Recent Advances in Petroleum Characterization by GC Field

Jul 24, 2002 - Recent Advances in Petroleum Characterization by GC Field Ionization Time-of-Flight High-Resolution Mass Spectrometry. Kuangnan Qian* a...
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Anal. Chem. 2002, 74, 3977-3983

Recent Advances in Petroleum Characterization by GC Field Ionization Time-of-Flight High-Resolution Mass Spectrometry Kuangnan Qian* and Gary J. Dechert

ExxonMobil Research & Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08502

Although mass spectrometry has been studied extensively in the past 50 years for analysis of petroleum and other fossil fuels, comprehensive characterization of total petroleum composition across a wide boiling range remains a challenge. In this work, we report our efforts in combining GC separation, field Ionization, and time-of-flight high-resolution mass spectrometry (GC-FI-TOF HRMS) for detailed analyses of petroleum products with a carbon number range of C6-C44. GC separates hydrocarbon molecules by boiling points. FI generates intact molecular ions for both aromatic and saturate petroleum molecules eluting from the GC. The elemental compositions of these molecular ions are consequently resolved and identified by TOF HRMS with a mass resolving power up to 7000 (M/∆Mfwhm) and mass accuracy of (3 mDa, which in turn yields chemical information (heteroatomic content, number of rings plus double bond, and carbon number) for the petroleum molecules. The ability to resolve sulfurcontaining hydrocarbons is greatly enhanced because of the unique combination. Petroleum samples are complicated hydrocarbon mixtures containing paraffins, cyclic paraffins, multiring aromatics, and various heteroatomic hydrocarbons (most commonly O, S, and N). Virgin petroleum crude oils contain molecules of a wide boiling point range from highly volatile C4 hydrocarbons to nonvolatile asphaltenes. Analysis of petroleum composition of various boiling ranges by mass spectrometry has been the subject of research over the past 50 years. During the 1950s and 1960s, mass spectrometry played a dominant role in petroleum characterization.1-9 The early methods largely depend on 50-70 eV electron impact ionization. Various “group type” analysis methods for gasoline, middle distillates, and gas oils were developed based on characteristic fragment ions of petroleum molecules.1-3 Some (1) Brown, R. A. Anal. Chem. 1951, 23, 430-437. (2) Lumpkin, H. E.; Johnson, B. H. Anal. Chem. 1956, 28, 1243-1247. (3) Robinson, C. J. Anal. Chem. 1971, 43, 1425-1434. (4) Robinson, C. J.; Cook, G. L. Anal. Chem. 1969, 41, 1548-1554. (5) Gallegos, E. J.; Green, J. W.; Lindeman, L. P.; Letourmeau, R. L.; Teeter, R. M. Anal. Chem. 1967, 39, 1833-1838. (6) Field, F. H.; Hasting, S. H. Anal. Chem. 1958, 28, 1248-1255. (7) Lumpkin, H. E. Anal. Chem. 1958, 30, 321-325. (8) Aczel, T. Rev. Anal. Chem. 1972, 1, 226-261. (9) Schmidt, C. E.; Sprecher, R. F.; Bratts, B. D. Anal. Chem. 1987, 59, 20272033. 10.1021/ac020166d CCC: $22.00 Published on Web 07/24/2002

