Anal. Chem. 2005, 77, 7916-7923
Laser-Induced Acoustic Desorption/Fourier Transform Ion Cyclotron Resonance Mass Spectrometry for Petroleum Distillate Analysis Kenroy E. Crawford,† J. Larry Campbell,†,‡ Marc N. Fiddler,† Penggao Duan,† Kuangnan Qian,§ Martin L. Gorbaty,§ and Hilkka I. Kentta 1 maa*,†
Department of Chemistry, Purdue University, West Lafayette, Indiana 47909, and ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801
Laser-induced acoustic desorption (LIAD) coupled with a 3-T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) allows the simultaneous analysis of both the nonpolar and polar components in petroleum distillates. The LIAD/FT-ICR method was validated by examining model compounds representative of the various classes of polar and nonpolar hydrocarbons commonly found in petroleum. LIAD successfully desorbs all the compounds as intact neutral molecules into the FTICR. Electron ionization (EI) at low energies (10 eV) and chemical ionization using cyclopentadienyl cobalt radical cation (CpCo•+) were employed to ionize the desorbed molecules. The EI experiments lead to extensive fragmentation of many of the hydrocarbon compounds studied. However, the CpCo•+ ion ionizes all the hydrocarbon compounds by producing only pseudomolecular ions without other fragmentation, with the exception of one compound (•CH3 loss occurs). Examination of two different petroleum distillate samples revealed hundreds of compounds. The most abundant components have an even molecular weight; i.e., they are likely to contain no (or possibly an even number of) nitrogen atoms. Rapid increase in energy demand combined with the ongoing depletion of light oil reserves makes it necessary for the petroleum industry to focus its attention toward the heavier fractions of crude oil. Effective processing of heavy feedstock will require the development of better analytical techniques for the characterization and identification of the heavier components of crude oil.1 The knowledge from this characterization can assist petroleum chemists and engineers in making decisions on refining and production procedures, thereby increasing the overall economic value of the products derived from the crude oil. Petroleum distillates derived from crude oil can be viewed as containing two major groups of compounds, nonpolar hydrocarbons (∼90%) and * To whom correspondence should be addressed. Fax: 765-494-0359. Email:
[email protected]. † Purdue University. ‡ Current address: Department of Biochemistry, University of Western Ontario, London, ON, Canada. § ExxonMobil Research and Engineering Co. (1) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; Marcel Dekker: New York, 1994.
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polar hydrocarbons (∼10%).1 Here, the term “nonpolar hydrocarbons” includes both saturated and aromatic molecules, such as alkanes (paraffins and isoparaffins), cycloalkanes (naphthenes), polyaromatic hydrocarbons, and sulfur-containing aromatic hydrocarbons. “Polar hydrocarbons” are compounds that contain one or more heteroatoms, most commonly nitrogen (basic components), oxygen (phenols and acids), Ni, and V (metalloporphyrins). Petroleum chemists acquire information on both the polar and nonpolar constituents of petroleum distillates by using a variety of techniques, including HPLC, NMR, IR, UV, and GPC.2-5 However, without isolation of the individual petroleum components, these techniques can only provide general information on the types of chemical classes or the presence of