Characterization of Heteroatom Compounds in a Crude Oil and Its

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Energy Fuels 2010, 24, 2545–2553 Published on Web 03/29/2010

: DOI:10.1021/ef901564e

Characterization of Heteroatom Compounds in a Crude Oil and Its Saturates, Aromatics, Resins, and Asphaltenes (SARA) and Non-basic Nitrogen Fractions Analyzed by Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Quan Shi,† Dujie Hou,‡ Keng H. Chung,§ Chunming Xu,*,† Suoqi Zhao,† and Yahe Zhang† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102200, People’s Republic of China, ‡ Faculty of Energy Resources, China University of Geosciences, Beijing 100083, People’s Republic of China, and § Well Resources, Inc., Edmonton, Alberta, Canada Received December 19, 2009. Revised Manuscript Received March 17, 2010

A Liaohe crude oil was separated as saturates, aromatics, resins, and asphaltenes (SARA) and neutral nitrogen fractions. The crude oil and its subfractions were analyzed by negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results show that neutral nitrogen and acidic heteroatom compounds in the crude oil contain 15-55 carbon atoms with double-bond equivalent (DBE) values of 1-27, containing N1, N2, N1O1, N1O2, N1O3, N1O4, O1, and O2 heteroatom classes. No molecules in the saturate fraction can be ionized by ESI. The aromatic fraction contains N1 and N1Ox compounds with high molecular weights but low DBE values. The resin and asphaltene fractions contain highly aromatic and acidic class species, which are enriched in oxygen- and nitrogen-containing compounds with lower molecular weights than those found in the aromatic fraction. The distribution patterns of N1, N1O1, and O1 class species in the resins and asphaltenes are similar. The mass spectrum of the neutral nitrogen fraction differs from those for the bulk crude oil and its SARA fractions; the neutral nitrogen fraction is enriched with N1 and N1O1 class species. Neutral nitrogen compounds with molecular weights lower than 200 were discriminated in the FT-ICR MS spectrum under the chosen operating conditions. However, the nitrogen species detected by gas chromatography only accounted for a small amount of that found in the neutral nitrogen fractions. Some of the neutral nitrogen species were entrained in asphaltenes during the deasphalting step of sample fractionation.

polarity. Compositional information on the thousands of species found in the saturate and aromatic fractions of petroleum characterized by the combination of GC and MS (GC-MS) techniques has been used to advance the science behind the refining processes and geochemistry.4 However, GC-MS techniques are inadequate to determine structural information on resins and asphaltenes, which have low volatility and a high tendency to form aggregates.5 Most of the chemical information on resins and asphaltenes has been derived from bulk properties, such as elemental composition

Introduction Crude oil is a complex mixture containing potentially billions of elemental compounds. There may be as many as 4.11  109 triacontane isomers in petroleum.1 Gas chromatography (GC) and mass spectrometry (MS) are the analytical tools often used to characterize petroleum species. However, most petroleum samples require fractionation prior to GC and MS analyses to isolate certain groups of compounds or to simplify the analyte composition, so that the peaks of the chromatogram and spectrum can be resolved and identified. A commonly used sample fractionation method is saturates, aromatics, resins, and asphaltenes (SARA) fractionation.2,3 In this method, asphaltenes are initially precipitated by the use of a nonpolar solvent followed by adsorption of the maltene on a Al2O3 and/or silica gel column. The saturate, aromatic, and resin fractions are obtained by eluting the adsorbed sample on the column with solvents with particular

(4) Peters, K. E.; Walters, C. C.; Moldown, J. M. The Biomarker Guide; Cambridge University Press: Cambridge, U.K., 2005; Vol. 2. (5) Betancourt, S. S.; Ventura, G. T.; Pomerantz, A. E.; Viloria, O.; Dubost, F. X.; Zuo, J.; Monson, G.; Bustamante, D.; Purcell, J. M.; Nelson, R. K.; Rodgers, R. P.; Reddy, C. M.; Marshall, A. G.; Mullins, O. C. Nanoaggregates of asphaltenes in a reservoir crude oil and reservoir connectivity. Energy Fuels 2009, 23 (3), 1178–1188. (6) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G.; Yen, A. Mass spectral analysis of asphaltenes. II. Detailed compositional comparison of asphaltenes deposit to its crude oil counterpart for two geographically different crude oils by ESI FT-ICR MS. Energy Fuels 2006, 20 (5), 1973–1979. (7) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Mass spectral analysis of asphaltenes. I. Compositional differences between pressure-drop and solvent-drop asphaltenes determined by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20 (5), 1965–1972. (8) Becker, C.; Qian, K.; Russell, D. H. Molecular weight distributions of asphaltenes and deasphaltened oils studied by laser desorption ionization and ion mobility mass spectrometry. Anal. Chem. 2008, 80 (22), 8592–8597.