© 2002 American Chemical Society

of these methods are still being used today to provide valuable crude assay properties. The availability of commercial highresolution mass spectrometers extended mass spectrometry to isobaric compound types and allowed direct identification of elemental composition of mass peaks.4,5 Because of the extensive fragmentation, these high-voltage methods do not provide information on carbon number distribution. Lowering electron ionization energy reduces the fragmentation of petroleum molecules and allows direct measurement of carbon number distribution of aromatic compounds in petroleum.6,7 The combination of lowvoltage electron impact ionization and high-resolution mass spectrometry (LVEI-HRMS) was extensively applied to analyses of aromatic compositions in petroleum and coal liquid.8,9 Saturate hydrocarbons needs to be analyzed separately. In recent years, many alternative “soft” ionization methods were explored for petroleum characterization, such as charge exchange chemical ionization using CS2 and benzene,10,11 Towsend discharge nitric oxide chemical ionization,12 thermospray ionization,13 matrix-assisted laser desorption-ionization,14,15 atmospheric pressure chemical ionization,16 and more recently electrospray ionization.17-20 Many of the ionization methods are selective toward aromatic compounds and polar molecules, such as crude acids and basic and neutral nitrogens. Characterization of total petroleum composition suffered from the lack of a “universal” soft ionization method that simultaneously satisfies the following criteria: (1) effective ionization of both saturate and aromatic petroleum molecules; (2) compatible with high-resolution mass (10) Hsu, C. S.; Qian, K. Anal. Chem. 1993, 65, 767-71. (11) Allgood, C.; Ma, Y. C.; Munson, B. Anal. Chem. 1991, 63, 721-5. (12) Dzidic, I.; Petersen, H. A.; Wadsworth, P. A.; Hart, H. V. Anal. Chem. 1992, 64, 2227-32. (13) Hsu, C. S.; Qian, K. Energy Fuels 1993, 7 (2), 268-72. (14) Li, C. Z.; Herod, A. A.; John, P.; Johnson, C. A. F.; Parker, J. E.; Smith, G. P.; Humphrey, P.; Chapman, J. R.; Rahman, M. Rapid Commun. Mass Spectrom. 1994, 8, 823-828. (15) Artok, L.; Su, Y.; Hirose, Y.; Hosokawa, M.; Murata, S.; Nomura, M. Energy Fuels 1999, 13, 287-296. (16) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217-223. (17) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492-498. (18) Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G.; Greaney, M. A.; Qian, K. Can. J. Chem. 2001, 79, 546-551. (19) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. Anal. Chem. 2001, 73, 4676-4681. (20) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505-1511.

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spectrometry to resolve isobaric masses over a broad mass range; and (3) compatible with on-line chromatography for separation of petroleum molecules that share the identical or very close exact masses. Field ionization (FI) has been a preferred soft ionization method for petroleum analysis. FI requires no reagent or matrix and yields a simple mass spectrum with intense M•+ molecular ions (or protonated molecular ions for certain polar molecules). FI was first introduced to mass spectrometry in 1954.21 Beckey and co-workers made significant contributions toward mass spectrometry applications of the technique.22,23 Practical use of FIMS (and its associate technique, field desorption MS) started in the early 1970s and was traditionally configured for magnetic sector instruments. The combination of FI with GC-HRMS using a magnetic sector instrument has been difficult for a number of reasons. First, ion yields by FI are considerably lower than that of EI or CI. Second, magnetic sector instruments require a longer scanning time to cover all masses in a spectrum, making it difficult to couple with a dynamic sample inlet, such as GC, where sample vapor pressure is not steady and ions must be generated in a very narrow time window. In general, FI is normally used in low mass resolution mode to provide molecular weight distribution or to provide the composition of fractionated petroleum samples (e.g., saturates and aromatic ring fractions by liquid chromatography separation). FI has been coupled to GC-quadrupole MS to provide the composition of diesels.24,25 The technology, however, has limitations for higher boiling petroleum molecules because of nominal mass overlaps. The rapid developments in TOF MS technology in recent years have made it possible to couple FI with fast GC and highresolution/accurate mass analysis. TOF with a multichannel plate (MCP) detector can simultaneously sample all masses with high sensitivity. Accurate mass measurement at elevated resolving power26-28 has been reported. In this work, the potential of combining boiling point separation, soft ionization, and exact mass measurement was demonstrated for characterization of both saturate and aromatic petroleum molecules with carbon numbers ranging from C6 to C44. The technology provided a new platform for petroleum compositional analysis with faster speed and enhanced detail. EXPERIMENTAL SECTION All experiments were carried out on a Micromass GCT instrument with an orthogonal acceleration TOF mass spectrometer coupled to a HP 6890 gas chromatograph. Gas Chromatography. The GC column used in this study is a 15-m DB-1 HT with 0.1-µm film thickness and 0.25-mm i.d. (J&W Scientific). This column separates petroleum molecules by boiling point because of its nonpolar nature. Petroleum samples were (21) Inghram, M. G.; Gomer, R. I. Chem. Phys. 1954, 22, 1279. (22) Beckey, H. D. Principles of Field Ionization and Field Desorption Mass Spectrometry; Pergamon Press: Oxford, 1977. (23) Beckey, H. D. Int. J. Mass Spectrom. Ion Phys. 1969, 2, 500-507. (24) Briker, Y.; Ring, Z.; Iacchelli, A.; McLean, N.; Rahimi, P. M.; Fairbridge, C.; Malhotra, R.; Coggiola, M. A.; Young, S. E. Energy Fuels 2001, 15, 23-37. (25) Briker, Y.; Ring, Z.; Iacchelli, A.; McLean, N.; Malhotra, R.; Coggiola, M. A.; Young, S. E. Energy Fuels 2001, 15, 996-1002. (26) Qian, K. Abstr. Pap.-Am. Chem. Soc. 2001, 221, GEOC-022. (27) Hsu, C. S.; Green, M. Rapid Commun. Mass Spectrom. 2001, 15 (3), 236239. (28) Green, M. Micromass technical brief, TB2; May 2000.