certain functional groups. In contrast, mass spectrometry can provide detailed molecular information for some petroleum components. Traditionally, mass spectrometric methods for petroleum analysis have relied on high-energy6-8 (70 eV) or low-energy9,10 (∼10 eV) electron ionization (EI), chemical ionization (CI),11-14 or field desorption (FD)/field ionization (FI).15-23 Each of these methods (2) Clutter, D. R.; Petrakis, L.; Stenger, R. L., Jr.; Jensen, R. K. Anal. Chem. 1972, 44, 1395-1405. (3) Coleman, J. H.; Hirsch, D. E.; Dooley, J. E. Anal. Chem. 1969, 41, 800804. (4) Gautam, K.; Jin, X.; Hansen, M. Appl. Spectrom. Rev. 1998, 33, 427-443. (5) Boduszynski, M. M. Energy Fuels 1988, 2, 597-613. (6) Brown, R. A. Anal. Chem. 1951, 23, 430-437. (7) Lumpkin, H. E. Anal. Chem. 1956, 28, 1946-1948. (8) Robinson, C. J. Anal. Chem. 1971, 43, 1425-1434. (9) Field, F. H.; Hastings, S. H. Anal. Chem. 1956, 28, 1248-1255. (10) Lumpkin, H. E.; Aczel, T. Anal. Chem. 1964, 36, 181-184. (11) Allgood, C.; Ma, Y. C.; Munson, B. Anal. Chem. 1991, 63, 721-725. (12) Hsu, C. S.; Qian, K. Anal. Chem. 1983, 65, 767-771. (13) Dzidic, I.; Petersen, H. A.; Wadsworth, P. A.; Hart, H. V. Anal. Chem. 1992, 64, 2227-2232. (14) Roussis, S. G. Rapid Commun. Mass Spectrom. 1999, 31, 1031-1051. (15) Bricker, Y.; Ring, Z.; Lacchelli, A.; McLean, N.; Malhortra, R.; Coggiola, M. A.; Yong, S. E. Energy Fuels 2001, 15, 996-1002. (16) Bricker, Y.; Ring, Z.; Lacchelli, A.; McLean, N.; Rahimi, P. M.; Fairbridge, C. C.; Malhortra, R.; Coggiola, M. A.; Yong, S. E. Energy Fuels 2001, 15, 23-37. (17) Qian, K.; Dechert; G. J. Anal. Chem. 2002, 74, 3977-3983. (18) Liang, Z.; Hsu, C. S. Energy Fuels 1998, 12, 637-643. (19) Mead, W. L. Anal. Chem. 1968, 40, 653-747. (20) Schaub, T. M.; Hendrickson, C. L.; Qian, K.; Quinn, J. P.; Marshall, A. G. Anal. Chem. 2003, 75, 2172-2176. (21) Schaub, T. M.; Linden, H. B.; Hendrickson, C. L.; Marshall, A. G. Rapid Commun. Mass Spectrom. 2004, 18, 1641-1644. 10.1021/ac0511501 CCC: $30.25
© 2005 American Chemical Society Published on Web 11/09/2005
Scheme 1
presents problems in data interpretation. High-energy EI and most CI methods generate significant amounts of fragment ions and hence cannot be used to obtain molecular weight (MW) information. Low-energy EI is a low-efficiency process and selective for aromatic hydrocarbons. These (EI/CI) methods also rely on thermal vaporization to bring molecules into the gas phase and, hence, have a limited ability to detect molecules with high boiling points. Presently, FD and FI are the primary soft ionization methods used in petroleum analysis because of their ability to ionize most saturated and aromatic hydrocarbons without fragmentation. However, branched hydrocarbons are problematic for FD/FI and typically yield abundant fragment ions.15,16 In addition, response factors for the different petroleum components can vary significantly in FI/FD, making quantification a difficult task. Electrospray ionization (ESI) has recently been demonstrated to successfully evaporate and ionize polar hydrocarbons.24-32 While ESI yields intact pseudomolecular ions without fragmentation, this technique can only account for that small portion (∼10%) of petroleum that can be easily protonated (basic compounds) or deprotonated (acidic compounds). However, the major fraction (∼90%) of petroleum distillates consists of nonpolar compounds (e.g., paraffins, cycloparaffins, polyaromatic and thiophenoaromatic hydrocarbons) that are neither basic nor acidic. These components go