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Resolution and Identification of Elemental Compositions for More than 3000 Crude Acids in Heavy Petroleum by Negative-Ion Microelectrospray High-Field Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Kuangnan Qian* and Winston K. Robbins ExxonMobil Research and Engineering, 1545 Route 22 East, Annandale, New Jersey 08801
Christine A. Hughey,† Helen J. Cooper, Ryan P. Rodgers, and Alan G. Marshall*,† Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310 Received May 17, 2001. Revised Manuscript Received July 13, 2001
Although crude acids are minor constituents in petroleum, they have significant implications for crude oil geochemistry, corrosion, and commerce. We have previously demonstrated that a single positive-ion electrospray ionization (ESI) high-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) experiment can resolve and identify 3000 chemically different elemental compositions of bases (basic nitrogen compounds) in a crude oil. Here, we show that negative-ion ESI high-field FT-ICR MS can selectively ionize and identify naphthenic acids without interference from the hydrocarbon background. When combined with prechromatographic separation, ESI FT-ICR MS reveals an even more detailed acid composition. An average mass resolving power, m/∆m50% g 80 000 (∆m50% is mass spectral peak full width at half-maximum peak height) across a wide mass range (200 < m/z < 1000), distinguishes as many as 15 distinct chemical formulas within a 0.26 Da mass window. Collectively, more than 3000 chemically different elemental compositions containing O2, O3, O4, and O2S, O3S, and O4S were determined in a South American heavy crude. Our data indicates that the crude acids consist of a mixture of structures ranging from C15-C55 with cyclic (1-6 rings) and aromatic (1-3 ring) structures. The acid composition appears to be simpler than that of the corresponding hydrocarbon analogues.
Introduction Crude acids are minor constituents in petroleum with special significance in geochemistry, corrosion, and commerce.1 Crude oils are considered acidic if their total acid number (TAN) exceeds 0.5 mg KOH/g by nonaqueous titration. Petroleum acids are found predominantly in immature, biodegraded, heavy crudes.2,3 The relationships between these acids and their hydrocarbon counterparts in crudes have been studied in petroleum formation and migration.4,5 In refineries, they distill into the gas oil and vacuum gas oil fractions and cause liquid-phase corrosion at process temperatures of 250* To whom correspondence may be addressed. † Department of Chemistry, Florida State University. (1) Brient, J.; Wessner, P. J.; Doyle, M. N. Naphthenic Acids, 4th ed.; Brient, J., Wessner, P. J., Doyle, M. N.; Ed.; 1995; Vol. 16, pp 1017-1029. (2) Ahsan, A.; Karlsen, D. A.; Patience, R. L. Mar. Pet. Geol. 1997, 14, 55-64. (3) Jaffe, R.; Gardinali, P.; Wolff, G. A. Org. Geochem. 1992, 18, 195201. (4) Jaffe, R.; Albrecht, P.; Oudin, J. L. Geochim. Cosmochim. Acta 1988, 52, 2599-2607. (5) Koike, L.; Reboucas, L. M. C.; Reis, F. D. M.; Marsaioli, A. J.; Richnow, H. H.; Michaelis, W. Org. Geochem. 1992, 18, 851-860.
