Desulfurized Fuels from Athabasca Bitumen and Their Polycyclic

Dec 6, 2012 - Institute of Inorganic and Analytical Chemistry, University of Muenster, ... desulfurized materials derived from Athabasca, Canada, bitu...
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Desulfurized Fuels from Athabasca Bitumen and Their Polycyclic Aromatic Sulfur Heterocycles. Analysis Based on Capillary Electrophoresis Coupled with TOF MS Thies Nolte,† Tjorben Nils Posch,‡ Carolin Huhn,‡ and Jan T. Andersson*,† †

Institute of Inorganic and Analytical Chemistry, University of Muenster, Corrensstrasse 30, D-48149 Muenster, Germany Forschungszentrum Juelich GmbH, Zentralabteilung für Chemische Analysen, D-52425 Juelich, Germany

‡

S Supporting Information *

ABSTRACT: Polycyclic Aromatic Sulfur Heterocycles (PASHs) are undesirable compounds in fuels and refined petroleumbased products. Capillary electrophoresis (CE) was investigated as an alternative to analyze the preisolated PASHs from partially desulfurized materials derived from Athabasca, Canada, bitumen. The sample complexity is considerably reduced by this preisolation by ligand exchange chromatography on a Pd(II) containing phase and subsequent ionization of the neutral PASHs through S-methylation to impart electrophoretic mobility. The PASH components are separated and characterized using CE with a time-of-flight mass spectrometer (TOF MS) as detector. An additional major advantage of the CE separation is that the drawbacks of HPLC (limited separation efficiency) and GC (volatility limitations) for separations of high molecular weight compounds can be circumvented. CE was demonstrated to be a highly efficient method in the separation of PASHs and up to 200,000 theoretical plates were obtained. The practicability of the method was shown by an extensive qualitative analysis of the PASH fraction of a hydrotreated middle distillate and a hydrotreated heavy gas oil. Additional information on the sample composition like the identity of parent ring systems, the alkyl substitution, and hydrogenated products from the hydrodesulfurization was gained through the use of CE-TOF MS and measuring standard PASHs expected to be present. The results are compared with GC-MS and high-resolution Orbitrap measurements of the same samples.

1. INTRODUCTION Measures have to be undertaken in many directions to solve the looming energy crisis, including major efforts to lessen the dependence on fossil fuels. Scenarios show that we are close to the point where the maximum rate of global petroleum extraction is reached. However, this peak oil may not have been reached yet due to extensive progress in exploitation and drilling technology. Not only new deposits but also those that were seen as noneconomic in the past, because the petroleum was too heavy or contained high concentrations of heteroatoms like sulfur and nitrogen, receive increased interest. Bitumen deposits like those found in the Athabasca region of Canada are now being commercially exploited on a large scale. Legal standards demand that these heavy materials be extensively desulfurized. The catalytic hydrodesulfurization (HDS) is one of the most important steps during this refining process, involving high hydrogen pressures and high temperatures to remove sulfur as hydrogen sulfide. Sulfur levels are routinely reduced from percents to parts per million. However, some sulfur compounds, especially sulfur aromatic compounds, are recalcitrant to the HDS and remain in desulfurized fuels.1 These recalcitrant compounds are undesirable because they may poison modern catalytic converters and form sulfur oxides during combustion in the engine. Therefore an efficient analytic monitoring of sulfur compounds in petroleum as well as in fuels is necessary. The biggest problem during the analysis of these fossil fuel samples is their high complexity. Petroleum is one of the most complex mixtures known, while the compounds of interest here, the PASHs, are only present in low concentrations. Especially © 2012 American Chemical Society

after desulfurization, the concentration of individual PASHs lies in the subppm range. Different methods, like the SARA fractionation, are used to decrease the complexity of such samples before the molecular characterization is done, usually employing gas chromatography (GC). However, most of the existing analytical methods reach their limits when polycyclic aromatic hydrocarbons (PAHs) and PASHs in high boiling materials are analyzed. Their volatility may be too low for GC and discrimination effects occur in the injector. HPLC does not provide the high resolution needed. An alternative may be capillary electrophoresis (CE) since it does not suffer from volatility constraints, involving separations in the condensed phase, and its resolution power is higher than that of HPLC. Following ionization of the PASHs and subsequent separation by capillary electrophoresis, this method can be used for simplification as well, because selectivity toward the sulfur compounds is obtained in the ionization step. Other heterocyclic and aromatic compounds are not methylated or result in unstable products so that only the sulfur containing heterocycles gain electrophoretic mobility. In the past, the CE separation of PAHs and PASHs, as recently reviewed,2 required that the compounds interact with charged surfactants to gain electrophoretic mobility. An alternative possibility for PASHs is the derivatization of the sulfur atom to Received: August 31, 2012 Revised: November 26, 2012 Published: December 6, 2012 97

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Figure 1. Overlay of a) methylated DBT to C7-DBT and b) C1- to C4-THDBT extracted ion electropherograms of aqueous CE-TOF MS of the middle distillate cut PASHs.

thiophenium ions. In previous studies3 the derivatization was performed by methylation4 or phenylation5,6 of the sulfur atom, and the subsequent separation was carried out in an aqueous phosphate buffer. Standard compounds as well as real world samples could be successfully separated, but the drawback of comigration remained when more complex samples were analyzed. To reduce the impact of this and to investigate the applicability of the method to more complex real world samples, CE separations of preisolated PASH fractions of a crude oil derived from the Athabasca region in Canada are described here for both aqueous and nonaqueous buffer systems with MS detection. In this paper we emphasize the separation characteristics of CE for this class of compounds. If a sufficiently good separation can be obtained, it can serve as a basis for a quantitative approach later. The samples are additionally analyzed by GC-MS and high-resolution Orbitrap-MS to support the results obtained by the CE-TOF MS method.

