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Energy & Fuels 2001, 15, 492-498
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 Kuangnan Qian* ExxonMobil Research and Engineering, Route 22 East, Annandale, New Jersey 08801
Ryan P. Rodgers, Christopher L. Hendrickson, Mark R. Emmett, and Alan G. Marshall*,† Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310 Received November 6, 2000
Extra heavy petroleum crude oil (50% of the mixture boils at >566 °C) has been analyzed directly, without prior fractionation, by a high-field (9.4 T) Fourier transform ion cyclotron resonance mass spectrometer coupled to an external micro-electrospray ion source. At an average mass resolving power, (m/∆m50% ≈ 50 000), a single wideband (250-1250 Da) mass spectrum exhibited ∼5000 resolved peaks with an average mass of 617 Da (e.g., up to 7-10 resolved peaks at each nominal mass). Their elemental compositions were positively identified by accurate mass measurement with an average deviation of less than 1 mDa from each assigned elemental composition. The number of elemental compositions at each nominal mass, the number of sulfur/ oxygen atoms in a molecule, and aromaticity each increase with increasing mass. On the basis of elemental composition alone, we resolve more than 3000 distinct chemical formulas (excluding 13C isotopic species). Of the 3000 unique elemental compositions, we identify 12 major heteroatomic “classes”; (e.g., molecules containing N, NS, NS2, NO, NOS, etc.); for the various “classes”, we identify more than 100 hydrocarbon “types” (e.g., molecules with the same number of rings plus double bonds); and for each “type”, we determine the carbon number distribution (20-80 carbons) to reveal the number of alkyl carbons appended to aromatic rings. The present results represent the most complete chemical characterization ever achieved for such a complex mixture, based on a single experimental data set.
Introduction As worldwide petroleum reserves evolve toward heavier crude oils with rich heteroatomic content, effective processing of heavy petroleum becomes increasingly important. Heavy petroleum is a complex mixture of hydrocarbon molecules containing multiple aromatic rings, some of which contain heteroatoms, such as N, S, and O. Variations in petroleum composition directly impact all refinery processes. For example, heteroatomic hydrocarbons, particularly those containing nitrogen, are known to play a key role in catalyst deactivation through coke formation on the catalyst surface.1-3 Compositions of low-boiling hydrocarbons, such as * Authors to whom correspondence should be addressed. † Also a member of the Department of Chemistry, Florida State University. (1) Guisnet, M.; Magnoux, P. Appl. Catal. 1989, 54, 1-27. (2) Qian, K.; Tomczak, D. C.; Rakiewicz, E. F.; Harding, R. H.; Yaluris, G.; Cheng, W.-C.; Zhao, X.; Peters, A. W. Energy Fuels 1997, 11, 596-601. (3) Harding, R. H.; Zhao, X.; Qian, K.; Rajagopalan, K.; Cheng, W.C. Ind. Eng. Chem. Res. 1996, 35, 2561-2569.
gasoline, diesel, and even gas oil, have been well characterized by high-resolution mass spectrometry,4-7 and by hyphenated mass spectrometric techniques, such as gas chromatography/mass spectrometry (GC/MS),8,9 liquid chromatography/mass spectrometry (LC/MS),10-12 and tandem mass spectrometry (MS/MS).13-16 In con(4) Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; LeTourneau, R. L.; Teeter, R. M. Anal. Chem. 1967, 39, 1833-1838. (5) Aczel, T. Rev. Anal. Chem. 1972, 1, 226-261. (6) Fisher, I. P.; Fischer, P. Talanta 1974, 21, 867-875. (7) Guan, S.; Marshall, A. G.; Scheppele, S. E. Anal. Chem. 1996, 68, 46-71. (8) Zadro, S.; Haken, J. K.; Pinczewski, W. V. J. Chromatogr. 1985, 323, 305-322. (9) Qian, K.; Peru, D. A.; Petti, T. F.; Zhao, X.; Yaluris, G.; Harding, R. H.; Cheng, W.-C.; Rajagopalan, K. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1998, 43, 169-171. (10) Qian, K.; Hsu, C. S. Anal. Chem. 1992, 64, 2327-2333. (11) Hsu, C. S.; Qian, K. Anal. Chem. 1993, 65, 767-771. (12) Hsu, C. S.; McLean, M. A.; Qian, K.; Aczel, T.; Blum, S. C.; Olmstead, W. N.; Kaplan, L. H.; Robbins, W. K.; Schulz, W. W. Energy Fuels 1991, 5, 395-398. (13) Roussis, S. G. Adv. Mass Spectrom. 1998, 14, D043330/1D043330/8. (14) Johnson, J. V.; Yost, R. A.; Wong, C. M. ASTM Spec. Technol. Publ. 1989, 1019, 47-58.
