Sulfur Speciation in Petroleum: Atmospheric Pressure Photoionization

Jul 24, 2007 - Molecular characterization of sulfur-containing species in petroleum is important because sulfur-containing compounds are detrimental t...
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Energy & Fuels 2007, 21, 2869-2874

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Sulfur Speciation in Petroleum: Atmospheric Pressure Photoionization or Chemical Derivatization and Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Jeremiah M. Purcell,† Priyanka Juyal,† Do-Gyun Kim,† Ryan P. Rodgers,*,† Christopher L. Hendrickson,† and Alan G. Marshall*,† National High Magnetic Field Laboratory, Florida State UniVersity, 1800 East Paul Dirac DriVe, Tallahassee, Florida 32310-4005 ReceiVed April 24, 2007. ReVised Manuscript ReceiVed June 14, 2007

Molecular characterization of sulfur-containing species in petroleum is important because sulfur-containing compounds are detrimental to the environment and the refining processes. In a recent report, the sulfur-containing compounds in a vacuum bottom residue (VBR) were methylated to enhance their detectability by electrospray ionization (ESI) mass analysis. The most abundant sulfur compounds exhibited relatively low double bond equivalents (4 < DBE < 12). Alternatively, atmospheric pressure photoionization (APPI) mass analysis can provide molecular characterization without chemical derivatization. Here, we compare the sulfur speciation of a petroleum vacuum bottom residue by ESI and APPI with a 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Even after methylation, ions produced by APPI extend to much higher DBE than by ESI. Moreover, analysis of the saturates and aromatics fractions of underivatized VBR by APPI shows comparable ionization efficiency across a broad DBE range. We conclude that methylation is hindered for high-DBE species (DBE > 20), so that methylation followed by ESI MS is not suitable for sulfur speciation of higher-boiling fractions from petroleum crude oil.

Introduction Despite recent interest in renewable energy sources, fossil fuels are projected to be the major source of energy for the next 50 years.1 The increase in global consumption of “sweet” crude oil reserves has led to an increase in the refinement of less desirable heavy crudes, as is evident by a steady trend in feed stock crude oils in the United States toward lower API gravity (heavier crude oils) and higher sulfur content.2 The heavier feedstocks contain a higher weight percent of sulfur, nitrogen, and oxygen heteroatoms. The heteroatom-containing compounds in heavy petroleum are harmful to the environment and are detrimental to hydrogen addition and carbon rejection processes in petroleum refineries and, therefore, must be removed.1 Even before the current need for low sulfur petroleum products, the refining industry had employed hydrodesulfurization for other reasons, e.g., to decrease corrosion, increase gasoline stability, and decrease smoke formation in kerosene.1 Therefore, the petroleum industry has significant experience in desulfurization processes but primarily for lighter feedstocks. With current feedstock trends and a desire to process atmo* To whom correspondence should be addressed: Ion Cyclotron Resonance Program, National High Magnetic Field Laboratory, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, FL 32310-4005. Telephone: 1-850-644-0529 or -2398. Fax: 1-850-644-1366. E-mail: [email protected] (A.G.M.) or [email protected] (R.P.R.). † Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-4390. (1) Speight, J. G. The Desulfurization of HeaVy Oils and Residua, 2nd ed.; Marcel Dekker, Inc.: New York, 2000; Vol. 78. (2) Swain, E. J. U. S. refining crude slates continue towards heavier feeds, higher sulfur content. Oil Gas J. 1998, 96 (40), 43-48.

spheric and vacuum bottom residue into marketable lighter petroleum products, there is a need to develop new refining technologies and processing methods better suited for the heavier feedstocks and residues. Crude oil is a complex mixture of hydrocarbons with varying amounts of nitrogen, sulfur, and oxygen and trace amounts of metal (iron, nickel, and vanadium). An industry standard for bulk characterization of crude oil is through the study of its fractional components. However, the fractional compositions vary significantly with laboratory isolation procedures, especially for heavier feedstocks.3 To develop more efficient refining processes for heavier petroleum feedstocks and residues, mass spectrometry can provide detailed compositional information on whole crude oils.4,5 Specifically, Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is capable of resolving >12 000 spectral peaks in a single mass spectrum (paramount for petroleum) and providing unambiguous molecular formulas based on mass accuracy and homologue series.6 Electrospray ionization (ESI) coupled to FT-ICR MS can provide elemental composition, CcHhNnOoSs, for ∼10 000 polar (3) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker, Inc.: New York, 1999; Vol. 76. (4) Marshall, A. G.; Rodgers, R. P. Petroleomics: The Next Grand Challenge for Chemical Analysis. Acc. Chem. Res. 2004, 37, 53-59. (5) Rodgers, R.; Schaub, T.; Marshall, A. Petroleomics: MS returns to its roots. Anal. Chem. 2005, 77 (1), 20A-27A. (6) Purcell, J. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Atmospheric Pressure Photoionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry for Complex Mixture Analysis. Anal. Chem. 2006, 78 (16), 5906-5912.