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normally diluted by CS2 into a 2% solution. About 0.5 µL of the sample was injected via a cold on-column (COC) injector or a program temperature vaporizer (PTV) injector in a splitless mode. The temperature of the GC-TOF interface is maintained at 350 °C. Initial GC oven temperature is typically controlled at -40 °C and ramped up to 375 °C at 10 or 15 °C/min to ensure the separation of low-boiling species and the elution of high-boiling molecules. Field Ionization. FI is used to ionize petroleum molecules eluting from the GC. The FI emitter (from CarboTech) consists of a 10-µm tungsten wire onto which carbon microneedles have been grown. The FI emitter is carefully aligned with the end of the GC capillary column so that effluent molecules pass near the tips of the carbon dendrites. The emitter (at ground voltage) is ∼1.5 mm away from a pair of extraction rods held at high potential (-12 kV), producing very high electric fields (∼10-7-10-8 V/cm) around the tips of the carbon dendrites. It is generally believed that, under the influence of these fields, an electron can be removed from the molecule via quantum tunneling effects, generating radical molecular ions with minimal fragmentation. FI emitter current was typically set at 0-5 mA during the scan. The emitter is flashed by a current of 12-20 mA during an interscan cycle (0.2 s) to regenerate the emitter. Time-of-Flight Mass Spectrometry. Ions generated by FI are accelerated and focused into a pusher region of the TOF. A voltage pulse of 960 V is applied, ejecting ions orthogonal to the original ion path. The ion packet drift through a TOF with an effective path length of 1.2 m. A reflectron reflects ions back to a dual microchannel plate detector. Ion arrivals are recorded using a time-to-digital converter (TDC) with a sampling rate of 3.6 GHz. The voltage pulse is applied at a frequency of 30 kHz. A full spectrum is generated every 33 µs. The mass range is normally set at 50-800 Da. The scan duration time or spectrum accumulation time is 1 s (i.e., every “scan” is an accumulation of 30 000 spectra). Calibration and Accurate Mass Measurement. Since FI generates mostly a single molecular ion, a mixture of compounds (normally halogenated hydrocarbons) was used to calibrate a wide mass range from 50 to 800 Da. A typical calibration mixture contains heptacosa, pentaflurorbenzene, hexafluorobenzene, pentafluoroiodobenzene, pentafluorochlorobenzene, perfluorotrimethylcyclohexane, xylene, and acetone. The calibrants were introduced into the ion source via a batch inlet and were pumped out after the calibration. During sample analyses, a single lock compound was introduced as an internal reference for accurate mass measurement. In our experiments, pentafluoroiodobenzene, with a monoisotopic mass of 293.896 Da, was used as the internal reference. Model Compounds and Samples. The n-paraffin mixture (C5-C40) was purchased from Agilent (Part No. 5080-8716). Other model compounds including C44 n-paraffin were obtained from Aldrich. All petroleum samples are from ExxonMobil. The middle distillate is a virgin petroleum distillate with a boiling point range of 400-650 °F. The aromatic petroleum fraction was isolated from a vacuum gas oil (VGO) with a boiling point range of 650-950 °F using a clay-gel separation scheme similar to that of ASTM D2007.

Figure 1. Total ion chromatogram and averaged mass spectrum of C6-C44 n-paraffins, demonstrating petroleum molecules of a wide boiling range can be analyzed by GC-FI-TOF MS. The masses of paraffin molecular ions were accurately determined.