*To whom correspondence should be addressed. Telephone: 86108973-3392. Fax: 8610-6972-4721. E-mail: [email protected]. (1) Beens, J.; Brinkman, U. A. T. The role of gas chromatography in compositional analyses in the petroleum industry. TrAC, Trends Anal. Chem. 2000, 19 (4), 260–275. (2) Raki, L.; Masson, J.-F.; Collins, P. Rapid bulk fractionation of maltenes into saturates, aromatics, and resins by flash chromatography. Energy Fuels 2000, 14 (1), 160–163. (3) Suatoni, J. C.; Swab, R. E. Rapid hydrocarbon group-type analysis by high-performance liquid chromatography. J. Chromatogr. Sci. 1975, 13 (8), 361–366. r 2010 American Chemical Society

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and acid and base reactivity. The characterization of asphaltene structures using high-resolution mass spectrometry has been published in recent years;6-12 however, determining the structure of asphaltenes still remains a difficult and challenging task. Most of the nitrogen and oxygen species are enriched in resins and asphaltenes, causing them to exhibit polar characteristics.13 The controversy on the molecular constituents of asphaltenes has lasted for some time. An important reason for this is that it is difficult to describe the molecular form of heteroatoms in resins and asphaltenes. The use of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in crude oil analysis has made it possible to characterize petroleum at the molecular level. FTICR MS is a new type of mass analyzer capable of probing the detailed composition of complex petroleum mixtures.14-17 The technique has the highest available broadband mass resolution, mass resolving power, and mass accuracy, which allows for the unambiguous assignment of a unique elemental composition to each peak of crude oils for ions up to ∼400 Da.16,18,19 Soft ionization techniques play an important role in the

FT-ICR MS application on petroleum analysis, which includes electrospray ionization (ESI),6,20-36 low-voltage electron impact (EI) ionization,28,37,38 atmospheric pressure chemical (23) Hemmingsen, P. V.; Kim, S.; Pettersen, H. E.; Rodgers, R. P.; Sj€ oblom, J.; Marshall, A. G. Structural characterization and interfacial behavior of acidic compounds extracted from a North Sea oil. Energy Fuels 2006, 20 (5), 1980–1987. (24) Hughey, C. A.; Galasso, S. A.; Zumberge, J. E. Detailed compositional comparison of acidic NSO compounds in biodegraded reservoir and surface crude oils by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. Fuel 2007, 86 (5-6), 758–768. (25) Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Czarnecki, J.; Wu, X. A.; Taylor, S. Detailed elemental compositions of emulsion interfacial material versus parent oil for nine geographically distinct light, medium, and heavy crude oils, detected by negative- and positiveion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2007, 21 (2), 973–981. (26) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Elemental composition analysis of processed and unprocessed diesel fuel by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2001, 15 (5), 1186–1193. (27) Muller, H.; Andersson, J. T.; Schrader, W. Characterization of high-molecular-weight sulfur-containing aromatics in vacuum residues using Fourier transform ion cyclotron resonance mass spectrometry. Anal. Chem. 2005, 77 (8), 2536–2543. (28) Fu, J.; Klein, G. C.; Smith, D. F.; Kim, S.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Comprehensive compositional analysis of hydrotreated and untreated nitrogen-concentrated fractions from syncrude oil by electron ionization, field desorption ionization, and electrospray ionization ultrahigh-resolution FT-ICR mass spectrometry. Energy Fuels 2006, 20 (3), 1235–1241. (29) Klein, G. C.; Rodgers, R. P.; Marshall, A. G. Identification of hydrotreatment-resistant heteroatomic species in a crude oil distillation cut by electrospray ionization FT-ICR mass spectrometry. Fuel 2006, 85 (14-15), 2071–2080. (30) Stanford, L. A.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Characterization of compositional changes in vacuum gas oil distillation cuts by electrospray ionization Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry. Energy Fuels 2006, 20 (4), 1664– 1673. (31) Pakarinen, J. M. H.; Ter€av€ainen, M. J.; Pirskanen, A.; Wickstr€ om, K.; Vainiotalo, P. A positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry study of Russian and North Sea crude oils and their six distillation fractions. Energy Fuels 2007, 21 (6), 3369–3374. (32) Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Czarnecki, J.; Wu, X. A. Compositional characterization of bitumen/water emulsion films by negative- and positive-ion electrospray ionization and field desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2007, 21 (2), 963–972. (33) Smith, D. F.; Rahimi, P.; Teclemariam, A.; Rodgers, R. P.; Marshall, A. G. Characterization of Athabasca bitumen heavy vacuum gas oil distillation cuts by negative/positive electrospray ionization and automated liquid injection field desorption ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2008, 22 (5), 3118–3125. (34) Smith, D. F.; Schaub, T. M.; Kim, S.; Rodgers, R. P.; Rahimi, P.; Teclemariam, A.; Marshall, A. G. Characterization of acidic species in Athabasca bitumen and bitumen heavy vacuum gas oil by negative-ion ESI FT-ICR MS with and without acid-ion exchange resin prefractionation. Energy Fuels 2008, 22 (4), 2372–2378. (35) Klein, G. C.; Angstrom, A.; Rodgers, R. P.; Marshall, A. G. Use of saturates/aromatics/resins/asphaltenes (SARA) fractionation to determine matrix effects in crude oil analysis by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20 (2), 668–672. (36) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G.; Yen, A.; Asomaning, S. Mass spectral analysis of asphaltenes. I. Compositional differences between pressure-drop and solvent-drop asphaltenes determined by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20 (5), 1965–1972. (37) Rodgers, R. P.; White, F. M.; Hendrickson, C. L.; Marshall, A. G.; Andersen, K. V. Resolution, elemental composition, and simultaneous monitoring by Fourier transform ion cyclotron resonance mass spectrometry of organosulfur species before and after diesel fuel processing. Anal. Chem. 1998, 70 (22), 4743–4750. (38) Fu, J.; Kim, S.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G.; Qian, K. Nonpolar compositional analysis of vacuum gas oil distillation fractions by electron ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2006, 20 (2), 661–667.