10.1021/ef700210q CCC: $37.00 © 2007 American Chemical Society Published on Web 07/24/2007

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constituents in petroleum7 and coal.8 Acidic molecular species, e.g., compounds with carboxylic acid or sulfonic acid groups, are deprotonated, and basic species, e.g., pyridinic nitrogen, are protonated in the ESI process to form [M-H]- or [M+H]+ ions. However, sulfur-containing compounds (e.g., thiophenes, which are not sufficiently acidic or basic) and hydrocarbons are not efficiently ionized by ESI. One possible analytical/characterization method for nonpolar sulfur species is ESI mass spectrometry of derivatized sulfur compounds. The derivatization chemistry involves electrophilic attack on sulfur by a strong alkylating (methylating) reagent to form S-alkyl (methyl) sulfonium salts in solution prior to ESI. As an alternative, atmospheric pressure photoionization (APPI) can efficiently ionize gas-phase nonpolar species (and polar species) through direct photon ionization9,10 or proton transfer.11,12 Thus, APPI precludes the need for derivatization. Recently, sulfur derivatization of a vacuum bottom residue followed by ESI FT-ICR MS analysis was reported.13 This analysis provided an elemental molecular characterization of a feedstock (∼900 species) before hydroprocessing and an effluent (∼1000 species) after hydroprocessing. The derivatization was preceded by a saturates-aromatics-resins-asphaltene (SARA) fractionation and a ligand exchange chromatographic procedure to enrich sulfur species. The results indicated that the bulk of the sulfur compounds for the feedstock and effluent of the vacuum bottom residue exhibited double bond equivalents (DBE, a value equal to the number of rings plus double bonds in the molecular structure, calculated from the elemental formula) between 4 and 12. However, from our experience with vacuum residues and heavy crude oils, the reported DBE ranges for the sulfur-containing species were much lower than previously observed by APPI and lower than that expected for species concentrated in a vacuum residue. In this report, we utilize ESI and APPI coupled to a 9.4 T FT-ICR mass spectrometer to speciate the sulfur-containing compounds in a vacuum bottom residue and identify similarities and/or differences in the species identified between the ionization techniques. We then compare the results (ESI and APPI) of the chemically derivatized vacuum bottom residue to highlight limitations of the derivatization process. Methods Vacuum Bottom Residue. Methyl iodide, silver tetrafluoroborate, 1,2- dichloroethane, methylene chloride, and acetonitrile were (7) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Resolution of 11 000 Compositionally Distinct Components in a Single Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrum of Crude Oil. Anal. Chem. 2002, 74, 4145-4149. (8) Wu, Z.; Jernstro¨m, 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, 946953. (9) Revel’skii, I. A.; Yashin, Y. S.; Kurochkin, V. K.; Kostyanovskii, R. G. Mass spectrometry with photoionization at atmospheric pressure and analysis of multicomponent mixtures without separation. ZaVod. Lab. 1991, 57 (3), 1-4. (10) Syage, J. A.; Evans, M. D. Photoionization Mass Spectrometry. Spectroscopy 2001, 16 (11), 14-21. (11) Robb, D. B.; Covey, T. R.; Bruins, A. P. Atmospheric pressure photoionization: an ionization method for liquid chromatography-mass spectrometry. Anal. Chem. 2000, 72 (15), 3653-9. (12) Purcell, J. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Atmospheric Pressure Photoionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: Proton Transfer Reactions in Complex Mixtures. J. Am. Soc. Mass Spectrom. 2007, submitted for publication. (13) Mueller, H.; Andersson, J. T.; Schrader, W. Characterization of High-Molecular-Weight Sulfur-Containing Aromatics in Vacuum Residues Using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry. Anal. Chem. 2005, 77 (8), 2536-2543.