400 °C.6-8 Commercial naphthenic acids extracted from gas oil find applications as specialty chemicals.1 Because naphthenic acids are surface active and marginally water-soluble, their release to wastewaters is closely monitored.9 Historically, crude oil acids have had to be isolated from the hydrocarbon matrix before they can be positively characterized by spectroscopic techniques. The acids isolated by amine-silica gel chromatography10 have been examined by FTIR11 and 13C NMR.12 Most characterization of acids, however, has come from a variety of mass spectrometric (MS) techniques. For example, exhaustive extraction and selective reduction to parent (6) Gutzeit, J. Mater. Perform. 1977, 16, 24-35. (7) Piehl, R. NACE Conf. 1987, paper no. 196. (8) Babaian-Kibala, E.; al., e. Mater. Perform. 1993, 50-55. (9) Lai, J. W. S.; Pinto, L. J.; Kiehlmann, E.; BendellYoung, L. I.; Moore, M. M. Environ. Toxicol. Chem. 1996, 15, 1482-1491. (10) Morrison, B.; DeAngelis, D.; Bonnette, L.; Wood, S. Presented at PittCon, New Orleans, 1992. (11) Li, K.; Zhang, J.; Zhao, X.; Luo, Y.; Xu, C. Acta Petrolei Sinica 1995, 6, 100-108. (12) Shinn, J.; Robinson, R.; Rechsteiner, C.; Tomczyk, N.; Winans, R. Presented at the World Petroleum Congress, Beijing, 1995; Forum 15, Poster 5.
10.1021/ef010111z CCC: $20.00 © 2001 American Chemical Society Published on Web 08/28/2001
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hydrocarbons have been combined with high-resolution MS to identify 3-ring and 4-ring steroid carboxylic acids in a California crude.13,14 A wider range of linear and acyclic isoprenoid acids has been esterified and identified by GC/MS.15,16 Isolated acid fractions have also been characterized by negative-ion mass spectrometry techniques to generate ring type and carbon number distributions. A range of acid molecules with 0-6 naphthenic rings and carbon numbers 10-35 has been reported by fluoride negative ion chemical ionization (NICI) MS of California crudes.17 Negative-ion fast atom bombardment (FAB) MS18 has been used to analyze acids isolated by ion exchange from California, Montana, and Louisiana crudes, showing a wider carbon range of naphthenic acids (C10-C50) with 0-6 rings. Aliphatic and naphthenic acids have been characterized by GC/MS following nonaqueous solid-phase extraction and methylation.19 Overall, MS data suggest that the naphthenic acids are closely related to the distribution of naphthenic hydrocarbons in their source oils. In addition, MS analyses reveal the presence of aromatic and hetero-aromatic acids (N, S, NOx, and SOx) at levels close to those for the “true” naphthenic acids in the California and Venezuelan crudes.14,20-22 Direct analysis of oxidized hydrocarbons (alcohol, ketone and acids) in lube oil has been accomplished by on-line liquid chromatography negative ion isobutane chemical ionization mass spectrometry.23 Finally, direct characterization of crude acids by negative ion atmospheric pressure chemical ionization (APCI) has been reported;24 however, the acid composition was not determined due to limited mass resolving power. In our previous efforts, we have explored the use of Electrospray Ionization (ESI) high field Fourier transform ion cyclotron resonance (FT-ICR) MS to characterize nitrogen-containing aromatics25 and petroporphyrins26 in crude oils. With high field positive-ion ESI FTICR MS, we were able to identify elemental compositions for more than 3000 basic nitrogen molecules in a South American heavy petroleum crude.25 In this work, we explore negative-ion ESI high field FT-ICR MS for characterizing the heavy petroleums. Acidic hydrocar(13) Seifert, W.; Teeter, R.; Howells, W.; Cantow, M. Anal. Chem. 1969, 41, 1639-1646. (14) Seifert, W. Fortzchr. Chem. Org. Naturst. 1975, 32, 1-49. (15) Green, J. B.; Stierwalt, B. K.; Thomson, J. S.; Treese, C. A. Anal. Chem. 1985, 57, 2207-2211. (16) Green, J. B.; Yu, S. K. T.; Vrana, R. P. HRC, J. High Resolut. Chromatogr. 1994, 17, 427-438. (17) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Anal. Chem. 1988, 60, 1318-1323. (18) Fan, T. P. Energy Fuels 1991, 5, 371-375. (19) Jones, D. M.; Watson, J. S.; Meredith, W.; Chen, M.; B., B. Anal. Chem. 2001, 73, 703-707. (20) Seifert, W.; Teeter, R. Analytical Chemistry 1970, 42, 750-758. (21) Green, J. B. Analysis of Heavy Oils: Method Development and Application to Cerro Negro Heavy Petroleum. U.S. D. O. E. Report No. DE0000200; Green, J. B., Ed.; U.S. Department of Energy: Washington, DC, 1989; Vol. NIPER Publication-452. (22) Tomczyk, N.; Winans, R.; Shinn, J. Identification of Acidic Constituents in a California Heavy Crude. Tomczyk, N., Winans, R., Shinn, J., Ed.; ACS Division of Fuel Preprints, American Chemical Society: Washington, DC, 1997; pp 339-343. (23) Qian, K.; Hsu, C.; Robbins, W.; Rose, K. In Proceedings of the 40th ASMS Conference on Mass Spectrometry and Allied Topics, Washington, DC, 1992; pp 758-759. (24) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217-223. (25) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492-498. (26) Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G.; Greaney, M.; Qian, K. Can. J. Chem. 2001, 79, 546-551.