10.1021/ef000255y CCC: $20.00 © 2001 American Chemical Society Published on Web 01/06/2001
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trast to light and middle distillates, knowledge of heavy petroleums (e.g., resid and asphaltene) is very limited, primarily due to analytical difficulties presented by the complex nature of the analyte. Analysis of heavy petroleum by mass spectrometry is difficult for three reasons. First, the high-boiling nature of heavy petroleum renders traditional vaporization and ionization methods ineffective. Second, the extraordinary complexity of the mixture demands ultrahigh-mass resolving power. Finally, characterization of heteroatomic hydrocarbons, such as nitrogen-containing aromatics, poses an even greater challenge due to the low concentrations of these molecules in the petroleum samples. Although traditional thermal vaporization (direct insertion probe, all-glass heated inlet system) followed by electron ionization (both high and low energy) remains an effective means for generating gas-phase ions of aromatic hydrocarbons (which may also contain sulfur or oxygen atoms),7,17,18 other ionization methods are required to extend mass spectrometry to the highboiling and/or more polar molecules (e.g., nitrogencontaining species). Field desorption/field ionization (FD/FI) has been widely used for the characterization of high-boiling nonpolar hydrocarbons by the petroleum industry.19-22 However, FD/FI has proven difficult to couple with high-resolution mass spectrometry for hydrocarbon-type differentiation. Thermospray (TSP) ionization can produce protonated molecular ions with a mass profile similar to FD. LC/MS coupled with TSP has been able to generate detailed hydrocarbon types for vacuum resid.23 Matrix-assisted laser desorption ionization (MALDI) recently generated hydrocarbons of mass up to 20 000 Da.24,25 However, it is not yet clear whether the ultrahigh molecular weight ions arise from noncovalent complexes formed in the ionization process. In other work, atmospheric pressure negative ion chemical ionization (APCI) has been found to detect naphthenic acids selectively in crude oil.26 Although electrospray ionization has been applied widely for analysis of molecules with highly polar functionalities, applications for hydrocarbon analysis are rare. Zhan and Fenn27 recently showed that ESI could ionize polar molecules in various petroleum distillates; however, their mass spectra were not interpreted due to insufficient mass resolving power. Miyabayashi (15) Laycock, J. D.; Yost, R. A.; Wang, L.; Quirke, J.; E., M. Energy Fuels 1995, 9, 1079-1085. (16) Holman, R. W. J. Coal Qual. 1990, 9, 120-124. (17) Hsu, C. S.; Liang, Z.; Campana, J. E. Anal. Chem. 1994, 66, 850-855. (18) Rodgers, R. P.; Andersen, K. V.; White, F. M.; Hendrickson, C. L.; Marshall, A. G. Anal. Chem. 1998, 70, 4743-4750. (19) Heine, C. E.; Geddes, M. M. Org. Mass Spectrom. 1994, 29, 277-282. (20) Pfeifer, S.; Beckey, H. D.; Schulten, H. R. Fresenius Z. Anal. Chem. 1977, 284, 193-195. (21) Malhotra, R.; McMillen, D. F.; Tse, D. S.; St. John, G. A.; Coggiola, M. J.; Matsui, H. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1989, 34, 330-338. (22) Rahimi, P. M.; Fouda, S. A.; Kelly, J. F.; Malhotra, R.; McMillen, D. F. Fuel 1989, 68, 422-429. (23) Hsu, C. S.; Qian, K. Energy Fuels 1993, 7, 268-272. (24) Li, C. Z.; Herod, A. A.; John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Humphrey, P.; Chapman, J. R.; Rahman, M. Rapid Commun. Mass Spectrom. 1994, 8, 823-828. (25) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1999, 13, 287-296. (26) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217-223. (27) Zhan, D. L.; Fenn, J. B. Int. J. Mass Spectrom. 2000, 194, 197208.
