Distribution of Acids and Neutral Nitrogen Compounds in a Chinese

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Energy Fuels 2010, 24, 4005–4011 Published on Web 06/28/2010

: DOI:10.1021/ef1004557

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 Quan Shi,*,† Suoqi Zhao,*,† Zhiming Xu,† Keng H. Chung,‡ Yahe Zhang,† and Chunming Xu† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, People’s Republic of China, and ‡ Well Resources, Incorporated, 3919-149A Street, Edmonton, Alberta T6R 1J8, Canada Received April 12, 2010. Revised Manuscript Received June 12, 2010

A Chinese crude oil was distilled into multiple narrow boiling fractions. The crude oil, 39 narrow distillate fractions (up to 560 °C), and atmospheric and vacuum residues were analyzed using negative electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). The heteroatoms, N1, N2, N1O1, N1O2, O1, and O2 class species, were identified and characterized by double-bond equivalent (DBE) values and carbon numbers. The composition of crude oil was correlated with increased boiling point. Most abundant O1 and O2 class species had DBE values and carbon numbers corresponding to biological skeleton structures, such as hopanoic acid, secohopanoic acid, and sterol. The distribution of acids and neutral nitrogen compounds in the various fractions were determined. At higher carbon numbers, the amount of the compounds and DBE values increased gradually with the boiling point for most oil fractions. The abundant N1 class species were centered at DBE values of 9, 12, 15, and 18. These were likely pyrrolic compounds with various numbers of aromatic rings. Species such as hopanoic acids and secohopanoic acids were highly abundant in fractions above 500 °C. Sterol-like compounds were enriched in the 460-500 °C fractions. These are likely the major species causing a high total acid number (TAN) in the crude oil.

the naphthenic acids7 present and the interaction of naphthenic acids with other compounds present in the crude oil.6 A feedstock database must be developed with the molecular composition of naphthenic acids in the crude oil and its derived streams, because corrosion problems in refinery operations are specific to processing streams and operating conditions. In addition, the characterization of acidic compounds is of interest to geochemical research and refinery wastewater treatment.8 Characterization of the molecular composition of crude oil distillates has been extensively published. Snyder et al.9,10 characterized three distillate fractions of a Californian crude oil by infrared and ultraviolet spectroscopy and low-voltage electron ionization (EI) high-resolution mass spectrometry.

Introduction Organic acids, such as naphthenic acids, comprised only a small amount of total crude oil. However, these acidic components are of great concern to the petroleum industry, because they cause corrosion problems in refinery operations.1-3 Naphthenic acids also form naphthenate deposits and cause emulsion problems in oil production facilities.4 The total acid number (TAN) is a key criterion in crude specifications used to determine the corrosivity of crude oil. TAN is defined as the amount of potassium hydroxide (KOH) (in milligrams of KOH) required to neutralize all acidic species in 1 g of oil sample.5 Crude oils with a TAN greater than 0.5 mg of KOH/g can cause severe corrosion problems to refinery operations.3 However, TAN is not directly correlated to the corrosivity of naphthenic acids.6 It depends upon the size and structure of

(7) 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. (8) 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 (23), 3968–3976. (9) Snyder, L. R.; Buell, B. E.; Howard, H. E. Nitrogen and oxygen compound types in petroleum. Total analysis of a 700-850 °F distillate from a California crude oil. Anal. Chem. 1968, 40 (8), 1303–1317. (10) Snyder, L. R. Nitrogen and oxygen compound types in petroleum. Total analysis of an 850-1000 °F distillate from a California crude oil. Anal. Chem. 1969, 41 (8), 1084–1094. (11) Boduszynski, M. M. Composition of heavy petroleums. 2. Molecular characterization. Energy Fuels 1988, 2 (5), 597–613. (12) Altgelt, K. H.; Boduszynski, M. M. Composition of heavy petroleums. 3. An improved boiling point-molecular weight relation. Energy Fuels 1992, 6 (1), 68–72. (13) Altgelt, K. H.; Boduszynski, M. M. Composition of heavy petroleums. 3. An improved boiling point-molecular weight relations. Energy Fuels 1992, 6 (5), 679–679.

