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Extended diamondoid assessment in crude oil using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry
Fuel 112 (2013) 125–133
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Extended diamondoid assessment in crude oil using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry Renzo C. Silva a,b,⇑, Raphael S.F. Silva a,c, Eustáquio V.R. de Castro b, Kenneth E. Peters d,e, Débora A. Azevedo a,⇑ a
Universidade Federal do Rio de Janeiro, Instituto de Química, Rio de Janeiro, RJ 21949-900, Brazil Universidade Federal do Espírito Santo, Departamento de Química, Vitória, ES 29075-710, Brazil c Instituto Federal de Educação, Ciência e Tecnologia do Rio de Janeiro, Núcleo de Ciências Químicas, Rio de Janeiro, RJ 20270-021, Brazil d Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA e Schlumberger Information Solutions, Mill Valley, CA 94941, USA b
h i g h l i g h t s GC GC-TOFMS offers new insights on diamondoid analysis. GC GC-TOFMS allows measurements needed for diamondoid and biomarker assessments. GC GC-TOFMS has the ability to derive diamondoid indexes. New diamondoids found lead to improved understanding of the diamondoid formation.
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 3 May 2013 Accepted 6 May 2013 Available online 18 May 2013 Keywords: Diamondoids GC GC-TOFMS Extended diamondoids Petroleum biomarkers
a b s t r a c t Comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC GC-TOFMS) was explored as a powerful tool to assess diamondoids in petroleum fluids. This work demonstrates the possibility to calculate diamondoid indexes, perform relative quantitations, and complete saturated biomarker assessments in one chromatographic run. Tetramantanes and triamantanes identification resulted in new discovered compounds that can be used for assessment (e.g., for new maturity indexes) depending on the position of alkyl-substitution. Methyl-protodiamantanes and alkyl-substituted diamondoids with chains bigger than ethyl were detected (except for tetramantanes) as a consequence of the improved resolution achieved by GC GC-TOFMS. Further studies using this method may lead to improved understanding of the formation mechanisms of diamondoids and highly mature petroleum fluid compositions. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Diamondoids were known to be present in petroleum fluids since the 1930s [1,2], but their usefulness as geochemical indicators was only developed in the last 20 years [3–9]. They differ from saturated hydrocarbons because of the structural rigidity of diamond-like three dimensional cages, which enhance resistance to thermal and microbial degradation [5,10–13]. Ultra-deep reservoirs are an important frontier for petroleum exploration, demanding both scientific and technological investments to achieve ⇑ Corresponding authors. Address: Av. Athos da Silveira Ramos, 149, Centro de Tecnologia, Bloco A, sala 603, Cidade Universitária, Rio de Janeiro, RJ 21949-900, Brazil. Tel.: +55 21 2260 3967. E-mail addresses: [email protected] (R.C. Silva), [email protected] (D.A. Azevedo). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.05.027
extended hydrocarbon recovery. For these mature and overmature oil accumulations, however, traditional biomarker parameters from saturated hydrocarbons are of limited use. Therefore diamondoid assessment is needed to understand such petroleum accumulations [3,6,14–16]. Many research groups worldwide are actively pursuing improved methods for diamondoid analysis. Diamondoids analyzed by GC–MS were used to assess biodegradation [10], source rock facies and lithology [15,17,18], and to evaluate several petroleum systems [11,16,19]. Recent literature shows the use of GC–MRM– MS (triple–quadrupole mass analyzer) to achieve quantitation limits as low as 0.08–0.37 lg/g for adamantanes and diamantanes [20]. This method was used to evaluate diamondoid abundances in a simulated oil cracking experiment, which suggested some diamondoid indexes for assessment of the late oil window [13].
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Fig. 1. GC GC-TOFMS contour plot (sample A). Intensities were selectively altered to show all classes in one figure. Calculated intensities are as follows: terpanes (m/z 19111), steranes (m/z 21710), adamantanes (m/z 135), diamantanes (m/z 187), triamantanes (m/z 2395), tetramantanes (m/z 291 60).
Recently, adamantanes and diamantanes from a Chinese condensate were investigated by GC GC-TOFMS [21]. Analyzing Chinese oil samples with a GC GC-TOFMS instrument, a study [22] recognized more than 100 diamondoid homologs of adamantane, diamantane and triamantane. Both works showed that GC GC techniques perform well for both target (usually analyzed) and non-target (previously unknown or neglected) compounds [23], as seen in several biomarker studies [24–31]. Since diamondoids are believed to be absent in modern sediments [15] and the lithology appears to play a role in diamondoid formation [15,17,18], understanding the underlying mechanisms might be useful to fine tune diamondoid parameters. For extended
diamondoids (triamantane up to hexamantane) this appears to be even more challenging [6]. It is important to identify and quantify diamondoids in petroleum fluids. Furthermore, even the late stages of cracking might be assessed by extended diamondoid parameters. The purpose of this work is to assess both diamondoids and saturated biomarkers in petroleum samples using GC GC-TOFMS. The work aims to gather data potentially useful in future studies on the formation mechanisms of diamondoids and/or the proposal of new diamondoid indexes. 2. Experimental 2.1. Samples In order to study the method, two Brazilian crude oil samples were selected. Sample A exhibits overmature characteristics, whilst sample B is believed to have been produced in the early stages of oil cracking. 2.2. Standards and chemicals
Fig. 2. Tetramantane isomers. Iso-tetramantane has three quaternary carbon atoms, while anti- and skew-tetramantane have two.
Twelve standard compounds purchased from Chiron AS (Stiklestadveien, Norway) were used as a reference for adamantane
Fig. 3. Chromatographic surface of tetramantanes. Intensities were calculated from the sum of m/z 291, 305, 319 and 333. Diagonal solid lines represent isomers of tetramantane (m/z 291 and 292), C1-tetramantanes (m/z 291 and 306), C2-tetramantanes (m/z 305 and 320) and C3-tetramantanes (m/z 319 and 334).
R.C. Silva et al. / Fuel 112 (2013) 125–133
and diamantane identifications. Internal deuterated standards (ndodecane-d26 and n-tetracosane-d50) also were used for relative quantification. For sample processing and analysis, chromatographic grade n-hexane (TediaBrazil, Rio de Janeiro, Brazil) was used.
2.3. Sample preparation Special precautions were taken to avoid the well-documented loss of volatile diamondoids when applying standard fractionation procedures for saturated biomarker analysis. Approximately 100 mg of each crude oil was dissolved in 0.5 mL of a hexane solution containing 20 lg/mL of deuterated standards (tetracosane-d50 and dodecane-d26), deposited on the top of an activated silica gel column and then eluted with 4.5 mL of n-hexane, similar to
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procedures published elsewhere [10,11,16]. Eluted solutions were immediately sealed and analyzed by GC GC-TOFMS.