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et al.28 applied ESI high-resolution FT-ICR MS (7 T) to analysis of an Arabian mix vacuum residue and observed primarily even-mass ions. However, the ions were incorrectly assigned as molecular ions rather than protonated ions. As a result, the even-mass ions were interpreted as (odd-electron) hydrocarbon molecules rather than as (even-electron, protonated) nitrogencontaining hydrocarbons (see below). Most recently, Rudzinski et al.29 discovered that when PdCl2 is cosprayed with a crude oil, sulfur heteorocycles can be selectively ionized due to complexation of organosulfur and Pd2+. Mass spectrometric characterization of nitrogen aromatics in petroleum has relied on prechromatographic separations to simplify the analyte compositions. Various nitrogen types have been isolated and identified by ion exchange chromatography followed by mass spectrometry and other spectroscopic techniques.30-32 However, the separations are typically time-consuming. Moreover, certain heteroatomic molecules may be stable in the petroleum matrix but may oxidize quickly if isolated. In this work, we demonstrate that the combination of electrospray ionization33 with high-field (9.4 T) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry34 offers a unique opportunity to characterize basic nitrogen aromatics directly in heavy petroleum at an unprecedented level of detail. Because the crude petroleum is analyzed directly without preseparation, chemical modifications of crude oil components are largely eliminated. This work thus represents a new approach for detailed characterization of heteroatomics in heavy petroleum. Experimental Methods Sample Preparation. The present extra heavy crude oil contains ∼50% of >566 °C boiling point hydrocarbons, and contains 4.02% sulfur and 0.65% nitrogen. For sample preparation, 0.9 mg of the crude was first dissolved in 2 mL of methylene chloride. A 1 mL volume of the methylene chloride concentrate was diluted with methanol to bring the final volume to 20 mL. The final solution was spiked with 0.5% acetic acid to promote electrospray ionization. Model compounds used in this study were obtained from Aldrich Chemical Co. A 1 mg volume each of phenanthrene, dibenzothiophene, carbazole, acridine, vanadyl octaethyl porphyrin (VOEP), and nickel octaethyl porphyrin (NiOEP) were dissolved in 10 mL of toluene. A 1 mL toluene solution was diluted 200 times with methanol. The final solution was then spiked with 0.5% of acetic acid. Electrospray Ionization High-Field FT-ICR Mass Spectrometry. The extra heavy oil was analyzed at the National High Magnetic Field Laboratory (NHMFL) with a home-built 9.4 T Fourier transform mass spectrometer.35 Ions were (28) Miyabahashi, K.; Suzuki, K.; Teranishi, T.; Naito, Y.; Tsujimoto, K.; Miyake, M. Chem. Lett. 2000, 172-173. (29) Rudzinski, W. E.; Aminabhavi, T. M.; Tarbox, T.; Sassman, S.; Whitney, K.; Watkins, L. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 2000, 45, 60-63. (30) Hsu, C. S.; Qian, K.; Robbins, W. K. J. High Resolut. Chromatogr. 1994, 17, 271-276. (31) Green, J. B.; Green, J. A.; Yu, S. K. T.; Grizzle, P. L. NIPER Report (NIPER-323; Order No. DE89000746), 1989, 103 pp. (32) McKay, J. F.; Weber, J. H.; Latham, D. R. Anal. Chem. 1976, 48, 891-898. (33) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64-71. (34) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Mass Spectrom. Rev. 1998, 17, 1-35.