*To whom correspondence should be addressed. Telephone: 8610-89733738. Fax: 8610-6972-4721. E-mail: [email protected] (Q.S.); sqzhao@ cup.edu.cn (S.Z.). (1) Derungs, W. A. Naphthenic acid corrosion an old enemy of the petroleum industry. Corros. J. 1956, 12 (2), 41–46. (2) Qu, D. R.; Zheng, Y. G.; Jing, H. M.; Yao, Z. M.; Ke, W. High temperature naphthenic acid corrosion and sulphidic corrosion of Q235 and 5Cr1/2Mo steels in synthetic refining media. Corros. Sci. 2006, 48 (8), 1960–1985. (3) Slavcheva, E.; Shone, B.; Turnbull, A. Review of naphthenic acid corrosion in oilrefining. Corros. J. 1999, 34 (2), 125–131. (4) Mohammed, M. A.; Sorbie, K. S. Naphthenic acid extraction and characterization from naphthenate field deposits and crude oils using ESMS and APCI-MS. Colloids Surf., A 2009, 349 (1), 1–18. (5) American Society for Testing and Materials (ASTM). ASTM D 664: Standard Test Method for Acid Number of Petroluem Products by Potentiometric Titration; ASTM International: West Conshohocken, PA. 2004.  (6) Laredo, G. C.; L opez, C. R.; Alvarez, R. E.; Cano, J. L. Naphthenic acids, total acid number and sulfur content profile characterization in Isthmus and Maya crude oils. Fuel 2004, 83 (11-12), 1689–1695. r 2010 American Chemical Society

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and electrospray ionization (ESI). Stanford et al.28 present the acidic polar heteroatomic molecular class composition of three distillate fractions of a vacuum gas oil (VGO). Acidic and basic species were detected by high-resolution negative- and positive-ion ESI FT-ICR MS. In recent studies, Athabasca bitumen heavy vacuum gas oil (HVGO) was distilled into eight fractions and characterized by ESI FT-ICR MS.32 Furthermore, atmospheric pressure photoionization (APPI) was used to determine the distributions of nonpolar and polar species, as a function of the HVGO boiling point.33,34 The results showed a relatively high abundance of non-aromatic O2 class species present, distributed over all of the HVGO distillate fractions. The amount of O2 class species with increased DBE value and carbon number increased as the boiling point of the HVGO fractions increased.

Boduszynski et al. showed the variations in molecular weight, hydrogen deficiency, and heteroatom concentrations as functions of atmospheric equivalent boiling point. Fu et al.15 analyzed three vacuum gas oil fractions with an external EI 7 T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS). The results of studies performed to date show that gas oils and atmospheric residue of crude oils have a broad molecular-weight distribution, high molecular weights, and double-bond equivalence (DBE) values. The extent of alkylation in hydrocarbons increases with an increased boiling point. However, traditional mass spectrometry is not adequate to resolve all mass peaks derived from crude oil. In addition, heteroatoms present in trace amounts cannot be detected in the presence of abundant hydrocarbons within the limited dynamic range of a mass spectrometer using conventional ionization sources. Petroleomic researchers are attempting to correlate chemical properties and behavior with the molecular composition of fossil fuels.16-18 In recent years, FT-ICR MS,19,20 which has ultra-high resolving power and mass accuracy, has been used to characterize acidic and basic components of crude oils. Naphthenic acids were characterized by FT-ICR MS with various selective ionization sources, such as chemical ionization21