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bons in the petroleum crude are directly speciated in extremely high detail. Isolated acid fractions from the crude were also analyzed by the same technique for comparison. Experimental Methods Samples. The South American heavy crude oil analyzed in this work has previously been characterized by positive-ion ESI FT-ICR mass spectrometry.25 It contains ∼50% of >566 °C boiling point hydrocarbons, and contains 4.02% sulfur and 0.65% nitrogen. It has a TAN number of 3.2. Acid fractions were isolated from the crude by solid-phase extraction (see below). Isolation of Acid Fractions. One hundred grams of the total crude oil was diluted with 700 mL (70:30) of toluene and methanol, and loaded onto 50 g of amino-propel silica (APS, Baker). After standing overnight, the bulk oil solution was removed by filtration and solvent wash. The acid-loaded APS was Soxhlet extracted with 30% acetic acid in toluene. The extract was water washed to remove residual acetic acid and then rotovapped to remove solvent. The residue was reextracted with hexane. The hexane-soluble fraction is hereafter designated as the “acid fraction” (2.34 g), and the hexaneinsoluble fraction as “acidic asphaltene” (0.37 g). ESI Sample Preparation. Crude oil samples were prepared by dissolving ∼10-20 mg in 3 mL toluene, and then diluting with 17 mL MeOH. Fifty to 100 µL of ammonium hydroxide solution (30%) was added to facilitate deprotonation of the acids and neutral nitrogen compounds to yield [M-H]ions. Less ammonium hydroxide (a few microliters) was added to the acidic asphaltene fraction to reduce the amount of precipitate that formed upon addition of the base. Electrospray Ionization High-Field FT-ICR Mass Spectrometry. The extra heavy oil was analyzed at the National High Magnetic Field Laboratory (NHMFL) with a home-built 9.4 T FT-ICR mass spectrometer.27 Ions were generated externally by a microelectrospray source28 and samples delivered by a syringe pump at a rate of 300 nL/min. 2.2 kV was applied between the capillary needle and ion entrance. The externally generated ions were accumulated in a short (45 cm) rf-only octopole for 5-10 s and then transferred via a 200 cm rf-only octopole ion guide to a Penning trap. Ions were excited by frequency-sweep (100-725 kHz at 150 Hz/µs at an amplitude of 200 Vp-p across a 10 cm diameter open cylindrical cell). The time-domain ICR signal was sampled at 1.28 Msample/s for 1.63 s to yield 2 Mword time-domain data. Two hundred data sets were co-added, zero-filled once, Hanning apodized, and fast Fourier transformed with magnitude computation. A continuous-wave 40 W CO2 laser (Synrad E48-2-115, Bothell, WA) was used to dissociate noncovalent ion complexes. Mass Calibration. The high field FT-ICR mass spectra were frequency-to-m/z calibrated internally, with respect to a #G2421A electrospray “tuning mix” from Agilent (high mass) and stearic acid (low mass). The full range mass spectrum was then converted to the Kendrick mass scale (see below) with high accuracy by use of identified sample peak(s): C19H33O2 (Kmass ) 292.9211) and C39H73O2 (Kmass ) 572.9211).