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generated externally by a micro-electrospray source36 and samples were delivered by a syringe pump at a rate of 300 nL/min. 2.2 kV was applied between the capillary needle and ion entrance. The externally generated ions were accumulated in a short (45 cm) rf-only octopole for 5-10 s and then transferred via a 200 cm rf-only octopole ion guide to a Penning trap. Ions were excited by frequency-sweep (100-725 kHz @ 150 Hz/µs at an amplitude of 200 Vp-p across a 10-cm diameter open cylindrical cell). The time-domain ICR signal was sampled at 1.28 Msample/s for 1.63 s to yield 2 Mword time-domain data. Ten data sets were co-added, zero-filled once, Hanning apodized, and fast Fourier transformed with magnitude computation. A continuous wave 40 W CO2 laser (Synrad E48-2115, Bothell, WA) was used to dissociate noncovalent ion complexes. Low-Resolution ESI Experiments. The relative ionization efficiencies of six petroleum model compounds were studied at the ExxonMobil Laboratory with a quadrupole mass spectrometer coupled with a Z-Spray ESI source (MicroMass Quattro II) with the following instrument conditions: cone voltage, 30 V; extraction voltage, 5 V; source block temperature, 120 °C, desolvation temperature, 200 °C; and 300 L/h nitrogen as a drying gas. Mass Calibration. Bovine ubiquitin (C378H629N105O118S1) was used as an internal reference for mass calibration.37,38 The compound formed primarily 7+ to 12+ multiply charged ions under our experimental conditions, spanning a 770-1220 Da mass-to-charge ratio range. Major homologous series were identified by accurate mass analysis (see Results and Discussion). The full-range mass spectrum was then recalibrated on the basis of identified sample peak(s) in the low-mass region and ubiquitin ion(s) in the high-mass region. Kendrick Mass for “Type” Analysis. It is helpful to convert the mass spectral data from the IUPAC mass scale (based on the 12C atomic mass as exactly 12 Da) to the Kendrick mass (KMass) scale to facilitate the identification of hydrocarbon homologues. Kendrick mass39 is obtained from IUPAC mass as shown in eq 1.
Kendrick mass ) IUPAC mass × (14/14.01565)
(1)
In other words, the Kendrick scale effectively converts the mass of CH2 from 14.01565 to exactly 14. Thus, the advantage of KMass for hydrocarbon analysis is that the members of a homologous series (namely, compounds with the same constitution of heteroatoms and number of rings plus double bonds, but different numbers of CH2 groups) will have identical Kendrick mass defect (KMD), as originally defined in eq 2 (see Table 1 for Kendrick “type” analysis of molecules of the N or NO “class”).
KMD ) (nominal mass - Kendrick mass) × 1000 (2) A list of KMD values is given in Table 2 for selected hydrocarbon types. It is thus a simple matter to scan a list of all Kendrick masses, and extract members of a homologue “type” series. The mass values for peaks ranging from 250 to 1250 Da with relative abundance greater than 2% noise threshold were exported to an Excel spreadsheet. The peaks were separated into hydrocarbon homologues by their nominal mass and KMD, (35) Senko, M. W.; Hendrickson, C. L.; Pasa-Tolic, L.; Marto, J. A.; White, F. M.; Guan, S.; Marshall, A. G. Rapid Commun. Mass Spectrom. 1996, 10, 1824-1828. (36) Senko, M. W.; Hendrickson, C. L.; Emmett, M. R.; Shi, S. D.H.; Marshall, A. G. J. Am. Soc. Mass Spectrom. 1997, 8, 970-976. (37) Ledford, E. B., Jr.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1984, 56, 2744-2748. (38) Shi, S. D.-H.; Drader, J. J.; Freitas, M. A.; Hendrickson, C. L.; Marshall, A. G. Int. J. Mass Spectrom. 2000, 195/196, 591-598. (39) Kendrick, E. Anal. Chem. 1963, 35, 2146-2154.
Table 1: 12C and Kendrick Masses of Two Hydrocarbon Homologues Sharing the Same Nominal Masses but with Different Kendrick Mass Defectsa C no.
