Molecular Characterization of Vacuum Resid and Its Fractions by

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Molecular Characterization of Vacuum Resid and Its Fractions by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry with Various Ionization Techniques Yahe Zhang,†,‡ Linzhou Zhang,† Zhiming Xu,† Na Zhang,† Keng H. Chung,§ Suoqi Zhao,† Chunming Xu,† and Quan Shi*,† †

State Key Laboratory of Heavy Oil Processing, and ‡College of Science, China University of Petroleum, Beijing 102249, People’s Republic of China § Well Resources, Incorporated, 3919-149A Street, Edmonton, Alberta T6R 1J8, Canada S Supporting Information *

ABSTRACT: Venezuela Orinoco extra-heavy-crude-oil-derived vacuum resid (VR) was subjected to supercritical fluid extraction and fractionation (SFEF) to prepare multiple narrow fractions. The SFEF fractions were analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with various ionization techniques, including positive-ion electrospray ionization (ESI), negative-ion ESI, positive-ion atmospheric pressure photoionization (APPI), and sulfur methylation followed by positive-ion ESI. The results showed that the SFEF separates the VR species by their molecular weights and degrees of molecular condensation. The mass ranges of compounds determined by various ionization techniques were comparable. The FT-ICR MS data were in agreement with the elemental analysis and molecular weight determined by gel permeation chromatography (GPC) and vapor pressure osmometry (VPO) for the extractable fractions. The molecular compositions of SFEF fractions determined by FT-ICR MS provide important clues for the understanding of the molecular composition for the unextractable end-cut (asphaltenes). Each ionization technique favors identification of certain compounds in heavy petroleum fractions and discriminates against others. APPI allows for a general overview of species present in heavy petroleum fractions, because of its ability to ionize a wide range of species. ESI is more selective toward polar species. A thorough characterization of species in heavy petroleum fractions cannot be achieved by using an ionization technique; however, it can be performed by combining various ionization techniques.

1. INTRODUCTION Heavy petroleum fractions are mixtures comprising millions of ill-defined complex hydrocarbon compounds.1 Hence, molecular characterization of heavy petroleum fractions is a challenging topic for petroleum chemists. It is unrealistic to expect individual compounds to be analyzable in a heavy petroleum, such as vacuum distillate resid (VR). A general approach for heavy petroleum compositional analysis is using pre-separation followed by high-resolution mass spectrometry analysis. Effective separation is essential to minimize interference components and facilitate improving the identification significantly. Supercritical fluid extraction fractionation (SFEF) with light alkanes is a choice for this purpose, which has been developed by the State Key Laboratory of Heavy Oil Processing (SKLHOP) in China.2 Scientists in SKLHOP have investigated dozens of heavy petroleum by the home-built SFEF instruments in past 2 decades. Heavy petroleum in the kilogram scale was separated into about a dozen extractable fractions and one end-cut, which facilitates the compositional or reaction behavior study on heavy petroleum and its fractions. Recent advances in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) enable the molecular analysis of complex hydrocarbon systems, including heavy petroleum residua, and determination of molecular formulas by accurate mass measurements.3−11 However, FT© 2014 American Chemical Society

ICR MS analysis requires the use of multiple ionization techniques to ionize and detect various types of components.3,4,9,12−19 The reported FT-ICR MS data were scattered and not systematic; some results, such as molecular weights of compounds in heavy petroleum fractions, were inconsistent.20−26 In this paper, we use FT-ICR MS with different ionization techniques to analyze a VR and its SFEF fractions. The major purpose is to investigate the molecular composition of heavy petroleum in different points of view and discuss the reliability of FT-ICR MS results for heavy petroleum analysis.