Experimental Section Sample Description. A crude oil sample from Bohai basin, China, was obtained from the PetroChina Dagang refinery and distilled using the American Society for Testing and Materials (ASTM) D1160 method into 39 narrow fractions. Distillate fractions were collected at initial boiling point (IBP)-65, 65-80, 80-100, and 100-350 °C with 10 °C increments, 350-395, 395425, 425-450, 450-460, and 460-520 °C with 20 °C increments, and 520-560 °C with 10 °C increments, respectively. A total of 1 mL of each narrow boiling oil sample was used for MS analysis. The remainder of each sample was combined into 26 fractions (Table 1), which were subjected to TAN analysis. Table 1 lists the properties of the crude oil and its distillate fractions. The crude oil contained 0.69 wt % heptaneinsoluble (n-C7) asphaltenes, 0.20 wppm vanadium, and 26.2 wppm nickel (analyzed by atomic absorption spectrometry). The total nitrogen and sulfur content were 0.31 and 0.19 wt %, respectively. GC and GC-MS Analysis. The crude oil sample was subjected to fractionation to obtain saturates, aromatics, resins, and asphaltenes (SARA) (Chinese Standard Analytical Method

(14) Boduszynski, M. M.; Altgelt, K. H. Composition of heavy petroleums. 4. Significance of the extended atmospheric equivalent boiling point (AEBP) scale. Energy Fuels 1992, 6 (1), 72–76. (15) Fu, J. M.; Kim, S.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G.; Qian, K. N. 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. (16) Marshall, A. G.; Rodgers, R. P. Petroleomics: The next grand challenge for chemical analysis. Acc. Chem. Res. 2004, 37 (1), 53–59. (17) Rogers, R. P.; Schaub, T. M.; Marshall, A. G. Petroleomics: MS returns to its roots. Anal. Chem. 2005, 77 (1), 20A–27A. (18) Marshall, A. G.; Rodgers, R. P. Petroleomics: Chemistry of the underworld. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (47), 18090–18095. (19) 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. (20) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Fourier transform ion cyclotron resonance mass spectrometry: A primer. Mass Spectrom. Rev. 1998, 17, 1–35. (21) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Naphthenic acids in crude oils characterized by mass spectrometry. Energy Fuels 2000, 14 (1), 217–223. (22) 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. (23) 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. (24) 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. (25) 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. (26) 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. (27) 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.

(28) 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. (29) 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. (30) 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 negativeion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry: The effect of suppression. Energy Fuels 2007, 21 (1), 266–273. (31) Mapolelo, M. M.; Juyal, P.; Rodgers, R. P.; Yen, A. T.; Debord, J. D.; Asomaning, S.; Marshall, A. G. Electrospray Ionization FT-ICR Mass Spectrometry of “ARN” Naphthenic Acids in Sodium and Calcium Naphthenate Deposits and Crudes: Characterization, Extraction and Quantification; American Institute of Chemical Engineers (AICE): New Orleans, LA, 2008. (32) 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. (33) McKenna, A. M.; Purcell, J. M.; Rodgers, R. P.; Marshall, A. G. Heavy petroleum composition. 1. Exhaustive compositional analysis of Athabasca bitumen HVGO distillates by Fourier transform ion cyclotron resonance mass spectrometry: A definitive test of the Boduszynski model. Energy Fuels 2010, 24 (5), 2929–2938. (34) McKenna, A. M.; Blakney, G. T.; Xian, F.; Glaser, P. B.; Rodgers, R. P.; Marshall, A. G. Heavy petroleum composition. 2. Progression of the Boduszynski model to the limit of distillation by ultrahigh-resolution FT-ICR mass spectrometry. Energy Fuels 2010, 24 (5), 2939–2946.

