Tracking Neutral Nitrogen Compounds in Subfractions of Crude Oil

Energy Fuels 2010, 24, 6321–6326 Published on Web 11/19/2010

: DOI:10.1021/ef1011512

Tracking Neutral Nitrogen Compounds in Subfractions of Crude Oil Obtained by Liquid Chromatography Separation Using Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Yahe Zhang,† Chunming Xu,† Quan Shi,*,† Suoqi Zhao,† Keng H. Chung,‡ and Dujie Hou§ † ‡

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China, Well Resources, Incorporated, 3919 149A Street Northwest, Edmonton, Alberta T6R 1J8, Canada, and § Faculty of Energy Resources, China University of Geosciences, Beijing 100083, China Received May 8, 2010. Revised Manuscript Received November 5, 2010

Neutral nitrogen compounds have been used as molecular markers for tracking secondary oil migration in geochemistry. However, the distribution of neutral nitrogen compounds in the separation process is not well-characterized because the conventional analytical technique, such as gas chromatography-mass spectrometry (GC-MS), is non-selective for neutral nitrogen and not capable of detecting non-volatile high-molecular-weight compounds. In this paper, a crude oil was subjected to the traditional two-step open-column liquid chromatography (LC) technique to prepare subfractions, which were characterized for their molecular composition of neutral nitrogen compounds by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). The results showed that the two-step LC technique produced a low yield of carbazole in the neutral nitrogen fraction. The neutral nitrogen fraction was enriched with low-molecular-weight neutral nitrogen compounds. Most N1 class species with relatively low double-bond equivalent (DBE) values and high carbon numbers were eluted into the aromatic fraction, and a portion of neutral nitrogen compounds was eluted into the amino fraction, which was not expected. Because the neutral nitrogen compounds detected by GC-MS analysis only account for a fraction of total neutral nitrogen compounds, caution should be exercised in interpreting the analytical data obtained from the two-step LC technique. The analytical bias of the separation technique could lead to erroneous geochemical interpretations when a low yield of carbazole in the neutral nitrogen fraction was derived. Negative-ion ESI FT-ICR MS was an effective tool to monitor and evaluate the separation technique for neutral nitrogen compounds in crude oil.

by forming coke on the catalyst surface.4 Neutral nitrogen compounds are used as molecular markers for tracking secondary oil migration in geochemistry.5,6 However, conducting a complete characterization of heteroatoms in crude oil is challenging because of the complexity and wide polarity range of the species present. Gas chromatography-mass spectrometry (GC-MS) is a commonly used analytical technique for identifying components in petroleum. Prior to GC-MS analysis, the petroleum sample must be separated into various functional classes of compounds by liquid chromatography (LC). It is difficult to determine the distribution of neutral nitrogen compounds in the subfractions of crude oil.2 Also, if neutral nitrogen compounds elute into the aromatic fraction, their trace quantities are often masked by the abundant aromatics in the chromatogram because of low mass resolving power and low ionization selectivity of conventional MS. Various techniques for isolating neutral nitrogen compounds from crude oils have been developed, including acid/base