(9) Pinkston, D. S.; Duan, P.; Gallardo, V. A.; Habicht, S. C.; Tan, X.; Qian, K.; Gray, M.; Mullen, K.; Kentta€amaa, H. I. Analysis of asphaltenes and asphaltene model compounds by laser-induced acoustic desorption/Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2009, 23 (11), 5564–5570. (10) McKenna, A. M.; Purcell, J. M.; Rodgers, R. P.; Marshall, A. G. Identification of vanadyl porphyrins in a heavy crude oil and raw asphaltene by atmospheric pressure photoionization Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Energy Fuels 2009, 23 (4), 2122–2128. (11) Purcell, J. M.; Merdrignac, I.; Rodgers, R. P.; Marshall, A. G.; Gauthier, T.; Guibard, I. Stepwise structural characterization of asphaltenes during deep hydroconversion processes determined by atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Energy Fuels 2010, DOI: 10.1021/ef900897a. (12) Chiaberge, S.; Guglielmetti, G.; Montanari, L.; Salvalaggio, M.; Santolini, L.; Spera, S.; Cesti, P. Investigation of asphaltene chemical structural modification induced by thermal treatments. Energy Fuels 2009, 23 (9), 4486–4495. (13) Jewell, D. M.; Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D. R. Ion-exchange, coordination, and adsorption chromatographic separation of heavy-end petroleum distillates. Anal. Chem. 1972, 44 (8), 1391–1395. (14) Marshall, A. G.; Rodgers, R. P. Petroleomics: The next grand challenge for chemical analysis. Acc. Chem. Res. 2003, 37 (1), 53–59. (15) Rodgers, R. P.; Marshall, A. G. Petroleomics: Advanced Characterization of Petroleum-Derived Materials by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS); Springer: New York, 2005; pp 63-93. (16) Marshall, A. G.; Rodgers, R. P. Petroleomics: Chemistry of the underworld. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (47), 18090–18095. (17) McLafferty, F. W. Mass spectrometry across the sciences. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (47), 18088–18089. (18) Senko, M. W.; Hendrickson, C. L.; Pasa-Toli, L.; Marto, J. A.; White, F. M.; Guan, S.; Marshall, A. G. Electrospray ionization Fourier transform ion cyclotron resonance at 9.4 T. Rapid Commun. Mass Spectrom. 1996, 10 (14), 1824–1828. (19) Rogers, R. P.; Schaub, T. M.; Marshall, A. G. Petroleomics: MS returns to its roots. Anal. Chem. 2005, 77 (1), 20A–27A. (20) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Resolution and identification of elemental compositions for more than 3000 crude acids in heavy petroleum by negative-ion microelectrospray high-field Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2001, 15 (6), 1505–1511. (21) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Reading chemical fine print: Resolution and identification of 3000 nitrogen-containing aromatic compounds from a single electrospray ionization Fourier transform ion cyclotron resonance mass spectrum of heavy petroleum crude oil. Energy Fuels 2001, 15 (2), 492– 498. (22) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Resolution of 11 000 compositionally distinct components in a single electrospray ionization Fourier transform ion cyclotron resonance mass spectrum of crude oil. Anal. Chem. 2002, 74 (16), 4145–4149.

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ionization (APCI), atmospheric pressure photoionization (APPI),10,39,42-44 atmospheric pressure laser ionization (APLI),39 matrix-assisted laser desorption (MALDI),39,45 and field desorption/field ionization (FD/FI).28,33,46-48 Among these ionization techniques, ESI is commonly used to determine compositional characteristics of petroleum distillates,26-31 crude oils,20-25 bitumens,32-34 polar crude oil fractions,6,35,36 and hydrotreated petroleum products.26,28,29 The advantages of ESI are ease of operation, relatively low equipment cost, and high selectivity for polar species in complex matrixes. While the polar species in crude oil can be analyzed by ESI FTICR MS directly without fractionation, separating the crude oil into separate fractions prior to analysis is a more effective manner to obtain greater insight into the structural nature of the components. Klein et al.35 compared the positive-ion ESI FT-ICR mass spectra of a South American crude oil to that of its SARA fractions to show the distribution of basic nitrogen compounds in the samples. However, acidic compounds cannot be characterized by the positive-ion ESI. In this study, neutral nitrogen and oxygen compounds in crude oil, its SARA fractions, and its neutral nitrogen fraction will be analyzed by negative-ion ESI.