Table 1. Field Ionization of Six Model Compounds Typically Present in Petroleumsa IM+(13C)/IM+(12C) compounds decane 1-decene 1-hexadecene n-butylcyclohexane n-butylbenzene dibenzothiophene

I

+ + M /(IM

+ ∑IF

0.97 0.94 0.98 1.00 1.00 1.00

+)

obs

calc

0.10 0.13 0.14 0.11 0.11 0.13

0.11 0.11 0.18 0.11 0.11 0.13

a All compounds show predominantly molecular ions. Minor fragmentation observed for decane, 1-decene, and 1-hexadecene. Overall, observed ratios of 13C/12C molecular ion intensity match well with the calculated values. IM+ represents molecular ion intensity (include 13C and 34S isotopes). IF+ denotes fragment ion intensity.

RESULTS AND DISCUSSION “Universal” Soft Ionization. Soft ionization is critical to petroleum characterization as it generates carbon number distribution for petroleum homologues. Among all soft ionization methods, FI is a more “universal” soft ionization method applicable to both saturates (i.e., paraffins, cyclic paraffins) and aromatics (i.e., alkylated benzenes, alkylated polynuclear aromatics, alkylated thiophenoaromatics, etc.) in petroleum products. Polar compounds, such as nitrogen-containing hydrocarbons and acids, could also be ionized but with a lower ionization efficiency. The characterization of the latter can be better handled by ESI17-20 and are not addressed in this work. The soft ionization of petroleum molecules by FI is demonstrated by Table 1, which shows the ratios of molecular ion intensity (include 13C and 34S isotopes) to the sum of total ion intensities (molecular ion plus fragment ions) for six model compounds, decane (paraffin), decene and 1-hexadecene (olefin), n-butylcyclohexane (naphthene), n-butylbenzene (aromatics), and dibenzothiophene (thiophenoaromatics). All compounds yield an intense singly charged molecular ion (M•+). Minor fragmentation was observed for decane, 1-decene, and 1-hexadecene with molecular ion/total ion

ratios of 0.97, 0.94, and 0.98, respectively. Other compounds show no detectable fragmentation. Table 1 demonstrates that FI generates very clean mass spectra for hydrocarbon molecules. It worth pointing out that FI is the only ionization method that generates primarily molecular ions for saturated hydrocarbons with minimal side products (such as adduct and fragment ions). Table 1 also provided the relative intensities of 13C isotope peaks as well as the theoretical calculation. Overall, the observed ratios of 13C/ 12C molecular ion intensity match well with the calculated values. The larger errors in isotope ratios of 1-decene and 1-hexadecene may be caused by ion-molecule reaction of the olefin molecules under FI conditions. To further illustrate that FI applies to a wide boiling point range of petroleums, we analyzed an n-paraffin mixture (C5-C44) in CS2. The total ion chromatogram (TIC) and averaged mass spectrum are shown in Figure 1a and b, respectively. C5 paraffin (pentane) coeluted with the CS2 solvent molecule and is not displayed. The data demonstrate that petroleum molecules of various volatilities (boiling points from 98 to 545 °C) can be separated by GC, efficiently ionized by FI and accurately mass analyzed by TOF MS. We noticed that C12, C14, and C16 paraffins (the three largest peaks in the chromatogram) show wider peak width than do the other species. One possibility is that FI emitter may absorb/desorb analyte molecules during the ionization process and cause the spread in retention time. The interaction is more pronounced for the higher concentration analytes. The averaged mass spectrum (Figure 1b) again shows predominantly molecular ions across the entire carbon number range. The fragment ions at low masses (m/z 85, 99, 113, etc.) are more pronounced because of accumulated contributions of fragments from larger paraffins. Overall the mass spectrum is considerably simpler than that by EI or CI. This simplification in mass spectrum greatly benefits both identification and quantification of hydrocarbon mixtures where thousands of molecules may be present in one sample. Identifications can be made based on the exact masses of the molecular ions (see below). Quantification can be made based on the abundance of the molecular ions with corrections of response Analytical Chemistry, Vol. 74, No. 16, August 15, 2002

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Table 2. Mass Resolving Power and Mass Accuracy of TOF MS formula

exp mass (Da)

calc mass (Da)

error (mDa)

rel error (ppm)

peak width ∆Mfwhm (Da)