Introduction Crude oils contain about 0.1-2 wt % of organic nitrogen compounds,1 consisting of mostly neutral pyrrolic and some basic pyridinic aromatic heterocycles.2 Organic nitrogen compounds are known to cause fuel instability during storage.3 Basic nitrogen compounds play a key role in catalyst deactivation *To whom correspondence should be addressed. Telephone: 86108973-3738. Fax: 8610-6972-4721. E-mail: [email protected]. (1) Boduszynski, M. M. Characterization of “heavy” crude components. Prepr. Pap.;Am. Chem. Soc., Div. Fuel Chem. 1985, 30 (5), 365–382. (2) 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. (3) Bauserman, J. W.; Mushrush, G. W.; Hardy, D. R. Organic nitrogen compounds and fuel instability in middle distillate fuels. Ind. Eng. Chem. Res. 2008, 47 (9), 2867–2875. (4) Qian, K. N.; Tomczak, D. C.; Rakiewicz, E. F.; Harding, R. H.; Yaluris, G.; Cheng, W.-C.; Zhao, X.; Peters, A. W. Coke formation in the fluid catalytic cracking process by combined analytical techniques. Energy Fuels 1997, 11 (3), 596–601. (5) Li, M.; Larter, S. R.; Stoddart, D.; Bjorøy, M. Fractionation of pyrrolic nitrogen compounds in petroleum during migration: Derivation of migration-related geochemical parameters. J. Geol. Soc. (London, U. K.) 1995, 86, 103–123. (6) Larter, S. R.; Bowler, B. F. J.; Li, M.; Chen, M.; Brincat, D.; Bennett, B.; Noke, K.; Donohoe, P.; Simmons, D.; Kohnen, M.; Allan, J.; Telnaes, N.; Horstad, I. Molecular indicators of secondary oil migration distances. Nature 1996, 383 (6601), 593–597. r 2010 American Chemical Society

(7) Merdrignac, I.; Behar, F.; Albrecht, P.; Briot, P.; Vandenbroucke, M. Quantitative extraction of nitrogen compounds in oils: Atomic balance and molecular composition. Energy Fuels 1998, 12 (6), 1342– 1355. (8) 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. 1981, 53 (11), 1612–1620.

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extraction, open-column LC, coordination chromatography,13 high-pressure liquid chromatography (HPLC),2 and solid-phase extraction (SPE).14-18 Among them, the two-step open-column LC is commonly used to isolate neutral compounds from crude oils and rock extracts. This technique was developed by Larter et al.8 and subsequently modified by Li et al.2 It consists of a separation of deasphalted crude oil or rock extract in a neutral alumina column followed by a silicic acid column. GC-MS is used to identify C0-C7 alkylcarbazoles and alkylbenzocarbazoles in crude oils and rock extracts by co-injecting model compounds as references.2,8,19 However, little is known about the technical merits of this technique, such as the material balance of species, repeatability, and sensitivity for various compounds. Electrospray ionization (ESI) is a technique used for detailed characterization of polar compounds in a petroleum system.20 The ultrahigh mass resolving power and mass accuracy of Fourier transform ion cyclotron resonance (FT-ICR) MS allow for the assignment of a unique elemental composition to each peak in the mass spectrum.21-23 ESI coupled with FT-ICR MS is a powerful analytical technique capable of detecting acids and pyrrolic nitrogen compounds, such as

carboxylic acids and carbazoles, present in trace quantities in crude oil and its products without prior sample pretreatment. Negative-ion ESI FT-ICR MS has been used to resolve and identify acids and neutral heteroatom compounds in crude oil,24-35 bitumen,36 oilsands,37 coal,38 asphaltenes,39 gas oil,40 (25) 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. (26) Klein, G. C.; Rodgers, R. P.; Teixeirac, M. A. G.; Teixeira, A. M. R. F.; Marshall, A. G. Petroleomics: Electrospray ionization FT-ICR mass analysis of NSO compounds for correlation between total acid number, corrosivity, and elemental composition. Prepr. Pap.;Am. Chem. Soc., Div. Fuel Chem. 2003, 48 (1), 14–15. (27) Rodgers, R. P.; Klein, G. C.; Nowlan, V.; Marshall, A. G. Petroleomics: ESI FT-ICR MS identification of hydrotreatment resistant neutral and acidic nigrogen species in crude oil. Prepr. Pap.;Am. Chem. Soc., Div. Fuel Chem. 2003, 48 (2), 574. (28) 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. (29) 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. (30) Hemmingsen, P. V.; Kim, S.; Pettersen, H. E.; Rodgers, R. P.; Sjoblom, 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. (31) 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. (32) 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. (33) Ter€av€ainen, M. J.; Pakarinen, J. M. H.; Wickstr€ om, K.; Vainiotalo, P. Comparison of the composition of Russian and North Sea crude oils and their eight distillation fractions studied by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry: The effect of suppression. Energy Fuels 2007, 21 (1), 266–273. (34) Purcell, J. M.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Speciation of nitrogen containing aromatics by atmospheric pressure photoionization or electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. J. Am. Soc. Mass Spectrom. 2007, 18 (7), 1265–1273. (35) Shi, Q.; Zhao, S.; Xu, Z.; Chung, K. H.; Zhang, Y.; Xu, C. Distribution of acids and neutral nitrogen compounds in a Chinese crude oil and its fractions: Characterized by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2010, 24 (7), 4005–4011. (36) 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. (37) Barrow, M. P.; Headley, J. V.; Peru, K. M.; Derrick, P. J. Fourier transform ion cyclotron resonance mass spectrometry of principal components in oilsands naphthenic acids. J. Chromatogr., A 2004, 1058 (1), 51–59. (38) Wu, Z. G.; Jernstrom, S.; Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Resolution of 10 000 compositionally distinct components in polar coal extracts by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2003, 17 (4), 946–953. (39) 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. (40) Kek€al€ainen, T.; Pakarinen, J. M. H.; Wickstro€ om, K.; Vainiotalo, P. Compositional study of polar species in untreated and hydrotreated gas oil samples by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FTICR-MS). Energy Fuels 2009, 23 (12), 6055–6061.