2.4. GC GC-TOFMS A Pegasus 4D (Leco, St. Joseph, MI, USA) system was used for all GC GC–TOFMS analysis. An Agilent Technologies 6890 GC (Palo Alto, CA, USA) with a secondary oven and a fixed quad-jet dualstage modulator was coupled to the TOF analyzer. Columns included a DB-5 (30 m 0.25 mm i.d. 0.25 lm df) for the first dimension (1D) and a BPX-50 (1.5 m 0.1 mm i.d. 0.1 lm df) for the second one (2D). An uncoated deactivated capillary column (0.5 m 0.25 mm i.d.) connected the second column to the TOF analyzer and SGE unions of SilTite metal ferrules (Austin, Texas, USA) connected all columns.
Fig. 4. Examples of mass spectra for tetramantane (top), C1-tetramantane (middle) and a C2-tetramantane (bottom). Based on the presence of both peaks m/z 305 and 320, the C2-tetramantane molecule represented herein is a dimethyl-tetramantane isomer.
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Table 1 Relative concentrations and selected data for tentatively identified tetramantanes in sample A.
a b c d
Compound class
Diagnostic ions (m/z)
1
tR (s)a
2
tR (s)b
Dimethyl-tetramantane Methyl-tetramantane Tetramantane (I) Trimethyl-tetramantane Dimethyl-tetramantane Methyl-tetramantane Dimethyl-tetramantane Dimethyl-tetramantane Methyl-tetramantane Tetramantane (II) Dimethyl-tetramantane Methyl-tetramantane Dimethyl-tetramantane Trimethyl-tetramantane Dimethyl-tetramantane Methyl-tetramantane Trimethyl-tetramantane Methyl-tetramantane Tetramantane (III) Dimethyl-tetramantane Trimethyl-tetramantane Dimethyl-tetramantane Dimethyl-tetramantane Methyl-tetramantane Methyl-tetramantane
305;320 291;306 292 319;334 305;320 291;306 305;320 305;320 291;306 292 305;320 291;306 305;320 319;334 305;320 291;306 319;334 291;306 292 305;320 319;334 305;320 305;320 291;306 291;306
2922 2934 2940 2952 2964 2976 3006 3024 3024 3024 3054 3054 3066 3072 3078 3078 3084 3102 3102 3108 3120 3120 3138 3144 3162
4.24 4.51 4.86 4.02 4.34 4.68 4.37 4.39 4.68 5.01 4.50 4.79 4.52 4.28 4.52 4.89 4.31 4.84 5.30 4.63 4.37 4.71 4.68 5.05 5.11
Quantitation ion (m/z)
S/Nc
Area
Concentration (ng/mg)d
305 291 292 319 305 291 305 305 291 292 305 291 305 319 305 291 319 291 292 305 319 305 305 291 291
67 123 259 14 34 36 21 12 85 158 32 38 20 10 18 71 11 10 84 13 10 28 20 31 19
42,502 81,116 137,784 10,546 32,614 20,464 14,154 13,220 56,046 119,527 76,144 40,342 23,292 7800 11,906 102,805 6539 4530 54,384 13,872 8144 36,878 28,514 28,561 13,883
1.8 3.5 5.9 0.5 1.4 0.9 0.6 0.6 2.4 5.1 3.3 1.7 1.0 0.3 0.5 4.4 0.3 0.2 2.3 0.6 0.3 1.6 1.2 1.2 0.6
First GC dimension retention time. Second GC dimension retention time. Signal to noise ratio calculated for the quantitation ion. Quantitation relative to ion m/z 66 of n-tetracosane-d50 mass spectrum.
acquisition rate of 200 Hz. ChromaTOF software version 2.32 (Leco Corp., St. Joseph, MI, USA) was used to acquire, process and analyze the data. 3. Results and discussion 3.1. Chromatographic aspects
Fig. 5. Chromatographic surface of triamantanes (sample A). Intensities were calculated from the sum of m/z 239, 240, 253 and 267.
The GC conditions were set to optimize chromatographic separations from the very light adamantanes to heavy C40 biomarkers. A 5 °C off-set was used between the dimensions during GC program, while the modulator was kept 30 °C higher than the first dimension. Initially stabilized at 50 °C, the first dimension temperature was raised at 4 °C/min until 350 °C, summing to 75 min for each analysis. The modulation period was chosen to be 6 s, with a 2 s hot pulse. Helium gas flow of 1.5 mL/min was kept constant. Sample was injected in the splitless mode (1 lL to 15 bar). Transfer line and ion source temperatures were held at 290 °C and 200 °C, respectively. Electron energy of 70 eV was applied, and the ions were detected (1700 V) in the range of m/z 50–700 at an
Chromatographic conditions were chosen after tests to determine sufficient resolution at the beginning of the chromatography run where adamantanes appear. The choice of one heating rate was to ensure the chromatographic structure of compound classes, where series elute in ordered patterns and are separated from other homologous series (roof-tile effect, common in GC GC analysis of petroleum fluids). Quality of biomarker assessment at the very end of the chromatographic run was an objective. Since one of the targets was the volatile adamantanes, evaporation steps after liquid chromatography were avoided, including the n-paraffin removal procedure. Leaving a high paraffin content in the sample is somewhat harmful for diamondoid assessment because some intense n-alkane peaks appear tailing along the second dimension. This interference can be filtered by the mass spectrometer. In these experiments, one can assess saturated biomarkers, diamondoids, and extended diamondoids in a sample using only one chromatographic run. Fig. 1 depicts the chromatographic surface where the studied compound classes were found. As the number of diamondoid cages increases, diamondoid interaction increases with the polar stationary phase of the second column. The intensity axis was modified to equalize class abundance. Diamondoid abundance decreases with the number of cages. The maximum observed tetramantane 2tR (second GC dimension retention time) value (5.36 s) was used to set the modulation period. Detected peaks from pentamantanes were so small (S/N < 10) that they are not discussed in this paper. At the end of the run, C35 pentakishomohopane (H35) and C39 tricyclic terpane (Tr39) were the last detected biomarkers.
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R.C. Silva et al. / Fuel 112 (2013) 125–133 Table 2 Relative concentrations and selected data for tentatively identified triamantanes in sample A.