Purcell et al. purchased from Sigma-Aldrich (high purity, St. Louis, MO). Established methodology was adopted for the derivatization chemistry.13-15 A sample of Canadian bitumen vacuum residue (13.5 mg) was dissolved in a (conical) vial containing 1,2dichloroethane (3 mL), methyl iodide (1 mmol), and a stir bar. During mixing, a solution of silver tetrafluoroborate (1 mmol) in 3 mL of 1,2-dichloroethane was added. A yellow-brown precipitate immediately formed upon addition. The solution was stirred vigorously on a magnetic stir plate at ambient temperature for 48 h. The precipitate was filtered and further washed with 1,2dichloroethane. The combined washings and filtrate were evaporated to dryness under reduced pressure to remove solvent and excess methyl iodide. The methylated samples were redissolved in a 1:1 (v/v) solution of methylene chloride/acetonitrile (10 mg/mL stock solution) for ESI analysis (1 mg/mL analysis concentration in methylene chloride/acetonitrile). The stock methylated solution was diluted (1:10) into toluene for APPI analysis. For comparison, untreated residue was prepared (1 mg/mL in 60:40 toluene:methanol and 1% formic acid) for ESI analysis and APPI analysis (1 mg/ mL in toluene). SARA Fractionation Solvents were purchased from Fisher Chemical (HPLC grade). A SARA fractionation16 of the vacuum bottom residue (VBR) was accomplished (517.1 mg). The sample was mixed with n-heptane (50 mL), stirred with a magnetic stir bar for 90 min, and stored in the dark overnight. A Whatman No. 1 filter paper was used to separate the n-heptane insolubles (asphaltenes) from the maltenes and the filter paper asphaltenes were dried at room temperature. The maltenes/n-heptane solution was rotary-evaporated to dryness under reduced pressure. The dry maltenes were redissolved in n-heptane (6 mL). The maltene solution was then adsorbed onto the surface of activated alumina (3 g), and the maltene alumina slurry was dried during stirring under a stream of nitrogen. A glass column (11 mm i.d. × 300 mm length) was packed with activated alumina adsorbent (6 g), and the adsorbed maltenes were packed on the top. In sequence, 40 mL of n-heptane, 80 mL of toluene, and 50 mL of a toluene:MeOH (8:2 v/v) mixture were used to elute the saturates, aromatics, and resins, respectively. The eluants were rotary-evaporated under reduced pressure until dry and then weighed. The recovered mass (presumably including some solvent) for each fraction and percent recovered follows: saturates, 122.5 mg (23.7%); aromatics, 150.0 mg (29.0%); resins, 150.1 mg (29.0%); asphaltenes, 172.6 mg (33.4%). The saturates and aromatics were diluted in toluene (1 mg/mL) for APPI analysis. CHNOS Analysis A flash elemental analyzer (C.E. Elantech, Inc.) model 1112 was used for bulk CHNS/O weight percent determination of the vacuum bottom residue. Quadruplicate samples (∼2 mg) were weighed for the CHNS analysis (combustion) and O analysis (pyrolysis). Calibration values were developed by use of sulfanilamide and 2,5-bis-(5-tert-butylbenzoxazol-2-yl)-thiophene (BBOT) standards. The percent composition was as follows (relative standard deviation (RSD)): carbon 81.3 ( 0.19% RSD, hydrogen 9.5 ( 0.28% RSD, nitrogen 0.75 ( 2.3% RSD, sulfur 5.7 ( 3.8% RSD, oxygen 1.5 ( 3.1% RSD. 9.4 T FT-ICR MS. A previously described 9.4 T FT-ICR mass spectrometer17,18 was utilized for the APPI and ESI analysis of the (14) Acheson, R. M.; Harrison, D. R. Synthesis, Spectra, And Reactions Of Some S-Alkylthiophenium Salts. J. Chem Soc., C 1970, 13, 1764. (15) Green, T. K.; Whitley, P.; Wu, K.; Lloyd, W. G.; Gan, L. Z. Structural characterization of sulfur compounds in petroleum by Smethylation and carbon-13 NMR spectroscopy. Energy Fuels 1994, 8, 244248. (16) Rudzinski, W.; Aminabhavi, T.; Sassman, S.; Watkins, L. Isolation and characterization of the saturate and aromatic fractions of a Maya crude oil. Energy Fuels 2000, 14 (4), 839-844. (17) Hakansson, K.; Chalmers, M. J.; Quinn, J. P.; McFarland, M. A.; Hendrickson, C. L.; Marshall, A. G. Combined electron capture and infrared multiphoton dissociation for multistage MS/MS in a Fourier transform ion cyclotron resonance mass spectrometer. Anal. Chem. 2003, 75 (13), 32563262. (18) 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.