undetected when ESI is employed. The atmospheric pressure chemical ionization/atmospheric pressure photoionization techniques33 are able to yield intact molecular ions for nonpolar and polar polyaromatic hydrocarbons, including those containing heteroatoms, but not for nonpolar saturated hydrocarbons. In addition, it generates both protonated molecules and molecular ions (radical cations), which significantly complicates spectral interpretation. An ideal desorption/ionization method for the mass spectrometric analysis of all components of petroleum distillates would allow (1) efficient desorption of all the components in petroleum into the gas phase without fragmentation, (2) fast ionization of all the components without fragmentation to yield stable product ions that are representative of the neutral hydrocarbons’ MWs, and (3) uniform efficiency for both desorption and ionization for
all components of the petroleum sample. To this end, we have developed a technique that employs laser-induced acoustic desorption34,35 (LIAD) to evaporate thermally labile components as intact neutral molecules into the gas phase, followed by gentle CI to produce pseudomolecular ions.36,37 Cyclopentadienyl cobalt radical cation (CpCo•+) was chosen as the CI reagent ion since it has been earlier demonstrated to rapidly ionize linear, branched, and cyclic alkanes with little to no fragmentation (Scheme 1).38-42 (22) Gross, J. H.; Weidner, S. M. Eur. J. Mass Spectrom 2000, 6, 11-17. (23) Montaudo, G.; Lattimer, R. P. Mass Spectrometry of Polymers; CRC Press: Boca Raton, FL, 2002. (24) Porter, D. J.; Mayer, P. M.; Fingas, M. Energy Fuels 2004, 18, 987994. (25) Wu, Z.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2004, 18, 1424-1428. (26) Marshall, A. G.; Rodgers, R. P. Acc. Chem. Res. 2004, 37, 53-59. (27) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492-498. (28) Qian, K.; Robbins, W. K.; Hughey, C. A.; Copper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505-1511. (29) Qian, K.; Edwards, K. E.; Diehl, J. H.; Green, L. A. Energy Fuels 2004, 18, 1784-1791. (30) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1186-1193. (31) Wu, Z.; Jernstro ¨m, S.; Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2003, 17, 946-953. (32) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 41454149. (33) Purcell, J. M.; Rodgers, R. P.; Hendrickson, C. L., Quinn, J. P.; Marshall, A. G 52nd Annual Meeting of the American Society for Mass Spectrometry Conference on Mass Spectrometry and Allied Topics; Nashville, TN, May 2327, 2004; Poster. (34) Golovlev, V. V.; Allman, S. L.; Garrett, W. R.; Taranenko, N. I.; Chen, C. H. Int. J. Mass Spectrom. Ion Processes 1997, 169/170, 69-78. (35) Lindner, B. Int. J. Mass Spectrom. Ion Processes 1991, 103, 203-218. (36) Pe´rez, J.; Ramı´rez-Arizmendi, L. E.; Petzold, C. J.; Guler, L. P.; Nelson, E. D.; Kentta¨maa, H. I. Int. J. Mass Spectrom. 2000, 198, 173-188. (37) (a) Pe´rez, J.; Petzold, C. J.; Watkins, M. A.; Vaughn, W. E.; Kentta¨maa, H. I. J. Am. Soc. Mass Spectrom. 1999, 10, 1105-1110. (b) Peltzold, C. J. Ph.D. Thesis, Purdue University, West Lafayette, IN, 2002. (38) Jacobson, D. B.; Freiser, B. S. J. Am. Chem. Soc. 1985, 107, 7399-7407. (39) Ekeberg, D.; Uggerud, E.; Lin, H.-Y.; Sohlberg, K.; Chen, H.; Ridge, D. P. Organometallics 1999, 18, 40-44. (40) Byrd, H. C. M.; Guttman, C. M.; Ridge, D. P. J. Am. Soc. Mass Spectrom. 2003, 14, 51-57. (41) Campbell, J. L.; Fiddler, M. N.; Crawford, K. E.; Gqamana, P. P.; Kentta¨maa, H. I. Anal. Chem. 2005, 77, 4020-4026.
