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Energy & Fuels 2006, 20, 668-672
Use of Saturates/Aromatics/Resins/Asphaltenes (SARA) Fractionation To Determine Matrix Effects in Crude Oil Analysis by Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Geoffrey C. Klein,† Annelie Angstro¨m,‡ Ryan P. Rodgers,† and Alan G. Marshall*,†,‡ Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State UniVersity, 1800 East Paul Dirac DriVe, Tallahassee, Florida 32310, and Department of Chemistry and Biochemistry, Florida State UniVersity, Tallahassee, Florida 32306 ReceiVed October 27, 2005. ReVised Manuscript ReceiVed January 14, 2006
We have previously demonstrated the ability of electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to resolve and identify the polar species found in all petroleum distillates. The ultrahigh resolving power and mass accuracy of FT-ICR MS allow for the identification of thousands of compounds in crude oils without prior chromatographic separation. Here, we compare positiveion ESI FT-ICR mass spectra of a South American crude oil with spectra of its saturates/aromatics/resins/ asphaltenes (SARA)-isolated asphaltenes, resins, and aromatics, to ascertain the effect of the other components on the relative mass spectral abundances of the polar aromatics. Saturates are unobservable by ESI. For the chosen oil, little to no signal was obtained for the asphaltenes and resins because of their mostly acidic nature. The mass distributions, heteroatom “class” distributions, “type” (rings plus double bonds) distributions, and carbon number distributions of the aromatic fraction and unfractionated crude oil were highly similar. Thus, the saturates, asphaltenes, and resins do not affect the relative abundances of polar aromatics observed by positive-ion electrospray FT-ICR MS. It is therefore not necessary to isolate the polar aromatic fraction to characterize its chemical composition in a petroleum crude oil.
Introduction Petroleum crude oils are among the most complex organic mixtures in the world, in terms of the number of chemically distinct constituents within a dynamic abundance range of 10 000-100 000. These crude oils are composed of saturated and unsaturated hydrocarbon molecules, polar components containing aromatic compounds with N, S, and O heteroatoms, and metals. Compositions of the saturated hydrocarbons have been well-characterized by gas chromatography mass spectrometry (GC-MS),1 two-dimensional gas chromatography coupled to mass spectrometry (GC×GC MS),2 high-resolution mass spectrometry,3,4 and liquid chromatography mass spectrometry (LC-MS).5-7 The unsaturated hydrocarbon fraction containing * Author to whom correspondence should be addressed. Telephone: 850-644-0529. Fax: 850-644-1366. E-mail:
[email protected]. † Department of Chemistry and Biochemistry, Florida State University. ‡ National High Magnetic Field Laboratory, Florida State University. (1) Zadro, S.; Haken, J. K.; Pinczewski, W. V., Analysis of Australian crude oils by high-resolution capillary gas-chromatography mass-spectrometry. J. Chromatogr. 1985, 323, 305-322. (2) Wang, F. C. Y.; Wan, K. N.; Green, L. A., GC×MS of diesel: A two-dimensional separation approach. Anal. Chem. 2005, 77, 2777-2785. (3) Fisher, I. P.; Fischer, P., Analysis of high-boiling petroleum streams by high-resolution mass-spectrometry. Talanta 1974, 21, 867-875. (4) Guan, S.; Marshall, A. G.; Scheppele, S. E. Resolution and chemical formula identification of aromatic hydrocarbons containing sulfur, nitrogen, and/or oxygen in crude oil distillates. Anal. Chem. 1996, 68, 46-71. (5) Qian, K. N.; Hsu, C. S. Molecular-transformation in hydrotreating processes studied by online liquid-chromatography mass-spectrometry. Anal. Chem. 1992, 64, 2327-2333. (6) Hsu, C. S.; Qian, K. G. Cs2 charge-exchange as a low-energy ionization technique for hydrocarbon characterization. Anal. Chem. 1993, 65, 767-771.