a b c d
Compound class
Diagnostic ions (m/z)
1
tR (s)a
2
tR (s)b
Quantitation ion (m/z)
S/Nc
Area
Concentration (ng/mg)d
Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Ethylmethyl-triamantane Ethylmethyl-triamantane Ethyl-triamantane Ethyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Propyl-triamantane Propyl-triamantane Triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane
253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;282 253;282 239;268 239;268 239;254 239;254 239;254 239;254 239;254 239;254 239;254 239;282 239;282 240 267;282 267;282 267;282 267;282 267;282 267;282 267;282 267;282 267;282 267;282
2454 2502 2520 2544 2556 2562 2568 2586 2628 2646 2670 2676 2688 2658 2742 2658 2682 2448 2454 2508 2520 2538 2556 2598 2790 2808 2442 2502 2568 2592 2610 2646 2658 2676 2694 2706 2730
3.85 4.01 4.04 4.00 4.04 4.13 4.13 4.20 4.16 4.23 4.41 4.34 4.35 3.92 4.05 4.17 4.20 4.13 4.10 4.27 4.29 4.30 4.29 4.50 4.12 4.18 4.42 3.74 3.90 3.87 3.89 4.00 4.02 4.04 4.04 4.09 4.15
253 253 253 253 253 253 253 253 253 253 253 253 253 253 253 239 239 239 239 239 239 239 239 239 239 239 240 267 267 267 267 267 267 267 267 267 267
763.7 1362.1 645.6 611.0 248.5 267.0 62.6 560.7 127.6 42.6 126.7 92.2 50.3 110.3 45.3 215.7 101.7 3672.5 822.9 1097.6 1608.4 560.2 325.9 369.7 65.0 25.2 4228.8 410.3 163.5 181.7 118.9 110.7 137.7 80.6 65.7 50.8 39.3
413,255 1,016,896 378,826 366,336 157,101 169,520 61,341 779,390 205,012 53,646 139,944 72,285 34,081 111,351 132,037 181,940 67,681 1,982,545 307,540 635,471 702,464 313,663 192,769 234,353 123,760 22,587 2,222,445 355,311 132,590 236,763 158,164 77,515 171,729 132,546 121,154 170,583 61,513
17.7 43.6 16.2 15.7 6.7 7.3 2.6 33.4 8.8 2.3 6.0 3.1 1.5 4.8 5.7 7.8 2.9 85.0 13.2 27.2 30.1 13.4 8.3 10.0 5.3 1.0 95.3 15.2 5.7 10.2 6.8 3.3 7.4 5.7 5.2 7.3 2.6
First GC dimension retention time. Second GC dimension retention time. Signal to noise ratio calculated for the quantitation ion. Quantitation relative to ion m/z 66 of n-tetracosane-d50 mass spectrum.
3.2. Tetramantanes Tetramantanes are the first pseudo homolog in the series of diamondoids: skew-, anti- and iso-tetramantane isomers, the latter having three quaternary carbon atoms instead of two (Fig. 2). Their first natural occurrence in petroleum was published in 1995 in a condensate from a Jurassic sandstone reservoir [6]. In that paper, 20 compounds were tentatively assigned as tetramantanes, even though coelutions noticeably affected the results. The tetramantane chromatographic surface is expanded in Fig. 3. At the top, three peaks on the diagonal tetramantane line show both m/z 291 and 292 as the most intense ions in their mass spectra, suggesting tetramantane isomers, even though their elution order is not known. C1-Tetramantanes (m/z 291 and 306) appear in a trend line below tetramantanes, summing to eight tentatively identified methyl-tetramantanes. C2-Tetramantanes are assumed to be dimethyl-tetramantanes because of the diagnostic ions m/z 305 and 320, rather than m/z 291 and 320, expected for ethyl-tetramantanes. By analogy, C3-tetramantanes are labeled as trimethyl-tetramantane (m/z 319 and 334), instead of propyl-tetramantane (m/z 291 and 334) or ethylmethyl-tetramantane (m/z 305, 319 and 334). In fact, alkylsubstitution on the observed tetramantanes did not show chains bigger than methyl, although this could be due to insufficient sensitivity. On Fig. 3, the first and most intense peak on the C1-tetramantanes line has a shape suggestive of coelution, and its 1tR (first
GC dimension retention time) is lower than the first tetramantane isomer. The same behavior is also seen for the first C2-dimethyltetramantane, which has 1tR lower than its lower mass homologs. As seen before [17] and also in the data presented below, a decrease in 1tR for more alkyl-substitution is not expected, at least for substituted adamantanes and diamantanes. Lack of authentic standards made it difficult to establish an elution order based on the positions of alkyl-substitution. There are coelutions in the first GC dimension that can be resolved using the additional dimension. However, GC GC-TOFMS may be incapable of deconvoluting two or more coeluting isomers, due to similarities in mass spectra. Twenty-five tetramantanes were tentatively identified based on mass spectra (Fig. 4 and Table 1): three tetramantanes, eight methyl-tetramantanes, ten dimethyl-tetramantanes and four trimethyl-tetramantanes. No tetramantanes were detected in sample B. 3.3. Triamantanes In the 14 years since the first recognition of triamantanes in petroleum [3] to the most comprehensive analysis of them by GC–MS [15] in 2006, thirteen new compounds have been identified. Recently, two triamantanes species were recognized by GC GC-TOFMS [22]. The region of triamantanes on the chromatographic surface is expanded and detailed in Fig. 5. Triamantanes are spread around 370 s in 1D, from triamantane (1tR 2454 s) to propyl-triamantane (1tR 2808 s). C1-Triamantanes include seven
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Fig. 6. Mass spectra for some triamantanes found in sample A. Note differences between dimethyl-triamantane (m/z 253 and 268) and ethyl-triamantane (m/z 239 and 268); and for trimethyl-triamantane (m/z 267 and 282) and propyl-triamantane (m/z 239 and 282).
Fig. 7. Chromatographic surface of diamantanes (sample A). Intensities correspond to the sum of ions m/z 187, 201, 215 and 229. Dashed line suggests the presence of methyl-protodiamantanes.
tentatively identified methyl-triamantanes (m/z 239 and 254), while previously five were detected and four were tentatively identified [15]. The same comparison can be made for dimethyltriamantanes: thirteen peaks were identified in this work,
compared to five peaks described elsewhere. For trimethyltriamantanes: ten were identified here compared to two from another publication [15]. In addition, two propyl-triamantanes, two ethyl-triamantanes and two ethylmethyl-triamantanes also were
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Fig. 8. Comparison between 1-methyl-diamantane mass spectrum and a typical spectrum from the protodiamantane region (dashed line in Fig. 7). Compounds were labeled as methyl-protodiamantanes based on recent discussion about protodiamondoids [32–35], which are assumed to have mass spectra similar to regular diamondoids.
Table 3 Methyl-protodiamantane and methyl-diamantane quantities found in samples A and B, based on ion m/z 187 intensities. Compound class
Methyl-protodiamantanes (pMD) Methyl-diamantanes (MD) pMD/MDa
Sum of areas Sample A
Sample B
839,287 1,015,3920 0.08
21,608 133,635 0.16
a Ratio between areas of methyl-protodiamantanes (pMD) and methyl-diamantanes (MD).
identified (Table 2 and Fig. 6). Alkyl-substitution with more than one carbon atom was not seen for tetramantanes, but is present in triamantanes. In total, 37 triamantanes were tentatively identified in Sample A. Sample B showed two tiny peaks for triamantane and methyltriamantane, but with signal-to-noise ratios lower than our requirements for further discussions (S/N < 10).
3.4. Adamantanes and diamantanes Analyses of adamantanes and diamantanes are well explored in the recent literature, achieving satisfactory limits of quantitation using gas chromatography coupled to a triple–quadrupole mass spectrometer [20]. A total of 22 adamantanes and 10 diamantanes were successfully identified and quantified. Another approaches to these classes used GC GC-TOFMS to analyze a Chinese condensate [21] and crude oils [22], where characterization of 100 species was effective. In the present study, authentic standards were used to check retention times of some adamantanes and diamantanes.
Fig. 9. Dahl’s diagram [5] displays the correlation between diamondoid appearance and biomarker (stigmastane) disappearance due to thermal stress. Sample A is in the zone of intense oil cracking because it exhibits high diamondoid content and low sterane concentration, while sample B is in the beginning of intense oil cracking due to low values of both steranes and diamondoids.