mass chromatograms of saturates and aromatics and selected ion mass chromatograms at m/z 191 and 217 (see Figure S-1 in the Supporting Information) revealed that the crude oil is virgin in nature, with well-preserved normal alkyls and regular aromatic compounds. These characteristics are typical to most crude oils from the Liaohe region. SARA Fractionation. The crude oil sample was subjected to SARA fractionation (Chinese Standard Analytical Method for Petroleum and Natural Gas Industry: SY/T 5119-2008). This separation method is commonly used in China for geochemical samples. Separation of asphaltenes from the sample was carried out by adding 30 mL of hexane to 50 mg of sample. The mixture was stirred, allowed to sit for 12 h, and then filtered. A piece of cotton-wool was placed at the mount of the funnel to filter the asphaltenes from the mixture. The asphaltenes on the cottonwool were washed with hexane until the filtrate was colorless. The asphaltenes were dissolved and eluted by washing with dichloromethane. The maltenes were adsorbed onto a packed column consisting of a 10 mm inner diameter column packed with 3 g of preactivated silica gel (80-100 mesh, activated at 150 °C for 4 h) and 2 g of neutral alumina (100-200 mesh, activated at 400 °C for 4 h and added 1 wt % water). A total of 25 mL of n-hexane was used to elute the saturate fraction, and a 15 mL solvent mixture of n-hexane/dichloromethane (1:1, v/v) was used to elute the aromatic fraction. A solvent mixture consisting of 10 mL of ethanol and 15 mL of chloroform was used to elute the resin fraction. Separation of Neutral Nitrogen Compounds. A modified twostep liquid column chromatography method49 was used to fractionate the neutral nitrogen fraction from the oil sample. Asphaltenes in the crude oil sample (150 mg) were separated using n-hexane, similar to the method described in the SARA separation. The maltenes were adsorbed onto 1 g of neutral alumina and put on a chromatographic column. The column consisted of a 20 mm inner diameter column packed with 6 g of neutral alumina (100-200 mesh, activated at 400 °C for 4 h and added 1 wt % water). The packed column was eluted using 50 mL of n-hexane, 30 mL of toluene, and 70 mL of a chloroform/ methanol mixture (98:2, v/v) in sequence to separate saturates, aromatics, and resins, respectively. The resins were further fractionated using a 10 mm inner diameter column packed with 2 g of silicic acid (100-200 mesh). The neutral nitrogen subfraction was obtained by eluting the column with 50 mL of n-hexane/toluene (1:1, v/v). ESI FT-ICR MS Analysis. The crude oil and its SARA and neutral nitrogen fraction test samples were dissolved using toluene at 10, 30, 30, 5, 5, and 4 mg/mL, respectively, to prepare them for ESI FT-ICR MS analysis. The selected amount of toluene solvent for each test sample was pre-determined to obtain a strong single peak in the mass spectrum. A total of 20 μL of each solution mixture was diluted with 1 mL of toluene/methanol (1:1) solution. All solvents were of analytical reagent grade, redistilled twice, and kept in glass bottles. Apart from the steel pistons of 10 μL Hamilton syringes, glassware was used for solvent handling and transfer. The crude oil and its fractions were analyzed using a Bruker apex-ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet. Sample solutions were infused via an Apollo II electrospray source at 180 μL/h with a syringe pump. Typical operating conditions for negative-ion formation were emitter voltage, 4.0 kV; capillary column introduce voltage, 4.5 kV; and capillary column end voltage, -320 V. Ions accumulated for 0.1 s in an hexapole with 2.4 V of direct current (DC) voltage and 400 Vp-p of radio-frequency (RF) amplitude. The

Experimental Section Materials. The crude oil used in this study was obtained from a drill stem test (3686.0-3679.5 m) at the Shuang 201 Well of the Liaohe oil field in Bohai Basin, China. The crude oil was originated from the Shahejie Formation, which is one of the key petroleum source rocks in China. The concentration of sulfur and nitrogen was 0.081 and 0.12 wt %, respectively. Total ion (39) Panda, S. K.; Andersson, J. T.; Schrader, W. Characterization of supercomplex crude oil mixtures: What is really in there? Angew. Chem., Int. Ed. 2009, 48 (10), 1788–1791. (40) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Naphthenic acids in crude oils characterized by mass spectrometry. Energy Fuels 1999, 14 (1), 217–223. (41) Rudzinski, W. E.; Rai, V. Detection of polyaromatic sulfur heterocycles in crude oil using postcolumn addition of tropylium and tandem mass spectrometry. Energy Fuels 2005, 19 (4), 1611–1618. (42) Haapala, M.; Purcell, J. M.; Saarela, V.; Franssila, S.; Rodgers, R. P.; Hendrickson, C. L.; Kotiaho, T.; Marshall, A. G.; Kostiainen, R. Microchip atmospheric pressure photoionization for analysis of petroleum by Fourier transform ion cyclotron resonance mass spectrometry. Anal. Chem. 2009, 81 (7), 2799–2803. (43) Purcell, J. M.; Juyal, P.; Kim, D.-G.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Sulfur speciation in petroleum: Atmospheric pressure photoionization or chemical derivatization and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2007, 21 (5), 2869–2874. (44) Purcell, J. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Atmospheric pressure photoionization Fourier transform ion cyclotron resonance mass spectrometry for complex mixture analysis. Anal. Chem. 2006, 78 (16), 5906–5912. (45) Limbach, P. A.; Macha, S. F.; Robins, C. Analysis of hydrocarbon materials by matrix-assisted laser desorption/ionization mass spectrometry: Searching for the perfect matrix: Advances in hydrocarbon characterization. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 2000, 45, 561–563. (46) Schaub, T. M.; Hendrickson, C. L.; Quinn, J. P.; Rodgers, R. P.; Marshall, A. G. Instrumentation and method for ultrahigh resolution field desorption ionization Fourier transform ion cyclotron resonance mass spectrometry of nonpolar species. Anal. Chem. 2005, 77 (5), 1317– 1324. (47) Schaub, T. M.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Green, L. A.; Olmstead, W. N. Speciation of aromatic compounds in petroleum refinery streams by continuous flow field desorption ionization FT-ICR mass spectrometry. Energy Fuels 2005, 19 (4), 1566–1573. (48) Stanford, L. A.; Kim, S.; Klein, G. C.; Smith, D. F.; Rodgers, R. P.; Marshall, A. G. Identification of water-soluble heavy crude oil organic-acids, bases, and neutrals by electrospray ionization and field desorption ionization Fourier transform ion cyclotron resonance mass spectrometry. Environ. Sci. Technol. 2007, 41 (8), 2696–2702.