heteroatoms has recently been characterized by electrospray ionization (ESI) ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS).8,9 The two key features of ultrahigh-resolution FT-ICR mass analysis are its ability to resolve species differing in elemental composition by C3 versus SH4 (0.0034 Da mass difference) and high mass accuracy ( 300 000 (in which ∆m50% is the magnitudemode mass spectral peak full width at half-maximum peak height) from 225 < m < 1000 Da,11-13 enabling the identification of thousands of polar species in a single mass spectrum. Electrospray was first introduced as an ionization source for (7) 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. Online liquid-chromatography mass-spectrometry for heavy hydrocarbon characterization. Energy Fuels 1991, 5, 395-398. (8) 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, 492-498. (9) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Resolution and identification of 3000 crude acids in heavy petroleum by negative-ion microelectrospray high field Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2001, 15, 1505-1511. (10) Senko, M. W.; Hendrickson, C. L.; Pasa-Tolic, L.; Marto, J. A.; White, F. M.; Guan, S.; Marshall, A. G. Electrospray ionization FT-ICR mass spectrometry at 9.4 T. Rapid Commun. Mass Spectrom. 1996, 10, 1824-1828.
10.1021/ef050353p CCC: $33.50 © 2006 American Chemical Society Published on Web 02/24/2006
SARA Fractionation in Crude Oil Analysis by ESI FT-ICR MS
the study of petroleum by Zhan and Fenn,14 who showed that ESI could ionize the polar molecules in various petroleum distillates. Although those polar species constitute less than 15 wt % of crude oil,15 they have significant implications in such areas as oil refining16 and fuel stability17 and their combustion produces sulfur and nitrogen oxides (SOx and NOx), known to produce acid rain and air pollution. Mass spectrometric characterization of crude oils has heretofore relied heavily on prechromatographic separations to simplify the analyte composition, because of the lack of resolving power and mass accuracy of conventional MS techniques needed to resolve and identify the thousands of components commonly found in a single crude oil. The most common method of separation is the saturates/aromatics/resins/ asphaltenes (SARA) fractionation, Jewell et al.18 separated the heavy end of crude oils into acid, base, neutral nitrogen, saturate, and aromatic fractions by use of a combination anion- and cation-exchange chromatography, coordination chromatography, and adsorption chromatography. Subsequently introduced separative techniques include high-pressure liquid chromatography (HPLC) containing silica and alumina columns19 and the development of a bonded stationary phase in HPLC columns.20-22 High-field ESI FT-ICR MS23 resolves and enables the assignment of elemental composition of thousands of chemically distinct species in petroleum24,25 and coal.26 However, ESI generates positive ions by protonation of neutrals in the original sample; ergo, the most basic constituents (e.g., aromatics with (11) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Elemental composition analysis of processed and unprocessed diesel fuel by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2001, 15, 1186-1193. (12) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K. N.; 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, 743759. (13) Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G.; Greaney, M. A.; Qian, K. Molecular characterization of petroporphyrins in crude oil by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Can. J. Chem. 2001, 79, 546-551. (14) Zhan, D. L.; Fenn, J. B. Electrospray mass spectrometry of fossil fuels. Int. J. Mass Spectrom. 2000, 194, 197-208. (15) Altgelt, K. H. B., Boduszynski, M. M. Composition and Analysis of HeaVy Petroleum Fractions. Marcel Dekker: New York, 1994. (16) Lavopa, V.; Satterfield, C. N. Poisoning of thiophene hydrodesulfurization by nitrogen-compounds. J. Catal. 1988, 110, 375-387. (17) Chmielowiec, J.; Fischer, P.; Pyburn, C. M. Characterization of precursors which cause light instability in hydroprocessed gas oils. Fuel 1987, 66, 1358-1363. (18) Jewell, D. M.; Weber, J. H.; Bunger, J. W.; Plancher, H.; Latham, D. R. Ion-exchange, coordination, and adsorption chromatographic separation of heavy-end petroleum distillates. Anal. Chem. 1972, 44, 1391-1395. (19) Suatoni, J. C.; Swab, R. E. Rapid hydrocarbon group-type analysis by high-performance liquid-chromatography. J. Chromatogr. Sci. 1975, 13, 361-366. (20) Miller, R. Hydrocarbon class fractionation with bonded-phase liquidchromatography. Anal. Chem. 1982, 54, 1742-1746. (21) Felix, G.; Thoumazeau, E.; Colin, J. M.; Vion, G. Hydrocarbon groups type analysis of petroleum-products by HPLC on specific stationary phases. J. Liq. Chromatog. 1987, 10, 2115-2132. (22) McLean, J. D.; Kilpatrick, P. K. Comparison of precipitation and extrography in the fractionation of crude oil residua. Energy Fuels 1997, 11, 570-585. (23) 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. (24) Marshall, A. G.; Rodgers, R. P. Petroleomics: The next grand challenge for chemical analysis. Acc. Chem. Res. 2004, 37, 53-59. (25) Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Petroleomics: Mass spectrometry returns to its roots. Anal. Chem. 2005, 77, 20A-27A. (26) Wu, Z. G.; Jernstrom, S.; Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Resolution of 10,000 compositionally distinct components in polar coal extracts by negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2003, 17, 946-953.