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vacuum bottom residue. Multiple (100-200) time-domain acquisitions were summed for each sample, Hanning-apodized, and zerofilled once before fast Fourier transform and magnitude calculation.19 All observed ions were singly charged, based on the unit m/z separation between 12Cn and 13C112Cn-1 isotopic variants of the same elemental composition.20 Therefore, mass spectral peak positions are reported in Dalton rather than as m/z. ESI Apparatus. One end of a 50 µm i.d. fused silica tube was ground in-house to a point and used as the microelectrospray source21 to produce ESI positive ions. A syringe pump delivered the solution at 400 nL/min. General conditions were as follows: needle voltage, 2 kV; tube lens, 350 V; and heated metal capillary operated at ∼10 W. Such conditions are optimal for generating the most complete characterization of petroleum by ESI.7 APPI Source. The APPI source was supplied by ThermoFisher Scientific. The vaporized analyte gas stream flows orthogonally to the mass spectrometer inlet (heated metal capillary) and the krypton vacuum UV lamp. The source was mounted to a custom-built interface to the first differentially pumped stage of the 9.4 T FTICR mass spectrometer through a heated metal capillary (stainless steel, 750 µm i.d.).6 A Harvard Apparatus stainless steel syringe (8 mL) and syringe pump delivered solution to the heated nebulizer of the APPI source. In the APPI source, the solvent flow rate was 50-100 µL/min; the nebulizer heater was operated at 250350 °C with carbon dioxide as the sheath gas at 550 kPa, and the auxiliary gas port was plugged. Kendrick Mass Analysis Because crude oil consists primarily of homologous series which differ by nCH2 (n is a positive integer), it is convenient to convert the IUPAC mass to a Kendrick mass.22,23 Kendrick mass ) IUPAC mass × (14/14.01565)

(1)

The Kendrick masses for members of a homologous alkylation series differ by increments of exactly 14 Da and have the same Kendrick mass defect (KMD). KMD ) (Kendrick nominal mass - Kendrick exact mass) × 1000 (2) The data may then be sorted by KMD to facilitate assignment of elemental composition. Furthermore, each homologous series is categorized by class, double bond equiValents24(DBE; see eq 3), and carbon number. The class designates the heteroatom content of the molecule, e.g., the S1 class denotes a class of molecules that contain only one sulfur atom and the remaining atoms are carbon and hydrogen. DBE is a measure of hydrogen saturation. A fully hydrogenated molecule has DBE ) 0. Every additional ring or double bond results in the loss of two hydrogen atoms. Within each class, there are many DBE values. For example, C42H59N1 belongs to the N1 class, with DBE ) 14, whereas C42H53N1 belongs to the same N1 class, but its DBE is 17. These two molecular species (19) Marshall, A. G.; Verdun, F. R. Fourier Transforms in NMR, Optical, and Mass Spectrometry: A User’s Handbook; Elsevier: Amsterdam, 1990; p 460. (20) Senko, M. W.; Beu, S. C.; McLafferty, F. W. Automated Assignment of Charge States from Resolved Isotopic Peaks for Multiply Charged Ions. J. Am. Soc. Mass Spectrom. 1995, 6, 52-56. (21) Emmett, M. R.; Caprioli, R. M. Micro-electrospray mass spectrometry: ultra-high-sensitivity analysis of peptides and proteins. J. Am. Soc. Mass Spectrom. 1994, 5 (7), 605-613. (22) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. Kendrick Mass Defect Spectrum: A Compact Visual Analysis for Ultrahigh-Resolution Broadband Mass Spectra. Anal. Chem. 2001, 73 (19), 4676-4681. (23) Kendrick, E. Mass scale based on CH2 ) 14.0000 for high-resolution mass spectrometry of organic compounds. Anal. Chem. 1963, 35 (13), 2146-54. (24) McLafferty, F. W.; Turececk, F. Interpretation of Mass Spectra, 4th ed.; University Science Books: Mill Valley, CA, 1993; p 371.