(49) Li, M.; Larter, S. R.; Stoddart, D.; Bjoroey, M. Liquid chromatographic separation schemes for pyrrole and pyridine nitrogen aromatic heterocycle fractions from crude oils suitable for rapid characterization of geochemical samples. Anal. Chem. 1992, 64 (13), 1337–1344.

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Figure 1. Broadband negative-ion ESI FT-ICR mass spectrum of crude oil.

optimized mass for Q1 was 250 Da. An argon-filled hexapole collision pool were operated at 5 MHz and 400 Vp-p of RF amplitude, in which ions accumulated for 4 s. The extraction period for ions from the hexapole to the ICR cell was 1.2 ms. The RF excitation was attenuated at 13 dB and used to excite ions over the range of 200-800 Da. A 2 M data set was acquired. A number of 128 scan FT-ICR data sets were co-added to enhance the signal-to-noise ratio and dynamic range. Mass Calibration and Data Analysis. The FT-ICR mass spectrometer was calibrated using an alkylcarbazole series, which contain a relative high abundance of negative-ion ESI mass spectra peaks. Peaks with the range of m/z 200-800 Da with relative abundance greater than 5 times the standard deviation of the baseline noise were exported to a spreadsheet. Data analysis was performed using custom software, which has been described elsewhere.50,51 In summary, the data analysis was performed by selecting a two-mass scale-expanded segment near the most abundant peak of the spectrum, followed by detailed identification of each peak. The peak of at least one of each heteroatom class species was arbitrarily selected as a reference peak. Each class species and its isotopes with different DBE and carbon number values were searched within a set (0.001 Kendrick mass defect (KMD) tolerance.50

Figure 2. Relative abundance of heteroatom class species in crude oil and its fractions.

in Figure 2. The abundant heteroatom class species include N1, N2, N1O1, N1O2, N1O3, N1O4, N2O1, O1, and O2. The N1 class species are the most abundant. A color-coded isoabundance plot of each class species based on DBE distribution is shown in Figure S-2 in the Supporting Information. In the figure, the inset shows the relative abundance of heteroatom species by excluding the N1 class species. The O1 compounds were more abundant than O2 class species. This is different from the results reported for most crude oils.20,25,52,53 Figure 3 shows the close-up view of expanded mass spectra for the crude oil and its fractions. The spectra on the left and right sides of Figure 3 are odd and even mass, respectively, of the negative-ion ESI FT-ICR spectra. The O3 and O4 class species were also identified in the resins and asphaltenes, in addition to the class species mentioned above. However, the resolving power of the equipment was not high enough to distinguish the O3 and O4 class species from the isotope peaks of N and NO class species. The unidentified class species that appeared on the spectrum of aromatics with low KMD values were presumed to be contaminants from sample preparation. This is because no apparent molecules composed of C, H, O, N,

Results and Discussion Distribution of Heteroatom Class Species. Figure 1 shows the negative ESI FT-ICR MS broadband (m/z 200-800) spectrum (128 co-added time-domain acquisitions) for the crude oil. The abundant peaks in the range of m/z 200-700 indicate a bimodal mass distribution of heteroatom class species. The even-mass peaks were found to be mostly single nitrogen compounds with a molecular-weight distribution peak at 300 Da. The odd-mass heteroatom species were O1 compounds with a peak at 450 Da. The resolving power (m/Δm 50%) was 250 000 at m/z 400. The relative abundance of various class species in a spectrum is defined as the magnitude of each peak divided by the sum of the magnitudes of all identified peaks (excluding the isotopic peaks) in the mass spectrum. While the relative abundance of a class species may be the same for two samples, the concentration of that class species may not be the same because it depends upon the amount of other class species present. The relative abundances of the negative-ion heteroatom class species for the crude oil are shown

(52) Barrow, M. P.; McDonnell, L. A.; Feng, X.; Walker, J.; Derrick, P. J. Determination of the nature of naphthenic acids present in crude oils using nanospray Fourier transform ion cyclotron resonance mass spectrometry: The continued battle against corrosion. Anal. Chem. 2003, 75 (4), 860–866. (53) Lu, X.; Shi, Q.; Zhao, S.; Gao, J.; Zhang, Y.; He, J. Composition and distribution of acidic compounds in Duba crude oil extracts: Revealed by negative electrospray ionization-Fourier transform ion cyclotron resonance-mass spectrometry. Chin. J. Anal. Chem. 2008, 36 (5), 614–618.

(50) Shi, Q.; Dong, Z. Y.; Zhang, Y. H.; Zhao, S. Q.; Xu, C. M. Data processing of high-resolution mass spectra for crude oil and its distillations. Chin. J. Instrum. Anal. 2008, 27 (Supplement 1), 246–248. (51) Shi, Q.; Xu, C.; Zhao, S.; Chung, K. H.; Zhang, Y.; Gao, W. Characterization of basic nitrogen species in coker gas oils by positiveion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2009, 24, 563–569.