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Figure 1. SARA fractionation and isolation scheme for the aromatic compounds found in South American crude oil. We consider here the aromatic fraction, asphaltenes, and resins because of their high content of polar species accessible by electrospray ionization.
at least one nitrogen atom) are preferentially observed in positive-ion ESI FT-ICR MS. An obvious question is the extent to which the relative abundances of protonated aromatics in the positive-ion ESI FT-ICR mass spectrum of crude oil are affected by the presence of other species (namely, saturates, resins, and asphaltenes). Therefore, in this paper, we perform a SARA procedure to isolate each fraction and then compare its positiVeion ESI FT-ICR mass spectrum to that of its parent crude oil. The present effort complements a prior analysis8 of the effect of other components on the relative abundances of fractionated acidic asphaltenes observed by negatiVe-ion ESI FT-ICR MS. Materials and Methods Fractionation of Aromatic Species. Figure 1 shows the SARA method used to fractionate the aromatic compounds found in crude oil. A modified version of the ASTM 2007 method was used to separate the different fractions. Approximately 1 g of South American crude oil was dissolved in 40.0 mL of toluene for introduction to a rotary evaporator (Model R-200, Buchi, New Castle, DE). After 1 h, nitrogen gas was bubbled through the remaining toluene/crude oil to remove toluene and volatile compounds. The volatile compounds comprised 11.2% (by weight) of the original crude oil. Asphaltenes were separated from the crude oil by the addition of 40 mL of hexane (HPLC grade) followed by vacuum filtration (Whatman #1 paper, Whatman International, Maidstone, U.K.). The asphaltenes were collected and stored in a glass vial. The asphaltenes constituted 7.1% of the original crude. A total of 8 mL of the hexane-soluble fraction was adsorbed onto 3 g of activated alumina (80-200 mesh, Fisher Scientific, Fairlawn, NJ). The activated alumina was then packed on top of 10 g of neutral alumina in a 22 × 400 mm column. The aliphatic components (24.8%) were fractionated by elution with 40 mL of hexane. The aromatic components (42.2%) were isolated by elution with 80 mL of toluene, whereas the resins (14.7%) were eluted with 50 mL of 80:20 (v/v) toluene/methanol. Sample Preparation for ESI FT-ICR MS. The crude oil sample and the aromatic, asphaltene, and resin fractions were each prepared by dissolving 20 mg in 3 mL of toluene and then diluting to 20 mL with methanol. There was no precipitation at that volumetric ratio of 8.5:1.5 methanol/toluene. A total of 3 µL of acetic acid was added to 1 mL of the final solution to facilitate protonation of the basic nitrogen compounds to yield [M + H]+ ions.
