Energy Fuels 2009, 23, 5564–5570 Published on Web 09/16/2009
: DOI:10.1021/ef9006005
Analysis of Asphaltenes and Asphaltene Model Compounds by Laser-Induced Acoustic Desorption/Fourier Transform Ion Cyclotron Resonance Mass Spectrometry )
David S. Pinkston,† Penggao Duan,†,^ Vanessa A. Gallardo,† Steven C. Habicht,† Xiaoli Tan,§ Kuangnan Qian,‡ Murray Gray,§ Klaus Mullen, and Hilkka I. Kentta¨maa*,†
)
† Department of Chemistry, Purdue University, 560 Oval Dr., West Lafayette, Indiana 47907, ‡ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801, §Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada, Max Planck Institute fur Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and ^Current address: Bruker Daltonics Inc. 40 Manning Rd., Billerica, Massachusetts 01821
Received June 12, 2009. Revised Manuscript Received August 21, 2009
Laser-induced acoustic desorption (LIAD)/electron ionization (EI) was used to study asphaltene model compounds and asphaltenes derived from North American crude oil in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer (MS). Successful desorption by LIAD of all model compounds (including a polyphenylated vanadoyl porphyrin) as intact neutral molecules into the mass spectrometer indicates that this method allows the evaporation of most if not all components of asphaltenes into mass spectrometers for further characterization. Electron ionization is a universal ionization method that ionizes all organic compounds. Hence, it is not surprising that all the model compounds studied were successfully ionized by using this method. Furthermore, this method yielded stable molecular ions for all model compounds studied. Because LIAD/EIMS provides MW information for these model compounds, this is almost certainly also true for all components of asphaltenes. Examination of asphaltene samples derived from North American crude oil by using this technique yielded a MW distribution of about 350-1050 Da and provided structural information for asphaltene components.
A deeper understanding of asphaltenes is required to effectively utilize these resources. The analysis of asphaltenes has proven to be a challenging endeavor due to their molecular complexity, high boiling points, limited solubility, and tendency to aggregate. Thus, the molecular compositions of asphaltenes remain poorly understood. Many studies have been dedicated just to probing the molecular weight (MW) distributions of asphaltenes.10-12 However, contradictory results have sparked a controversy over the correct distributions. The reported MW distributions of petroleum asphaltenes span from a low mass distribution with a range of about 400-1500 Da to high mass distributions that extend past 106 Da. Several recent reviews focus on this controversial topic.13-15 Time-resolved fluorescence depolarization (TRFD) was the technique that first provided evidence in support of a “low” mass distribution for asphaltenes.16-18 Moreover, this technique provided evidence for an “island” (one aromatic core) molecular model of asphaltenes rather than the “archipelago”
Introduction Asphaltenes are typically defined as a petroleum fraction soluble in toluene and insoluble in n-heptane.1 They are known as the heaviest components with the highest boiling points in crude oil. They have a high degree of aromaticity and can contain nitrogen, oxygen, sulfur, and metal atoms. Removal of contaminants in asphaltenes during the processing of crude oil is necessary to avoid catalyst fouling and hence lower liquid yields.2-5 Also, asphaltenes can precipitate in the equipment, thus increasing maintenance costs.5 Hence, the achievement of a better understanding of asphaltenes’ composition has become important.6-8 Furthermore, in the years to come, the significance of asphaltenes is poised to increase as oil industry shifts to the use of heavier crude oils that contain high concentrations (∼15%) of asphaltenes.9 *To whom correspondence should be addressed. E-mail: hilkka@ purdue.edu. (1) Mullins, O. C.; Sheu, E. Y.; Hammami, A.; Marshall, A. G., Asphaltenes, Heavy Oils, and Petroleomics; Springer: New York, 2007. (2) Ancheyta, J.; Betancourt, G.; Centeno, G.; Marroquin, G.; Alonso, F.; Garciafigueroa, E. Energy Fuels 2002, 16, 1438–1443. (3) Ancheyta, J.; Betancourt, G.; Marroquin, G.; Centeno, G.; Castaneda, L. C.; Alonso, F.; Munoz, J. A.; Gomez, M. T.; Rayo, P. Appl. Catal., A 2002, 233, 159–170. (4) Ancheyta, J.; Betancourt-Rivera, G.; Marroquin-Sanchez, G.; Perez-Arellano, A. M.; Maity, S. K.; Cortez, T.; del Rio-Soto, R. Energy Fuels 2001, 15, 120–127. (5) Trejo, F.; Centeno, G.; Ancheyta, J. Fuel 2004, 83, 2169–2175. (6) Klein, G. C.