2. EXPERIMENTAL SECTION 2.1. Vacuum Resid (VR) and Its SFEF Fractions. The feedstock used in this work was a VR (500 °C+ material) derived from Venezuela Orinoco extra-heavy crude oil. The VR was subjected to SFEF separation to prepare 13 extractable narrow fractions (SFEF1, SFEF2, ..., and SFEF13) and an unextractable end-cut. The supercritical solvent was normal pentane, and the pressure was programmed from 4 to 12 MPa with an increasing gradient of 1.0 MPa/h. The temperatures were 230, 240, and 250 °C for the bottom, center, and top of the extraction tower, respectively. The detailed operation of SFEF has been described elsewhere.2 The VR, SFEF1, Received: September 24, 2014 Revised: November 24, 2014 Published: November 30, 2014 7448

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Table 1. Yields and Bulk Properties of the VR and Its SFEF Fractions Na (wt %) Ob (wt %) Sc (wt %) Hd (wt %) Cd (wt %) Nie (wppm) Ve (wppm) CCRf (wt %) density at 20 °Cg (g/cm3) extract pressure (MPa) yield (wt %) saturatesi (wt %) aromaticsi (wt %) resinsi (wt %) asphaltenesi (wt %)

VR

SFEF1

SFEF4

SFEF7

SFEF10

SFEF13

end-cut

0.98 1.60 4.80 9.70 82.7 175.5 751.7 26.2 1.05

0.47 1.41 3.40 11.1 83.8 4.0 21.0 4.6 0.97 6 5.24 50.51 33.62 14.92 0.95

0.50 1.30 3.90 10.9 83.8 6.7 20.2 7.8 0.98 7.2 5.08 21.4 61.03 17.32 0.24

0.58 1.58 4.40 10.5 83.7 22.6 78.5 12.1 1.01 8.2 5.06 7.37 65.72 26.71 0.19

0.81 1.65 4.90 10.1 83.1 66.4 253 18.4 1.02 9.6 4.62 0.52 59.60 39.60 0.27

1.07 1.48 5.20 9.5 82.6 145 605 28.8 1.05 12 3.48 0 31.50 67.27 1.23

1.78 1.62 5.70 8.50 83.0 313 1383 49.3 0.44h

7.98 35.38 40.70 15.94

34.38 1.56 7.82 21.85 68.77

a

ASTM D5762 (Antek 7000 elemental analyzer). bASTM D5622 (flash EA 1112 analyzer). cASTM D5453 (Antek 7000 elemental analyzer). ASTM D5291 (flash EA 1112 analyzer). eASTM D5708 (Vista-PRO simultaneous ICP−OES). fASTM D189 (Shanghai Yutong Instrument YT30011). gASTM D1480 (pyonometer). hThe density is the stacking density. iASTM D2007-11.

d

Figure 1. Broadband FT-ICR mass spectra of VR and its SFEF fractions with various ionization techniques (−ESI, negative-ion ESI; +ESI, positiveion ESI; +MeESI, S methylation followed by positive-ion ESI; and +APPI, positive-ion APPI). SFEF4, SFEF7, SFEF10, SFEF13, and end-cut were subjected to elemental and bulk property analyses and FT-ICR MS analysis.27 The description and bulk property analysis of the VR and its SFEF fractions are shown in Table 1. 2.2. Methylation of Sulfur Compounds and Electrospray Ionization (ESI) FT-ICR MS Analysis. The oil sample (100 mg) was diluted with 2 mL of dichloromethane (CH2Cl2). The diluted solution was added with 50 μL of methyl iodide and 2 mL of 0.2 mol/L silver tetrafluoroborate in 1,2-dichloroethane solution. The mixture solution in a beaker was covered with an aluminum foil and immersed in an ultrasonic bath for 5 min and then allowed to react in the dark at room temperature for 24 h. The reaction was repeated 3 times to enhance the conversion. The solvents remaining in the reaction system were

evaporated under a nitrogen atmosphere. Toluene was used to remove the unreacted oil and obtain the methylsulonium salts. Methylsulfonium salts (10 mg) were diluted with 1 mL of CH2Cl2. The methylsulfonium salt solution (2 μL) was diluted with 1 mL of toluene/methanol/CH2Cl2 (3:3:4) solution prior to positive-ion ESI analysis.28,29 2.4. Positive-Ion Atmospheric Pressure Photoionization (APPI) FT-ICR MS Analysis. The oil sample was dissolved in toluene to yield a 10 mg/mL solution. The diluted oil was further diluted with toluene to yield a 0.4 mg/mL solution. The diluted oil sample was injected into a spray needle with a syringe pump at 180 μL/h. The APPI source was purchased from Bruker Daltonics. Nitrogen was used as the drying and nebulizing gas. The nebulizing gas temperature was 350 °C for the VR and its lighter extractable fractions and 400 °C for 7449