670 Energy & Fuels, Vol. 20, No. 2, 2006 Mass Spectrometry. Mass analysis was carried out with a homebuilt FT-ICR mass spectrometer10 equipped with a 22 cm horizontal room-temperature bore 9.4 T magnet (Oxford Instruments American, Inc., Concord, MA). Data were collected and processed by a modular ICR data acquisition system (MIDAS).27 Positive ions were generated from a microelectrospray source equipped with a 50 µm i.d. fused silica micro-ESI needle.28 Samples were infused at a flow rate of 400 nL/min. Typical ESI conditions were needle voltage, 2.1 kV; tube lens, 370 V; and heated capillary current, 6 A. Ions were accumulated externally in a linear octopole ion trap for 30 s and transferred through rf-only multipoles to a 10 cm diameter, 30 cm long open cylindrical Penning ion trap.29 Multipole ion guides were operated at 1.7 MHz at 100 Vp-p rf amplitude. Broadband frequency-sweep (chirp) dipolar excitation (∼70-641 kHz at a sweep rate of 150 Hz/µs and peak-peak amplitude of 190 V) was followed by direct-mode image current detection that yielded 4 Mword time-domain data. A total of 200 co-added time-domain data sets were Hanning-apodized, followed by a single zero-fill before fast Fourier transformation and magnitude calculation. Data Processing. Mass values for singly charged ions between 225 and 1000 Da with FT-ICR mass spectral magnitude greater than 3σ of baseline noise were converted from IUPAC mass to Kendrick mass as previously described.30-32 The Kendrick masses were then sorted on the basis of Kendrick mass defect and nominal z value and imported into an Excel spreadsheet. Homologous series (those compounds containing a particular heteroatom composition and number of rings plus double bonds but differing by multiples of CH2) could then be identified on the basis of their identical Kendrick mass defect and nominal z values. Molecular formulas of species less than ∼400 Da in a series could be assigned uniquely based solely on mass measurement to (1 ppm. A molecular formula calculator program was limited to molecular formulas consisting of up to 100 12C, 2 13C, 200 1H, 5 14N, 10 16O, 5 32S, and 1 34S assigned elemental compositions. Because members of a homologous series differ only by integer multiples of CH2, the assignment of a single member of such a series usually sufficed to identify all higher mass members of that series.32
Klein et al.
Figure 2. Broadband positive-ion electrospray ionization FT-ICR mass spectra of a South American crude oil and its aromatic fraction derived from SARA fractionation (see Figure 1). Note the high similarity of the two spectra, evidenced by their similar number- and weight-average molecular weights, Mn and Mw. The average mass resolving power ranges from 350 000 < m/∆m50% < 450 000 for each spectrum.
Results and Discussion The aromatic fraction, asphaltenes, and resins were isolated from the heavy crude oil to allow us to determine their effect on electrospray ionization efficiency for polar aromatic species. Figure 2 shows the broadband (250 < m/z < 900) mass spectra of a South American crude oil and its aromatic fraction isolated by the SARA method. Little to no signal was obtained for the asphaltenes and resin fractions. Asphaltenes are acidic in nature and contain few basic components and are thus more appropriately observed as negative ions, for which acidic species are preferentially ionized (see our previous paper).8 All ions are singly charged, as evident from the m/z 1.0034 spacing (27) Senko, M. W.; Canterbury, J. D.; Guan, S.; Marshall, A. G. A highperformance modular data system for FT-ICR mass spectrometry. Rapid Commun. Mass Spectrom. 1996, 10, 1839-1844. (28) Emmett, M. R.; White, F. M.; Hendrickson, C. L.; Shi, S. D.-H.; Marshall, A. G. Application of micro-electrospray liquid chromatography techniques to FT-ICR MS to enable high sensitivity biological analysis. J. Am. Soc. Mass Spectrom. 1998, 9, 333-340. (29) Senko, M. W.; Hendrickson, C. L.; Emmett, M. R.; Shi, S. D.-H.; Marshall, A. G. External accumulation of ions for enhanced electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. J. Am. Soc. Mass Spectrom. 1997, 8, 970-976. (30) Kendrick, E., A mass scale based on CH2 ) 14.0000 for highresolution mass spectrometry of organic compounds. Anal. Chem. 1963, 35, 2146-2154. (31) Hsu, C. S.; Qian, K.; Chen, Y. C. An innovative approach to data analysis in hydrocarbon characterization by on-line liquid chromatographymass spectrometry. Anal. Chim. Acta 1992, 264, 79-89. (32) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. N. Kendrick mass defect spectrum: A compact visual analysis for ultrahigh-resolution broadband mass spectra. Anal. Chem. 2001, 73, 4676-4681.