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MS has been described elsewhere. A sample solution was injected into the electrospray source at 150 μL/h using a syringe pump. The operating conditions for negative-ion formation consisted of a 4.0 kV emitter voltage, 4.5 kV capillary column introduce voltage, and -320 V capillary column end voltage. Ions accumulated in the ion source for 0.1 s in a hexapole. All of the ions passed through a single quadrupole, accumulated in an argonfilled hexapole collision pool, in which ions accumulated for 1 s. The delay was set to 1.2 ms to transfer the ions from the collision pool to an ICR cell by electrostatic focusing of transfer optics. The mass range was set at m/z 200-1000. The data size was set to 2 M words, and time-domain data sets were co-added with 256 acquisitions. Mass Calibration and Data Analysis. The FT-ICR mass spectra were internally calibrated for a mass range of 150-1000 using a sodium formate aqueous solution and recalibrated with the alkylcarbazole mass series from the mass spectra of the crude oil. The alkylcarbazole mass series is commonly present in relatively high abundance in negative-ion ESI mass spectra.35 The m/z values between 200 and 800 Da with relative abundance greater than 6 times the standard deviation of the baseline noise value were exported to a spreadsheet. Data analysis was performed using custom software.36 Measured masses were converted from the International Union of Pure and Applied Chemistry (IUPAC) mass scale to the Kendrick mass scale. The Kendrick mass defect (KMD) was calculated. Molecular formulas of two neighboring even and odd normal masses were assigned on the basis of mass measurement to (1.5 ppm. Formulas were also confirmed/eliminated unequivocally by the presence/absence of the corresponding nuclide containing one 13C. For an assigned class species, compound types with various DBE values were identified by the difference of integer multiples of H2. The range of DBE values of 0-50 was allowed. Members of a homologous series differ by integer multiples of CH2. Each homologous series was identified by the assigned single members, with an additional limit of KMD tolerance of 0.0015. For each series, elemental compositions were assigned by a mass calculator program limited to molecular formulas consisting of up to 100 12C, 2 13C, 200 1H, 2 14N, 5 16O, 2 32S, and 1 34S atoms.

Table 1. Properties of the Crude Oil and Its Distillation Cuts boiling ID point (°C) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

crude oil IBP-65 65-80 80-100 100-120 120-140 140-160 160-180 180-200 200-220 220-240 240-260 260-280 280-300 300-320 320-340 340-350 350-395 395-425 425-450 450-460 460-480 480-500 500-520 520-540 540-560 >560

weight percent 0.62 0.33 1.03 1.04 1.29 1.45 1.51 1.49 1.93 2.08 2.55 2.70 2.97 3.22 3.14 1.97 6.18 5.98 5.03 5.95 3.08 3.09 1.58 3.85 3.25 32.60

weight percent density at TAN (mg of (cumulative) 20 °C (g/cm3) KOH/g) 0.62 0.95 1.98 3.02 4.31 5.76 7.27 8.76 10.69 12.77 15.32 18.02 20.99 24.21 27.35 29.32 35.50 41.48 46.51 52.46 55.54 58.63 60.21 64.06 67.31 99.91

0.8882 0.6641 0.7060 0.7294 0.7419 0.7563 0.7675 0.7815 0.7956 0.8162 0.8265 0.8380 0.8462 0.8511 0.8471 0.8599 0.8653 0.8645 0.8787 0.8957 0.9038 0.9095 0.9111 0.9049 0.9067 0.9115 0.9294

0.91 0.037 0.024 0.025 0.010 0.012 0.018 0.070 0.078 0.10 0.11 0.26 0.27 0.28 0.30 0.59 0.66 0.70 0.86 1.18 1.67 0.77 0.72 0.99 0.20