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EXPERIMENTAL SECTION Chemicals. All individual hydrocarbons, cyclopentadienyl cobalt dicarbonyl, and carbon disulfide were purchased from Sigma Aldrich (St. Louis, MO) and used without further purification. The two petroleum samples used in this study are distillation cuts obtained from ExxonMobil Research and Engineering Co. (Annandale, NJ). Petroleum 350 (boiling range 348-457 °C) has an estimated average MW of 350. Petroleum 500 (boiling range 377-588 °C) has an estimated average MW of 500. Ti foil was purchased from Alfa-Aesar (Ward Hill, MA). Sample Preparation. Individual Hydrocarbons. Some of the hydrocarbons (tetracosane, 5-R-cholestane, squalane, eicosene, coronene, 1,3-diphenylisobenzofuran, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) were introduced into the mass spectrometer by using LIAD. The samples were prepared by dissolving 1 mg of the hydrocarbon in 5 mL of hot (46 °C) carbon disulfide. Approximately 1 mL of this solution was deposited on a 12.7-µmthick, 2-cm-diameter round Ti foil situated on a hot plate heated to ∼50 °C. The solvent was allowed to evaporate, leaving a thin film of the hydrocarbon on the surface of the foil. This sample preparation method is fast and simple and requires no matrix or sample pretreatment. The Ti foil with sample was mounted onto an end of the LIAD probe and inserted into the FT-ICR mass spectrometer. All other hydrocarbons (phenyldodecane, 4-phenyl1-butene, benzocyclohexane, thiofuran, and cyclopentylacetic acid) were introduced via leak valves or a thermal probe (provides a temperature gradient of 25-300 °C). Model Hydrocarbon Mixture. A model hydrocarbon mixture was prepared that consisted of five compounds representative of various classes of hydrocarbons commonly found in petroleum distillates: tetracosane (linear alkane), 5-R-cholestane (cycloparaffin), squalane (branched alkane), coronene (nonpolar polycyclic aromatic hydrocarbon), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (polar polycyclic aromatic hydrocarbon). Equimolar amounts of each hydrocarbon were dissolved in 10 mL of hot carbon disulfide. Approximately 1 mL of this solution was applied to a 12.7-µm Ti foil by using the method described for the individual hydrocarbons. Petroleum Distillates. Petroleum distillate solutions were prepared by dissolving ∼1 mg of the distillate in 5 mL of hot CS2 (46 °C). Approximately 1 mL of the distillate solution was deposited onto a 12.7-µm Ti foil by using the method described for the individual hydrocarbons. Instrumentation. Laser-Induced Acoustic Desorption. Each LIAD experiment involved desorption of neutral hydrocarbon molecules by using 50-200 laser shots applied in a circular pattern on the backside of a Ti foil (the side opposite to where the sample was deposited). Acoustic waves were generated by firing a pulse of laser light (Continuum minilite Nd:YAG laser, light of 532-nm wavelength, 3-5-ns pulse width, 10-Hz repetition rate) at the thin Ti foil (12.7 µm). A laser shot desorbs molecules from an area of 10-3 cm2 on the Ti foil (laser power density at the metal surface was ∼108-109 W/cm2). Approximately 5% of the foil’s total surface area was irradiated when the foil was rotated 360°. Since ∼1 mg of sample was deposited on the entire foil, ∼50 µg of material was desorbed into the FT-ICR cell. The LIAD probe37b was positioned 5 mm from the FT-ICR cell’s trapping plate. (42) Campbell, J. L.; Crawford, K. E.; Kentta¨maa, H. I. Anal. Chem. 2004, 76, 959-963.
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FT-ICR Mass Spectrometry. Experiments were conducted in a dual-cell Nicolet model FTMS-2000 FT-ICR mass spectrometer described previously.41,42 The CpCo•+ reagent ion (m/z 124) was generated by electron ionization (20-eV electron energy, 6-µA emission current, 1-s ionization time) of cyclopentadienylcobalt dicarbonyl that was introduced through a batch inlet into one cell at a nominal pressure of 1.5 × 10-8 Torr. Ions were trapped by using a 2-V trapping potential and transferred into the other cell through a 2-mm hole in the conductance limit plate by grounding this plate for 110 µs. Prior to transfer, a negative potential of -3.5 V was applied for 12 ms to the remote trapping plate of the receiving cell to purge this cell of unwanted ions. At all other times, the three trapping plates were kept at 2 V. The ion-transfer efficiency was enhanced by quadrupolar axialization43 (1 s, excitation at 373 kHz (on resonance), 4 Vp-p) that was implemented by introducing helium via a pulsed valve assembly (the nominal peak pressure in the cell was ∼1 × 10-5 Torr). The transferred ions were cooled for 1 s by IR emission, and collisions with argon gas pulsed into the cell (peak nominal pressure ∼1 × 10-5 Torr). Subsequent ejection of unwanted ions was performed using stored-waveform inverse Fourier