(9) Ford, C. D.; Holmes, S. A.; Thompson, L. F.; Latham, D. R. Separation of nitrogen compound types from hydrotreated shale oil products by adsorption chromatography on basic and neutral alumina. Anal. Chem. 1981, 53 (6), 831–836. (10) Dorbon, M.; Ignatiadis, I.; Schmitter, J.-M.; Arpino, P.; Guiochon, G.; Toulhoat, H.; Huc, A. Identification of carbazoles and benzocarbazoles in a coker gas oil and influence of catalytic hydrotreatment on their distribution. Fuel 1984, 63 (4), 565–570. (11) Frolov, Y. B.; Vanyukova, N. A.; Sanin, P. I. Selective isolation of carbazoles from crude oil. Pet. Chem. U.S.S.R. 1987, 27 (2), 114–120. (12) Bakel, A. J.; Philp, R. P. The distribution and quantitation of organonitrogen compounds in crude oils and rock pyrolysates. Org. Geochem. 1990, 16 (1-3), 353–367. (13) Oliveira, E. C.; Vaz de Campos, M. C.; Rodrigues, M. R. A.; P
erez, V. F.; Melecchi, M. I. S.; Vale, M. G. R.; Zini, C. A.; Caram~ao, E. B. Identification of alkyl carbazoles and alkyl benzocarbazoles in Brazilian petroleum derivatives. J. Chromatogr., A 2006, 1105 (1-2), 186–190. (14) Bennett, B.; Chen, M.; Brincat, D.; Gelin, F. J. P.; Larter, S. R. Fractionation of benzocarbazoles between source rocks and petroleums. Org. Geochem. 2002, 33 (5), 545–559. (15) Briker, Y.; Ring, Z.; Iacchelli, A.; McLean, N. Miniaturized method for separation and quantification of nitrogen species in petroleum distillates. Fuel 2003, 82 (13), 1621–1631. (16) Bastow, T. P.; van Aarssen, B. G. K.; Chidlow, G. E.; Alexander, R.; Kagi, R. I. Small-scale and rapid quantitative analysis of phenols and carbazoles in sedimentary matter. Org. Geochem. 2003, 34 (8), 1113– 1127. (17) Huang, H.; Bowler, B. F. J.; Zhang, Z.; Oldenburg, T. B. P.; Larter, S. R. Influence of biodegradation on carbazole and benzocarbazole distributions in oil columns from the Liaohe basin, NE China. Org. Geochem. 2003, 34 (7), 951–969. (18) Zhang, Y.; Shi, Q.; He, J.; Hu, W.; Zhu, L. Separation and determination of carbazoles in crude oil by SPE/GC-MS method. J. Instrum. Anal. 2008, 27 (Supplement 1), 252–255 (in Chinese). (19) Bowler, B. F. J.; Larter, S. R.; Clegg, H.; Wilkes, H.; Horsfield, B.; Li, M. Dimethylcarbazoles in crude oils: Comment on “Liquid chromatographic separation schemes for pyrrole and pyridine nitrogen aromatic heterocycle fractions from crude oils suitable for rapid characterization of geochemical samples”. Anal. Chem. 1997, 69 (15), 3128–3129. (20) Zhan, D.; Fenn, J. B. Electrospray mass spectrometry of fossil fuels. Int. J. Mass Spectrom. 2000, 194 (2-3), 197–208. (21) Marshall, A. G.; Rodgers, R. P. Petroleomics: The next grand challenge for chemical analysis. Acc. Chem. Res. 2004, 37 (1), 53–59. (22) Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Petroleomics: MS returns to its roots. Anal. Chem. 2005, 77 (1), 20A–27A. (23) Marshall, A. G.; Rodgers, R. P. Petroleomics: Chemistry of the underworld. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (47), 18090–18095. (24) Qian, K. N.; 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.