for Petroleum and Natural Gas Industry, SY/T 5119-2008).35 A Thermo-Finnigan Trace GC 2000 coupled with a DSQ mass detector and flame ionization detector (FID) was used to analyze the composition of the crude oil and its saturate and aromatic fractions. The mass spectrometer was equipped with an electron impact (EI) source at 70 eV ionization energy and set to scan from 35 to 500 Da in 1 s. A HP-5MS (60 m0.25 mm0.25 μm) fused silica capillary column was used for GC-MS analysis. The oven temperature for GC-MS analysis of saturates was held at 50 °C for 1 min, increased to 120 °C at 20 °C/min, then increased to 250 °C at 4 °C/min, further increased to 310 °C at 3 °C/min, and held constant at 310 °C for 30 min. The oven temperature for GC-MS analysis of aromatics was held at 50 °C for 1 min, increased to 120 °C at 20 °C/min, further increased to 300 °C at 3 °C/min, and held constant at 300 °C for 25 min. The injector was maintained at 300 °C in split-less mode. The injection volume was 1 μL. The sample concentration was 8 mg/mL. A HP-1 (60 m 0.25 mm 0.25 μm) fused silica capillary column was also used for GC analysis. The oven temperature for GC analysis was held at 50 °C for 10 min, increased to 120 °C at 2 °C/min, increased to 300 °C at 4 °C/min, and held constant at 300 °C for 30 min. The operating conditions for the injector were the same as that for GC-MS analysis. The detector temperature was at 300 °C. FT-ICR MS Analysis. The crude oil and its distillate fractions were dissolved in toluene to produce a 10 mg/mL solution for ESI FT-ICR MS analysis. A total of 20 μL of the solution was further diluted with 1 mL of toluene/methanol (1:1, v/v) solution. All solvents were analytical-reagent-grade, which were distilled twice and kept in a glass bottle. Glassware was used for solvent handling and transfer, except for the steel pistons of 10 μL Hamilton syringes. The MS analysis were performed using a Bruker Apex ultra FT-ICR MS equipped with a 9.4 T superconducting magnet. The operating procedure for the negative ESI analysis by FT-ICR

Results and Discussion Bulk Crude Oil Sample. Figure 1 shows chromatograms for the crude oil. The bottom and top diagrams are total ion chromatograms of the saturate and aromatic fractions, respectively. The two mass chromatograms in the middle are those for the hopanes and steranes. The inset in the middle left of Figure 1 shows the FID gas chromatogram of crude oil. The results shown in Figure 1 indicate that this crude oil is different from other high TAN crude oils reported.26,37 The presence of highly abundant C27 20R and C29 20R steranes and the low odd carbon number predominance indicates that the crude oil is relatively immature in nature. Abundant longchain normal alkanes are characteristics of a paraffinic crude oil. The compound 25-norhopane, which is often used as an indicator for crude oil biodegradation,38 was not detected. This suggests that the crude oil has not been severely biodegraded. However, a TAN value of 0.91 is considered high (36) 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. (37) Li, M.; Cheng, D.; Pan, X.; Dou, L.; Hou, D.; Shi, Q.; Wen, Z.; Tang, Y.; Achal, S.; Milovic, M.; Tremblay, L. Characterization of petroleum acids using combined FT-IR, FT-ICR-MS and GC-MS: Implications for the origin of high acidity oils in the Muglad Basin, Sudan. Org. Geochem. 2010, in press. (38) Peters, K. E.; Moldowan, J. M.; McCaffrey, M. A.; Fago, F. J. Selective biodegradation of extended hopanes to 25-norhopanes in petroleum reservoirs. Insights from molecular mechanics. Org. Geochem. 1996, 24 (8), 765–783.

(35) 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, 2545–2553.

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Figure 1. Chromatograms of the crude oil. C2-Na, C2-naphthalene; P, phenanthrene; MP, methyl phenanthrene; Pr, pristane; Ph, phytane. The chromatogram of the crude oil in the middle left is obtained with a FID.

for non-biodegraded crude oils. As a results, the acidic compounds in this crude oil may not be the same as those found in the biodegraded oils. The relatively low concentration of vanadium and a high ratio of nickel/vanadium in the crude indicates that the crude oil is likely derived from continental source rock.39 Acidic Species Identified in the Crude Oil and Distillate Residues. Figure 2 shows the negative ESI FT-ICR MS spectra of the crude oil, its atmospheric residue (AR), and vacuum residue (VR). The crude oil and its AR have lower and narrower mass distributions (200 < m/z < 650) than the VR (300 < m/z < 1000). The distribution peak of the crude oil and its AR were centered at m/z 400 and 450, respectively. While the mass distributions of molecules in the crude oil and its AR are similar, the mass distributions of molecules in the crude oil and the VR are different. The mass spectra in Figure 2 suggest that the acidic compounds in the crude oil are concentrated in the AR. Once the light fraction was separated, large molecular acidic compounds show high relative abundance in the mass spectrum. Hence, the large molecular acidic compounds present at low concentrations can be easily identified. The inset in Figure 2 is the close-up view of the expanded mass spectrum obtained under a mass resolving power of 250 000 (m/Δm 50% at m/z 350). For the crude oil, more than 5601 peaks (>6σ of baseline noise) at 200-800 Da were detected, of which 3905 were assigned with the molecular formulas by exact masses. The mass resolving power (m/Δm 50%) was not sufficient to resolve all of the species present in the crude oil. For example, the mass difference of 13C1C22H30 and C20H33O3 is 1.7 mDa. The mass of N1 class species with a 13C