transform44 excitation pulses. The isolated CpCo•+ reagent ions were then allowed to react with individual hydrocarbons, their mixtures, or petroleum distillates introduced via a leak valve inlet, thermal desorption probe, or LIAD. Excitation of the ions for detection was achieved by a fast broadband rf sweep (from ∼2 kHz to 3 MHz, 121 Vp-p, 2000 Hz/µs). The transients (0.065 536 s), recorded as 128K data points, were subjected to Hanning apodization, followed by augmentation of the data by one zero fill prior to Fourier transformation. The resolution was typically 6800 at m/z 400. A background spectrum (no laser shots) was subtracted from each mass spectrum to lessen contributions from minor ion impurities and electronic noise. Control of the experiments and data acquisition were performed by using a Sun workstation running the Odyssey software version 4.0. Product ion branching ratios were obtained by taking the ratio of a particular ion’s signal intensity to the total signal intensity of all of the product ions. These ratios have a precision of (3% (which is the standard deviation for multiple measurements of the product branching ratios). RESULTS AND DISCUSSION In all experiments, LIAD desorption of the analytes from the Ti foil (Figure 1) was confirmed by visual inspection. The regions of the foil where the laser had struck the backside showed an apparently clean Ti surface, suggesting that most of the analyte molecules had evaporated.41 The desorbed neutral analytes were ionized via either EI or CI. LIAD/EI of Hydrocarbons. Petroleum distillates are complex mixtures of aliphatic, naphthenic, and polyaromatic hydrocarbons, including various heteroatom-containing (e.g., S, N, O) hydrocarbons. Olefins may be present in the processed refinery streams. Aliphatic and many naphthenic hydrocarbons possess ionization energies (IE) in the range of 10-13 eV, while olefinic and (43) Schweikhard, L.; Guan, S.; Marshall, A. G. Int. J. Mass Spectrom. Ion Processes 1992, 120, 71-83. (44) Chen, L.; Wang, T.-C. L.; Ricca, T. L.; Marshall, A. G. Anal. Chem. 1987, 59, 449-454.
Figure 1. LIAD process: (1) laser irradiation, (2) propagation of an acoustic wave through Ti foil, and (3) desorption of petroleum distillate molecules.
Figure 2. Comparison of LIAD/EI (top) and LIAD/CpCo•+ (bottom) mass spectra of a five-component model hydrocarbon mixture (∼1:1:1:1:1) containing tetracosane (MW 338), 5-R-cholestane (MW 372), coronene (MW 300), squalane (MW 422), and 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (MW 360).
polyaromatic hydrocarbons have IE < 10 eV.45 Hence, 70-eV EI ionizes all the hydrocarbons. However, a large portion of the molecular ions (of both aliphatic hydrocarbons and alkylated aromatic hydrocarbons) undergoes fragmentation, which compli(45) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. Ion Energetics Data. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg MD, 20899, March 2003, (http://webbook.nist.gov).
cates the interpretation of the mass spectra.6-8 The interpretation of 10-eV EI mass spectra is much simpler than the standard 70eV mass spectra due to reduction in the extent of fragmentation and predominant ionization of aromatic hydrocartbons.10 Since only a small amount of energy is transferred into the hydrocarbon by low-energy EI, often mostly intact molecular ions are formed. At 10 eV, the detected molecular ions predominately correspond to polyaromatic hydrocarbons (IE < 10 eV 9,10), while ion signals Analytical Chemistry, Vol. 77, No. 24, December 15, 2005
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Figure 3. Petroleum distillate 350 desorbed by LIAD and ionized by 70-, 10-, and 10-eV EI with removal of ions m/z 40-200.
for saturated hydrocarbons (IE > 10 eV) are very weak or nonexistent.46 However, there are several disadvantages associated with using low-energy EI, such as significant losses in ion yields because of low electron density and smaller cross section.47 Increasing the energy even slightly above 10 eV results in greater ion signals and ionization of saturated hydrocarbons. However, the extent of fragmentation also increases greatly, which makes MW determination based on molecular ions impractical. To evaluate the ability of the LIAD/EI method to produce useful mass spectra, a mixture of model hydrocarbons similar to the different compounds found in petroleum was studied. The mixture contained tetracosane (a linear alkane or paraffin), 5-Rcholestane (a cycloparaffin or naphthene), squalane (a branched alkane), coronene (a polycyclic aromatic hydrocarbon), and 2,9dimethyl-4,7-diphenyl-1,10-phenanthroline (a heteroatom-containing polar polycyclic aromatic hydrocarbon). The NIST 70-eV EI mass spectral database shows that 70-eV EI of the five individual (46) Lumpkin, H. E. Anal. Chem. 1958, 30, 321-325. (47) Crable, G. F.; Kearn, G. L.; Norris, M. S. Anal. Chem. 1960, 32, 13-17.