12C
mass
Kendrick mass
20 30 40 50 60 70 80 90
282.2222 422.3787 562.5352 702.6917 842.8482 983.0047 1123.1612 1263.3177
CcH2c-13N + H+ 281.907 421.907 561.907 701.907 841.907 981.907 1121.907 1261.907
30 40 50 60 70 80 90
422.2484 562.4049 702.5614 842.7179 982.8744 1123.0309 1263.1874
CcH2c-29NO + H+ 421.7769 561.7769 701.7769 841.7769 981.7769 1121.7769 1261.7769
nominal mass
KMD
282 422 562 702 842 982 1122 1262
93 93 93 93 93 93 93 93
422 562 702 842 982 1122 1262
223 223 223 223 223 223 223
a For this “type” series, successive entries in the table differ by 10 CH2 groups, corresponding to IUPAC mass difference of 10(14.01565) ) 140.1565 Da, or a Kendrick mass difference of 10(14) ) 140. Note the identical Kendrick mass defect in the third and fifth columns for all members of the series.
Table 2: A Partial List of Kendrick Mass Defects (KMD) for Protonated Nitrogen-Containing Hydrocarbon Types Z
X
KMD (mDa)
Z
X
KMD (mDa)
-1 -3 -5 -5 -15 -7 -7 -17 -19 -9 -19 -29 -21 -21
N NO NO2 NS N NO3 NOS NO NO2 NS2 NS N NO3 NOS
12.6 48.9 85.2 103.0 106.4 121.6 139.3 142.7 179.0 193.4 196.8 200.2 215.4 233.1
-31 -33 -23 -33 -43 -35 -35 -45 -47 -37 -47 -49 -49
NO NO2 NS2 NS N NO3 NOS NO NO2 NS2 NS NO3 NOS
236.5 272.8 287.2 290.6 294.0 309.2 326.9 330.3 366.6 381.0 384.4 403.0 420.7
following a multiple sorting procedure.40 Molecular compositions of hydrocarbon homologues were identified by their unique combination of nominal mass and KMD, and were expressed as a general chemical formula, CcH2c+ZX, in which c is the carbon number, Z is often referred to as the hydrogen deficiency index, and X denotes the constituent heteroatoms (N, S, O) in a hydrocarbon molecule. For convenience, we denote a given hydrocarbon “type” and “class” according to its Z and X components. For example, CcH2c-13N is abbreviated as -13N, whereas CcH2c-29NO is listed as -29NO. The Z value is directly related to the number of rings plus double bonds in the molecule. For example, the number of rings plus double bonds in a molecule of chemical formula, CcHhNnOo, is (c h/2 + n/2 + 1).41 For C12H9N, for example, the number of rings plus double bonds is 12-9/2 + 1/2 + 1 ) 9, for which an isomer with three rings and 6 double bonds is shown at the far left in Figure 5, and 2c + Z ) 9, or Z ) 9 - 2(12) ) -15 for this molecular “type”. Members of a given “type” series each have the same Z-value (i.e., same number of rings plus double bonds), but may differ by multiples of 14.01565 in IUPAC mass (and by multiples of exactly 14 in Kendrick mass) according to the number of CH2 groups appended to the rings. Members of such a series can therefore be recognized from their identical Kendrick mass defects (e.g., Table 1). (40) Hsu, C. S.; Qian, K.; Chen, Y. C. Anal. Chim. Acta 1992, 264, 79-89. (41) McLafferty, F. W.; Turecek, F. Interpretation of Mass Spectra; University Science Books: Sausalito, CA, 1993.
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Figure 1. Full-range positive-ion ESI FT-ICR mass spectrum of a heavy crude oil containing a bovine ubiquitin internal standard at an average mass resolving power, m/∆m50% ≈ 50 000 over the entire mass range (∼5000 peaks from 250 < m/z < 1250). From the crude oil molecular weight distribution, the average mass was determined to be ∼617 Da. The atmospheric equivalent boiling point calculated from this average mass correlated well with the experimentally determined specification that the extra heavy oil contains 50% of >566 °C hydrocarbon components.