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Figure 3. Expanded scale of odd (left) and even (right) mass of the negative-ion ESI FT-ICR spectra of crude oil and its fractions. The resolution power is not high enough to distinguish C27H43O3 and 13CC29H40N and C26H39O4 and 13CC28H36NO. The peak marked with an asterisk denotes an unknown species.

Figure 4. Iso-abundance plots of DBE versus carbon number of N1 class species in crude oil and its fractions. The size of circles corresponds to the relative abundance of N1 species in the spectra.

and S matched the peaks. Also, these peaks do not exhibit periodic mass series in the full mass range. The mass recovery for SARA was 73.34, 10.09, 5.05, and 6.94%, respectively. Figure 2 shows the relative abundance of negative-ion heteroatom class species for the SARA and neutral nitrogen fractions. The O3 and O4 class species were in trace amounts and not included in the relative abundance calculation. The O1 and O2 species had high abundance on the odd mass spectra of the crude oil and its fractions, except for the aromatics. The aromatic fraction has a high concentration of N1 class species. The N1Ox class species appeared to be relatively abundant in the aromatics; however, the absolute intensity of all species in aromatics was very low compared to those in other fractions. Some of species present in the aromatics could be from contaminants in the solvent used during sample preparation. To provide detailed compositional information, the data from FT-ICR MS analysis were used to construct iso-abundance

dot-size-coded plots and the differences in DBE and carbon number distributions for each class species in the crude oil and its fractions were examined. DBE versus Carbon Number for N1 Class Species. Figure 4 shows the iso-abundance plots of DBE versus carbon number for the N1 class species in the crude oil and its fractions. The N1 nitrogen species in the crude oil were spread over a wide range of DBE (6-24) and carbon number (15-55). The highest relative abundance of N1 class species was at DBE of 9, 12, 15, and 18. These are most likely carbazoles, benzocarbazoles, dibenzocarbazoles, and benzonaphthocarbazoles, respectively.54 The lowest carbon numbers at 12 (not shown), 16, and 20 of species at DBE of 12, 15, and 18, respectively, (54) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Robbins, W. K. Identification of acidic NSO compounds in crude oils of different geochemical origins by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2002, 33 (7), 743–759.

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Figure 5. Iso-abundance plots of DBE versus carbon number of N1O1 class species in crude oil and its fractions.

crude oil contained the N1O1 class species with DBE of 6-23 and carbon number at 15-50. The intervals at three DBE units corresponding to the addition of an additional aromatic ring fused to an aromatic core. Figure 4 shows that the N1 class species with DBE of 15 and 18 were abundant and homologous to pyrrole-based nitrogen molecules with a total of 4 and 5 fused benzene rings, respectively. The N1O1 class species with DBE of 15 and 18 were also present at high relative abundance in crude oil, resins, and asphaltenes. The oxygen atom in these N1O1 class species were determined to be a hydroxyl rather than furan-like cyclic ether because of the following reasons: (1) The high relative abundance of N1O1 class species at DBE of 18 is consistent with the DBE value of hydroxyl-benzonaphthocarbazole. (2) The lowest carbon numbers of various abundant N1O1 class species at 20, 24, and 28 are in agreement with the number of carbon atoms in the cores of hydroxylcarbazole-type molecules. (3) Hydroxyl compounds have a higher polarity than furan-like compounds. It is more likely that highly polar hydroxyl compounds elute to resins rather than aromatics. (4) On the basis of the knowledge of the type of abundant carbazole compounds present in the neutral nitrogen species in petroleum,49 hydroxylcarbazole compounds may be present in petroleum. (5) If oxygen were in the form of a furan skeleton, it would be eluted to the aromatics. However, high DBE (>16) N1O1 class species were not found in the aromatics. All N1O1 species in the aromatics had a relatively low DBE and a very low abundance in the crude oil. Because no O1 compounds were found in the aromatics, the oxygen atom in these N1O1 class species could not be in the form of a hydroxyl. DBE versus Carbon Number for N1O2 Class Species. Figure 6 shows the iso-abundance plots of DBE versus carbon number for the N1O2 class species in the crude oil and its fractions. The N1O2 class species were spread over a wide range of DBE (6-26) with carbon number at 15-45. It is difficult to distinguish the predominant N1O2 species. Pyrrole, hydroxy, and

also support the assignment for the four proposed nitrogencontaining compounds. Despite the presence of a high relative abundance of N1 class species in the crude oil and its fractions, the distributions of DBE and carbon number for the crude oil and its fractions were quite different. The most abundant N1 class species in the aromatics were at DBE of 9 with a normal carbon number distribution centered at 32, indicating that the N1 class species in the aromatics had relatively low aromaticity and with either long or multisubstituted alky side chains. The N1 class species in the aromatics with a DBE value of 9 should be alky carbazole homologues based on prior knowledge of neutral nitrogen compounds found in petroleum.49,55 The highest relative abundance of N1 class species was at DBE of 15 and carbon number at 22, which is consistent with C2-dibenzocarbazoles. The N1 class species in the resins and asphaltenes had similar DBE and carbon number distributions, with relatively high DBE values and low carbon numbers. The O1 and N1O1 class species also had identical signals between these fractions. Although the similar picture in the Kendrick plots may not imply similar structural composition, it is likely that N1, N1O1, and O1 class species in resins and asphaltenes have similar characteristics because of poor solubilities. If lowmolecular-weight polar compounds deposit on asphaltenes, even in very small concentrations, it could lead to similar pictures between the asphaltenes and maltanes. This does not imply that the separation into different fractions was poor but that compositional differences exist among various deasphalting methods. DBE versus Carbon Number for N1O1 Class Species. Figure 5 shows the iso-abundance plots of DBE versus carbon number for the N1O1 class species in the crude oil and its fractions. The (55) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K. N.; Mankiewicz, P. Acidic and neutral polar NSO compounds in smackover oils of different thermal maturity revealed by electrospray high field Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2004, 35 (7), 863–880.