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Figure 2. Compound class distribution in VR and its SFEF fractions from FT-ICR mass spectra with various ionization techniques. SFEF13 and the end-cut, respectively. The nebulizing gas flow rate was at 1 L/min. The drying gas was 200 °C at 2 L/min. The MS analysis was performed using a Bruker Apex-ultra FT-ICR mass spectrometer equipped with a 9.4 T superconducting magnet. The capillary column end voltage was 320 V. The skimmer voltage was set to 30 V. Ions were accumulated for 0.001 s in a hexapole with 2.4 V direct current (DC) voltage and 500 Vp−p radio-frequency (RF) amplitude. The optimized mass for Q1 was 200 Da. An argon-filled hexapole collision pool was operated at 5 MHz and 1000 Vp−p RF amplitude, in which ions accumulated for 0.001 s. The extraction period for ions from the hexapole to the ICR cell was set to 1.4 ms. The RF excitation was attenuated at 11 dB and used to excite ions over the range of 250−1200 Da. Spectra comprising 4 MW data points were collected. The signal-to-noise (S/N) ratio was enhanced by summing 256 time domain transients. Mass spectra were internally calibrated using an extended homologous alkylation series (molecular ion of aromatic hydrocarbons and thiophenes) of high relative abundance in a mixed heavy oil within the mass range of 300−1000 Da. The typical mass resolving power, m/ Δm50% > 600 000, at m/z 400 with 0.4 ppm mass error, was achieved. Mass spectrum peaks with a relative abundance greater than 6 times the standard deviation of the baseline noise were exported to a spreadsheet. Data analysis was performed using a custom software. The detail of data processing has been described elsewhere.29,30

compounds detected by negative-ion ESI exhibited different m/ z distribution patterns for various SFEF fractions. As the SFEF fraction became heavier, the mass range became broader and shifted to a larger value. Peaks in positive-ion ESI spectra were mainly basic nitrogen compounds, which exhibited similar mass distribution as that in negative-ion ESI spectra. However, the variation of mass range shifting was not distinct. A oligomeric series of compounds found in heavy fractions and the end-cut were identified as contaminants (emulsifying agent).27 The sulfur compounds formed sulfonium cations and facilitated the ionization for positive-ion ESI analysis. Figure 1 also shows the S-methylation (+MeESI) spectra of VR and its SFEF fractions. The mass range of +MeESI spectra was close to that of positive-ion ESI for each SFEF fraction, indicating that sulfur compounds and basic nitrogen compounds had a similar molecular mass range in the VR. It is known that the majority of sulfur and basic nitrogen compounds in heavy petroleum fractions are thiophenes and pyridines, respectively,17,28,29,31 which are aromatic heteroatom compounds with similar molecular skeleton structures. The abundant peaks in the negative-ion ESI mass spectra of “light” SFEF fractions were naphthenic acids, which had relatively low molecular weights compared to the nitrogen- and sulfur-containing compounds, and were easily extracted from the VR by supercritical pentane. Hydrocarbons are usually the dominant components of petroleum fractions; however, ESI is not capable of ionizing neutral hydrocarbons. APPI is a technique of choice for heavy petroleum characterization, which ionizes both polar and

3. RESULTS AND DISCUSSION 3.1. FT-ICR MS with Various Ionization Techniques. Figure 1 shows the FT-ICR mass spectra of VR and its SFEF fractions with various ionization techniques. Positive- and negative-ion ESI results were obtained from a recent publication.27 Acidic oxygen compounds and neutral nitrogen 7450

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Figure 3. Expanded mass scale spectra of positive-ion APPI FT-ICR MS at m/z 499.