Data Analysis and Interpretation It is helpful to convert the mass spectral data from the IUPAC mass scale (based on the 12C atomic mass as exactly 12 Da) to the Kendrick mass (KMass) scale to facilitate the identification of petroleum homologues. (27) Senko, M. W.; Hendrickson, C. L.; Pasa-Tolic, L.; Marto, J. A.; White, F. M.; Guan, S.; Marshall, A. G. Rapid Commun. Mass Spectrom. 1996, 10, 1824-1828. (28) Senko, M. W.; Hendrickson, C. L.; Emmett, M. R.; Shi, S. D.H.; Marshall, A. G. J. Am. Soc. Mass Spectrom. 1997, 8, 970-976.
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Kendrick mass29 is obtained from IUPAC mass as shown in eq 1.
Kendrick mass ) IUPAC mass(14/14.01565) (1) The Kendrick scale effectively converts the mass of CH2 from 14.01565 to exactly 14. Thus, the advantage of KMass for petroleum analysis is that the members of a homologous series (namely, compounds with the same constitution of heteroatoms and number of rings plus double bonds, but different numbers of CH2 groups) will have identical Kendrick mass defect, defined in eq 2.
Kendrick mass defect ) (nominal mass - Kendrick mass) (2) Theoretically, nominal mass is defined as the sum of the number of protons and neutrons in a chemical formula.30 Practically, nominal masses are readily obtained by rounding-off Kendrick masses. The data analysis procedure has been explained in our previous publication.31 Briefly, the mass values for peaks ranging from 200 to 1000 Da with signal-to-noise ratio greater than 3:1 were exported to an Excel spreadsheet. The peaks were separated into petroleum homologues by their nominal mass and Kendrick mass defect following a multiple sorting procedure.31 Molecular compositions of petroleum homologues were identified by their unique combination of nominal mass and Kendrick mass defect, and were expressed as a general chemical formula, CcH2c+ZX, in which c is the carbon number, Z is often referred to as the hydrogen deficiency index, and X denotes the constituent heteroatoms (N, S, or O) in a molecule. For convenience, we denote a given “type” and “class” according to its Z and X components. For example, CcH2c-4O2 is abbreviated as -4 O2, whereas CcH2c-8O2S is listed as -8 O2S. Such abbreviations always denote neutral molecules. Members of a given “type” series each have the same Z value (i.e., same number of rings plus double bonds), but may differ by multiples of 14.01565 in IUPAC mass (and by multiples of exactly 14 in Kendrick mass) according to the number of CH2 groups appended to the rings. Members of such a series can therefore be recognized from their identical Kendrick mass defects. Results and Discussion We previously reported that positive-ion ESI could selectively detect bases (basic nitrogen) in heavy petroleum without interference from the hydrocarbon matrix. High field FT-ICR MS resolved and allowed assignment of elemental compositions for more than 3000 basic nitrogen molecules.25 Those findings stimulated our interest in negative-ion ESI FT-ICR mass spectral analysis of petroleum. Preliminary work at ExxonMobil had shown that carboxylic acids could be selectively ionized under negative-ion conditions.24 However, the spectra were extremely complex and high-resolution mass spectrometry was necessary to resolve and identify the acid composition. (29) Kendrick, E. Anal. Chem. 1963, 35, 2146-2154. (30) Scheppele, S. E.; Chung, K. C.; Hwang, C. S. Int. J. Mass. Spectrom. Ion Phys. 1983, 49, 143-178. (31) Hsu, C. S.; Qian, K. N.; Chen, Y. N. C. Anal. Chim. Acta 1992, 264, 79-89.
Figure 1. Full range negative-ion ESI FT-ICR mass spectra of a heavy crude oil (top) and its acid fraction (bottom) at an average mass resolving power, m/∆m50% ≈ 80 000-100 000 (∆m50% is mass spectral peak full width at half-maximum peak height), over the entire mass range. Note: the mass scale for each displayed mass spectrum is expressed in Kendrick, not IUPAC, mass.