RP M/∆Mfwhm

C3H6O CF3 C8H10 C6HF5 C6F6 C6F5Cl C6F5I C8F15 C9NF18 C9NF20 C12NF25

58.0412 68.9965 106.0744 167.9986 185.9894 201.9602 293.8955 380.9756 463.9772 501.9736 632.9706

58.0419 68.9952 106.0783 167.9998 185.9904 201.9609 293.8965 380.9760 463.9743 501.9711 632.9632

-0.7 1.3 -3.9 -1.2 -1 -0.7 -1 -0.4 2.9 2.5 7.4

-12.1 18.8 -36.8 -7.1 -5.4 -3.5 -3.4 -1.0 6.3 5.0 11.7

0.017 0.018 0.022 0.033 0.036 0.038 0.054 0.066 0.074 0.079 0.092

3414 3833 4821 5090 5166 5314 5443 5772 6270 6354 6880

factors. Collectively, Figure 1 and Table 1 demonstrate that petroleum samples containing various hydrocarbon types of a wide range of volatility can be analyzed by this technique. Mass Resolving Power and Accuracy of TOF. One problem introduced by soft ionization is the loss of structural information that may be provided by fragmentation patterns. Many petroleum molecules share the same nominal mass but differ in exact masses (isobars). The differentiation of these molecules needs high resolution and accurate mass analysis capability. As an example, Figure 2 illustrates four common petroleum molecules that overlap in nominal masses. The two most common overlaps, C/H12 and C2H8/S doublets, have mass differences of 94 and 90 mDa, respectively. and require a mass resolving power (RP) of ∼3000 for separation at mass 274. C3/SH4 doublet has a mass difference of only 3.4 mDa and requires an RP of ∼80 000 for separation at the same mass. The typical RP (M/∆Mfwhm) and mass accuracy of the GC-FI-TOF MS is illustrated in Table 2. The RP of the TOF MS increases from ∼3500 to ∼7000 as the mass increasees from ∼50 to ∼650 Da, which is sufficient to resolve C/H12 and C2H8/S doublets across the mass range that is of interest to petroleum analysis. The RP, however, is insufficient to directly resolve closer doublets, such as C3/SH4 (3.4 mDa), O/CH4 (36.4mDa), and N/13CH (8.1 mDa). We shall show that TOF in conjunction with gas chromatography (see below) can resolve these doublets. The mass accuracy of TOF MS shown in Table 2 is very impressive. The average and relative average errors in mass measurement are less than 3 mDa and 15 ppm, respectively. The accuracy is comparable with (or even better than) that of a high-resolution magnetic sector instrument. Resolution and Identification of Petroleum Molecules. Most saturate hydrocarbon molecules (e.g., paraffins and one- to three-ring cyclic paraffins) in a petroleum sample can be directly resolved and determined by TOF MS without sample fractionation (i.e., separation of saturate and aromatics). The nominal mass overlap between saturates and aromatics is largely due to the 12C/ 12H doublet and can be easily discerned by TOF MS. Table 3 illustrates the unique matches in elemental compositions of paraffins and cycloparaffins of a petroleum middle distillate determined by GC-FI-TOF. The exact masses of both paraffins and cycloparaffins match extremely well with the theoretical values across the entire homologue series. It worth noting that the mass accuracies in Table 3 were obtained while molecules are “on-fly” from GC. The accuracies are comparable with that obtained under a constant pressure (Table 2). Isomers (such as olefin versus 3980 Analytical Chemistry, Vol. 74, No. 16, August 15, 2002

Table 3. Accurate Mass Measurements of Paraffins and Cycloparaffins in a Petroleum Middle Distillate by GC-FI-TOF MSa exp mass (Da)

rel abund

142.169 156.1867 170.202 184.2189 198.2345 212.2507 226.2659 240.2827 254.2956 268.313 282.3279 296.3392 310.3622 324.3755

1.03 2.18 3.69 6.71 11.3 15.25 16.14 13.15 11.1 9.02 7.84 4.74 2.35 1.31

126.1498 140.1623 154.1748 168.189 182.2032 196.219 210.2352 224.2508 238.2669 252.2824 266.2968 280.3127 294.3276 308.3423 322.3575 336.3747 350.39 364.3989 378.4206 392.4295