Identification of most components by GC GC-TOFMS has been conducted [21,22] and this work does not intend to be repetitive, although some details require attention. Ion m/z 187 in diamantane region contains different groups of molecules highlighted in Fig. 7. Their mass spectra have m/z 187 and 202 as the most intense peaks, similar to those from methyl-diamantane (Fig. 8). Considering that diamantane belongs to the D3d symmetry group, having three
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Table 4 Diamondoid indexes from GC GC-TOFMS analyses. Indexa
MAI MDI DMAI DMDI EAI TMA
Sample A
B
0.38 0.39 0.48 0.22 0.40 0.31
0.67 0.45 0.57 0.45 0.65 0.24
a MAI = (1-methyl-adamantane)/(1-methyl-adamantane + 2-methyl-adamantane); MDI = (4-methyl-diamantane)/(4-methyl-diamantane + 1-methyl-diamantane + 3-methyl-diamantane); DMAI = (1,3-dimethyl-adamantane)/(1,2-dimethyladamantane + 1,3-dimethyl-adamantane); DMDI = (3,4-dimethyl-diamantane)/ (4,9-dimethyl-diamantane + 3,4-dimethyl-diamantane); EAI = (2-ethyl-adamantane)/(1-ethyl-adamantane + 2-ethyl-adamantane); TMAI = (1,3,5-trimethyl-adamantane)/(1,3,5-trimethyl-adamantane + 1,3,4-trimethyl-adamantane).
possible methyl-substituted isomers: 1-, 3-, and 4-methyl-diamantanes, these findings suggest the presence of protodiamondoids, which presumably are intermediates in diamondoid formation [32–35]. Quantification of these methyl-protodiamantanes from Table 3 shows that m/z 187 areas compute to 8% and 16% of methyldiamantanes in samples A and B, respectively. These results need more refinement from additional study. For example, since sample A is more mature, the percentage of methyl-protodiamantanes appears to decrease with maturity. These results highlight the usefulness of GC GC-TOFMS for non-target compound analysis. Previous work [33] claims that alkyl-adamantanes with alkyl groups longer than the ethyl have not been found in oil, supporting the idea of alkyl-adamantane production via catalytic isomerization of tricyclic hydrocarbons on activated clays or aluminosilicates. Based on mass spectra (base peaks at m/z 135 and 178) and retention time, one propyl-adamantane was tentatively identified in sample A, whilst two propyl-adamantanes were found in sample B. Traditional diamondoid analysis focuses either on quantitative data to explore thermal maturity of oil sample sets [11,13,16,20,35] or gathering diamondoid indexes for other purposes [10,15]. Both goals are achieved by the GC GC-TOFMS analysis proposed herein. Dahl’s diagram [5] plots stigmastane versus 3-+4-methyl-diamantanes to evaluate thermal maturity, and Fig. 9 shows locations of samples A and B. The capability to create such plots using only one chromatographic run for each sample is advantageous. Another advantage of GC GC-TOFMS is the ability to derive diamondoid indexes used for maturity assessments and to distinguish source facies, such as methyl-adamantane index (MAI), methyl-diamantane index (MDI), dimethyl-adamantane index (DMAI), dimethyl-diamantane index (DMDI) and so on [4,10,15] (Table 4). In this study, diamondoid data support maturity measurements based on other metrics (Table S1 in the Supplementary material). 4. Conclusions GC GC-TOFMS was explored as a tool for detailed diamondoid analysis using two crude oil samples. This paper demonstrates the power of this technique to evaluate diamondoids in oil samples, thus allowing improved geochemical assessments. Some key observations include: (a) GC GC-TOFMS is a powerful tool to study diamondoids. (b) To improve mechanistic proposals of diamondoid formation, one might consider information obtained using GC GCTOFMS, such as the existence of propyl-triamantanes and evidence of methyl-protodiamantanes in crude oils.
(c) GC GC-TOFMS allows measurements needed for traditional diamondoid and biomarker assessments (Dahl’s diagram, diamondoid indexes and saturated biomarker parameters) using only one chromatographic run.
Acknowledgements The authors thank CNPq (Brazilian research council) for fellowships and Petrobras S/A for financial support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2013.05.027. References [1] Landa S, Machacek M, Mzourek M, Landa M. Chim Ind 1933; Publ No 506; Chem Abstr 27, 5949. [2] Landa S, Machacek V. Coll Czech Chem Commun 1933;5:1. [3] Wingert WS. GC–MS analysis of diamondoid hydrocarbons in Smackover petroleums. Fuel 1992;71:37–43. [4] Chen J, Fu J, Sheng G, Liu D, Zhang J. Diamondoid hydrocarbon ratios: novel maturity indices for highly mature crude oils. Org Geochem 1996;25:179–90. [5] Dahl JE, Moldowan JM, Peters KE, Claypool GE, Rooney MA, Michael GE, et al. Diamondoid hydrocarbons as indicators of natural oil cracking. Nature 1999;399:54–7. [6] Lin R, Wilk AZ. Natural occurrence of tetramantane (C22H28), pentamantane (C26H32) and hexamantane (C30H36) in a deep petroleum reservoir. Fuel 1995;74:1512–21. [7] Schoell M, Carlson RMK. Diamondoids and oil are not forever. Nature 1999;399:15–6. [8] de Araujo PLB, Mansoori GA, de Araujo ES. Diamondoids: occurence in fossil fuels, applications in petroleum exploration and fouling in petroleum production. Int J Oil Gas Coal Technol 2012;5:316–67. [9] Nekhaev AI, Bagril EI, Maximov AL. Petroleum nanodiamonds: new in diamondoid naphthenes. Petrol Chem 2011;51:86–95. [10] Grice K, Alexander R, Kagi RI. Diamondoid hydrocarbon ratios as indicators of biodegradation in Australian crude oils. Org Geochem 2000;31:67–73. [11] Springer MV, Garcia DF, Gonçalves FTT, Landau L, Azevedo DA. Diamondoid and biomarker characterization of oils from the Llanos Orientales Basin. Colombia Org Geochem 2010;41:1013–8. [12] Zhang S, Su J, Wang X, Zhu G, Yang H, Liu K, et al. Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China: Part 3. Thermal cracking of liquid hydrocarbons and gas washing as the major mechanisms for deep gas condensate accumulations. Org Geochem 2011;42:1394–410. [13] Fang C, Xiong Y, Liang Q, Li Y. Variation in abundance and distribution of diamondoids during oil cracking. Org Geochem 2012;47:1–8. [14] Peters KE, Walters CC, Moldowan JM. The biomarker guide. 2nd ed. Cambridge: Cambridge University Press; 2005. [15] Wei Z, Moldowan JM, Paytan A. Diamondoids and molecular biomarkers generated from modern sediments in the absence and presence of minerals during hydrous pyrolysis. Org Geochem 2006;37:891–911. [16] Azevedo DA, Tamanqueira JB, Dias JCM, Carmo APB, Landau L, Gonçalves FTT. Multivariate statistical analysis of diamondoid and biomarker data from Brazilian basin oil samples. Fuel 2008;87:2122–30. [17] Wei Z, Moldowan JM, Dahl JE, Goldstein TP, Jarvie DM. The catalytic effects of minerals on the formation of diamondoids from kerogen macromolecules. Org Geochem 2006;37:1421–36. [18] Schulz LK, Wilhelms A, Rein E, Steen AS. Application of diamondoids to distinguish source rock facies. Org Geochem 2001;32:365–75. [19] Zhang S, Huang H, Xiao Z, Liang D. Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China. Part 2: Maturity assessment. Org Geochem 2005;36:1215–25. [20] Liang Q, Xiong Y, Fang C, Li Y. Quantitative analysis of diamondoids in crude oils using gas chromatography–triple quadrupole mass spectrometry. Org Geochem 2012;43:83–91. [21] Li S, Hu S, Cao J, Wu M, Zhang D. Diamondoid characterization in condensate by comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry: The Junggar Basin of northwest China. Int J Mol Sci 2012;13:11399–410. [22] Wang G, Shi S, Wang P, Wang T-G. Analysis of diamondoids in crude oils using comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry. Fuel 2013;107:706–14. [23] Eiserbeck C, Nelson RK, Grice K, Curiale J, Reddy CM. Comparison of GC–MS, GC–MRM–MS, and GC GC to characterise higher plant biomarkers in Tertiary oils and rock extracts. Geochim Cosmochim Acta 2012;87:299–322. [24] Silva RSF, Aguiar HGM, Rangel MD, Azevedo DA, Aquino Neto FR. Comprehensive two-dimensional gas chromatography with time of flight
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[30] Silva RSF, Tamanqueira JB, Dias JCM, Passarelli FM, Bidart AMF, Aquino Neto FR, et al. Comprehensive two-dimensional gas chromatography with time of flight mass spectrometry applied to analysis of Fischer-Tropsch synthesis products obtained with and without carbon dioxide addition to feed gas. J Braz Chem Soc 2011;22:2121–6. [31] Avila BMF, Pereira R, Gomes AO, Azevedo DA. Chemical characterization of aromatic compounds in extra heavy gas oil by comprehensive twodimensional gas chromatography coupled to time-of-flight mass spectrometry. J Chromatogr A 2011;1218:3208–16. [32] Gordadze GN, Giruts MV. Synthesis of adamantane and diamantane hydrocarbons by high-temperature cracking of higher n-alkanes. Petrol Chem 2008;48:414–9. [33] Gordadze GN. Geochemistry of cage hydrocarbons. Petrol Chem 2008;48:241–53. [34] Giruts MV, Gordadze GN. Generation of adamantanes and diamantanes by thermal cracking of polar components of crude oils of different genotypes. Petrol Chem 2007;47:12–22. [35] Berwick L, Alexander R, Pierce K. Formation and reactions of alkyl adamantanes in sediments: carbon surface reactions. Org Geochem 2011;42:752–61.
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Fuel journal homepage: www.elsevier.com/locate/fuel
Extended diamondoid assessment in crude oil using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry Renzo C. Silva a,b,⇑, Raphael S.F. Silva a,c, Eustáquio V.R. de Castro b, Kenneth E. Peters d,e, Débora A. Azevedo a,⇑ a
Universidade Federal do Rio de Janeiro, Instituto de Química, Rio de Janeiro, RJ 21949-900, Brazil Universidade Federal do Espírito Santo, Departamento de Química, Vitória, ES 29075-710, Brazil c Instituto Federal de Educação, Ciência e Tecnologia do Rio de Janeiro, Núcleo de Ciências Químicas, Rio de Janeiro, RJ 20270-021, Brazil d Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA e Schlumberger Information Solutions, Mill Valley, CA 94941, USA b
h i g h l i g h t s GC GC-TOFMS offers new insights on diamondoid analysis. GC GC-TOFMS allows measurements needed for diamondoid and biomarker assessments. GC GC-TOFMS has the ability to derive diamondoid indexes. New diamondoids found lead to improved understanding of the diamondoid formation.
a r t i c l e
i n f o
Article history: Received 23 January 2013 Received in revised form 3 May 2013 Accepted 6 May 2013 Available online 18 May 2013 Keywords: Diamondoids GC GC-TOFMS Extended diamondoids Petroleum biomarkers
a b s t r a c t Comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC GC-TOFMS) was explored as a powerful tool to assess diamondoids in petroleum fluids. This work demonstrates the possibility to calculate diamondoid indexes, perform relative quantitations, and complete saturated biomarker assessments in one chromatographic run. Tetramantanes and triamantanes identification resulted in new discovered compounds that can be used for assessment (e.g., for new maturity indexes) depending on the position of alkyl-substitution. Methyl-protodiamantanes and alkyl-substituted diamondoids with chains bigger than ethyl were detected (except for tetramantanes) as a consequence of the improved resolution achieved by GC GC-TOFMS. Further studies using this method may lead to improved understanding of the formation mechanisms of diamondoids and highly mature petroleum fluid compositions. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Diamondoids were known to be present in petroleum fluids since the 1930s [1,2], but their usefulness as geochemical indicators was only developed in the last 20 years [3–9]. They differ from saturated hydrocarbons because of the structural rigidity of diamond-like three dimensional cages, which enhance resistance to thermal and microbial degradation [5,10–13]. Ultra-deep reservoirs are an important frontier for petroleum exploration, demanding both scientific and technological investments to achieve ⇑ Corresponding authors. Address: Av. Athos da Silveira Ramos, 149, Centro de Tecnologia, Bloco A, sala 603, Cidade Universitária, Rio de Janeiro, RJ 21949-900, Brazil. Tel.: +55 21 2260 3967. E-mail addresses: [email protected] (R.C. Silva), [email protected] (D.A. Azevedo). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.05.027
extended hydrocarbon recovery. For these mature and overmature oil accumulations, however, traditional biomarker parameters from saturated hydrocarbons are of limited use. Therefore diamondoid assessment is needed to understand such petroleum accumulations [3,6,14–16]. Many research groups worldwide are actively pursuing improved methods for diamondoid analysis. Diamondoids analyzed by GC–MS were used to assess biodegradation [10], source rock facies and lithology [15,17,18], and to evaluate several petroleum systems [11,16,19]. Recent literature shows the use of GC–MRM– MS (triple–quadrupole mass analyzer) to achieve quantitation limits as low as 0.08–0.37 lg/g for adamantanes and diamantanes [20]. This method was used to evaluate diamondoid abundances in a simulated oil cracking experiment, which suggested some diamondoid indexes for assessment of the late oil window [13].
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Fig. 1. GC GC-TOFMS contour plot (sample A). Intensities were selectively altered to show all classes in one figure. Calculated intensities are as follows: terpanes (m/z 19111), steranes (m/z 21710), adamantanes (m/z 135), diamantanes (m/z 187), triamantanes (m/z 2395), tetramantanes (m/z 291 60).