Figure 2. 14 Da mass segments of the negative-ion ESI FTICR mass spectra of Figure 1. The similarity in the two spectra demonstrates the selectivity of negative ion ESI toward acids. Odd mass ions predominate, suggesting that most acids do not contain a nitrogen atom.
A purified acid fraction was isolated from the heavy crude to help establish a baseline reference for crude acid analysis. The acid fraction was also used to optimize ESI and FT-ICR MS conditions. Unlike basic nitrogen compounds in the positive ion mode, the acid molecules may form noncovalent complexes, in which case infrared laser multiphoton dissociation (IRMPD) may be applied to dissociate the ion complexes. Figure 1 shows broadband mass spectra of crude and acid fraction (200 < m/z < 1000). All species observed are singly charged, as evident from the absence of peaks at integral fractions of 1 Da between the nominal mass peaks. By visual examination, the mass spectrum of the crude appears to contain more peaks at both low and high mass, suggesting that some light and heavy components may be lost during the isolation process. The number-average and weight-average molecular weights, Mn and Mw, of the two samples are similar: Mn/Mw are 454/491 and 440/461 for the crude and the acid fraction, respectively. These values are considerably lower than those for the basic nitrogen and hydrocarbon analogues (Mn,Mw ≈ 600). This result is not especially surprising, because acids in crude oil are formed by
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Table 1. Identification of Singly Charged Anions at Nominal Mass, 363, Observed in Acid Asphaltene (Figure 6)a peak no.
observed Kmass (Da)
rel abundance
resolving power
theoretical Kmass (Da)
1 2 3 4 5 6 7
362.9211 362.9125 362.8846 362.8611 362.8485 362.8299 incompletely resolved 362.8146
100.0 5.4 2.8 1.8 11.8 22.1
81318 81320 81326 81331 81334 46480
362.9211
0.0
362.8847
-0.1
362.8485 362.8307 362.8341
0.0 -0.8
5.8
36152
362.8136
1.0
7.4 67.1 2.0 2.8 14.1 6.2 1.8
54228 81346 81351 46489 81359 81367 81371
362.8095 362.7943 362.7733 362.7580 362.7369 362.7039 362.6856
0.7 0.1 0.9 -0.1 0.1 -0.1 -0.1
8 9 10 11 12 13 14 15
362.8102 362.7944 362.7742 362.7579 362.7370 362.7038 362.6855
error (mDa)
formula C24H43O2 UnID C23H39O3 UnID C22H35O4 C22H35O2S C25H31O2
Z
X
-4
O2
-6
O3
-8 -8 -18
O4 O2S O2
-18 -10 -20 -12 -22 -14
S O3S OS O4S O2S O3S2
13C
peak from even mass C25H31S C21H31O3S C24H27OS C20H27O4S C23H23O2S C19H23O3S2 UnID
a Similar ions were also found in crude and acid fraction (see Figure 3). Note that the listed masses and formulas refer to the observed (deprotonated) species, but that the “Z” values are calculated for the corresponding neutral.
Figure 3. Ultrahigh-resolution segments of the mass spectra of Figure 2, allowing for resolution and elemental composition assignment (based on accurate mass measurement) of 5 (top) and 9 (bottom) chemically distinct species at a single nominal mass. Detailed identifications are listed in Table 1. Structures are intended as illustrative, not literal. Although masses and structures denote deprotonated ions, “Z” values denote corresponding neutrals (data reported similarly in Figure 6).