corresponded to hydrocarbons and sulfur- and nitrogencontaining compounds. Even though various class species were identified by FT-ICR MS with various ionization techniques, the compositions of the corresponding compounds are different. Negative-ion ESI ionizes acidic compounds and some neutral nitrogen compounds, such as carboxylic acids (O2), phenolic compounds (O1), and pyrrole compounds (N1). Positive-ion ESI ionizes basic nitrogen compounds, sulfoxides, and sulfones. Positive-ion MeESI ionizes methylsulfonium salts and basic nitrogen compounds, which are not separated from the salt solution or in the form of its methylation derivatives. Positiveion APPI could ionize most components in the heavy petroleum, including hydrocarbons and heteroatoms, except for compounds with a low DBE value, such as linear paraffins. Regardless of the ionization efficiency for various class species, the APPI results shown in Figure 2 revealed an overall perspective of compounds in the VR and its fractions. The nonpolar species, HC and Sx compounds, were dominant. These species cannot be detected in conventional ESI analysis. The relative abundance of S1, S2, and S3 class species determined by APPI and the differences of relative abundance among these fractions were generally in agreement with those by +MeESI. This suggested that the compositions of sulfur compounds in the methylsulfonium salts determined by +MeESI are representatives of those in the parent oil sample. In other words, the analytical ability of positive-ion APPI for sulfur-containing compounds is comparable to that of +MeESI. The O2X class species, which were the abundant species containing a carboxylic acid function group determined by negative-ion ESI analysis, were not detected by +APPI analysis. This indicates that a combination of various ionization techniques is needed to achieve a comprehensive characterization of petroleum molecular composition. In general, the detection of various class species is dependent upon the

nonpolar species. The broadband positive-ion APPI FT-ICR mass spectra of VR and its SFEF fractions are shown in the last column of Figure 1. The mass spectrum of VR ranged from m/ z 400 to 1000, centered at m/z 550. The molecular weights determined by APPI were slightly higher than those by ESI. ESI and APPI primarily reveal the composition of polar and nonpolar components, respectively. The mass range and maximum peak of the SFEF fraction increased slightly as the fraction became heavier. Boduszynski et al. concluded that the composition of heavy petroleums increased gradually and continuously with regard to molecular weight, hydrogen deficiency, and heteroatom concentration, which were a function of the atmospheric equivalent boiling point.32 This implies that the Boduszynski mode, which was established on the basis of the distillate fractions and supported by the results by FT-ICR MS,33 is also appropriate for the SFEF fractions. The spectra of SFEF13 and the end-cut were distinct compared to those of other SFEF fractions. A mass series of compounds with the base peak of m/z 530 was mostly abundant, which corresponded to vanadyl porphyrins with double bond equivalent (DBE) of 17−21 based on the accurate mass value and previous studies on porphyrins.15,27,34−37 The results indicated that vanadyl porphyrins were easily detected by APPI in the SFEF end-cut, which is a supercritical C5-derived fraction containing 68.77 wt % C7 asphaltenes. The distributions of class species in VR and its SFEF fractions determined by FT-ICR MS with various ionization techniques are shown in Figure 2. A total of 13 class species were identified from the FT-ICR MS: HC, N1, N1O1, N1O1S1, N1S1, N1S2, O1, O1S1, O1S2, S1, S2, S3, and N4O1V1. Because vanadium species were weak or undetectable in most spectra, the N4O1V1 class species were not shown in Figure 2. The relative abundance value of each class species was nomarlized according to the total assigned class species. The highly abundant classes were HC, S1, S2, N1, and N1S1, which 7451

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Figure 4. Plots of DBE as a function of the carbon number for the hydrocarbon species in VR and its SFEF fractions from positive-ion APPI FT-ICR mass spectra.