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coker gas oil (CGO), vacuum gas oil (VGO), emulsion interfacial material,44 soil,45 metal naphthenate deposit,46,47 and asphaltene inhibitor.48 Compounds can be characterized by class (number of N, O, and S heteroatoms), type [doublebond equivalent (DBE) values, i.e., rings plus double bonds], and carbon number. The compositional differences of petroleum samples obtained by ESI FT-ICR MS can be used to identify the geochemical origins of crude oils25 and determine the degree of thermal maturity28 and biodegradation.29,31,45 In this study, ESI FT-ICR MS analysis was used to track the distribution of neutral nitrogen compounds in subfractions of crude oil obtained by the two-step open-column LC separation technique. The objective of this study is to show the analytical bias of the separation technique, which was overlooked by previous workers and could lead to erroneous geochemical interpretations.

Figure 1. Two-step open-column LC separation process for nitrogen compounds.

crude oil sample was adsorbed onto a packed column with 8 g of neutral alumina. A total of 50 mL of n-hexane was used to elute the saturate fraction, followed by 50 mL of toluene to elute the aromatic fraction and then 70 mL of chloroform/methanol (98:2, v/v) solution to elute the nitrogen-enriched fraction. The nitrogen-enriched fraction was further separated into three subfractions by adsorption onto a 2 g packed silicic acid column. Neutral (pyrrolic), amino, and pyridinic nitrogen fractions were eluted sequentially using 50 mL of n-hexane/toluene (1:1, v/v) solution, 30 mL of toluene, and 50 mL of toluene/anhydrous diethyl ether (1:1, v/v) solution. The neutral nitrogen fraction was characterized by GC-MS. Neutral nitrogen compounds were identified using published data from Li et al.2 and Bowler et al.19 The d8-carbazole was added to the crude oil sample before deasphalting and used as a reference to estimate the yield of neutral nitrogen compounds. N-Phenylcarbazole was used as an internal standard in the GC for quantifying the neutral nitrogen compounds. The crude oil sample and subfractions were diluted with dimethylbenzene to 0.5 mL. A total of 5 μL of each sample was injected to a nitrogen analyzer (ANTEK model 7000) to determine the total nitrogen content and evaluate the elemental balance of nitrogen. Sample Preparation for Negative-Ion ESI FT-ICR MS Analysis. A total of 10 mg of the oil sample was dissolved in 1 mL of toluene. A total of 20 μL of the solution was further diluted with 1 mL of toluene/methanol (1:1, v/v) solution. A total of 15 μL of 28% NH4OH was added to facilitate deprotonation of acid species and neutral nitrogen compounds to yield [M - H]ions.24 Toluene and methanol used were analytical-reagent-grade solvents and were distilled twice and kept in glass bottles with ground glass stoppers. FT-ICR MS Analysis. The samples were analyzed using Bruker Apex IV FT-ICR MS equipped with a 7.0 T superconducting magnet. The sample solution was infused by an Apollo electrospray source at 150 μL/h using a syringe pump. The conditions for negative-ion formation were 4.5 kV emitter voltage, 4.0 kV capillary column front end voltage, and -150 V capillary column end voltage. Ions accumulated for 3 s in a hexapole. The delay was set to 1.3 ms to transfer the ions to an ICR cell by electrostatic focusing of transfer optics. The ICR was operated at the 150-1500 Da mass range and 1 M acquired data size. Timedomain data sets were co-added from 64 data acquisitions. Mass Calibration and Data Analysis. The mass spectra were calibrated using a known and highly abundant homologous series of nitrogen-containing compounds (alkyl carbazoles). Peaks with relative abundance greater than 6 times the standard deviation of the baseline noise level were exported to a spreadsheet. Data analysis was performed using custom software, which has been described elsewhere.50 In general, the data analysis was performed by selecting a two-mass scale-expanded segment in the middle of the mass spectrum, followed by detailed identification