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Figure 2. A mass scale-expanded segment (bottom) of the full range heavy crude mass spectrum of Figure 1, showing the presence of species at every nominal mass, with clear periodicities repeating at every 14 nominal mass units. A further mass scale expansion (top) reveals the tendency of the ESI process to form more abundant ions of even nominal mass (marked with *). The mass scale is plotted in Kendrick mass units.
Our objective is to apply ESI FT-ICR MS to direct speciation of polar molecules in heavy petroleum. Specifically, we seek to resolve and identify different heteroatomic “classes” (e.g., molecules containing N, NS, NS2, NO, NOS, etc.); for each “class”, to identify various “types” (molecules with the same number of rings plus double bonds); and for each “type”, the carbon number distribution (i.e., number of alkyl carbons appended to aromatic rings). The procedures for determining elemental composition, “class”, “type”, and carbon number from accurate mass measurements are provided in the Experimental Methods section. ESI is known to be sensitive toward molecules with polar functionalities. High field FT-ICR furnishes the ultrahigh-mass resolving power needed to resolve multiple isobaric mass overlaps in the high-mass region. Figure 1 shows a wide band ESI FT-ICR mass spectrum of extra heavy crude oil. Mass spectral peaks range from 250 to 1250 in mass-to-charge ratio with numberaverage and weight-average molecular weights of 616 and 647. (Except for the ubiquitin calibrant, all other species are singly charged, as evident from the absence of peaks at integral fractions of 1 Da between the nominal mass peaks.) Noncovalent Complexes. The presence of high molecular weight hydrocarbons in mass spectra (e.g., field desorption, matrix-assisted laser desorption, etc.) has been questioned due to potential formation of noncovalent ion complexes. In a separate study,42 under similar experimental conditions, we have observed that petroporphyrin molecules indeed form dimer ions in the ESI process. Infrared multiple photon dissociation (IRMPD) (20 W CO2 laser irradiation for 1 s) dissociates petroporphyrin dimers without fragmenting molecular
ions. In the present experiments, however, IRMPD had little effect on the mass distribution of the extra heavy crude oil, indicating that hydrocarbon ions do not form noncovalent complexes as easily as petroporphyrins. The average mass (617 Da) of the crude would have an atmospheric equivalent boiling point (AEBP) of roughly 550 °C,43 in agreement with the specification that the extra heavy oil contains 50% of >566 °C AEBP materials. Even- vs Odd-Mass Species. In the mass scale expanded spectrum in Figure 2, even-mass ions are clearly more abundant than odd-mass ions. In positiveion mode, electrospray often yields quasimolecular ions, i.e., even-electron species consisting of intact molecules with protons or other cation adducts. According to the well-known “nitrogen rule,”41 such even-mass ions contain an odd number of nitrogens. Considering that nitrogen-containing molecules are a minor component of crude oil, the hydrocarbon matrix (e.g., paraffins, hydrocarbon aromatics, thiophenoaromatics, etc.) is evidently not effectively ionized by electrospray. In fact, detailed mass analysis (see below) reveals that most odd-mass ions are 13C isotopic species of the even-mass ions (rather than zero- or even-number of nitrogens in even-electron molecules whose carbons are all 12C). Elemental Composition from Accurate Mass Measurement. Figure 3 illustrates why ultrahigh-mass resolving power and ultrahigh-mass accuracy are necessary for mass analysis of heavy hydrocarbon mixtures. Nine chemically distinct components are detected and resolved (a new record) in a narrow mass window (477.7-477.9 Da) spanning a single nominal mass of 478 Da. Mass resolving power, m/∆m50% (in which ∆m50% is the magnitude-mode mass spectral peak full width at half-maximum peak height), ranged from 40 00070 000. (It is worth noting that such ultrahigh-mass resolving power is achieved throughout the whole mass range simultaneously by FT-ICR MS, whereas double-
(42) Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G.; Greaney, M.; Qian, K. Can. J. Chem., in press.
(43) Boduszynski, M. M.; Altght, K. H. Energy Fuels 1992, 6, 7276.