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hydrocarbons yields only a few or no molecular ions, but abundant fragment ions.45 Hence, low-energy (10 eV) EI was used to evaluate the LIAD/EI method. This experiment produced abundant fragment ions for three of the five components in the mixture (branched alkane, linear alkane, and cycloparaffin) (Figure 2, top). Only the polycyclic aromatic and the polar polycyclic aromatic hydrocarbons yielded intense molecular ions. LIAD/EI of Petroleum Distillate Samples. The mass spectra of two petroleum distillate samples obtained under low(10 eV) and high-energy (70 eV) EI conditions display an abundance of fragment ions of low m/z values (Figures 3 and 4, top and center). The m/z values are consistent with typical aliphatic fragment ions (C3H7, C4H7, C4H9, C5H9, C6H9, etc.). A close examination of the low-energy EI mass spectra of both petroleum distillate samples revealed the presence of higher mass ions at low abundances. The low signal for the higher mass ions may be due to space charging (ion-ion repulsion).48 Removal of the very abundant low-mass ions (e.g., from m/z 40 to 400) by a frequency sweep makes the peaks due to the high-mass ions more (48) Hsu, C. S.; Liang, Z.; Campana, J. E. Anal. Chem. 1994, 66, 850-855.
Figure 4. Petroleum distillate 500 desorbed by LIAD and ionized by 70-, 10-, and 10-eV EI with removal of ions m/z 40-300.
visible (Figures 3 and 4, bottom). These ions of even mass values are presumed to be molecular ions of polyaromatic compounds (IE e 10 eV) containing no (or possibly an even number of) nitrogen atoms. LIAD/CpCo•+ CI of Individual Hydrocarbons and A Model Hydrocarbon Mixture. The combination of EI with LIAD was demonstrated to be inadequate for obtaining MW information for model hydrocarbon mixtures and actual petroleum distillate samples since fragment ions dominate the mass spectra. Therefore, chemical ionization methods were considered. Previous studies have shown that cyclopentadienyl cobalt radical cation, CpCo•+ (m/z 124), reacts rapidly with small linear, branched, and cyclic alkanes in the gas phase to produce mostly adduct ions that have lost one or two molecules of hydrogen, with little to no other fragmentation (Scheme 1).38-42 To evaluate the potential of the CpCo•+ ion for use as a chemical ionization reagent ion in petroleum distillate analysis, the reactivity of CpCo•+ toward various classes of hydrocarbons known to be present in petroleum distillates was examined. The CpCo•+ ion was allowed to react with several individual hydrocarbons to determine what product ions each compound class is likely to form (Table 1). Each of the hydrocarbons examined yields stable addition products, addition products that have lost one or two hydrogen molecules, or both products. Two compounds were found to react slightly differently with the CpCo•+
ion. 5-R-Cholestane (cyclic alkane) reacts by addition accompanied by the loss of a methyl radical and two hydrogen molecules. Similar products have been seen for small cyclic branched hydrocarbons.49 Cyclopentylacetic acid (naphthenic acid) reacts with CpCo•+ to form a stable addition product, addition product that has lost one hydrogen molecule, and addition product that has lost water, carbon monoxide, and one hydrogen molecule. Similar products have also been observed for reactions of the CpCo+• ion with peptides.50 Based on the results shown in Table 1, it is concluded that CpCo•+ ionizes various types of hydrocarbons commonly found in petroleum to form pseudomolecular ions without major fragmentation. In most cases, information on the molecular weight of the hydrocarbon compounds is retained. The same five-component model hydrocarbon mixture that was employed in the evaluation of the LIAD/EI method was also subjected to the LIAD/CpCo•+ CI method. Only pseudomolecular ions, no fragment ions, were produced for most of the components of the mixture (Figure 2, bottom). Hence, the LIAD/CpCo•+ CI method yields mass spectra that reveal the molecular weight for all but one of the hydrocarbons (5-R-cholestane) in the mixture. Even highly branched hydrocarbons yield solely pseudomolecular ions when exposed to CpCo•+ chemical ionization. (49) Gqamana, P. P.; Campbell, J. L.; Williams, K. P.; Nash, J. J.; Kentta¨maa, H. I. In preparation. (50) Crawford, K. E.; Campbell, J. L.; Kentta¨maa, H. I. In preparation.