similar to those for hydrocarbons (Figure 4). In general, the APPI results were in agreement with the ESI results. 3.3. Molecular Weight of Heavy Petroleum Fractions. Molecular weight is an important concern in designing the petroleum processing system. However, the molecular weights of asphaltenes and heavy petroleum fractions have been debated for decades because of the complexity of the heavy petroleum system.23,24 One of the functions of MS is to determine the molecular weight, which is theoretically the most accurate molecular weight. However, the MS molecular weights for heavy petroleum systems are inconsistent because of the varying ionization efficiencies for various compounds.24 Furthermore, MS exhibits mass discrimination in the lower or upper range of the molecular weight of the petroleum system.23,24,39,40 The number-average molecular weight (MW) can be determined from MS using the following equation:

ionization source. Hence, the results obtained from various ionization techniques are complementary. 3.2. Molecular Composition of SFEF Fractions. Figure 3 shows the expanded mass scale spectra of positive-ion APPI FT-ICR MS at m/z 499, indicating complex molecular compositions of VR and its SFEF fractions. The predominant peaks in VR were hydrocarbons and sulfur- and nitrogencontaining compounds. The resolving power (m/Δm50%) at m/ z 400 was 632 K, which enables most compounds to exhibit distinct peaks and facilitates assignment of molecules based on accurate masses. Distinct disparity in compositions among the SFEF fractions and the end-cut was observed from the mass spectra, indicating the compositional separation efficiency of SFEF for VR. Hydrocarbons and single heteroatom compounds, such as S1 and N1, were dominant in the light SFEF fractions. Heavy SFEF fractions and the end-cut were enriched with multi-heteroatom compounds, such as N1S1, S2, and N4O1V1 class species. The different compositions of various SFEF fractions shown in Figure 3 were indicative of the difference in saturates, aromatics, resins, and asphaltenes (SARA) composition of SFEF fractions (shown in Table 1), which was also supported by the evidence in earlier publications.29,38 Figure 4 shows the plots of DBE as a function of the carbon number for the hydrocarbon species in VR and its SFEF fractions from positive-ion APPI FT-ICR mass spectra. The DBE and carbon number distribution varied over a wide range, with DBE of 2−24 and carbon numbers of 25−70, and overlapped extensively among the SFEF fractions (even though the SFEF fractions were not adjacent to each other). The abundant species with the lowest DBE value of 4 corresponded to alkylbenzenes. This indicates that APPI favors aromatics over saturates. The molecules with the same DBE value and carbon number in various SFEF fractions have the same molecular composition, but their molecular structures may be different. The DBE value and carbon number of the molecules increased as the SFEF fraction became heavier. The results for N1, N1S1, S1, and S2 class species (see the Supporting Information) were

MW =

∑ (MI )/∑ I

(1)

where M is the measured mass-to-charge ratio and I is the intensity of mass peaks. The individual mass peak with a relative abundance greater than 6 times the standard deviation of the baseline noise was considered. Figure 5 shows the MW obtained from the mass spectra of FT-ICR MS by −ESI, +ESI, +APPI, +MeESI, vapor pressure osmometry-hindered stepwise aggregation (VPO−HSA) model,41,42 and gel permeation chromatography (GPC) for SFEF fractions and the end-cut. The MW of the light SFEF fraction varied slightly with the ionization technique used because of the varying ionization efficiencies for various compounds, as discussed above. However, the MW of light SFEF fractions determined by each ionization technique increased as the fraction became heavier. The mean molecular weights of light SFEF fractions obtained by FT-ICR MS with various ionization techniques were also in agreement with those by VPO−HSA and GPC. This showed that compounds in 7452

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content of asphaltenes exhibit molecular aggregation, which is not detected by MS at m/z < 2000.26 As a result, the mean molecular weights of heavy SFEF fractions obtained by FT-ICR MS with various ionization techniques were much lower than those by VPO−HSA and GPC. However, regardless of the actual MW of asphaltenes, the MS data showed that at least the asphaltenes (end-cut) contain low-molecular-weight molecules. To validate the molecular compositions of SFEF fractions obtained by MS with various ionization techniques, the elemental compositions of SFEF fractions were compared to those of elemental analysis. The carbon content (Cw, in wt %) of the SFEF fraction can be calculated by the following equation: Cw = Figure 5. Average molecular weights from MS with various ionization methods, VPO, and GPC for SFEF fractions.