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2006, 20, 1965–1972. (7) Mitra-Kirtley, S.; Mullins, O. C.; Vanelp, J.; George, S. J.; Chen, J.; Cramer, S. P. J. Am. Chem. Soc. 1993, 115, 252–258. (8) George, G. N.; Gorbaty, M. L. J. Am. Chem. Soc. 1989, 111, 3182– 3186. (9) Head, I. M.; Jones, D. M.; Larter, S. R. Nature 2003, 426, 344–352. r 2009 American Chemical Society
(10) Tanaka, R.; Sato, S.; Takanohashi, T.; Hunt, J. E.; Winans, R. E. Energy Fuels 2004, 18, 1405–1413. (11) Hortal, A. R.; Hurtado, P.; Martinez-Haya, B.; Mullins, O. C. Energy Fuels 2007, 21, 2863–2868. (12) Badre, S.; Goncalves, C. C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1–11. (13) Strausz, O. P.; Safarik, I.; Lown, E. M.; Morales-Izquierdo, A. Energy Fuels 2008, 22, 1156–1166. (14) Trejo, F.; Ancheyta, J.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2007, 21, 2121–2128. (15) Herod, A. A.; Bartle, K. D.; Kandiyoti, R. Energy Fuels 2007, 21, 2176–2203. (16) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237– 11245. (17) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677–684. (18) Groenzin, H.; Mullins, O. C. Pet. Sci. Technol. 2001, 19, 219–230.
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model (multiple aromatic centers bridged by alkyl chains). Following these investigations, many other researchers corroborated their results by using Taylor dispersion,19 NMR translational diffusion,20 and/or field correlation spectroscopy.21,22 Mass spectrometric measurements have also produced contradictory results for the molecular weights of asphaltenes. Experiments utilizing electrospray ionization (ESI),23 atmospheric pressure chemical ionization (APCI),24 and field desorption/field ionization25 (FD/FI) have yielded results that agree with the low mass distribution, with detected MWs starting at 500 Da and extending to 3000 Da, although the results appear to be very sensitive to the experimental conditions used (e.g., sample concentration, solvent, and temperature). However, other mass spectrometric techniques, such as matrix-assisted laser desorption/ionization (MALDI), have indicated the presence of molecules with much higher MWs, even over 10 000 Da.14 Laser desorption/ionization (LDI) has yielded varied results.10,11,26,27 This discrepancy concerning the MW distributions of asphaltenes has been suggested to arise from their propensity to aggregate.27 Evidence in support of this hypothesis has been obtained from LDI experiments.27,28 In one such experiment, higher energy laser pulses and/or higher asphaltene concentrations were found to result in a broader MW distribution and greater measured MWs, presumably due to aggregation.27 In another experiment, ion mobility mass spectrometry was used to distinguish between monomers and aggregates formed from asphaltene samples upon LDI.28 When using a higher pressure, molecules with higher molecular weights were observed. This was explained by collisional cooling of the desorbed aggregate ions at higher pressures. More recently, a new laser-based technique, called two-step laser mass spectrometry, was used to obtain a MW distribution for asphaltenes with a maximum near 600 Da and the highest MW within 1000-1500 Da.29,30 This experiment involves the use of a short pulse of low-energy photons (with energies well below the ionization energies of asphaltenes) to quickly thermally desorb all molecules. After desorption, the neutral molecules are ionized using a second, higher energy laser pulse. The authors believe that this approach eliminates aggregation since the desorption step involves neutral molecules as opposed to ions, and desorbed