Recently, adamantanes and diamantanes from a Chinese condensate were investigated by GC GC-TOFMS [21]. Analyzing Chinese oil samples with a GC GC-TOFMS instrument, a study [22] recognized more than 100 diamondoid homologs of adamantane, diamantane and triamantane. Both works showed that GC GC techniques perform well for both target (usually analyzed) and non-target (previously unknown or neglected) compounds [23], as seen in several biomarker studies [24–31]. Since diamondoids are believed to be absent in modern sediments [15] and the lithology appears to play a role in diamondoid formation [15,17,18], understanding the underlying mechanisms might be useful to fine tune diamondoid parameters. For extended
diamondoids (triamantane up to hexamantane) this appears to be even more challenging [6]. It is important to identify and quantify diamondoids in petroleum fluids. Furthermore, even the late stages of cracking might be assessed by extended diamondoid parameters. The purpose of this work is to assess both diamondoids and saturated biomarkers in petroleum samples using GC GC-TOFMS. The work aims to gather data potentially useful in future studies on the formation mechanisms of diamondoids and/or the proposal of new diamondoid indexes. 2. Experimental 2.1. Samples In order to study the method, two Brazilian crude oil samples were selected. Sample A exhibits overmature characteristics, whilst sample B is believed to have been produced in the early stages of oil cracking. 2.2. Standards and chemicals
Fig. 2. Tetramantane isomers. Iso-tetramantane has three quaternary carbon atoms, while anti- and skew-tetramantane have two.
Twelve standard compounds purchased from Chiron AS (Stiklestadveien, Norway) were used as a reference for adamantane
Fig. 3. Chromatographic surface of tetramantanes. Intensities were calculated from the sum of m/z 291, 305, 319 and 333. Diagonal solid lines represent isomers of tetramantane (m/z 291 and 292), C1-tetramantanes (m/z 291 and 306), C2-tetramantanes (m/z 305 and 320) and C3-tetramantanes (m/z 319 and 334).
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and diamantane identifications. Internal deuterated standards (ndodecane-d26 and n-tetracosane-d50) also were used for relative quantification. For sample processing and analysis, chromatographic grade n-hexane (TediaBrazil, Rio de Janeiro, Brazil) was used.
2.3. Sample preparation Special precautions were taken to avoid the well-documented loss of volatile diamondoids when applying standard fractionation procedures for saturated biomarker analysis. Approximately 100 mg of each crude oil was dissolved in 0.5 mL of a hexane solution containing 20 lg/mL of deuterated standards (tetracosane-d50 and dodecane-d26), deposited on the top of an activated silica gel column and then eluted with 4.5 mL of n-hexane, similar to
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procedures published elsewhere [10,11,16]. Eluted solutions were immediately sealed and analyzed by GC GC-TOFMS.
2.4. GC GC-TOFMS A Pegasus 4D (Leco, St. Joseph, MI, USA) system was used for all GC GC–TOFMS analysis. An Agilent Technologies 6890 GC (Palo Alto, CA, USA) with a secondary oven and a fixed quad-jet dualstage modulator was coupled to the TOF analyzer. Columns included a DB-5 (30 m 0.25 mm i.d. 0.25 lm df) for the first dimension (1D) and a BPX-50 (1.5 m 0.1 mm i.d. 0.1 lm df) for the second one (2D). An uncoated deactivated capillary column (0.5 m 0.25 mm i.d.) connected the second column to the TOF analyzer and SGE unions of SilTite metal ferrules (Austin, Texas, USA) connected all columns.
Fig. 4. Examples of mass spectra for tetramantane (top), C1-tetramantane (middle) and a C2-tetramantane (bottom). Based on the presence of both peaks m/z 305 and 320, the C2-tetramantane molecule represented herein is a dimethyl-tetramantane isomer.
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Table 1 Relative concentrations and selected data for tentatively identified tetramantanes in sample A.
a b c d
Compound class
Diagnostic ions (m/z)
1
tR (s)a
2
tR (s)b
Dimethyl-tetramantane Methyl-tetramantane Tetramantane (I) Trimethyl-tetramantane Dimethyl-tetramantane Methyl-tetramantane Dimethyl-tetramantane Dimethyl-tetramantane Methyl-tetramantane Tetramantane (II) Dimethyl-tetramantane Methyl-tetramantane Dimethyl-tetramantane Trimethyl-tetramantane Dimethyl-tetramantane Methyl-tetramantane Trimethyl-tetramantane Methyl-tetramantane Tetramantane (III) Dimethyl-tetramantane Trimethyl-tetramantane Dimethyl-tetramantane Dimethyl-tetramantane Methyl-tetramantane Methyl-tetramantane
305;320 291;306 292 319;334 305;320 291;306 305;320 305;320 291;306 292 305;320 291;306 305;320 319;334 305;320 291;306 319;334 291;306 292 305;320 319;334 305;320 305;320 291;306 291;306
2922 2934 2940 2952 2964 2976 3006 3024 3024 3024 3054 3054 3066 3072 3078 3078 3084 3102 3102 3108 3120 3120 3138 3144 3162
4.24 4.51 4.86 4.02 4.34 4.68 4.37 4.39 4.68 5.01 4.50 4.79 4.52 4.28 4.52 4.89 4.31 4.84 5.30 4.63 4.37 4.71 4.68 5.05 5.11
Quantitation ion (m/z)
S/Nc
Area
Concentration (ng/mg)d
305 291 292 319 305 291 305 305 291 292 305 291 305 319 305 291 319 291 292 305 319 305 305 291 291
67 123 259 14 34 36 21 12 85 158 32 38 20 10 18 71 11 10 84 13 10 28 20 31 19
42,502 81,116 137,784 10,546 32,614 20,464 14,154 13,220 56,046 119,527 76,144 40,342 23,292 7800 11,906 102,805 6539 4530 54,384 13,872 8144 36,878 28,514 28,561 13,883
1.8 3.5 5.9 0.5 1.4 0.9 0.6 0.6 2.4 5.1 3.3 1.7 1.0 0.3 0.5 4.4 0.3 0.2 2.3 0.6 0.3 1.6 1.2 1.2 0.6
First GC dimension retention time. Second GC dimension retention time. Signal to noise ratio calculated for the quantitation ion. Quantitation relative to ion m/z 66 of n-tetracosane-d50 mass spectrum.
acquisition rate of 200 Hz. ChromaTOF software version 2.32 (Leco Corp., St. Joseph, MI, USA) was used to acquire, process and analyze the data. 3. Results and discussion 3.1. Chromatographic aspects
Fig. 5. Chromatographic surface of triamantanes (sample A). Intensities were calculated from the sum of m/z 239, 240, 253 and 267.