Experimental Section Two-Step Open-Column LC Separation Method. A drill stem test (DST) crude oil sample, which contained 0.11 wt % nitrogen, 0.10 wt % sulfur determined by an organic elemental analyzer (OEA), and 4 wt % asphaltenes, was obtained from the Liaohe oilfield in Bohai Basin, China. It was subjected to the separation process illustrated in Figure 1 to obtain the various functional subfractions. The separation scheme is a minor modification of that used by Li et al.2 The asphaltenes in 150 mg of crude oil sample were precipitated by n-hexane (Chinese Standard Analytical Method for Petroleum and Natural Gas Industry, SY/T 5119-2008).49 The remainder of the deasphalted (41) Fu, J. M.; 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. (42) 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. (43) 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. (44) 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. (45) Hughey, C. A.; Minardi, C. S.; Galasso-Roth, S. A.; Paspalof, G. B.; Mapolelo, M. M.; Rodgers, R. P.; Marshall, A. G.; Ruderman, D. L. Naphthenic acids as indicators of crude oil biodegradation in soil, based on semi-quantitative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Rapid Commun. Mass Spectrom. 2008, 22, 3968–3976. (46) Brandal, O.; Hanneseth, A. M.; Hemmingsen, P. V.; Sjoblom, J.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Isolation and characterization of naphthenic acids from a metal naphthenate deposit: Molecular properties at oil-water and air-water interfaces. J. Dispersion Sci. Technol. 2006, 27 (3), 295–305. (47) Mapolelo, M. M.; Stanford, L. A.; Rodgers, R. P.; Yen, A. T.; Debord, J. D.; Asomaning, S.; Marshall, A. G. Chemical speciation of calcium and sodium naphthenate deposits by electrospray ionization FT-ICR mass spectrometry. Energy Fuels 2009, 23 (1), 349–355. (48) Smith, D. F.; Klein, G. C.; Yen, A. T.; Squicciarini, M. P.; Rodgers, R. P.; Marshall, A. G. Crude oil polar chemical composition derived from FT-ICR mass spectrometry accounts for asphaltene inhibitor specificity. Energy Fuels 2008, 22 (5), 3112–3117. (49) Shi, Q.; Hou, D.; Chung, K. H.; Xu, C.; Zhao, S.; Zhang, Y. 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. Energy Fuels 2010, 24 (4), 2545–2553.

(50) Shi, Q.; Dong, Z.; Zhang, Y.; Zhao, S.; Xu, C. Data processing of high-resolution mass spectra for crude oil and its distillations. J. Instrum. Anal. 2008, 27 (Supplement 1), 246–248 (in Chinese).