Figure 1. Raw vacuum bottom unmethylated Canadian bitumen residue heteroatom class distribution. All classes ionized by ESI and APPI above 1% relative abundance are represented. The nonpolar classes, e.g., S1, S2, and HC (hydrocarbon), are not detected by ESI. APPI analysis detects both polar and nonpolar species.

have the same number and type of heteroatoms but differ by 3 DBE. The DBE is equal to the number of rings plus double bonds in the molecular structure and is calculated from double bond equivalents ) c - h/2 + n/2 + 1

(3)

in which c, h, and n are the numbers of carbon, hydrogen, and nitrogen atoms in the molecular formula.

Results and Discussion Although crude oil is the most compositionally complex organic mixture, ESI FT-ICR MS has enabled the detailed speciation of its polar constituents.4,5 In addition, APPI can produce ions from nonpolar sulfur-containing petroleum compounds and, when combined with the ultrahigh mass resolving power and unmatched mass accuracy of FT-ICR MS, can achieve sulfur speciation of petroleum. Petroleum fractionation methods can be employed to reduce the sample complexity. Mueller et al.13 previously fractionated a vacuum bottom residue (VBR) before mass analysis; we choose here to compare raw VBR (for simpler sample preparation) and methylated raw VBR by ESI and APPI coupled to a 9.4 T FT-ICR mass spectrometer. Raw Vacuum Bottom Residue. The residue analysis (raw, unmethylated) yielded approximately 5800 and 2000 unique elemental formulas for positive ion APPI and ESI, respectively. The elemental formulas were sorted by class, DBE, and carbon number. Figure 1 represents the summed relative abundances of each class (>1% relative abundance). The nitrogen class is the most abundant for both ionization techniques. ESI efficiently ionizes the basic pyridinic N1 class25 at twice the relative abundance of the APPI N1 class. For other polar classes, e.g., N1S1 and N1O1, the relative ionization efficiency for the two ionization methods is comparable. For the nonpolar species, e.g., S1, S2, and HC (hydrocarbon), only APPI produces mass spectral signals. Additional information is derived from isoabundance contours for a plot of DBE vs carbon number, for a given heteroatom class. Figure 2 shows DBE vs carbon number distributions (25) Purcell, J. M.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Speciation of nitrogen containing aromatics by atmospheric pressure photoionization or electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. J. Am. Soc. Mass Spectrom. 2007, 18, 12651273.

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Figure 2. Isoabundance-contoured DBE versus carbon number images for S1 and N1S1 ion classes from raw vacuum bottom Canadian bitumen residue. Relative ion abundance is scaled within each heteroatom class. The ESI and APPI N1S1 classes (as well as other classes, not shown) have similar carbon number and DBE distributions. Nonpolar S1 species (e.g., dibenzothiophenes) are not detected by ESI.

Figure 3. Heteroatom class distribution for the raw methylated vacuum bottom Canadian bitumen residue. Note the dramatic increase in the relative abundance of S1 class ions generated by ESI. The S2, HC, and O1 classes are also detected by ESI only after methylation (compare to Figure 1). The APPI heteroatom class distribution is otherwise similar to that in Figure 1.