neutral molecules are not expected to form aggregates as readily as ionic molecules.31 In 2008, Marshall et al. published a comprehensive paper on mass spectrometric determination of the molecular weight distributions of petroleum asphaltenes that considered all the above results as well as some new ones.32 They concluded that all mass spectrometric measurements, if conducted appropriately, yield the same MW distribution for petroleum asphaltenes, and this distribution has a maximum at 750 ((200 Da) and a full-width half-maximum of 500-1000 Da. Although the issue on the general molecular weight distribution of petroleum asphaltenes may have been resolved, we still lack a method that easily yields reliable and quantitative molecular weight information for asphaltenes of different origins, as well as bitumens. Most if not all mass spectrometric methods discussed above have a bias for selected components of asphaltenes. For example, conventional ESI is most sensitive toward the most basic, N-containing components, and may not ionize substantially less-basic compounds when Ncontaining analytes are present.33 The same is almost certainly true for MALDI and other methods that are usually (but not always) based on proton transfer ionization. The two-step laser mass spectrometry method discussed above is based on resonance ionization and hence is likely to have different response factors for asphaltene components with different structures, as for example different numbers of aromatic rings.29 As a result, the MW distributions measured using these methods may not provide an accurate description of all the components in asphaltenes. To examine the MW distributions of petroleum asphaltenes by using yet a different method, and to test the potential of this method for providing reliable quantitative molecular level information on asphaltenes, we have employed laser-induced acoustic desorption34-37 (LIAD) to evaporate asphaltenes into a dual-cell Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) where the neutral asphaltene molecules were ionized by electron bombardment, a universal ionization method that is known to ionize all organic compounds. One of the benefits of LIAD is that it does not use direct irradiation of the sample like the other laser techniques discussed above. Instead, LIAD is based on firing laser pulses on the backside of a thin Ti foil that has a sample deposited on the opposite side. This results in the generation of acoustic waves that propagate through the foil and evaporate the sample as neutral, internally and kinetically low-energy molecules.38-40 These neutral molecules can then be ionized by various methods, such as electron bombardment and chemical ionization, within the mass spectrometer.
(19) Wargadalam, V. J.; Norinaga, K.; Lino, M. Fuel 2002, 81, 1403– 1407. (20) Goncalves, S.; Castillo, J.; Fernandez, A.; Hung, J. Fuel 2004, 83, 1823–1828. (21) Schneider, M. H.; Andrews, A. B.; Mitra-Kirtley, S.; Mullins, O. C. Energy Fuels 2007, 21, 2875–2882. (22) Guerra, R. E.; Ladavac, K.; Andrews, A. B.; Mullins, O. C.; Sen, P. N. Fuel 2007, 86, 2016–2020. (23) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 4145–4149. (24) Cunico, R. L.; Sheu, E. Y.; Mullins, O. C. Pet. Sci. Technol. 2004, 22, 787–798. (25) Qian, K. N.; Edwards, K. E.; Siskin, M.; Olmstead, W. N.; Mennito, A. S.; Dechert, G. J.; Hoosain, N. E. Energy Fuels 2007, 21, 1042–1047. (26) Acevedo, S.; Gutierrez, L. B.; Negrin, G.; Pereira, J. C.; Mendez, B.; Delolme, F.; Dessalces, G.; Broseta, D. Energy Fuels 2005, 19, 1548– 1560. (27) Hortal, A. R.; Martinez-Haya, B.; Lobato, M. D.; Pedrosa, J. M.; Lago, S. J. Mass Spectrom. 2006, 41, 960–968. (28) Becker, C.; Qian, K. N.; Russell, D. H. Anal. Chem. 2008, 80, 8592–8597. (29) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. J. Am. Chem. Soc. 2008, 130, 7216–7217. (30) Pomerantz, A. E.; Hammond, M. R.; Morrow, A. L.; Mullins, O. C.; Zare, R. N. Energy Fuels 2009, 23, 1162–1168. (31) Maechling, C. R.; Clemett, S. J.; Engelke, F.; Zare, R. N. J. Chem. Phys. 1996, 104, 8768–8776.