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Table 1. CpCo•+ Reactions with Model Hydrocarbons Representative of Compounds Found in Petroleum Distillates
a
Introduction via LIAD. b Introduction via heated inlet or thermal probe.
Figure 5. LIAD/CpCo•+ mass spectrum of petroleum distillate 350. CpCo•+ provides a simple mass spectrum that indicates no fragmentation. The signal appears at ∼120 mass units greater m/z values than in the EI spectra, suggesting incorporation of CpCo•+ into the hydrocarbons. No peaks were seen below m/z 300. 7922 Analytical Chemistry, Vol. 77, No. 24, December 15, 2005
the presence of homologous series of ions differing by multiples of 14 Da (i.e., CH2), confirming the presence of homologous hydrocarbons with increasing alkyl chain lengths. The major peaks correspond to even m/z values that are most likely formed from hydrocarbons with no (or possibly an even number of) nitrogen atoms. From the visual inspection of the mass spectra of both distillates, it can be concluded that the petroleum distillate 500 is more complex than the petroleum distillate 350. Compositional analysis of the various hydrocarbon components present in both petroleum distillates is currently underway.
Figure 6. LIAD/CpCo•+ mass spectrum of petroleum distillate 500. The signal appears at ∼120 mass units greater m/z values than in the EI spectra, suggesting incorporation of CpCo•+ into the hydrocarbons. No peaks were seen below m/z 300.
While the relative ion abundances observed in the CI mass spectra (Figure 2, bottom) do not exactly match the relative molar concentration of each hydrocarbon in the sample (1:1:1:1:1 molar ratio), the relative responses are remarkably good compared to results obtained by using other ionization methods (such as EI and FI/FD), This is especially notable when considering the fact that the compositions and structures of the model compounds are significantly different from each other. This result represents a significant advantage in practical application as it is impossible to determine response factors for all petroleum compounds. LIAD/CpCo•+ CI of Petroleum Distillate Samples. Finally, the two petroleum distillate samples discussed above were subjected to the LIAD/CpCo•+ CI analysis. The resulting mass spectra display shifts in the m/z values of ∼120 mass units compared to the high-mass peaks in the low-energy (10 eV) EI mass spectra (Figures 5 and 6). These results agree with the LIAD/CpCo•+ CI studies of individual hydrocarbons and the model hydrocarbon mixture and suggest that the CpCo•+ reagent ion (m/z 124) was incorporated into the molecules present in the petroleum distillates. Further, the CpCo•+ CI ionization method produces mass spectra that indicate no fragmentation for the components of the petroleum distillates. The mass spectra indicate
CONCLUSIONS This study demonstrates that the combination of LIAD/CI with FT-ICR mass spectrometry offers a fast and simple way to analyze both the polar and nonpolar components of petroleum distillates in a single experiment. LIAD allows the desorption of the compounds as intact neutral molecules into the gas phase. While electron ionization causes extensive fragmentation for some of the compounds of interest, chemical ionization with CpCo•+ only yields pseudomolecular ions (no fragment ions) for most of the compounds, including highly branched saturated hydrocarbons (e.g., squalane). This lack of fragment ions provides further support to the above statement that the compounds were evaporated as intact molecules during LIAD. Hence, LIAD/CI with the CpCo•+ reagent ion yields information on the composition of the hydrocarbon mixture and the molecular weight for the mixture components. When combined with FT-ICR mass spectrometry,26-28,30,31 the LIAD/CI approach allows the implementation of high-resolution and accurate mass measurements in order to determine the elemental compositions of petroleum components. ACKNOWLEDGMENT This work was supported by ExxonMobil Research & Engineering Co. The authors also thank Mark Carlsen, Phil Wyss, Weldon Vaughn, Tim Selby, Doug Matthews, and Dr. Hartmut Hedderich for their invaluable assistance with instrumentation and helpful discussions. Received for review June 28, 2005. Accepted September 15, 2005. AC0511501
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