Figure 3. Narrow segment of the ultrahigh-resolution mass spectra in Figure 2 at a representative even mass, 470 Da. Monoisotopic evenelectron [(M + H)+] species correspond to compounds that contain an odd number of nitrogen atoms (nitrogen rule), such as N, NO, NS, NOS, NS2, and N3. Asterisks denote species containing one (*) or two (**) 13C atoms in place of 12C. Vertical dashed lines denote species found in the aromatic fraction.
between 12Cn and 13C12Cn-1 ions of otherwise identical elemental composition. The mass spectra of the crude oil and its aromatic fraction are visually similar and exhibit similar number- and weight-average molecular weights, Mn and Mw (Mn/Mw ) 560/ 580 and 570/590, respectively.) The somewhat lower signalto-noise ratio for the aromatic fraction accounts for the decrease in the number of peaks with a magnitude larger than 3σ of the baseline noise. Figure 3 shows mass scale-expanded segments of the FTICR mass spectra of Figure 2, at a representative even mass, 470 Da. Mass resolving power, m/∆m50%, ranges from 350 000450 000 throughout the mass range. All 21 different elemental compositions for the South American crude oil have been identified. A total of 16 of 17 peaks were identified in the corresponding nominal mass of the aromatic fraction. The similarity between the two spectra is evident by the 16 common peaks (indicated by the dashed lines). At least some of the 5 peaks identified in the crude oil that are not seen in the aromatic fraction may well be present but are not identified because of the lower signal-to-noise ratio for that fraction.
SARA Fractionation in Crude Oil Analysis by ESI FT-ICR MS
Figure 4. Relative abundances of positive ions in the mass spectra of Figure 2, listed by compound class. Only classes with a relative abundance > 1% are shown. The high similarity between the two distributions shows that electrospray ionization efficiency for polar species is relatively unaffected by the presence of other species in the crude oil.
According to the “nitrogen rule”,33 even-mass ions contain an odd number of nitrogen atoms, for the even-electron (M + H)+ ions typically observed in positive-ion electrospray. For example, N1 species observed at (even) nominal mass (470 Da in Figure 3) include N, NO, NS, NOS, and NS2. Conversely, elemental compositions with an even number of nitrogen atoms at an even mass must contain one 13C rather than 12C (indicated by an asterisk in Figure 3). Finally, even-mass ions can contain one nitrogen and two 13C, as seen for a few species (indicated by a double asterisk) in Figure 3. Similarly, at odd masses (not shown), we observed only N2S and N2, along with some evenmass 12Cn compositions with 13C12Cn-1. The deviation between assigned and calculated mass is very small (0-1 mDa), confirming the elemental assignment of each ion. Elemental composition immediately yields the “class” (namely, the numbers of heteroatoms, NnOoSs) and “type” (number of rings plus double bonds, reported as double-bond equivalents, DBE) for each compositionally distinct component. Figure 4 lists the seven most abundant classes found in both the South American crude oil and its aromatic fraction. The percent relative abundance is the summed mass spectral peak magnitudes for all members of each class divided by the sum of all peak magnitudes in the spectrum. Only those compounds with a percent relative abundance > 1% are included; they are N, NO, NS, NS2, NOS, N2, and N2S. The near-identical percent relative abundances for crude oil and aromatic fraction classes (e.g., 49.7 versus 50.3% for the N class) for each of the seven classes clearly demonstrate that the nonaromatic fractions (saturates, resins, and asphaltenes) do not affect the relative ionization efficiencies for the aromatics. For an elemental composition, CcHhNnOoSs, it can be shown that33
rings + double bonds ) DBE ) c - h/2 + n/2 + 1 (1) Figure 5 shows the DBE distribution for the class of compounds that contain one nitrogen atom (CcH2c+ZN), in which Z is called the hydrogen deficiency. Z is a measure of aromaticity: when the value becomes more negative, the species becomes more aromatic. DBE is also a measure of aromaticity: when the DBE (33) McLafferty, F. W.; Turecek, F. Interpretation of Mass Spectra. University Science Books: Sausalito, CA, 1993; p 371.