(32) Mullins, O. C.; Martinez-Haya, B.; Marshall, A. G. Energy Fuels 2008, 22, 1765–1773. (33) Duan, P. G.; Fu, M. K.; Pinkston, D. S.; Habicht, S. C.; Kentta¨maa, H. I. J. Am. Chem. Soc. 2007, 129, 9266–9267. (34) Golovlev, V. V.; Allman, S. L.; Garrett, W. R.; Taranenko, N. I.; Chen, C. H. Int. J. Mass Spectrom. Ion Processes 1997, 169, 69–78. (35) Lindner, B. Int. J. Mass Spectrom. Ion Process. 1991, 103, 203– 218. (36) Perez, J.; Petzold, C. J.; Watkins, M. A.; Vaughn, W. E.; Kentta¨maa, H. I. J. Am. Soc. Mass Spectrom. 1999, 10, 1105–1110. (37) Perez, J.; Ramirez-Arizmendi, L. E.; Petzold, C. J.; Guler, L. P.; Nelson, E. D.; Kentta¨maa, H. I. Int. J. Mass Spectrom. 2000, 198, 173– 188. (38) Shea, R. C.; Petzold, C. J.; Campbell, J. L.; Li, S.; Aaserud, D. J.; Kentta¨maa, H. I. Anal. Chem. 2006, 78, 6133–6139. (39) Shea, R. C.; Petzold, C. J.; Liu, J. A.; Kentta¨maa, H. I. Anal. Chem. 2007, 79, 1825–1832. (40) Shea, R. C.; Habicht, S. C.; Vaughn, W. E.; Kentta¨maa, H. I. Anal. Chem. 2007, 79, 2688–2694.
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Figure 1. LIAD/EI (70 eV, 12 mJ) mass spectra of model compound A (top, 1 laser pulse), B (middle, 1 laser pulse), and C (bottom, 10 laser pulses, each on a new spot).
Experimental Section
Sample Preparation. Asphaltene solutions were prepared by dissolving ∼1 mg of the sample in ∼ 10 mL of CH2Cl2 or CS2. The CH2Cl2 solution was electrospray deposited on a 12.7 μm thick, 2 cm diameter round Ti foil situated on a platform. Alternatively, a drop of the hot CS2 solution was placed onto a Ti foil on a hot plate (∼50 C). Both methods of sample deposition left a thin film of the asphaltene on the surface of the foil. Each Ti foil with the sample was mounted onto a LIAD probe38-40 and inserted into the FT-ICR mass spectrometer. Model compounds A, B, and C were prepared for analysis by using the electrospray deposition technique described above, and model compounds D and E were analyzed by using the CS2 technique described above. Instrumentation. FT-ICR Mass Spectrometry. All experiments were conducted in a dual-cell Nicolet model FTMS2000 FT-ICR mass spectrometer described previously.43,44
Chemicals. Asphaltene samples derived from the vacuum distillation bottom (1000 Fþ) of a North American crude oil were provided by ExxonMobil Research and Engineering Company (EMRE, Annandale, NJ). Approximately 20 g of the material was dissolved in 200 mL of n-heptane. The solution was equilibrated overnight at ambient temperature. The insoluble (n-C7 asphaltene) was then filtered through a medium porosity Buchner funnel and dried in a vacuum oven at 100 C for several hours. The asphaltene accounts for ∼28 wt % of the sample. Asphaltene model compounds A and C were provided by Professor Murray Gray from the University of Alberta.41 Asphaltene model compound B was provided by Professor Klaus Mullen from the Max Planck Institute, Germany.42 Model compounds D and E were purchased from Sigma Aldrich (St. Louis, MO) and were used as received. (41) Tan, X. L.; Fenniri, H.; Gray, M. R. Energy Fuels 2008, 22, 715– 720. (42) Watson, M. D.; Fechtenkotter, A.; Mullen, K. Chem. Rev. 2001, 101, 1267–1300.
(43) Crawford, K. E.; Campbell, J. L.; Fiddler, M. N.; Duan, P.; Qian, K.; Gorbaty, M. L.; Kentta¨maa, H. I. Anal. Chem. 2005, 77, 7916–7923. (44) Campbell, J. L.; Crawford, K. E.; Kentta¨maa, H. I. Anal. Chem. 2004, 76, 959–963.
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Figure 2. LIAD/EI (26 mJ, 70 eV, one laser pulse) mass spectrum of model compound D. The spectrum is an average of five scans.