derived from ESI and APPI for the S1 and N1S1 classes of the raw VBR. The APPI S1 class exhibits a carbon distribution, 22 < c < 45, and, more interestingly, a DBE distribution of 6 < DBE < 35, whereas ESI S1 class ions are absent altogether. The most abundant APPI S1 ions have DBE values between 20 and 30. The ESI and APPI N1S1 class ions have similar abundance at low DBE, but the APPI N1S1 class ions extend to higher DBE (similar to that for the APPI S1 class); note that the APPI aromatic nitrogen species include both pyridinic and pyrrolic aromatics, whereas positive-ion ESI shows only pyridinic aromatics.25 Similarly, the most abundant ions from other mutually common classes (ESI and APPI, Figure 1) display comparable carbon number and low-range DBE values, but APPI-generated species extend to higher DBE values. Finally, vacuum residues are not highly alkylated (note the diagonal shape of the DBE vs. carbon number plot of Fig. 2 (bottom left), showing that the increase in carbon number is predominantly associated with additional aromatic rings rather than additional alkylation). Methylated Raw Vacuum Bottom Residue. Positive-ESI and APPI FT-ICR MS analyses were performed for the

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Figure 4. Isoabundance-contoured DBE versus carbon number images for S1 and N1S1 ion classes from raw methylated vacuum bottom Canadian bitumen residue. Comparison to Figure 2 shows that the N1S1 images are similar to those for the unmethylated sample, whereas S1 class ESI species (which are absent altogether by ESI) are rendered much more abundant following methylation.

methylated raw vacuum bottom residue and yielded the class distributions shown in Figure 3. The APPI class distribution (Figure 3) for the methylated raw VBR is nearly identical to that for the raw (underivatized) VBR in Figure 1. In contrast, for the ESI classes, there is a dramatic shift in the highest relative abundance from the N1 class (Figure 1) to the S1 (and S2 and O1S1 classes, presumably because the methylated thiophene moiety is already positively charged before electrospray). Furthermore, the methylation reaction followed by ESI generates hydrocarbon (HC) and O1 class ions not previously detected in the raw VBR ESI data. The ESI O1 class most likely contains furans that can also react with the derivatization reagent to form oxonium ions, and even aromatic hydrocarbons can react.26,27 Figure 4 presents isoabundance-contoured plots of DBE vs carbon number for the APPI and ESI S1 and N1S1 class ions from methylated VBR. The APPI S1 and N1S1 images are similar to those for unmethylated VBR (Figure 2), except that species of slightly lower DBE values in the raw VBR S1 image (APPI) are absent from the methylated VBR (APPI). Similarly, the ESI N1S1 image of Figure 4 is comparable to the ESI N1S1 image of Figure 2. And, of course, methylation renders ESI-observable S1 species that are absent from the unmethylated sample. However, the differences between the ESI and APPI S1 class images for the derivatized VBR sample are remarkable. Although the carbon number distributions are similar, the DBE distributions are well separated with little overlap. If the methylation reaction is hindered for larger DBE species, the low DBE S1 species may represent only a small percentage (low mass fraction) of the total raw VBR sulfur species but nevertheless appear highly abundant in the ESI mass spectrum due to their preferential methylation efficiency (compared to high-DBE S1 species) in the derivatization step. Saturates and Aromatic Fraction of the Vacuum Bottom Residue. Conversely, it is possible that APPI more efficiently ionizes larger DBE species and thereby biases the DBE images to high values. We therefore examine the APPI ionization efficiency for species of widely different aromaticity (DBE), by fractionation of VBR into saturates and aromatics fractions (26) Olah, G. A.; Prakash, G. K. S. Carbocation Chemistry; John Wiley and Sons Inc.: Hoboken, NJ, 2004. (27) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; John Wiley and Sons Inc.: New York, 1985.

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Figure 5. Heteroatom class distribution for ions produced by APPI from the saturates, aromatics, and an equal mass/volume mixture of saturates and aromatics extracted from Canadian bitumen vacuum bottom residue.

Figure 6. DBE distribution for APPI-generated S1 class positive ions from the saturates, aromatics, and the saturates-plus-aromatics fractions extracted from Canadian bitumen vacuum bottom residue. Half-integer DBE values correspond to protonated compounds [M+H]+, and integer DBE values correspond to molecular radical ions, M+•.