The GC conditions were set to optimize chromatographic separations from the very light adamantanes to heavy C40 biomarkers. A 5 °C off-set was used between the dimensions during GC program, while the modulator was kept 30 °C higher than the first dimension. Initially stabilized at 50 °C, the first dimension temperature was raised at 4 °C/min until 350 °C, summing to 75 min for each analysis. The modulation period was chosen to be 6 s, with a 2 s hot pulse. Helium gas flow of 1.5 mL/min was kept constant. Sample was injected in the splitless mode (1 lL to 15 bar). Transfer line and ion source temperatures were held at 290 °C and 200 °C, respectively. Electron energy of 70 eV was applied, and the ions were detected (1700 V) in the range of m/z 50–700 at an
Chromatographic conditions were chosen after tests to determine sufficient resolution at the beginning of the chromatography run where adamantanes appear. The choice of one heating rate was to ensure the chromatographic structure of compound classes, where series elute in ordered patterns and are separated from other homologous series (roof-tile effect, common in GC GC analysis of petroleum fluids). Quality of biomarker assessment at the very end of the chromatographic run was an objective. Since one of the targets was the volatile adamantanes, evaporation steps after liquid chromatography were avoided, including the n-paraffin removal procedure. Leaving a high paraffin content in the sample is somewhat harmful for diamondoid assessment because some intense n-alkane peaks appear tailing along the second dimension. This interference can be filtered by the mass spectrometer. In these experiments, one can assess saturated biomarkers, diamondoids, and extended diamondoids in a sample using only one chromatographic run. Fig. 1 depicts the chromatographic surface where the studied compound classes were found. As the number of diamondoid cages increases, diamondoid interaction increases with the polar stationary phase of the second column. The intensity axis was modified to equalize class abundance. Diamondoid abundance decreases with the number of cages. The maximum observed tetramantane 2tR (second GC dimension retention time) value (5.36 s) was used to set the modulation period. Detected peaks from pentamantanes were so small (S/N < 10) that they are not discussed in this paper. At the end of the run, C35 pentakishomohopane (H35) and C39 tricyclic terpane (Tr39) were the last detected biomarkers.
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R.C. Silva et al. / Fuel 112 (2013) 125–133 Table 2 Relative concentrations and selected data for tentatively identified triamantanes in sample A.
a b c d
Compound class
Diagnostic ions (m/z)
1
tR (s)a
2
tR (s)b
Quantitation ion (m/z)
S/Nc
Area
Concentration (ng/mg)d
Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Dimethyl-triamantane Ethylmethyl-triamantane Ethylmethyl-triamantane Ethyl-triamantane Ethyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Methyl-triamantane Propyl-triamantane Propyl-triamantane Triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane Trimethyl-triamantane
253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;268 253;282 253;282 239;268 239;268 239;254 239;254 239;254 239;254 239;254 239;254 239;254 239;282 239;282 240 267;282 267;282 267;282 267;282 267;282 267;282 267;282 267;282 267;282 267;282
2454 2502 2520 2544 2556 2562 2568 2586 2628 2646 2670 2676 2688 2658 2742 2658 2682 2448 2454 2508 2520 2538 2556 2598 2790 2808 2442 2502 2568 2592 2610 2646 2658 2676 2694 2706 2730
3.85 4.01 4.04 4.00 4.04 4.13 4.13 4.20 4.16 4.23 4.41 4.34 4.35 3.92 4.05 4.17 4.20 4.13 4.10 4.27 4.29 4.30 4.29 4.50 4.12 4.18 4.42 3.74 3.90 3.87 3.89 4.00 4.02 4.04 4.04 4.09 4.15
253 253 253 253 253 253 253 253 253 253 253 253 253 253 253 239 239 239 239 239 239 239 239 239 239 239 240 267 267 267 267 267 267 267 267 267 267
763.7 1362.1 645.6 611.0 248.5 267.0 62.6 560.7 127.6 42.6 126.7 92.2 50.3 110.3 45.3 215.7 101.7 3672.5 822.9 1097.6 1608.4 560.2 325.9 369.7 65.0 25.2 4228.8 410.3 163.5 181.7 118.9 110.7 137.7 80.6 65.7 50.8 39.3
413,255 1,016,896 378,826 366,336 157,101 169,520 61,341 779,390 205,012 53,646 139,944 72,285 34,081 111,351 132,037 181,940 67,681 1,982,545 307,540 635,471 702,464 313,663 192,769 234,353 123,760 22,587 2,222,445 355,311 132,590 236,763 158,164 77,515 171,729 132,546 121,154 170,583 61,513
17.7 43.6 16.2 15.7 6.7 7.3 2.6 33.4 8.8 2.3 6.0 3.1 1.5 4.8 5.7 7.8 2.9 85.0 13.2 27.2 30.1 13.4 8.3 10.0 5.3 1.0 95.3 15.2 5.7 10.2 6.8 3.3 7.4 5.7 5.2 7.3 2.6
First GC dimension retention time. Second GC dimension retention time. Signal to noise ratio calculated for the quantitation ion. Quantitation relative to ion m/z 66 of n-tetracosane-d50 mass spectrum.
3.2. Tetramantanes Tetramantanes are the first pseudo homolog in the series of diamondoids: skew-, anti- and iso-tetramantane isomers, the latter having three quaternary carbon atoms instead of two (Fig. 2). Their first natural occurrence in petroleum was published in 1995 in a condensate from a Jurassic sandstone reservoir [6]. In that paper, 20 compounds were tentatively assigned as tetramantanes, even though coelutions noticeably affected the results. The tetramantane chromatographic surface is expanded in Fig. 3. At the top, three peaks on the diagonal tetramantane line show both m/z 291 and 292 as the most intense ions in their mass spectra, suggesting tetramantane isomers, even though their elution order is not known. C1-Tetramantanes (m/z 291 and 306) appear in a trend line below tetramantanes, summing to eight tentatively identified methyl-tetramantanes. C2-Tetramantanes are assumed to be dimethyl-tetramantanes because of the diagnostic ions m/z 305 and 320, rather than m/z 291 and 320, expected for ethyl-tetramantanes. By analogy, C3-tetramantanes are labeled as trimethyl-tetramantane (m/z 319 and 334), instead of propyl-tetramantane (m/z 291 and 334) or ethylmethyl-tetramantane (m/z 305, 319 and 334). In fact, alkylsubstitution on the observed tetramantanes did not show chains bigger than methyl, although this could be due to insufficient sensitivity. On Fig. 3, the first and most intense peak on the C1-tetramantanes line has a shape suggestive of coelution, and its 1tR (first
GC dimension retention time) is lower than the first tetramantane isomer. The same behavior is also seen for the first C2-dimethyltetramantane, which has 1tR lower than its lower mass homologs. As seen before [17] and also in the data presented below, a decrease in 1tR for more alkyl-substitution is not expected, at least for substituted adamantanes and diamantanes. Lack of authentic standards made it difficult to establish an elution order based on the positions of alkyl-substitution. There are coelutions in the first GC dimension that can be resolved using the additional dimension. However, GC GC-TOFMS may be incapable of deconvoluting two or more coeluting isomers, due to similarities in mass spectra. Twenty-five tetramantanes were tentatively identified based on mass spectra (Fig. 4 and Table 1): three tetramantanes, eight methyl-tetramantanes, ten dimethyl-tetramantanes and four trimethyl-tetramantanes. No tetramantanes were detected in sample B. 3.3. Triamantanes In the 14 years since the first recognition of triamantanes in petroleum [3] to the most comprehensive analysis of them by GC–MS [15] in 2006, thirteen new compounds have been identified. Recently, two triamantanes species were recognized by GC GC-TOFMS [22]. The region of triamantanes on the chromatographic surface is expanded and detailed in Fig. 5. Triamantanes are spread around 370 s in 1D, from triamantane (1tR 2454 s) to propyl-triamantane (1tR 2808 s). C1-Triamantanes include seven
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Fig. 6. Mass spectra for some triamantanes found in sample A. Note differences between dimethyl-triamantane (m/z 253 and 268) and ethyl-triamantane (m/z 239 and 268); and for trimethyl-triamantane (m/z 267 and 282) and propyl-triamantane (m/z 239 and 282).