Figure 3. LIAD/EI (26 mJ, 70 eV, one laser pulse) mass spectrum of model compound E. The spectrum is an average of five scans.
model compounds studied (A with MW = 763, B with MW = 1532, and C with MW = 612, Figure 1; D with MW = 599, Figure 2; and E with MW = 679, Figure 3) contain several aromatic rings, and two of them also have long alkyl chains, as expected for petroleum molecules. After evaporation by LIAD (for model compounds A, B, and C, 12 mJ laser energy was used; for model compounds D and E, 26 mJ laser energy was used since laser energies lower than about 18 mJ proved to be inefficient for desorption), the model compounds were ionized via electron ionization (EI) at 70 eV electron energy. The LIAD/EI mass spectra of model compounds A and B show a predominant molecular ion along with some fragment ions (atmospheric pressure photoionization of compound B also yields no fragment ions45). This was a surprising discovery since the long alkyl chains of these compounds were expected to be cleaved relatively easily, as is the case, for example, for the molecular ion of butylbenzene46,47 and the porphyrin model compound with ethyl side chains (D) (studied here). The reason for the lack of extensive fragmentation of the model compounds A and B in these high-energy electron bombardment experiments is likely their many internal degrees of freedom due to their long alkyl chains, which slow down their molecular ions’ unimolecular fragmentation and hence allow cooling by infrared light emission.48 In sharp contrast, the LIAD/EI mass spectrum of model compound C shows a minor molecular ion peak of m/z 612 (about 20% of the
The three trapping plates were kept at 2 V. Asphaltene and model compounds were evaporated into the cell via LIAD and ionized using electron bombardment. Excitation of the ions for detection was achieved by a fast broadband rf sweep (from ∼2 kHz to 3 MHz, 121 Vp-p, 3200 Hz/μs or ∼15 to 924 kHz, 121 Vp-p, 800 Hz/μs). The transients (0.065536 or 0.024576 s), recorded as 64k data points (data system limit), were subjected to Hanning apodization, followed by augmentation of the data by one zero fill prior to Fourier transformation. For some experiments, a broadband excitation pulse was used to eject low mass ions. All the spectra were corrected by subtracting background spectra from them. Background spectra were recorded by performing exactly the same experiment except not firing the laser. The experiments were performed and data acquisition carried out under the control of a Sun workstation running the Odyssey software version 4.0. LIAD Technique. The LIAD technique employs laser-generated acoustic waves to evaporate thermally labile and/or nonvolatile samples as intact neutral molecules into the mass spectrometer from the front side (sample side) of a 12.7 μm thick Ti foil when the backside of the foil is irradiated by pulses of laser light. A high-power LIAD probe (“fiberless” probe) was used in this work.40 The LIAD probe was positioned 5 mm from one of the FT-ICR cell’s trapping plates. Each LIAD experiment involved desorption of molecules by using 1-20 laser shots (Continuum Minilite Nd:YAG laser, 532 nm wavelength light, 3 ns pulse width, 10 Hz repetition rate) applied in a circular pattern on the backside of a Ti foil (the side opposite to where the sample was deposited). Each laser shot desorbs molecules from an area of 10-3 cm2 on the Ti foil (laser power density at the metal surface ranged from ∼4 to 9 109 W/cm2). Approximately 5% of the foil’s total surface area was irradiated when the foil was rotated 360o. The input laser energy was set from 12 to 26 mJ which correlates to ∼6 to 13 mJ at the backside of the foil.
(45) McKenna, A. M.; Purcell, J. M.; Rahimi, P.; Rodgers, R. P.; Marshall, A. G. Proceedings of the 235th American Chemical Society National Meeting and Exposition, New Orleans, LA, April 6-10, 2008; WOM 99069. (46) Lightner, D. A.; Quistad, G. B.; Irwin, E. Appl. Spectrosc. 1971, 25, 253–258. (47) Gray, M. R.; McCaffrey, W. C. Energy Fuels 2002, 16, 756–766. (48) Ho, Y. P.; Dunbar, R. C. Int. J. Mass Spectrom. Ion Process. 1996, 154, 133–144.