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Figure 6. Iso-abundance plots of DBE versus carbon number of N1O2 class species in crude oil and its fractions.

carboxy compounds can all be ionized under a negative ESI source. Hence, it is difficult to deduce with high certainty the molecular structure of the N1O2 class species by DBE values alone. The N1O2 class species with 16 DBE are likely dibenzocarbazoles with a carboxyl or two hydroxyl groups attached to the core structure. Carbon atoms for dihydroxy dibenzocarbazole and dibenzocarbazole carboxylic acid are 20 and 21, respectively. This is in agreement with the presence of these compounds in the crude oil and asphaltenes at the lowest carbon number of 20. Because N1 and O2 class species commonly coexist in high relative abundances in petroleum-derived hydrocarbons, the oxygen atoms in the N1O2 class species are likely in carboxyl form. However, functional groups other than hydroxyl and carboxyl are likely found in the N1O2 class species in the aromatics, because no O1 and O2 class species were identified. DBE versus Carbon Number for N2 Class Species. Figure 7 shows the iso-abundance plots of DBE versus carbon number for the N2 class species in the crude oil and its resin and asphaltene fractions. The N2 class species had low relative abundance in the crude oil and its resins and asphaltenes. No N2 class species were found in the aromatics. The N2 class species in resins with high DBE values resemble those found in the crude oil. These N2 class species can be divided into two series: DBE of 17, 20, and 23 and DBE of 18, 21, and 24. The three DBE increments are characteristic of an additional aromatic ring fused to the existing aromatic cores. The N2 class species with DBE of 17 likely have two fused carbazole cores. The series with DBE of 18 is likely a benzocarbazole fused with a quinoline molecule or two carbazoles joined by a bridge bond. Although a naphthenic ring attached to the species with DBE of 17 has a DBE value of 18, it could not account for the lower abundance for multiple naphthenic rings. For N2 class species, one of the two nitrogen atoms in the pyrrole compound can be ionized by the ESI source, regardless of the form of the other nitrogen atom. DBE versus Carbon Number for O1 and O2 Class Species. Figure 8 shows the iso-abundance plots of DBE versus

carbon number for members of the O1 and O2 class species in the crude oil and its resin and asphaltene fractions. The O1 class species are the second most relatively abundant heteroatoms found in crude oil. The O1 class species had DBE of 4-22 and carbon number at 15-50. The relative abundance of O1 class species decreased as the DBE increased. The normal distribution of relative abundance was centered at a carbon number of 30-35. The pattern of the iso-abundance plots of the crude oil and its fractions are similar, except for the aromatics, in which O1 class species were not detected. The iso-abundance plots of resins and asphaltenes do not yield additional molecular information on the O1 class species, except that these showed that the oxygen-containing species are highly polar from the SARA fractionation. The low DBE value of 4 suggests that the O1 class species are phenolics instead of hydroxyl naphthenes (no species with DBE of 1-3 were found). The O2 class species were spread over a wide range of DBE and carbon number. However, only a few O2 class species were found in relatively high abundance. The O2 class species with DBE of 1 are fatty acids and were in high relative abundance at carbon numbers of 16 and 18. These compounds are commonly found in non-biodegraded crude oils.56 They can also be introduced during sample preparation. The O2 class species with DBE of 5 were the species with the second highest relative abundance in the crude oil and had the highest relative abundance in the resins. These species are likely mono-aromatic acids, instead of four-ring naphthenic acid, because no species with a low DBE value were found in the iso-abundance plot. The iso-abundance plots of O2 class species in resins and asphaltenes were significantly different from all of the other heteroatoms. The data suggest that fatty (56) Kim, S.; Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K.; Wenger, L. M.; Mankiewicz, P. Microbial alteration of the acidic and neutral polar NSO compounds revealed by Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2005, 36 (8), 1117–1134.

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Figure 7. Iso-abundance plots of DBE versus carbon number of N2 class species in crude oil and its resin and asphaltene fractions.

Figure 8. Iso-abundance plots of DBE versus carbon number of O1 and O2 class species in crude oil and its resin and asphaltene fractions.

acids have a higher propensity to form precipitates and/or are more readily adsorbed irreversibly on the neutral alumina column than aromatic acids. Heteroatom Class Species in the Neutral Nitrogen Fraction. Neutral nitrogen compounds in petroleum-derived streams have been used as geochemical markers for tracking the migration of hydrocarbons.57-60 In traditional analyses, the isolated neutral nitrogen compounds are subjected to GCMS. However, there has not been a universally accepted standardized protocol developed for this. It is difficult to have adequate experimental repeatability because of the isolation of neutral nitrogen compounds from the complex oil mixture and the required multi-step analytical technique. GC-MS analysis indicated that the neutral nitrogen fraction was enriched in carbazole and benzocarbazole homologues (see Figure S-1 in the Supporting Information). However, the yield of each nitrogen-containing compound is not known because of the inability of GC-MS to provide quantitative compositional information. Using the FT-ICR MS analysis, neutral nitrogen compounds with up to 50 carbons can be determined in comparison to compounds