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of each peak. The peak of at least one of each heteroatom class species was arbitrarily selected as a reference. Species with the same heteroatom class and its homologues with different values of DBE and carbon number were searched within a set of (0.002 Kendrick mass defect (KMD) tolerance.51 GC-MS Analysis. Thermo-Finnigan Trace DSQ GC-MS equipped with a HP-5 MS column (30 m  0.25 mm  0.25 μm) was used to analyze the composition of the neutral nitrogen compounds in the neutral nitrogen fraction. The GC oven was held at 80 °C for 1 min, increased at a rate of 15 °C/min to 150 °C, increased by 3 °C/min to 270 °C, and held constant for 10 min. The sample was injected at 300 °C with splitless injection by an AS3000 autosampler. The electron impact (EI) ionization source was operated under 70 eV ionization energy in selected ion monitoring (SIM) mode. The ion source temperature was 250 °C, and the ion current was 100 μA.

Table 1. Yield of Total Nitrogen for Each Fraction and Total Nitrogen Recovery (%) saturates aromatics neutral amino pyridinic asphaltenes recovery UDa a

15.4

21.0

2.7

18.9

6.9

64.9

Undetected.

which is a characteristic of most crude oils from the Liaohe oilfield in Bohai Basin, China. The relative abundances of heteroatom class species are different for the crude oil and the subfractions. The N1, N1O1, N1O2, O1, O2, and O3 class species were identified and accounted for more than 85% of the total mass peak abundance for the crude oil. On the basis of findings from previous studies, the N1 class species were likely the neutral nitrogen compounds.24,25 The N1 class species were detected using negative-ion ESI in all subfractions, except the pyridinic fraction. In the aromatic fraction, N1 class species (neutral compounds) are the dominant species, comprising more than 90% of the total. These are followed by O1 and O2 class species (likely phenols and carboxylic acids), which comprise 7% of the total. N1O1, N1O2, and O3 class species were not detected. The N1 class species are also dominant in the neutral nitrogen fraction, consisting of more than 62%, followed by O1 and O2 class species, which accounted for 22 and 8% of the total species present, respectively. The neutral nitrogen fraction had a low abundance of N1O1, N1O2, and O3 class species. For the amino and pyridinic nitrogen fractions, the O2 class species dominate with more than 54% abundance, followed by the O1 class species. The abundance of O3 class species is similar to that of O1 class species in the pyridinic nitrogen fraction. The dominant homologous series in the amino fraction are O1 class species. A DBE value of 4 implies that these compounds are likely alkyl phenols. Although the pyridinic nitrogen fraction has a relatively high abundance of nitrogen compounds, basic nitrogen compounds could not be ionized by negative-ion ESI. The mass spectrum indicates that neutral nitrogen compounds were effectively isolated from this fraction. The O2 and O3 class species, which are likely acidic compounds, are highly abundant in the amino and pyridinic fractions. Previous studies have shown that the O2 class species, such as C16 and C18 fatty acids, are often present in the negative-ion ESI mass spectra as contaminants.33 A distinctive group of O3S class species (C17-19 and DBE value of 4) is present in the spectrum of the pyridinic fraction. These compounds were also identified in the crude oil but appear to concentrate in the pyridinic fraction. Muller et al.53 indicated that the O3S class species are alkylbenzenesulfonates, which are commonly used as oilfield chemicals, such as asphaltene suppressants, detergents, and fouling control additives. DBE versus Carbon Number for N1 Class Species. The isoabundance maps of DBE as a function of the carbon number for the N1 class species in the crude oil and its subfractions are shown in Figure 4. The N1 class species in the crude oil were spread over a wide range of DBE values (9-20) and carbon numbers (12-55). Species with the DBE value of 9 are likely carbazoles,25,28 with a bimodal relative abundance distribution (carbon numbers of 12-19, centered at 15 carbons; carbon numbers of 20-55, centered at 30 carbons). The highest relative abundant N1 class species have a DBE value of 12.