bacterial degradation which tends to attack paraffinic chains preferably, leaving short chains on naphthenic and aromatic rings. Mass scale-expanded 14 Da segments (Figure 2) of the two spectra of Figure 1 are very similar, suggesting that acids in crude may indeed be ionized selectively without hydrocarbon interference. In negative-ion ESI, the most common ionization mechanism is deprotonation, resulting primarily in [M-H]- anions. Because we observed predominantly odd mass peaks, the “nitrogen rule”32 implies that most acids do not contain one (or more generally, an odd number of) nitrogen atom. The relative magnitudes for the even-mass ions closely track the 13C isotope contributions for the odd-mass ions, a finding further corroborated by accurate mass analysis (see below). We did observe low levels of neutral nitrogens (i.e., the pyrrole benzologues) in the crude. The data on neutral nitrogens will not be discussed here as it exceeds the scope of this paper. It is important to note that the mass spectrum of the acid fraction exhibits higher signal-to-noise ratio than does the crude sample, (32) McLafferty, F. W.; Turecek, F. Interpretation of Mass Spectra; University Science Books: Sausalito, CA, 1993.
Figure 4. Type distribution for CcH2c+ZOoS acids identified in the ESI FT-ICR mass spectrum of Figure 1. For each heteroatom composition (O2, O3, O4, O2S, and O3S), there is a distribution of species differing in Z value (note that Z decreases by 2 for each additional ring or double bond). Structures are illustrative
indicating that preseparation could greatly facilitate detection of low level components in crude. Detailed scrutiny of the same 0.26 Da mass spectral segment (Figure 3) of Figure 1 reveals a wealth of components: at least five in the crude and at least nine in the acid fraction. Accurate mass analysis positively identifies the elemental compositions of these compounds (peak identifications are listed in Table 1). In this spectrum, a dicyclic acid (C24H43O2, -4 O2) is the major component, followed by a sulfur-containing tetracyclic acid (C22H35O2S, -8 O2S), a dicyclic diaromatic acid (C25H31O2, -18 O2), and a sulfur-containing triaromatic acid (C23H23O2S, -22 O2S). We also detected species containing O4, O3, and O3S in the acid fraction, but not in direct crude analysis. An average resolving power, m/∆m50% ≈ 100 000 (in which ∆m50% is the mass spectral peak full width at half-maximum peak height), was achieved for the crude sample and m/∆m50% ≈ 80 000 for the acid fraction across a wide mass range (200-1000 Da). The chemical complexity of acid fraction increases dramatically with molecular weight. The compound type distributions for the crude and the isolated acid fraction (Figure 4) again exhibit high similarity. A total of 32 acid types could be identified,
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Table 2. Possible Acid Structures (O2) for Species with Various Z values
Table 3. Possible Sulfur-Containing Acid Types (O2S)
each with carbon number ranging from C15 to C55. Hydrogen deficiency values, -2 > Z > -22, were observed for the acids (O2), vs -6 > Z > -26 for sulfurcontaining acids (O2S). There is a sudden drop in O2 compound types after the -16 O2 series in the acid
fraction sample, due to insufficient mass resolution to distinguish C3/SH4 doublets. The main differences between the two samples are that the acid fraction sample contains more O3S, O4 and O3, presumably due to the enrichment factor (∼45 times) from the acid isolation.
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Figure 5. Carbon number distribution for four compound types (-4 O2, -8 O2S, -18 O2, and -22 O2S). Note that -8 O2S and -18 O2 require mass resolving power higher than 100 000 for complete resolution.
Figure 6. Ultrahigh-resolution mass spectral segment (m/z ) 363) of an asphaltene sample. All 15 peaks resolved in the 0.26 mass window are identified as listed in Table 1. Structures are illustrative.