and subsequent analysis by APPI. An equal mixture of saturates and aromatics should thus reveal any gross differences in ionization efficiency between low-DBE S1 species (in the saturates fraction) and high-DBE S1 species (in the aromatics fraction). Figure 5 is the APPI heteroatom class distribution for the saturates, aromatics, and a combined mixture (1:1 v/v of equal w/v solutions) of the two. Most notably, the saturates fraction is high in S1 and low in N1 class ions, suggesting that the saturates fraction in a SARA isolation procedure is highly enriched in sulfur-containing compounds. As expected, the combined saturates and aromatics mixture heteroatom class distribution is similar to that for APPI of the raw VBR distribution (Figure 1). Figure 6 shows the S1 class positive-ion DBE distribution for each of the three solutions. Because each calculated DBE value (eq 3) is for the cation (not the neutral), a protonated analyte, [M+H]+, yields a half-integer calculated DBE due to its additional proton, whereas a radical molecular ion, M+•, yields an integer calculated DBE value. For the saturates, the DBE values are low, whereas, for the aromatics fraction, DBE values are high. As expected, the combined saturates and

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Figure 7. Isoabundance-contoured image for the S1 class of APPIgenerated positive ions from the saturates, aromatics, and combined saturates/aromatics fractions extracted from Canadian bitumen vacuum bottom residue. As for the DBE distribution (Figure 6), the lower DBE species are found in the saturates fraction, and higher DBE species in the aromatic fraction.

aromatics mixture yields a broad distribution that is essentially the sum of those for the individual saturates and aromatics fractions. Figure 7 shows iso-abundance contoured DBE vs carbon number plots for the saturates, aromatics, and combined fractions S1 class APPI ions. As for the DBE distributions alone (Figure 6), the carbon number distribution for the combined mixture is essentially the sum of those for the saturates and aromatics fractions. Thus, it appears from both Figures 6 and 7 that APPI ionizes analytes spanning a wide range of DBE values with comparable efficiency. Thus, the increased abundance of low DBE S1 species observed by ESI of methylated VBR (Figure 4, top left) and whose presence in the unmethylated VBR is confirmed by APPI mass spectral analysis of the SARA fractions (Figures 6 and 7) evidently results from enhanced methylation reactivity for low-DBE S1 species and not from DBE-specific APPI ionization efficiency. Therefore, the lowDBE S1 species in the saturates fraction (Figures 6 and 7) must be present at low relative abundance (reasonable for a vacuum bottom residue) and are not observed because they are simply below the detection limit. Although chemical derivatization does allow for nonpolar sulfur speciation by ESI, high-DBE sulfur species are not efficiently methylated. Thus, methylation does not provide accurate sulfur speciation for petroleum materials that contain high DBE (>10) sulfur species and is inherently limited to analysis of light distillate fractions. We are exploring other derivatization reactions and conditions to see if DBEdependent reactivity can be overcome. The APPI ionization mechanisms (charge exchange and proton transfer) result in efficient ionization of structurally diverse compounds. One is thus led to ask if the relative magnitudes of APPI mass spectral peaks can quantitatively represent the relative abundances of their neutral precursor compounds in the original sample. For the raw unmodified APPI VBR, the spectral peak magnitude-weighted elemental analysis is as follows: C 84.8%, H 7.5%, N 2.1%, O 0.6%, and S 5.0%, i.e., roughly comparable to the bulk elemental analysis. More specifically, the carbon, hydrogen, and sulfur match well, but the nitrogen-weighted elemental MS analysis is high, and the oxygen is low. Those differences may be attributed to different ionization efficiency, i.e., that ionization efficiency depends more on acidic or basic moieties than on an aromatic core structure. For example, pyridinic nitrogen compounds are efficiently protonated (accounting for the higher nitrogen abundance derived from mass spectral magnitudes). Similarly,

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much of the oxygen is present as carboxylic acids that can lose a proton to form negative ions not seen by positive ion mass analysis, thus accounting for the lower MS-based oxygen estimate. A similar calculation from the APPI positive-ion mass spectrum of the methylated ESI VBR sample yields an apparent elemental composition: C 82.4%, H 11.0%, N 5.6%, O 4.4%, and S 5.5%. Thus, alhough the sulfur species are overrepresented (Figure 3) in the mass spectrum, the MS-based, sulfur-weighted elemental abundance is comparable to that for the bulk sample. In contrast, the nitrogen and oxygen MS-based

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elemental abundances are several orders of magnitude higher than that for the bulk sample, due to higher ionization efficiency for those species. Acknowledgment. This work was supported by the NSF National High Field FT-ICR Facility (DMR 00-84173), Florida State University, and the National High Magnetic Field Laboratory in Tallahassee, FL. EF700210Q