with fewer than 20 carbons by GC-MS analysis. Moreover, only a fraction of total neutral nitrogen compounds pass through the GC capillary column. Nitrogen compounds with high DBE values and molecular weights likely form coke in the GC capillary column, which also reduces the life span of the column. Figure 9 shows the iso-abundance plots of DBE versus carbon number for N1, N2, N1O1, N1O2, O1, and O2 class species in the neutral nitrogen fraction of the crude oil. The pattern of N1 class species in the neutral nitrogen fraction was similar to that for the resins and asphaltenes. Because the neutral nitrogen fraction was isolated from the maltenes, it was not known whether neutral nitrogen compounds were lost during the deasphalting step prior to sample preparation. Li et al.49 suggested that deasphalting is needed to solve the problem in the alumina step of the method by Later et al.,61 which has coelution of the nitrogen compounds with “asphaltene-like” species. However, the asphaltene content and its properties vary with the separation method and also sometimes with the individual technique. Factors such as contact time, solvent/oil ratio, and temperature influence asphaltene precipitation.62 The results of this study show that low-molecular-weight nitrogen species are present in asphaltenes. The information on neutral nitrogen compounds derived from maltenes is incomplete, because the amount of neutral nitrogen compounds lost in the asphaltenes is not known.

(57) Silliman, J. E.; Li, M.; Yao, H.; Hwang, R. Molecular distributions and geochemical implications of pyrrolic nitrogen compounds in the permian phosphoria formation derived oils of Wyoming. Org. Geochem. 2002, 33 (5), 527–544. (58) Li, M.; Yao, H.; Stasiuk, L. D.; Fowler, M. G.; Larter, S. R. Effect of maturity and petroleum expulsion on pyrrolic nitrogen compound yields and distributions in duvernay formation petroleum source rocks in central Alberta, Canada. Org. Geochem. 1997, 26 (11-12), 731– 744. (59) Wang, T. G.; Li, S. M.; Zhang, S. C. Oil migration in the Lunnan region, Tarim Basin, China based on the pyrrolic nitrogen compound distribution. J. Pet. Sci. Eng. 2004, 41 (1-3), 123–134. (60) Hwang, R. J.; Heidrick, T.; Mertani, B.; Qivayanti, Li, M. Correlation and migration studies of north central Sumatra oils. Org. Geochem. 2002, 33 (12), 1361–1379.

(61) Later, D. W.; Lee, M. L.; Bartle, K. D.; Kong, R. C.; Vassilaros, D. L. Chemical class separation and characterization of organic compounds in synthetic fuels. Anal. Chem. 2002, 53 (11), 1612–1620. (62) Alboudwarej, H.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W. Sensitivity of asphaltene properties to separation techniques. Energy Fuels 2002, 16 (2), 462–469.

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Figure 9. Iso-abundance plots for DBE as a function of the carbon number of heteroatom class species in the neutral nitrogen fraction.

The pattern of Ox (O1 and O2) class species in the neutral nitrogen fraction is similar to that of the crude oil and asphaltenes, indicating that Ox class species coeluted with neutral nitrogen compounds (similar to the N1 class species) and were partly lost during the deasphalting step. The N2 and N1Ox class species in the neutral nitrogen fraction had relative high DBE values compared to those of the crude oil. The reason for the shift to higher DBE values of N2 and N1Ox class species in the neutral nitrogen fraction is the absence of basic nitrogen compounds, such as pyridines.

found in the aromatic fraction. The distribution patterns of N1, N1O1, and O1 class species in resins and asphaltenes were similar. The carbazole and benzocarbazole species detected by GC accounted for a small amount of that found in the neutral nitrogen fraction. Some of the neutral nitrogen species were entrained in asphaltenes during the deasphalting step of sample preparation. This study examined the distribution of acidic heteroatoms in SARA fractions for a Liaohe crude oil. It is not known whether other crude oils would exhibit similar characteristics. ESI FT-ICR MS is a useful analytical tool to identify the distribution of heteoatom species present in the subfractions derived from various separation techniques, which are often used in petroleum and geological chemistry.

Conclusions The compositional analysis of the crude oil and its SARA fractions as well as neutral nitrogen fraction by negative ESI FT-ICR MS was conducted in this study. The results provide valuable information for the understanding of the detailed composition of heteroatom compounds in crude oils. Neutral nitrogen and acidic heteroatom compounds in the crude oil have 15-55 carbon atoms with DBE values of 1-27, containing N1, N2, NO, NO2, NO3, NO4, O1, and O2 heteroatoms. The aromatic fraction contains N1 and NOx compounds with high molecular weight but relatively low DBE values. The resin and asphaltene fractions contain highly aromatic and acidic class species with enriched oxygen- and nitrogen-containing compounds with lower molecular weight than those

Acknowledgment. The authors thank Dr. Maowen Li (the Geological Survey of Canada, Calgary, Alberta, Canada) for helpful discussions. This work was supported by the National Natural Science Foundation of China (40972097) and the National Basic Research Program of China (2010CB226901). Supporting Information Available: Chromatograms of the crude oil and its fractions (Figure S-1) and relative abundance of assigned heteroatom class species in the negative ESI FT-ICR MS spectrum of the crude oil (Figure S-2). This material is available free of charge via the Internet at http://pubs.acs.org.

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