Table 4. Estimated Acid Structure Populations backbone
O2
O2S
O3S
naphthenic mono-aromatic di-aromatic tri-aromatic
41.9 25.5 8.3 0.7
4.3 8.5 8.5 1.8
0.5
Both the O2 and O2S classes illustrate bimodal Z distributions, suggesting the presence of at least two different core structures in the acids. Despite overlaps in exact masses of acid isomers, high resolution MS data, combined with prior knowledge of crude acids, may be used to deduce acid structures. It is generally believed that polycyclic acids contain fewer than six rings.12-19,33 Sulfur is normally incorporated in crude as five-member ring cyclic sulfides or thiophenes (especially dibenzothiophenes) and higher benzologues.34 Proposed acid structures and their Z values for O2 and O2S classes are summarized in Tables 2 and 3. Note that naphthenic and aromatic acids can have identical Z values and overlap in exact masses. These isomers are not resolved by mass spectrometry alone. Assuming the most likely structures (*) for each Z group, we can estimate the acid structure population distribution summarized in Table 4. Our data suggest that true “naphthenic acids” account for less than half of the total acids. Most of the acids contain at least one aromatic ring or one sulfur in their structures. Up to 3-ring aromatics are observed. Figure 5 shows the carbon number distributions for four homologue series, -4 O2, -8 O2S, -18 O2, and -22 O2S, of the crude sample. These four homologues share the same nominal masses that would not be resolved by low-resolution mass spectrometry. The close C3/SH4 doublets (between the - 8 O2S and -18 O2 homologues) are completely or partially resolved throughout the entire carbon number range, performance accessible only by means of FT-ICR MS. Carbon number ranges from C15 to C55 were detected. The average carbon number shifts to higher values as the Z and number of heteroatoms increase (or the core size increases). (33) Schmitter, J. M.; Arpino, P.; Guiochon, G. J. Chromatogr. 1978, 167, 149-158. (34) Orr, W. L., Damste, J. S. S., Orr, W. L., Damste, J. S. S., Eds. American Chemical Society: Washington, DC, 1990; Vol. 429, Chapter 1.
Figure 7. Compound type distributions for the acidic asphaltene. Large Z values imply large aromatic structures. Up to nine aromatic rings may be present.
The composition of the “acidic asphaltene” is extremely complex. Figure 6 shows the same 0.26 Da mass segment as in Figure 3, but this time with resolution of 15 peaks at a single nominal mass. Most of the peaks were identified with high confidence and are listed in Table 1. Figure 7 shows the Z distributions (-6 > Z > -48) for the O2S, O3S, and O4S classes from acidic asphaltene. The asphaltene sample evidently contains mainly aromatic acids (probably up to 9 aromatic rings). The most dramatic difference between the spectrum in Figure 6 and those in Figure 3 is the large -10 O3S component (Figure 6), which we ascribe to an oxidized analogue of a sulfur-containing acid. These highly polar molecules are significantly enriched in the “acidic asphaltene” sample due to their lower solubility. About 100 homologues containing mainly O2, O2S, O3S, O4, and O4S heteroatoms were found in the mass range, 2001000 Da. Conclusions In conclusion, we demonstrate that, under negative ion conditions, acidic hydrocarbons (mostly crude acids), can be ionized selectively by electrospray ionization without interference from the hydrocarbon matrix. Moreover, isolation of the acid fraction allows us to
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detect species present at much lower level. We have determined detailed elemental compositions of acid and acidic asphaltenes by high field FT-ICR MS accurate mass measurements. Collectively, over one hundred acid homologues, ∼3000 chemical formulas containing O2, O3, O4, O2S, O3S, and O4S with carbon numbers ranging from 15 to 55, have been identified. Acknowledgment. The authors thank Christopher L. Hendrickson, Mark R. Emmett, and John P. Quinn for helpful discussions, and Daniel McIntosh for machining all of the custom parts required for the 9.4 T
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instrument construction. This work was supported by the NSF National High-Field FT-ICR Mass Spectrometry Facility (CHE 99-09502), Florida State University, and the National High Magnetic Field Laboratory at Tallahassee, Florida. K.Q. and W.K.R. wish to thank Drs. William N. Olmstead, Larry A. Green, and Frank C. McElroy of ExxonMobil Research and Engineering Company for valuable discussions. We also thank ExxonMobil for supporting this exploratory research and for permission to publish the data. EF010111Z