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Figure 5. Type distribution for the basic N compounds identified in the ESI FT-ICR mass spectra of South American crude oil and its aromatic fraction. DBE represents the DBE and indicates the number of rings plus double bonds for species containing one nitrogen. The high similarity between the two distributions ranging from 4 < DBE < 28 shows that it is not necessary to isolate the aromatic fraction to identify its polar components.
Figure 6. Carbon number distributions for one of the most abundant basic N1-class compound types (DBE ) 9 for the crude oil and its aromatic fraction). The carbon number range (∼20-65) indicates the degree of alkylation. The similar carbon number distributions suggest that the ionization efficiency and detected degree of alkyl substitution are relatively unaffected by the presence of other species in the crude oil.
value becomes higher, the compound will contain more rings plus double bonds. Both the range, 3 < DBE < 26, and the percent relative abundance for each DBE value are strikingly similar. For the N1 class, for both crude oil and its aromatic fraction, the highest abundance is for DBE ) 8 (most likely diaromatic), consistent with a prior suggestion that most neutral and basic nitrogen compounds contain at least one aromatic ring or one sulfur. Figure 6 shows the carbon number distributions for N1 compounds that contain nine rings plus double bonds. The carbon number, which indicates the degree of alkyl substitution, ranges from 20 to ∼64 for both the crude oil and its aromatic fraction. Again, the percent relative abundances for each carbon number match well between the crude oil and its aromatic fraction, further demonstrating the lack of matrix effects from other species found in crude oil on the basic, polar components. Another way to graphically display species within a particular “class” and “type” is by plotting DBE versus the carbon number,
672 Energy & Fuels, Vol. 20, No. 2, 2006
Figure 7. DBE versus the carbon number for those species containing the heteroatoms, N and S. The signal is color-coded according to the percent relative abundance. Note the high similarity in both DBE distribution and carbon number distribution for South American crude oil and its aromatic fraction.
with color-coding according to percent relative abundance. Figure 7 is one such plot for those components that contain the heteroatoms, N and S. A comparison of the range in both DBE and carbon number distribution clearly illustrates the similarity between these two samples. The South American crude oil and its aromatic fraction both have a DBE range of 6-27, with the highest abundance centered within DBE ) 9-15. The carbon number distributions are approximately the same, ranging from 20 to ∼60 carbon atoms. Figure 8 is also a similar plot of DBE versus the carbon number for species containing two nitrogens. The similarity in both the DBE range and the carbon number distribution again supports the lack of matrix effects of the saturates, resins, and asphaltenes on the basic, polar species typically seen by positive-ion ESI. A similar analysis (not shown) for all classes found in both samples exhibits the same trend. This similarity of the “class”, “type”, and carbon number distributions clearly demonstrates that the ionization of the basic aromatic species generated by positive-ion electrospray is unaffected by the saturates, resins, and asphaltenes also present in crude oil samples.
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Figure 8. DBE versus the carbon number for those species containing two nitrogens, for South American oil and its aromatic fraction.
Conclusions The present results for positive-ion ESI FT-ICR MS complement and support prior results for negative-ion ESI FT-ICR MS.8 In both cases, compounds such as the saturates, resins, and nonpolar aromatics do not affect the ionization efficiency for the polar species. Thus, it is not necessary to fractionate crude oil to analyze its polar components. Elimination of the need for fractionation improves the ultimate prospects for quantitation, because the relative amounts of various constituents are not altered before analysis. Finally, ionization efficiency is expected to be relatively constant for members of any given heteroatom class. In the future, we shall attempt to determine the relative ionization efficiency differences between different heteroatom classes, a comparison rendered more difficult because of a lack of representative model compounds. Acknowledgment. The authors thank Christopher L. Hendrickson, Christine A. Hughey, and John P. Quinn for helpful discussions and Daniel McIntosh for machining all of the custom parts required for the 9.4 T instrument construction. This work is supported by the NSF National High Field Mass Spectrometry Facility (CHE99-09502), Florida State University, and the National High Magnetic Field Laboratory in Tallahassee, FL. EF050353P