Figure 2. Negaive-ion electrospray FT-ICR mass spectra of the crude oil (top), its AR (middle), and VR (bottom). The inset shows the expanded 350 mDa mass scale at m/z 415, indicating the complexity of the acidic compound composition.

atom is close to that of O3 class species, Hence, these two series of class species tend to overlap as the molecular mass increases. As well, the O3 class species have a very low relative abundance at the low mass ranges (m/z value lesser than 380), and this may also be the case for the high mass range. To avoid ambiguity, the O3 class species were not identified as part of this work. The class species derived from the spectra of the crude oil and its residue were N1, N2, N1O1, N1O2, O1, and O2. The relative abundance of each class species with various DBE values are presented in Figure 3.

(39) Peters, K. E.; Walters, C. C.; Moldown, J. M. The Biomarker Guide; Cambridge University Press: New York, 2005; Vol. 2, p 88.

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acids and/or aromatic acids. The data in Figure 5 show that the high abundances of species are at DBE values of 5 and 6 and a carbon number of 31. Crude oils containing 4 and 5 ring naphthenes with special biologic skeleton structures, such as hopane and sterane, are commonly used as biomarkers in the field of geochemical science.41 The species with a DBE value of 6 are likely hopanoic acids, and the species with a DBE value of 5 are likely secohopanoic acids, because carboxylic steranes have not been identified in crude oils.41,42 Species with a DBE of 5, 6, or higher values and with low carbon numbers are likely aromatic acids. A distribution map of various acid types is shown in Figure 5. The N1 class species are another key species shown in negative ESI mass spectra. Figure 6 shows the iso-abundance maps of DBE as a function of the carbon number for the N1 class species in the 530-540 °C distillate fraction of the crude oil. The N1 class species are abundant at DBE values of 12-15 series. These are likely benzocarbazoles and dibenzocarbazoles. The N1 class species with a DBE value of 9 are likely carbazoles. The data in Figure 6 shows an inverse correlation between DBE and the carbon number. This indicates that the species with high DBE values and low carbon number have a similar boiling point to those with low DBE values and high carbon numbers. The abundant N1O1 and N2 class species have high DBE values, indicating that these are highly aromatic compounds. The O1 class species are concentrated at a DBE value of 4. The relative abundance of O1 class species decreases gradually as the DBE value increases. It is likely that the O1 class species are phenolics instead of hydroxyl compounds. However, this does not explain the abundance pattern of species with DBE values of 4 and 5 and carbon numbers of 27 and 28. These compounds are likely cholesterols and 5,6-dihydrocholesterols, coincident with the origin of the crude oil, which is associated with Cenozoic source rock. The presence of abundant O2 class species, such as hopanoic acids, with special biologic skeleton structures also supports this deduction. Acidic Species Distribution in Narrow Distillate Fractions. O1 and O2 Class Species. Table 1 shows that the distillate fractions at 200-280 °C and VR have low TAN. For the distillate fractions above 300 °C, TAN increased with an increasing boiling point and reached a maximum value at 480500 °C. A similar study was performed on Athabascabitumen-derived heavy virgin gas oil fractions.32 Qian et al.43 demonstrated that ESI-MS can be used as a means for rapid and micro-scale measurement of TAN and TAN boiling point distributions for petroleum crude and products. A relationship between the TAN and mass response was proposed for high TAN (>0.9) crude oils. It is difficult to obtain an adequate FT-ICR MS mass spectrum for acidic species in distillate fractions below 250 °C, because of the low concentrations of acidic species and the

Figure 3. Heteroatom class (number of heteroatoms) and type (DBE) distribution derived from negative-ion ESI FT-ICR mass spectrum of the crude oil (top), its AR (middle), and VR (bottom).