∑ ((CcI /(Cc + Hh + Nn + Oo + Ss))/∑ I (2)

where C, H, N, O, and S are the atomic mass of carbon, hydrogen, nitrogen, oxygen, and sulfur, respectively, and c, h, n, o, and s are the number of atoms of C, H, N, O, and S in the molecular formula. Isotopic peaks were also considered in the calculation. Similarly, other element contents, such as hydrogen (Hw), nitrogen (Nw), oxygen (Ow), and sulfur (Sw) were calculated. The elemental compositions of SFEF fractions obtained by MS with various ionization techniques are listed in Table 2. Because each ionization technique exhibits an inherent discrimination for certain compounds, the MS elemental composition of SFEF fractions may be deviated. Moreover, the element contents were normalized according to eq 2. The

SFEF fractions ionized by various ionization techniques have similar molecular weight distribution. As for the heavy SFEF fractions (SFEF13 and the end-cut), the MW values determined by MS were distinctly different from those by VPO−HSA and GPC. Although the MW values by FT-ICR MS with various ionization techniques were consistent and similar, they were not credible based on the low-quality mass spectra of these heavy SFEF fractions. Asphaltene components cannot be completely ionized.43−45 Hence, MS just reveals the compositions of a fraction of asphaltenes. In addition, the heavy SFEF fractions with a higher

Table 2. Comparison of Elemental Composition of VR and Its SFEF Fractions between Elemental Analysis (EA) and FT-ICR MS with APPI and ESI

VR

SFEF1

SFEF4

SFEF7

SFEF10

SFEF13

end-cut

EA +APPI −ESI +ESI EA +APPI −ESI +ESI EA +APPI −ESI +ESI EA +APPI −ESI +ESI EA +APPI −ESI +ESI EA +APPI −ESI +ESI EA +APPI −ESI +ESI

N (wt %)

O (wt %)

S (wt %)

H (wt %)

C (wt %)

H/C

1.0 0.4 1.6 2.6 0.5 0.4 1.9 2.6 0.5 0.4 2.0 2.5 0.6 0.5 2.1 2.5 0.8 0.9 2.0 2.7 1.1 1.2 2.4 2.9 1.8 5.3 2.8

1.6 0.3 2.2 0.6 1.4 0.3 2.9 0.4 1.3 0.3 2.6 0.6 1.6 0.4 2.0 0.5 1.7 0.4 1.7 0.7 1.5 0.5 1.4 0.6 1.6 1.4 1.3

4.8 4.1 2.2 2.3 3.4 3.6 1.7 1.8 3.9 3.6 2.3 2.6 4.4 3.7 2.3 2.7 4.9 4.8 3.1 3.2 5.2 4.8 3.3 3.3 5.7 2.3 3.3

9.7 10.4 10.1 10.4 11.1 10.9 9.6 10.6 10.9 10.6 9.5 10.2 10.5 10.0 9.1 9.8 10.1 9.4 9.2 9.1 9.5 8.6 8.5 8.5 8.5 8.3 8.1

82.7 84.7 83.9 84.1 83.8 84.8 83.8 84.6 83.8 85.1 83.6 84.2 83.7 85.3 84.5 84.5 83.1 84.6 84.1 84.4 82.6 84.9 84.4 84.7 83.0 82.7 84.6

1.4 1.5 1.4 1.5 1.6 1.5 1.4 1.5 1.6 1.5 1.4 1.5 1.5 1.4 1.3 1.4 1.5 1.3 1.3 1.3 1.4 1.2 1.2 1.2 1.2 1.2 1.1

7453

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in VR and its SFEF fractions from positive-ion APPI FT-ICR mass spectra, plots of DBE versus carbon number for S1 (Figure S5) and S2 (Figure S6) species in VR and its SFEF fractions from S-methylation positive-ion ESI FT-ICR mass spectra, and plots of time domain signals for the FT-ICR mass spectra of VR and its SFEF fractions by various ionization techniques (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