2.05 4.44 7.04 12.05 25.37 43.36 70.65 100 91.71 85.22 75.44 61.34 39.07 25.74 15.12 8.95 6.17 3.32 2.57 1.61

calc mass (Da)

error (mDa)

rel error (ppm)

formula

Paraffins 142.1722 -3.2 156.1878 -1.1 170.2035 -1.5 184.2191 -0.2 198.2348 -0.3 212.2504 0.3 226.2661 -0.2 240.2817 1 254.2974 -1.8 268.313 0 282.3287 -0.8 296.3443 -5.1 310.36 2.2 324.3756 -0.1

-22.2 -7 -8.5 -1.1 -1.3 1.4 -0.7 4.2 -6.9 0 -2.7 -17.2 7.2 -0.3

C10H22 C11H24 C12H26 C13H28 C14H30 C15H32 C16H34 C17H36 C18H38 C19H40 C20H42 C21H44 C22H46 C23H48

Cycloparaffins 126.1409 8.9 140.1565 5.8 154.1722 2.6 168.1878 1.2 182.2035 -0.3 196.2191 -0.1 210.2348 0.4 224.2504 0.4 238.2661 0.8 252.2817 0.7 266.2974 -0.6 280.313 -0.3 294.3287 -1.1 308.3443 -2 322.36 -2.5 336.3756 -0.9 350.3913 -1.3 364.4069 -8 378.4226 -2 392.4382 -8.7

70.9 41.4 17.2 7.1 -1.4 -0.5 2.1 1.8 3.6 2.8 -2.1 -1.1 -3.6 -6.5 -7.6 -2.7 -3.6 -22 -5.2 -22.2

C9H18 C10H20 C11H22 C12H24 C13H26 C14H28 C15H30 C16H32 C17H34 C18H36 C19H38 C20H40 C21H42 C22H44 C23H46 C24H48 C25H50 C26H52 C27H54 C28H56

a Relative abundance and elemental composition were determined simultaneously.

cycloparaffins, tetracyclic paraffins versus alkylated benzenes, etc.) cannot be discerned by TOF MS but may be resolved chromatographically by their differences in boiling point. Resolution and identification of aromatic and sulfur-containing aromatic molecules is much more challenging. Figure 3 shows a TIC and averaged mass spectrum of an aromatic fraction isolated from a VGO. Because of the large number of analyte molecules,

Figure 2. Mass differences of common doublets in petroleum and mass resolving powers needed to resolve the isobars.

Figure 3. Total ion chromatogram and averaged mass spectrum of an aromatic fraction from a VGO distillate.

the total ion chromatogram (top) is completely unresolved. The averaged mass spectrum (bottom) contains thousands of mass peaks with severe nominal mass overlaps. A detailed view of mass 274 shows the presence of three mass peaks (Figure 4). Accurate mass analyses provided possible matches of elemental compositions for the three peaks with a mass tolerance of (15 mDa. Mass 274.267 has a unique match, C20H34. However, mass 274.172 has two possible matches, C18H26S and C21H22. Mass 274.071 also has two possible matches, C16H18S2 and C19H14S. All are different by 3.4 mDa in exact masses. The combination of gas chromatography and mass spectrometry relieves the requirements of both mass and chromatographic resolving powers for the resolution of petroleum molecules. Figure 5 shows the selected ion chromatograms (SIC) of the three exact masses given in Figure 4, revealing a total of five components at nominal mass 274. The compounds with indistinguishable masses, e.g., C18H26S at m/z 274.176 and C21H22 at m/z 274.172, are separated chromatographically. In the meantime, the compounds that coelute in chromatography, e.g., C20H34 and C18H26S, are separated by mass spectrometry. The elution sequence help to

assign the correct molecular formula to the SIC peaks. C18H26S elutes earlier than does C21H22 because the former is less condensed than the latter (see Figure 2). Based on an empirical equation proposed by Boduszynski and Altght,29 the atmospheric pressure equivalent boiling point (AEBP) of the two hard-toresolve formulas, C18H26S/C21H22 and C16H18S2/C19H14S, are estimated to be 346/382 °C and 374/427 °C, respectively. Although each formula contains a large number of isomeric structures as evidenced by the broad SIC peaks, the boiling point differences are sufficiently large to separate these pairs. Carbon Number and Compound-Type Distribution. Petroleum samples are commonly characterized in terms of carbon number and compound-type distributions. Compound types (or homologues) are those molecules containing the same core structure but different alkylation. They are normally expressed in a general chemical formula, CnH2n+ZX, where n is the carbon number and Z is the hydrogen deficiency determined by the number of double bonds, rings, and heteroatoms (X) in a (29) Boduszynski, M. M.; Altgelt, K. H. Energy Fuels 1992, 6, 72-76.