The N1, O1, and O2 class species are dominant in the negativeion ESI mass spectra. Figure 4 shows the iso-abundance map of DBE as a function of the carbon number for N1, O1, and O2 class species in the crude oil, AR, and VR. Each class species is distributed over a wide range of DBE values and carbon numbers. The abundant O2 class species are at DBE of 1-3 and 5-6, whereas the N1 class species shift to higher DBE values of 9, 12, and 15. Structural Information of Acidic Species. While an accurate molecular mass allows for the assignment of a unique elemental composition to each peak in the mass spectrum, this does not provide the molecular structure of the species. Negative-ion ESI selectively ionizes the acidic components that contain functional groups, such as carboxylic acids, phenols, and neutral pyrrolic nitrogen. In combination with the molecular elemental composition determined by FT-ICR MS, molecular structures of the species can be obtained along with the DBE values and carbon numbers. Acidic class species corresponding to the specific mass spectra have been discussed elsewhere.35 Figure 5 shows the iso-abundance maps of DBE as a function of the carbon number for O2 class species in the 530540 °C distillate fraction of the crude oil. The O2 class species are distributed over a wide range of DBE and carbon number, suggesting that the molecular structures of individual O2 class species are significantly different. The most common O2 class species are carboxylic acids, even though the hydroxyl O1 class species are also present in the negative ESI mass spectrum. In Figure 4, the O2 class species with DBE of 1 are the fatty acids. In Figure 5, the relative abundance value for C16 fatty acid is not definite, because C16 and C18 fatty acids act as contaminants in the negative ESI analysis of samples with lower molecular polarity, such as crude oil.30 Species with DBE values of 2-7 are naphthenic acids with 1-6 naphthenic rings, which have been identified by GC-MS, and MS with various ionization techniques.21,24,40 Species with higher DBE values are likely multi-ring naphthenic

(41) Jaffe, R.; Gallardo, M. T. Application of carboxylic acid biomarkers as indicators of biodegradation and migration of crude oils from the Maracaibo Basin, Western Venezuela. Org. Geochem. 1993, 20 (7), 973–984. (42) Nascimento, L. R.; Rebouc-as, L. M. C.; Koike, L.; de A.M Reis, F.; Soldan, A. L.; Cerqueira, J. R.; Marsaioli, A. J. Acidic biomarkers from Albacora oils, Campos Basin, Brazil. Org. Geochem. 1999, 30 (9), 1175–1191. (43) Qian, K.; Edwards, K. E.; Dechert, G. J.; Jaffe, S. B.; Green, L. A.; Olmstead, W. N. Measurement of total acid number (TAN) and TAN boiling point distribution in petroleum products by electrospray ionization mass spectrometry. Anal. Chem. 2008, 80 (3), 849–855.

(40) Jones, D. M.; Watson, J. S.; Meredith, W.; Chen, M.; Bennett, B. Determination of naphthenic acids in crude oils using nonaqueous ion exchange solid-phase extraction. Anal. Chem. 2001, 73 (3), 703–707.

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Figure 4. Plots of DBE versus the carbon number for O2, N1, and O1 class species from the negative-ion ESI FT-ICR MS of the crude oil, its AR, and VR. The DBE values and carbon numbers refer to the compounds in their neutral states.

Figure 5. Plots of DBE versus carbon number for O2 class species from the negative-ion ESI FT-ICR mass spectra of the 530-540 °C distillate cut.