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Figure 3. A further mass scale expanded segment of the full range heavy crude mass spectrum of Figure 1 allows for resolution and elemental composition assignment (based on accurate mass measurement) of 9 chemically distinct species at a single nominal mass. The tabulated data for the 9 peaks show an average mass error of ∼1.5 ppm for the proposed assignments. The chemical “type” (i.e., number of rings plus double bonds) is classified according to “Z” value in the chemical formula at the top of the diagram (see text), and the heteroatom content is also shown in the table.
Figure 4. A series of zoom mass segments (408, 548, 688, and 828 Da) demonstrate the increasing chemical complexity of heavy crude oil as mass increases. The number of resolved elemental compositions increases from 4 species at 408 Da to 9 species at 828 Da. The compound type and heteroatom content are listed next to each peak.
focusing mass analyzers can achieve high-mass resolving power only by scanning very slowly across a very narrow mass window at a time.) The most probable elemental compositions of the mass peaks are identified in the inset of Figure 3. The deviation between assigned and calculated mass is very small (0-2 mDa), confirming that each of these ions contains one nitrogen atom. Mass Dependence of Chemical Composition. As molecular weight increases, so do (a) the complexity of the nitrogen-containing molecules, (b) the hydrogen deficiency (or aromaticity), and (c) the number of sulfur atoms per molecule (see Figure 4). Moreover, FT-ICR mass resolving power (m/∆m50%) varies approximately inversely with increasing mass.44 It thus becomes more difficult to resolve broader and more numerous peaks (44) Marshall, A. G.; Comisarow, M. B.; Parisod, G. J. Chem. Phys. 1979, 71, 4434-4444.
Qian et al.
Figure 5. Low-resolution positive-ion electrospray mass spectra of various species of the same type found in crude oil. This test mixture consists of 500 ppb each of phenanthrene, dibenzothiophene, carbazole, acridine, VOEP, and NOEP in methanol. The most abundant ions (protonated acridine at 180 Da) correspond to the most basic compound in the mixture. This spectrum supports the assignment of positive-ion ESI species from heavy crude oil as quasimolecular (protonated) cations.
at higher mass. For example (peaks no. 4 and no. 5 in the inset table in Figure 3), of compound type, -17N and -27NS (see Methods section for this notation) differ by only 3.4 mDa in mass. At 450 Da or less, the two compound types are almost completely resolved in the direct-mode34 broadband spectrum. Similar ambiguities arise for NS vs NS2, NS2 vs NS3, and NO vs NOS compound types. For these difficult-to-resolve pairs in the high-mass region (>550 Da), our assignments were made on the best-matched elemental compositions. Neutral vs Basic Nitrogen. Accurate mass analysis alone cannot distinguish isomeric molecules (in this case, nitrogen-containing aromatics). Petroleum products are known to contain two major categories of N compounds, i.e., “neutral” nitrogen (pyrollic benzologs) and basic nitrogens (pyridinic benzologs). The neutral and basic nitrogens cannot be differentiated by their molecular weights. For example, naphthenocarbazole and propyl acridine belong to the same compound type (-17 N) and have the same exact mass. (Actually, even isomers may in principle be resolved according to their different heats of formation, but the required mass resolution would need to be sub-nanoDalton, which is not yet experimentally feasible.) To determine the nature of the nitrogen types preferably ionized by ESI, we evaluated relative electrospray efficiencies of selected aromatic compounds commonly found in petroleum, by use of a low-resolution quadrupole mass spectrometer (MicroMass Quattro II) equipped with an ESI source. Figure 5 shows the ESI quadrupole mass spectrum of a mixture containing approximately 500 ppb (w/v) each of phenanthrene, dibenzothiophene, carbazole, acridine, vanadyl octaethylporphyrin (VOEP), and nickel octaethylporphyrin (NiOEP) in methanol. The peak at 180 Da (protonated acridine) dominates the spectrum, confirming that the efficiency of positive-ion ESI strongly favors more basic molecules. The responses of the other five molecules are at background level. A more detailed quantitative survey of ESI efficiency of various petroleum compound types is in progress at
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Figure 6. Type distribution for CcH2c+ZNSs aromatic compounds identified in the ESI FT-ICR mass spectrum of Figure 1. For each heteroatom composition (N, NS, NS2, NS3), there is a distribution of species differing in Z-value (note that Z decreases by 2 for each additional ring or double bond).