Results and Discussion Nitrogen Content of the Subfractions. The amount of nitrogen in each subfraction of the crude oil sample was determined and expressed as a percentage of the total nitrogen in the crude oil. The relative standard deviation (RSD) of nitrogen yield data is less than 6% based on three replicate measurements conducted for each sample. The results listed in Table 1 show the distribution of the total nitrogen in the crude oil subfractions. Initially, it was expected that the nitrogen compounds would enrich in the polar fraction. However, a significant amount of nitrogen compounds eluted into the aromatic fraction. The recovery of total nitrogen was quite low (64.9 wt %). Nitrogen was lost most likely from irreversible adsorption of nitrogen compounds on the neutral alumina, and low concentration of nitrogen compounds below the detection limit of the nitrogen analyzer. Because the chemical properties and response factor of d8carbazole are similar to those of carbazole, the concentration of carbazole in crude oil can be quantified using the reference d8carbazole.18 The yield of carbazole was 32%. The low yield of carbazole suggested that the amount of neutral nitrogen compounds in the neutral nitrogen fraction was not the same as that in the crude oil. Distribution of Heteroatom Class Species in Crude Oil and Its Subfractions. Figure 2 shows the negative-ion ESI FTICR MS broadband (150-900 Da) spectra (64 co-added time-domain acquisitions) of the crude oil and its subfractions. The abundant peaks with even masses in the molecular-weight range of 200-700 Da for aromatic and neutral fractions are N1 class species, while those with odd masses for amino and pyridinic fractions are O1, and O2 class species, respectively. The relative abundances of heteroatom class species for the crude oil and its subfractions are shown in Figure 3. Relative abundance is defined as the magnitude of each peak divided by the sum of magnitudes of all identified peaks, excluding the isotopic peaks in the mass spectrum. Even if the relative abundance of a given class species is the same for two samples, the absolute abundance of that class species may not be the same because it depends upon the abundances of other class species in each sample.52 No sulfur compounds were identified in the spectra as a published paper reported,49 (51) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. Kendrick mass defect spectrum: A compact visual analysis for ultrahigh-resolution broadband mass spectra. Anal. Chem. 2001, 73 (19), 4676–4681. (52) 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 2010, 24 (1), 563–569.

(53) Muller, H.; Pauchard, V. O.; Hajji, A. A. Role of naphthenic acids in emulsion tightness for a low total acid number (TAN)/high asphaltenes oil: Characterization of the interfacial chemistry. Energy Fuels 2009, 23 (3), 1280–1288.

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Figure 2. Negative-ion ESI FT-ICR mass spectra of the crude oil and its subfractions: aromatic, neutral (pyrrolic), amino, and pyridinic fractions.

Figure 4. Plots of DBE as a function of the carbon number for N1 class species from negative-ion ESI FT-ICR mass spectra of crude oil and its subfractions. No isotopic peaks were included. Figure 3. Relative abundances of heteroatom class species in the crude oil and its subfractions. No isotopic peaks were included.

with long alkyl side chains and/or multi-substitute homologues likely have a polarity similar to aromatic hydrocarbons. The neutral nitrogen fraction was enriched with neutral nitrogen compounds, which are highly condensed molecules with lower carbon numbers than that in the crude oil and aromatics. This suggests a highly biased distribution of neutral nitrogen in subfractions obtained by the two-step separation. The neutral nitrogen fraction only accounts for a fraction of all of the neutral nitrogen compounds in the crude oil. However, the biased distribution of neutral nitrogen compounds is consistent with the geochemical application that uses GC-MS to analyze nitrogen compounds with lower carbon numbers in the neutral nitrogen fraction as molecular markers for

These are most likely benzocarbazoles.25,28 The relative abundance distributions of N1 class species were quite different for the crude oil compared to its subfractions. The aromatic fraction contains N1 class species with relatively low DBE values of 9-16, in which the DBE value of 9 series was the most abundant. Neutral nitrogen compounds with a DBE value of 9 are carbazoles, which have 12-65 carbon numbers and exhibit a normal distribution for the carbon number. The relative abundance distribution pattern of N1 class species in this sample was similar to that of an aromatic fraction of a Liaohe crude oil.49 Carbazoles 6325

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: DOI:10.1021/ef1011512

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nitrogen fraction. Some of the neutral nitrogen compounds may also have been lost during the deasphalting step.49 The yield of nitrogen compounds was low, and the neutral nitrogen compounds had a different distribution in the crude oil and its subfractions. Because the composition of the subfraction varies and depends upon the activity, surface area, and pore size of the adsorbents used and the loading density of the columns, the results in this study may not be comparable to those obtained by Li et al.2 However, this study shows the characteristics of heteroatom composition of the crude oil and the dependency of analytical results of subfractions upon the two-step separation process. Therefore, if a low yield of low-molecular-weight neutral nitrogen compounds is obtained, it is necessary to examine the loss of neutral nitrogen compounds in the various chromatography fractions because this could lead to erroneous geochemical interpretations.