Fig. 7. Chromatographic surface of diamantanes (sample A). Intensities correspond to the sum of ions m/z 187, 201, 215 and 229. Dashed line suggests the presence of methyl-protodiamantanes.
tentatively identified methyl-triamantanes (m/z 239 and 254), while previously five were detected and four were tentatively identified [15]. The same comparison can be made for dimethyltriamantanes: thirteen peaks were identified in this work,
compared to five peaks described elsewhere. For trimethyltriamantanes: ten were identified here compared to two from another publication [15]. In addition, two propyl-triamantanes, two ethyl-triamantanes and two ethylmethyl-triamantanes also were
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Fig. 8. Comparison between 1-methyl-diamantane mass spectrum and a typical spectrum from the protodiamantane region (dashed line in Fig. 7). Compounds were labeled as methyl-protodiamantanes based on recent discussion about protodiamondoids [32–35], which are assumed to have mass spectra similar to regular diamondoids.
Table 3 Methyl-protodiamantane and methyl-diamantane quantities found in samples A and B, based on ion m/z 187 intensities. Compound class
Methyl-protodiamantanes (pMD) Methyl-diamantanes (MD) pMD/MDa
Sum of areas Sample A
Sample B
839,287 1,015,3920 0.08
21,608 133,635 0.16
a Ratio between areas of methyl-protodiamantanes (pMD) and methyl-diamantanes (MD).
identified (Table 2 and Fig. 6). Alkyl-substitution with more than one carbon atom was not seen for tetramantanes, but is present in triamantanes. In total, 37 triamantanes were tentatively identified in Sample A. Sample B showed two tiny peaks for triamantane and methyltriamantane, but with signal-to-noise ratios lower than our requirements for further discussions (S/N < 10).
3.4. Adamantanes and diamantanes Analyses of adamantanes and diamantanes are well explored in the recent literature, achieving satisfactory limits of quantitation using gas chromatography coupled to a triple–quadrupole mass spectrometer [20]. A total of 22 adamantanes and 10 diamantanes were successfully identified and quantified. Another approaches to these classes used GC GC-TOFMS to analyze a Chinese condensate [21] and crude oils [22], where characterization of 100 species was effective. In the present study, authentic standards were used to check retention times of some adamantanes and diamantanes.
Fig. 9. Dahl’s diagram [5] displays the correlation between diamondoid appearance and biomarker (stigmastane) disappearance due to thermal stress. Sample A is in the zone of intense oil cracking because it exhibits high diamondoid content and low sterane concentration, while sample B is in the beginning of intense oil cracking due to low values of both steranes and diamondoids.
Identification of most components by GC GC-TOFMS has been conducted [21,22] and this work does not intend to be repetitive, although some details require attention. Ion m/z 187 in diamantane region contains different groups of molecules highlighted in Fig. 7. Their mass spectra have m/z 187 and 202 as the most intense peaks, similar to those from methyl-diamantane (Fig. 8). Considering that diamantane belongs to the D3d symmetry group, having three
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Table 4 Diamondoid indexes from GC GC-TOFMS analyses. Indexa
MAI MDI DMAI DMDI EAI TMA
Sample A
B
0.38 0.39 0.48 0.22 0.40 0.31
0.67 0.45 0.57 0.45 0.65 0.24
a MAI = (1-methyl-adamantane)/(1-methyl-adamantane + 2-methyl-adamantane); MDI = (4-methyl-diamantane)/(4-methyl-diamantane + 1-methyl-diamantane + 3-methyl-diamantane); DMAI = (1,3-dimethyl-adamantane)/(1,2-dimethyladamantane + 1,3-dimethyl-adamantane); DMDI = (3,4-dimethyl-diamantane)/ (4,9-dimethyl-diamantane + 3,4-dimethyl-diamantane); EAI = (2-ethyl-adamantane)/(1-ethyl-adamantane + 2-ethyl-adamantane); TMAI = (1,3,5-trimethyl-adamantane)/(1,3,5-trimethyl-adamantane + 1,3,4-trimethyl-adamantane).
possible methyl-substituted isomers: 1-, 3-, and 4-methyl-diamantanes, these findings suggest the presence of protodiamondoids, which presumably are intermediates in diamondoid formation [32–35]. Quantification of these methyl-protodiamantanes from Table 3 shows that m/z 187 areas compute to 8% and 16% of methyldiamantanes in samples A and B, respectively. These results need more refinement from additional study. For example, since sample A is more mature, the percentage of methyl-protodiamantanes appears to decrease with maturity. These results highlight the usefulness of GC GC-TOFMS for non-target compound analysis. Previous work [33] claims that alkyl-adamantanes with alkyl groups longer than the ethyl have not been found in oil, supporting the idea of alkyl-adamantane production via catalytic isomerization of tricyclic hydrocarbons on activated clays or aluminosilicates. Based on mass spectra (base peaks at m/z 135 and 178) and retention time, one propyl-adamantane was tentatively identified in sample A, whilst two propyl-adamantanes were found in sample B. Traditional diamondoid analysis focuses either on quantitative data to explore thermal maturity of oil sample sets [11,13,16,20,35] or gathering diamondoid indexes for other purposes [10,15]. Both goals are achieved by the GC GC-TOFMS analysis proposed herein. Dahl’s diagram [5] plots stigmastane versus 3-+4-methyl-diamantanes to evaluate thermal maturity, and Fig. 9 shows locations of samples A and B. The capability to create such plots using only one chromatographic run for each sample is advantageous. Another advantage of GC GC-TOFMS is the ability to derive diamondoid indexes used for maturity assessments and to distinguish source facies, such as methyl-adamantane index (MAI), methyl-diamantane index (MDI), dimethyl-adamantane index (DMAI), dimethyl-diamantane index (DMDI) and so on [4,10,15] (Table 4). In this study, diamondoid data support maturity measurements based on other metrics (Table S1 in the Supplementary material). 4. Conclusions GC GC-TOFMS was explored as a tool for detailed diamondoid analysis using two crude oil samples. This paper demonstrates the power of this technique to evaluate diamondoids in oil samples, thus allowing improved geochemical assessments. Some key observations include: (a) GC GC-TOFMS is a powerful tool to study diamondoids. (b) To improve mechanistic proposals of diamondoid formation, one might consider information obtained using GC GCTOFMS, such as the existence of propyl-triamantanes and evidence of methyl-protodiamantanes in crude oils.
(c) GC GC-TOFMS allows measurements needed for traditional diamondoid and biomarker assessments (Dahl’s diagram, diamondoid indexes and saturated biomarker parameters) using only one chromatographic run.
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