Results and Discussion Model compounds thought to be representative of the molecules present in asphaltenes were first examined. The five 5567
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Figure 4. LIAD/EI mass spectra of an asphaltene sample measured by using 12 mJ (top), 18 mJ (middle), and 26 mJ (bottom) laser energy. Each spectrum was obtained using 60 eV electron energy and 20, 30, and 1 laser pulses, respectively. Each laser pulse irradiates a new spot on the Ti foil. Ions of m/z 40-300 were ejected prior to detection.
peak due to the protonated molecule at m/z 613) and major fragmentation. Due to the lack of long alkyl chains that act as a heat sink for the molecular ions of A and B, thus enabling cooling via IR emission, the unimolecular fragmentation rate of the molecular ion of C is likely substantially greater than the rate of IR emission, which could explain the more extensive fragmentation in this case (as well as for many other organic molecules of this size and smaller; the dissociation behavior of the protonated model compound C caused by multiple IR photons has been reported earlier49). The same is likely to be the case for model compounds D and E that tend to fragment by losses of alkyl or phenyl groups, respectively (upon ESI, model compound D was found to produce a dimer,50 which is not the case for LIAD/EI). Model compound E undergoes predominant fragmentation by the loss of H2O (to yield the ion of m/z 661; Figure 3). This was quite surprising and is under further investigation. The fact that all of the model compounds studied were successfully evaporated by LIAD and yield a stable molecular
ion or protonated molecule upon EI suggests that the LIAD/ EI experiment should allow evaporation and ionization of all components of asphaltenes and yield information on the MW distributions of asphaltenes. Therefore, an asphaltene derived from North American crude oil was analyzed by LIAD/EI under the same experimental conditions (12 mJ laser energy and 60 eV electron energy; Figure 4, top) as used for most of the model compounds. Low-mass ions were ejected out of the cell during this experiment in order to reduce space-charge effects51,52 (later experiments demonstrated that this ejection had no influence on the spectra; see Figures 5 and 6 for comparison). The majority of the peaks in the mass spectrum correspond to ions that have m/z values ranging from 350 to 750 Da. This finding supports the expectation that mainly stable molecular ions are generated from asphaltenes in this experiment. However, the detected upper MW is much smaller than expected. Since no ions greater in mass than 750 Da were observed in the above experiment, the laser energy was increased from 12 to 18 mJ to facilitate evaporation of larger molecules (Figure 4, middle). This resulted in an increase in the detected
(49) Ehrmann, B. M.; Schaub, T. M.; Rodgers, R. P.; Marshall, A. G. Proceedings of the 55th American Society for Mass Spectrometry Annual Conference on Mass Spectrometry, Indianapolis, IN, June 3-7, 2007; Abstract No. 370; MPI140. (50) Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G.; Greaney, M.; Qian, K. N. Can. J. Chem. 2001, 79, 546–551.
(51) Guan, S. H.; Marshall, A. G. J. Am. Soc. Mass Spectrom. 1994, 5, 64–71. (52) Li, G. Z.; Werth, G. Int. J. Mass Spectrom. Ion Process. 1992, 121, 65–75.
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Figure 5. LIAD/EI mass spectra of an asphaltene sample measured by using 15 eV (A), 20 eV (B), 40 eV (C), and 60 eV electron energies (D). Each spectrum was obtained using laser energy of 22 mJ and one laser pulse. No ions were ejected.
side chains,53-55 types of bridges between aromatic or cyclic groups,56,57 and the chemical structure of the pendant groups.56,58 On average, the aromatic carbon content is in the range 40-60% of the total carbon53,59 hence, although the model compounds examined in this study could potentially exist in an actual asphaltenes mixture, they fall outside the typical range. Compounds C, D, and E are much more aromatic than the norm, whereas A and B are below the median, with 28 and 37% aromatic carbon, respectively. Although the side chains on A and B are within the observed range of 1-30 carbons, petroleum asphaltenes have an average length of only ca. 6 carbons.53. To get closer to the normal range of aromatic carbon content of asphaltenes, however, we
highest mass value from 750 to about 1050 Da. Further increase in the laser energy to 26 mJ (Figure 4, bottom) and eventually to 30 mJ (not shown) did not increase the highest detected mass value. Hence, 18 mJ is concluded to be a high enough energy to desorb even the biggest components of this asphaltene sample. This result suggests that the heaviest components in this asphaltene sample have a MW of about 1050 Da. The same result was obtained upon the examination of other asphaltene samples derived from North American crude oil. To examine the influence of the ionizing electrons’ energy on the highest mass value detected for the asphaltene samples, the electron energy was varied. The mass spectra obtained using electron energies between 15 and 60 eV are essentially identical (Figure 5). This finding suggests that the molecular ions generated from this asphaltene sample over a wide range of electron energies are stable toward fragmentation. Hence, most of the asphaltene components appear to feature enough alkyl side chains of sufficient length to slow down dissociation of the molecular ions to the extent that energy loss via emission of IR light becomes competitive. It should be noted, though, that in addition to a distribution of molecular weight, the components of the asphaltene fraction will exhibit distributions of the number of side chains per molecule, length of
(53) Nali, M.; Calemma, V.; Montanari, L. Org. Mass Spectrom. 1994, 29, 607–614. (54) Savage, P. E.; Klein, M. T.; Kukes, S. G. 1985, 24, 1169-1174. (55) Strausz, O. P.; Lown, E. M., The Chemistry of Alberta Oil Sands, Bitumens, and Heavy Oils; AERI: Calgary, AB, 2003. (56) Hofmann, I. C.; Hutchison, J.; Robson, J. N.; Chicarelli, M. I.; Maxwell, J. R. Org. Geochem. 1992, 19, 371–387. (57) Peng, P.; Morales-Izquierdo, A.; Hogg, A.; Strausz, O. P. Energy Fuels 1997, 11, 1171–1187. (58) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M.; Kowalewski, I.; Behar, F. Energy Fuels 1999, 13, 228–247. (59) Gray, M. R. Energy Fuels 2003, 17, 1566–1569.