Figure 5. Gas chromatogram of the neutral nitrogen fraction of the crude oil.

tracking secondary oil migration. The GC-MS analysis is only capable of detecting low-molecular-weight nitrogen compounds.54 Figure 4 shows that the relative abundance of benzocarbazoles was much higher than that of carbazoles. This was different from the results obtained by GC-MS, as shown in Figure 5. Although the relative abundances of carbazoles and benzocarbazoles shown in Figure 5 are semiquantitative, they are representative for compounds with low molecular weight. Analyses were also performed using another FT-ICR MS with a higher superconducting magnet (Bruker Daltonics apex-ultra FT-ICR MS equipped with a 9.4 T superconducting magnet). Both results indicated that FT-ICR MS appeared to be biased toward compounds with a molecular weight greater than 200 Da. Low-molecular-weight compounds, such as carbazoles with short alkyl chains, are severely discriminated in the presence of high-mass compounds. This could be due to the inherent ion-transfer system of FT-ICR MS and/or the different volatility and ionization efficiencies of benzocarbazoles and carbazoles. As a result, negative-ion ESI FT-ICR MS has a relatively low response for low-molecular-weight carbazoles. The iso-abundance map of N1 class species in the amino nitrogen fraction is similar to that in the neutral nitrogen fraction. However, it exhibits slightly higher DBE values and lower carbon numbers (13-30). The pyridinic nitrogen fraction has N1 class species with DBE values of 11-13 and carbon numbers of 17-22 at a very low intensity (not shown). Neutral and basic nitrogen compounds were poorly separated in the amino nitrogen fraction. Given the low yield of carbazole at 32 wt %, the neutral nitrogen compounds that could not be accounted for are likely present in the amino

Conclusions Negative-ion ESI coupled to FT-ICR MS was used to characterize the neutral nitrogen composition of a crude oil and its subfractions obtained by the two-step open-column LC technique. The two-step LC technique produced a low yield of carbazole in the neutral nitrogen fraction. This separation technique also significantly affected the distributions of neutral nitrogen compounds in various functional subfractions of crude oil. Some of the neutral nitrogen compounds with relatively low DBE values of 9-16 and long alkyl side chains (12-65 carbon numbers) were eluted into the aromatic fraction. The composition and distribution of carbazoles in the neutral nitrogen fraction were different from those in the crude oil. Because the compounds detected by GC-MS analysis only account for a portion of total neutral nitrogen compounds, caution should be exercised in interpreting the data obtained from the two-step LC technique. The analytical bias of the separation technique, which was overlooked by previous workers, could lead to erroneous geochemical interpretations when a low yield of carbazole in the neutral nitrogen fraction was derived. Negative-ion ESI FT-ICR MS was an effective tool to monitor and evaluate the separation technique for neutral nitrogen compounds in crude oil. Acknowledgment. The authors thank Dr. Maowen Li (Geological Survey of Canada) for helpful discussion, Dr. Yongming Xie (Bruker Daltonics, Inc.) and Professor Changfeng Hu (Health Science Center of Peking University) for assisting in the FT-ICR MS analysis, and Mr. Shengbao Shi for assisting in the sample separation. This work was supported by the National Natural Science Foundation of China (40972097) and the National Basic Research Program of China (2010CB226901).

(54) Liu, C. J.; Zhang, G. L. Non-hydrocarbon Compounds in Petroleum and Its Productions; China Petrochemical Press: Beijing, China, 1991.

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