ExxonMobil lab. Figure 5 also demonstrates that hydrocarbon molecules form predominantly protonated molecular ions. Petroporphyrins, such as VOEP, form both protonated and Na adduct ions.42 N and NS, NS2, and NS3 Species. On the basis of the model compound results, it is reasonable to assume that the species observed in Figure 1 arise mainly from basic nitrogen molecules. Analysis of the entire mass spectrum (250-1250 Da) revealed 12 major heteroatomic classes, over 100 compound types, and more than 3000 molecules of different elemental composition (excluding 13C isotopic species). For example, molecules containing N and NSs (s ) 1-3) account for ∼79% of the total ions. Figure 6 shows the N and NSs hydrocarbon types as a function of Z value, in which Z is defined by the chemical formula, CcH2c+ZX (see Methods). The N compound class exhibits Z-values ranging from -5 to -29, peaking at -17, corresponding to one (pyridine) to five (dibenzoacridine) aromatic rings with three rings (acridines) as the most abundant type. The number of sulfur atoms increases with Z value, suggesting that sulfurs are incorporated in aromatic structures, such as thiophene. NO and NOS, NOS2, and NOS3 Species. NO and NOSs (s ) 1-3) account for ∼10% of the total ions. Figure 7 shows NO and NOSs aromatics as a function of Z value. The nature and the origin of NO compound types have long been controversial, because of possible oxidation or other chemical changes during isolation/ separation. Because our analysis does not involve any chromatographic separations, the possibility that NO compound types derive from oxidation during the analytical process is minimized. Compared to N compound type distribution, the NO compound types are shifted to higher Z values, suggesting that oxygen atoms, like sulfurs, are incorporated in aromatic rings. Both furan and amide structures are possible. In that respect, infrared spectroscopic data have previously suggested the presence of amide structures in crude oil.30,32 NO2 and NO3 compound types were also observed but present at much lower levels. Odd-Mass Species. For odd-mass species, two compound classes, N2 and SO2, were positively identified. The Z value for N2 ranges from 28 to 54, or about twice
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Figure 7. Type analysis for CcH2c+ZNOSs aromatic compounds identified in the ESI FT-ICR mass spectrum of Figure 1. For each heteroatom composition (NO, NOS, NOS2, NOS3), there is a distribution of species differing in Z-value (note that Z decreases by 2 for each additional ring or double bond).
Figure 8. Carbon number distribution for each of the four compound types shown in Figure 6. (Note that addition of CH2 groups does not change the number of rings and double bonds for a given compound type.)
that for N compound classes, and corresponds to 6-10 aromatic rings. Considering that highly condensed aromatic rings are rare in unprocessed crude oil, we speculate that N2 compounds are formed by bridging small N compounds with biaryl or alkyl bonds. SO2 species could be sulfides formed by oxidation of organosulfur compounds in the crude oil during storage. Carbon Number Distributions. The carbon number (c) for each member of a given compound type may be determined from the molecular weight (MW), Z, and the combined heteroatomic mass (X):
c ) (MW - X - Z)/14
(3)
Figure 8 displays the carbon number distributions for each of four compound types (-15N, -19NS, -33NS, and -37NS2). These four compound types share the same nominal mass. The average carbon numbers for -15N, -29NS, -33NS, and -37NS2 are 40, 44, 51, and 55, indicating the changes in aromatic core structures. Carbon numbers up to 80 are observed. Such high carbon numbers suggest extensive alkyl substitutions. Acknowledgment. The authors thank Daniel McIntosh for machining all of the custom parts required for the 9.4 T instrument construction and John P. Quinn
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for many helpful discussions. This work was supported by the NSF National High-Field FT-ICR Mass Spectrometry Facility (CHE 99-09502), Florida State University, and the National High Magnetic Field Laboratory at Tallahassee, FL. The authors thank ExxonMobil
Qian et al.
for supporting this exploratory research of heavy petroleum and the permission to publish the data.
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