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Figure 6. Top: LIAD/EI (60 eV, 18 mJ, 30 laser pulses, each on a new spot) mass spectrum of an asphaltene sample. Bottom: LIAD/EI (60 eV, 18 mJ, 5 laser pulses, each on a new spot) mass spectrum of an asphaltene sample spiked with model compound B (MW 1532). Ions of m/z 40-300 were ejected prior to detection.
would need to add ca. 5 n-hexyl side chains to model compound C and reduce the average length of the side chains in compounds A and B. Several studies have indicated the presence of bridges of polymethylene groups between aromatic and other pendant structures55,58,60 as well as ether and sulfide bridges, that all should be easier to fragment than alkyl chains.56,57 The mass spectra of Figure 5 suggest either that none of these bridged compounds are present in the sample asphaltene at detectable concentrations, or that the presence of alkyl side chains on bridged compounds confers stability to fragmentation up to electron energies of 60 eV. Additional studies of alkylated model compounds with bridges are required to define their behavior. To verify that no secondary instrumentation related effects, such as a limited dynamic range or space-charge effects, prevent the observation of the highest MW components of the asphaltene samples, two experiments were carried out. The abundant detected ions formed from the asphaltene sample studied in above experiments may prevent the detection of even higher mass ions (>1000 units) due to space-charge effects.51,52 This issue can be addressed by ejecting out the detected ions. When all ions in the mass range 300-1050 were ejected after ionization of the asphaltene sample, no peaks were visible in the spectrum. This finding suggests that the sample does not contain components with MW above 1050. The dynamic range issue was addressed by spiking the asphaltene sample with the largest model compound (B) and analyzing this mixture via LIAD/EI. The LIAD/EI (18 mJ;
60 eV) mass spectrum clearly shows the presence of both the asphaltenes and the model compound (Figure 6). This finding suggests that the dynamic range of the FT-ICR mass spectrometer is not hindering the analysis of this asphaltene sample Conclusions Several experiments performed under different conditions indicate that high-energy LIAD (g18 mJ) successfully evaporates all the components of the asphaltene samples studied as neutral molecules into a mass spectrometer. Examination of LIAD/EI mass spectra of model compounds demonstrated that this technique desorbs and ionizes all these compounds. This is not surprising since EI is known as a universal ionization method that ionizes all organic compounds exposed to it, as opposed to, for example, ESI, PI, and MALDI. However, it was somewhat unexpected that these LIAD/EI experiments also provided a stable molecular ion, and hence MW information, for all the model compounds studied. LIAD/EI mass spectra measured for asphaltene samples under many different experimental conditions demonstrate that the MW distribution of these samples ranges from 350 to 1050 Da, with a maximum around 750 Da. Furthermore, the lack of formation of abundant low-mass fragment ions for the asphaltenes, even when using high-energy electrons for ionization, suggests that the molecular ions of most of the components of these asphaltene samples (derived from North-American crude oil) are stabilized by the presence of long alkyl side chains. Acknowledgment. The authors would like to thank ExxonMobil for financial support of this work and for the asphaltene sample.
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