Figure 6. Plots of DBE versus the carbon number for N1 class species from the negative-ion ESI FT-ICR mass spectra of the 530-540 °C distillate cut.

discrimination of ions at the low mass range for the FT-ICR MS.44 The iso-abundance maps of DBE as a function of the carbon number for the O2 class species in the distillate fractions above 250 °C of the crude oil shown in Figures S1 and S2 in the Supporting Information). The abundance of O2 class species shift to higher DBE values and carbon numbers

as the boiling point of the distillate fraction increases. For the distillate fractions below 300 °C, the most dominant O2 class species are C12-C16 carboxylic acids with 1-2 naphthenic rings, followed by fatty acids. The narrow distillate fractions in sequence at increased temperature ranges correspond to an increment of 2-3 carbon numbers. For the distillate fraction above 300 °C, the O2 class species are likely multi-ring naphthenic acids and/or aromatic carboxylic acids. Hopanoic and secohopanoic acids are likely in the distillate fractions between 500 and 560 °C.

(44) 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.

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The iso-abundance maps of DBE as a function of the carbon number for O1 class species in a few selected distillate fractions are shown in Figure S3 in the Supporting Information. The relative abundance of the O1 class species with DBE values greater than 5 exhibits a normal distribution. However, the relative abundance of the O1 class species with DBE values of 4 and 5 decreases and then increases as the carbon number increases. The O1 class species with 27 and 28 carbon atoms have high relatively abundance in the 450-460 °C fraction and are further enriched in the 460-480 and 480500 °C fractions. This coincides with a relatively high TAN of 460-500 °C distillate fractions. Because negative ESI is less effective for ionization of hydroxyl compounds compared to carboxyl acids,23 the high abundance of O1 class species are likely due to high concentrations of sterol-like compounds in the crude oil. Therefore, high TAN in the 450-500 °C distillate fractions is primarily due to the sterollike compounds in the crude oil. N1 Class Species. The iso-abundance maps of DBE as a function of the carbon number for N1 class species in distillate fractions are shown in Figure S4 in the Supporting Information. The N1 class species were not detected in the distillate fraction below 300 °C. Methylcarbazoles and dimethylcarbazoles are dominant in 300-320 °C distillate fractions, and benzocarbazoles are dominant in 350460 °C fractions. Dibenzocarbazoles are in the distillate fraction above 450 °C, and highly aromatic N1 class species are in distillate fractions above 500 °C. The N1 class species with a DBE value of 18 are benzonaphthocarbazoles, which have the highest relative abundance. These species have not been previously identified using GC or GC-MS techniques.

Conclusions Detailed molecular compositions of acids and neutral nitrogen compounds in a Chinese crude oil, its distillate fractions, and residue were characterized by negative ESI FT-ICR MS. The most abundant O1 and O2 class species correspond to biologic skeleton structures, such as hopanoic acids, secohopanoic acids, and sterols. The distributions of acidic compounds in various distillate fractions were determined. Most of the acidic compounds were present in the vacuum distillate fractions. The distribution of acidic compounds at higher carbon numbers and DBE values increased gradually with the boiling point of the oil fraction. The abundant N1 class species were centered at DBE values of 9, 12, 15, and 18; these were likely pyrrolic compounds with various numbers of aromatic rings. It seems that compounds, such as hopanoic acids and secohopanoic acids, were highly abundant in the more than 500 °C fractions. Sterol-like compounds were enriched in the 460-500 °C fractions. These were likely the major species causing high TAN in the crude oil. Acknowledgment. This work was supported by the National Basic Research Program of China (2010CB226901). Supporting Information Available: Plots of DBE versus carbon number for O2 class species from the negative-ion ESI FT-ICR mass spectra of 250-425 °C distillate cuts (Figure S1), plots of DBE versus carbon number for O2 class species from the negative-ion ESI FT-ICR mass spectra of 340-560 °C distillate cuts (Figure S2), plots of DBE versus carbon number for O1 class species from the negative-ion ESI FT-ICR mass spectra of 395-550 °C distillate cuts (Figure S3), and plots of DBE versus carbon number for N1 class species from the negative-ion ESI FT-ICR mass spectra of 310-560 °C distillate cuts (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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