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Figure 4. Close view of nominal mass 274 in Figure 3 revealing three mass peaks. Elemental compositions are obtained from matching exact masses.

Figure 5. Selected ion chromatograms of exact masses ((0.05 mDa) shown in Figure 4. A total of five components are revealed. The compounds having indistinguishable masses were resolved chropmatographically.

molecule. Each additional double bond or ring makes the Z value more negative by 2 unitsstherefore, the more negative the Z value, the more aromatic the molecule. For convenience, we normally abbreviate molecular formulas according to their Z value and heteroatomic components. For example, the formula for alkylated dibenzothiophenes, CnH2n-16S (structure shown in Figure 7), would be abbreviated as -16 S. Because FI is a soft ionization technique, carbon number distribution can be obtained from the molecular ion intensities of each species. Resolution of petroleum molecules shown in Figure 5 can be carried out across the whole mass range. Integration of SIC peaks in high-resolution mode ((0.05 Da) results in carbon number distributions for the targeted homologues. Figure 6 shows the carbon number distributions of -22 and -12 S compound types obtained by this approach. The members of two compound types have very close exact masses (3.4 mDa difference) due to the C3/SH4 doublet and cannot be distinguished by TOF MS alone. Figure 6 demonstrated that the combination of GC separation and TOF MS with a moderate resolving power (RP ∼3500-7000) can effectively resolve the C3/ SH4 doublet for nonpolar petroleum molecules with a carbon 3982

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number range of C6-C44 (mass range ∼80-650 Da). It represents significant progress in differentiating aromatics from thiophenoaromatics. Compound-type distribution (or Z-distribution) can be consequently obtained by summation of members of each homologue series. Figure 7 summarizes the total compound-type distributions in the aromatic VGO distillate. A total of 24 compound types (oneto four-ring aromatics and naphthenoaromatics, two- to four-ring aromatic thiophenes and disulfur compounds) were resolved and identified. The fact that half of the compound types were resolved using the combination of GC and TOF MS demonstrates the significant advantage of the technique in detailed petroleum analysis. Core structures of petroleum molecules can be proposed (as illustrated in Figure 7) based on Z values and heteroatomic compositions. The detailed composition provides important information to understand petroleum chemistry and process. CONCLUSIONS We demonstrate that the combination of boiling point separation, field ionization, and accurate mass analysis at elevated

Figure 6. Carbon number distributions of -12 S and -22 compound types obtained by integration of high-resolution SIC peaks across the whole mass range. The overlapping members that cannot be distinguished by TOF MS alone are displayed in stacked bars. Structures shown are for illustrative purposes.

Figure 7. Determination of 24 compound types for the aromatic VGO distillate. Note that half of the compound types were resolved using a combination of GC-FI-TOF MS. Structures shown are for illustrative purposes.

resolving power provides a powerful tool for comprehensive speciation of complex petroleum molecules with a wide carbon number range. The fact that saturate and aromatic molecules can be ionized and accurately mass measured in a “single analysis” offers a new platform to analyze petroleum samples that are small in quantity and are difficult to distill or fraction by chromatography. The ability to resolve sulfur-containing hydrocarbon molecules is greatly enhanced because of the unique combination. Carbon number and compound-type distribution can be directly measured by the technique.

ACKNOWLEDGMENT The authors thank Drs. L. A. Green, S. C. Blum, W. N. Olmstead, and J. M. Brown for their technical input and suggestions during the development of the GC-FI-TOF technology. Many thanks to Drs. R. L. Rucker and F. C. McElroy for their review and comments of the manuscript. Received for review March 13, 2002. Accepted May 31, 2002. AC020166D

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