a biodegraded Colombian crude oil using SMIM.
Figure 2 illustrates the SMIM sterane distributions from a hydrocarbon extract of a sediment sample (Coupvray) at 2100 m from the Paris Basin. The nominal m / z 217 mass fragmentogram and resulting sterane distribution of this same extract sample have been elucidated by Mackenzie et al. (3). Comparison with the sum of the metastable parent ion transitions shown in Figure 2a and the Mackenzie et al. (3) SIM distribution indicate a close correlation suggesting that SMIM is comparable to SIM in respect to relative abundances of various sterane components. However, in the SMIM technique the individual sterane homologue distributions are much more clearly defined (Figure 2b-d). Figure 3 is a comparison between medium resolution SIM of the m / z 217 ion and SMIM of another crude oil from the Williston Basin. Again, the two traces are very similar. The two oils shown in Figures 4 and 5 are from Colombia, South America. Both have the same origin (12),except that one oil (Figure 5) was subsequently severely biodegraded by bacteria in the reservoir (after generation and perhaps migration) while the other oil (Figure 4)has not been significantly altered. It has been reported (13)that the diasteranes are relatively more resistant to severe biodegradation than the regular steranes. On comparison of Figures 4 and 5, it can be seen that the Colombia oil diasteranes have survived biodegradation more effectively than the regular steranes. (Diasteranes elute prior to regular steranes of the same molecular weight as indicated in Figures 4 and 5.) Also, the distributions of specific sterane homologues (i.e., either CZ7, C2*,or CZ9),whether regular or rearranged steranes, are ap-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
126
-
1W 1
trace (3000 resolution) with the metastable ion transition 372+ 217' trace of 100 pg of a cholestane standard (50 pg of 5/3(H)- and 50 pg of Sa(H)-cholestane). It is clear that SMIM is more sensitive to sterane detection than medium resolution SIM.
1WW = 72404
SIM 217 1966 TRACE
ACKNOWLEDGMENT We thank J. M. Heard for chromatographic isolation of crude oil aliphatic hydrocarbon fractions and J. R. Maxwell for providing the Coupvray extract from the Paris Basin. We also acknowledge M. J. Leenheer, A. S. Mackenzie, J. R. Maxwell, S. E. Palmer, Z. Sofer, and R. S. Stradling for helpful comments and suggestions.
LITERATURE CITED
100 3 100% = 91356
1
1 METASTABLE ION TRANSITION
1
372*-
217*
I
60 -
I
I\ I1
2500
2660
2700
I
I1
2800
2900
3000
Flgure 6. A sensitivity comparison between medium resolution SIM and SMIM using 100 pg of a cholestane standard (50p g of 5/3(Hb and 50 pg of 5a(H)-cholestane).
parent from b, c, and d in these figures. Figure 6 is a comparison of the conventional SIM m f z 217
(1) Tissot, B. P.; Welte, D. H. "Petroleum Formation and Occurrence"; Springer-Verlag: New York, 1978; Chapter 3. ( 2 ) Seifert, W. K.; Moldowan, J. M. Geochim. Cosmochlm. Acta 1978, 42, 77-95. (3) Mackenzie, A. S.; Patience, R. L.; Maxwell, J. R.; Vanderbroucke, M.; Durand, 8. Geochim. Cosmochlm. Acta 1980, 44, 1709-1721. (4) Seifert, W. K.; Moldowan, J. M. Geochim. Cosmochlm. Acta 1981, 45, 783-794. (5) Mulheirn, L. J.; Ryback, G. Nature (London) 1975, 256, 301-302. ( 6 ) Mulheirn, L. J.; Ryback, G. I n "Advances in Organic Geochemistry"; Campos, R., GoAi, J., Eds.; ENADIMSA:, Madrld, Spain, 1975; pp 173-192. (7) Gallegos, E. J. "High Performance Mass Spectrometry"; American Chemical Society: Washington DC, 1978; Chapter 15. (8) Gaskell, S. J.; Miilington, D. S. Blamed. Mass Spectrom. 1978, 5 , 557-558. (9) Finlay, E. M. H.; Gaskell, S. J. Clln. Chem. (Winston-Salem, NC) 1981, 27. 1165-1 170. (10) Jennings, K. R. "Some Aspects of Metastable Transitions, Mass Spectrometry Techniques and Application"; Miine, G. W. H., Ed.; Wiley-Interscience: New York, 1971. (11) Mackenzle, A. S.;Maxwell, J. R., University of Bristol, personal communlcation, 1982. (12) Zumberge, J. E. 26th International Geological Congress, Paris, 1980; Abstract, p 806. (13) Seifert, W. K.; Moldowan, J. M. Oeochim. Cosmochim. Acta 1979, 43. 111-126.
RECEIVED for review August 16, 1982. Accepted October 6, 1982.