errors associated with any element content will impact the values of other element contents. Hence, the element contents obtained from MS seem meaningless. However, the pseudovalue is valuable for the interpretation of the MS data. For example, the oxygen and sulfur contents of VR from elemental analysis are 1.6 and 4.8%, respectively, and from −ESI, both are 2.2%. The relative content of oxygen from elemental analysis is lower than that from −ESI, but it is reversed for sulfur. Considering that −ESI selectively ionizes phenolic and carboxylic compounds instead of sulfur compounds, the MS results are reasonable. Otherwise, we should suspect that the MS analysis did not detect the dominant components. Relative to the element content, the H/C ratio is more valuable for the understanding of heavy petroleum composition. As shown in Table 2, the H/C ratio from MS and elemental analysis are generally close for the VR and most fractions, indicating that the detectable components by various ionization techniques have a similar molecular condensation degree. It is an important clue for the molecular composition investigation of undetectable components in heavy petroleum by MS. For example, both H/C ratios of the end-cut are 1.2 for +APPI and elemental analysis, and the APPI result has revealed that porphyrins are dominant in the mass spectrum; therefore, the undetectable components in the end-cut should have a similar average molecular condensation degree to that of detected porphyrins. If the undetectable components have a very large molecular weight, they must have a very large DBE value to keep the H/C ratio at a constant value. In this case, the MS results could be further interpreted for the understanding of heavy petroleum molecular composition, although not all of the species were ionized in the ion source.

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Corresponding Author

*Telephone: +86-10-8973-3738. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Sha Chen for preparing the SFEF samples. This work was supported by the Union Fund of the National Natural Science Foundation of China (NSFC) and the China National Petroleum Corporation (CNPC) (U1162204) and the NSFC Funds (21236009 and 21376262).

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REFERENCES

(1) Obiosa-Maife, C.; Shaw, J. M. Toward identification of molecules in ill-defined hydrocarbons using infrared, Raman, and nuclear magnetic resonance (NMR) spectroscopy. Energy Fuels 2010, 25 (2), 460−471. (2) Yang, G.; Wang, R. A. The supercritical fluid extractive fractionation and the characterization of heavy oils and petroleum residua. J. Pet. Sci. Eng. 1999, 22 (1−3), 47−52. (3) 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. (4) 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. (5) Marshall, A. G.; Rodgers, R. P. Petroleomics: Chemistry of the underworld. Proc. Natl. Acad. Sci. U. S. A. 2008, 105 (47), 18090− 18095. (6) Rodgers, R. P.; Marshall, A. G. Petroleomics: Advanced characterization of petroleum-derived materials by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer: New York, 2007; pp 63−93. (7) Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Petroleomics: MS returns to its roots. Anal. Chem. 2005, 77 (1), 20A−27A. (8) Marshall, A. G.; Rodgers, R. P. Petroleomics: The next grand challenge for chemical analysis. Acc. Chem. Res. 2004, 37 (1), 53−59. (9) Hsu, C. S.; Hendrickson, C. L.; Rodgers, R. P.; McKenna, A. M.; Marshall, A. G. Petroleomics: Advanced molecular probe for petroleum heavy ends. J. Mass Spectrom. 2011, 46 (4), 337−343. (10) 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. (11) 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

4. CONCLUSION The VR and its SFEF fractions were characterized by FT-ICR MS with different ionization techniques. Each method favors certain compounds and discriminates against others. APPI allows for a more thorough overview of the heavy petroleum samples because it can ionize more and different species, while ESI has a high degree of selectivity for polar species. There is no one method that allows for a heavy petroleum sample to be truly described, but it is possible to analyze and characterize it using the combination of these methods. Each ionization method leads to a different mass spectrum, with the mass peaks corresponding to different molecular compositions. However, the mass range detected by various ionization methods is generally close. The compositional results derived from different ionization methods are complementary and are supported mutually. The SFEF separates compounds in the heavy petroleum by molecular weight and molecular condensed degree, indicating that the VR sample has a large range of dispersities in molecular composition. The FT-ICR MS data are in agreement with the elemental analysis and molecular weight determined by GPC and VPO−HSA for the extractable fractions. The compositional information derived from FT-ICR MS analysis is credible for extractable fractions and provides an important clue for the understanding of the molecular composition of the end-cut (asphaltenes).

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

Plots of DBE versus carbon number for S1 (Figure S1), S2 (Figure S2), N1 (Figure S3), and N1S1 (Figure S4) class species 7454

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Energy & Fuels

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dx.doi.org/10.1021/ef502162b | Energy Fuels 2014, 28, 7448−7456