Hunter B.03.01 software via the Agilent CE-MS Adapter Kit G1603A and the CE-ESI-MS Sprayer Kit G1607A. The flow of sheath liquid was operated with an Agilent Isocratic Pump 1260. The aqueous CE-MS buffer was prepared by titrating a 300 mmol/L acetic acid solution to pH 4.0 with ammonia. The buffer was mixed with isopropanol (1:1). Fused-silica capillaries with an o.d. of 364 μm and i.d. of 50 μm (Polymicro Technologies, Phoenix, AZ) with a length of 59.5 cm were used for aqueous CE. A solution of 4% acetic acid in methanol with 70 mM ammonium formate was used as nonaqueous CE background electrolyte and similar capillaries as above but of length 74.5 cm. The sheath liquid was a mixture of isopropanol with water (1:1) containing 1% acetic acid. The solution was pumped into the CE-MS interface by an isocratic pump at a flow rate of 4.5 μL/min. The pressure of the nebulizer gas was 2 psi for the first 20 s and then increased to 6 psi to avoid sucking air into the inlet when sample and buffer vials were changed. Dry gas was applied at 5 L/min and 325 °C. The Agilent QTOF MS was operated in the positive ion mode in the mass range 100− 1700 amu (Low Mass Range Mode) GC-MS. GC-MS measurements were performed on a Finnigan MAT GCQ coupled to a Finnigan MAT GCQ Polaris MS with a Supelco SLB5 ms column (30 m × 0.25 mm × 0.25 μm). The instrument was operated with electron ionization at 70 eV in the full scan mode from 50 to 600 amu. The injected volume was 1 μL. The injection was performed in the splitless mode. The injector and transfer line temperatures were held at 300 °C. The temperature program was 60 °C for 1 min, ramped at 5 °C/min to 300 °C, and kept for 15 min. 2.3. Sample Preparation. 2.3.1. CE-TOF MS. Standards. A variety of standard compounds was used within this study. The compounds were either purchased (dibenzothiophene (DBT), 4-methylDBT, 4,6-

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. All chemicals were of analytical reagent grade. Standards not commercially available were synthesized in our laboratories.7−9 Some PASHs are known to be carcinogenic and should be handled with care. 2.2. Procedures and Conditions. CE-MS. The measurements were performed on an Agilent 7100 Capillary Electrophoresis System controlled by the software ChemStation B.04.02. The CE was coupled to an Agilent 6520 Accurate-Mass Q-TOF MS controlled by the Mass 98

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Figure 2. Overlay of a) methylated DBT to C7-DBT and b) C1- to C5-THDBT extracted ion electropherograms of nonaqueous CE-TOF MS of the middle distillate cut PASHs. dimethylDBT) or synthesized in our laboratories according to straightforward procedures (1-, 2-methyl and 3-methylDBT, 4,6diethylDBT, 1,2,3,4-tetrahydroDBT, 4-methyltetrahydroDBT).7−9 For CE-analysis, 2.0 mg of each PASH was dissolved in 2 mL of 1,2dichloroethane, 100 μL of methyl iodide was added, and the vial wrapped with aluminum foil to exclude light (Figure S1). Sixty mg of silver tetrafluoroborate was added, and the mixture was stirred overnight. (Later research has shown that the reaction is complete within minutes, and this time can be shortened correspondingly.) The precipitated silver iodide was removed by filtration, and the filtrate was washed with 1,2-dichloroethane. The thiophenium salts were obtained by evaporation of the filtrate. They were dissolved in 1.5 mL of acetonitrile and kept in the freezer at −18 °C. The products were checked by mass spectrometry. The samples were diluted 1:1000 in acetonitrile for CE-MS measurements. The sum of the number of carbon atoms in the side chains of the aromatic compounds will be referred to as follows, exemplified using dibenzothiophene: C1-DBT are the methyl-DBT, C2-DBT ethyl- or dimethyl-DBT etc., with all possible isomers of DBT with the stated number of carbon atoms in the side chains being included. Real World Samples. The PASHs of two desulfurized fuels were used as real world samples. The first one was a hydrotreated middle distillate cut derived from Athabasca bitumen with a residual sulfur concentration of 822 ppm (middle cut), and the second one was a hydrotreated heavy gas oil derived from Athabasca bitumen with a residual sulfur concentration of 2960 ppm (heavy gas oil). The middle cut was derived from the heavy gas oil by fractionated distillation. The heavy gas oil distilled in the range (5−95%) 256−530 °C and the middle cut 342− 511 °C.

The PASHs were isolated from the fuel matrix by means of ligand exchange chromatography (LEC). The stationary phase contained palladium(II) bonded to mercaptopropanosilica gel (Pd-MPSG9). Ten g of Pd-MPSG was washed into a glass column of 3 cm i.d. with cyclohexane. Two mL of the sample was applied, and nonsulfur compounds were washed off the column with 80 mL of a cyclohexanedichloromethane mixture (9:1 v/v). The PASH fraction was eluted with 200 mL of cyclohexane-dichloromethane (7:3 v/v) containing 1 vol.% of isopropanol. The solvent was removed on the aspirator, and the residue was dissolved in 2 mL of 1,2-dichloroethane. The derivatization was carried out as described above for standard compounds. The samples were diluted 1:10 before the CE-MS measurements. The standard compounds were used to identify compounds in the samples through the criteria identical migration times in CE and correct mass spectra. 2.3.2. GC-MS. The PASHs were isolated from the samples by LEC as described above. After removal of the solvent, the PASH fraction was redissolved in cyclohexane for GC-MS analysis. 2.3.3. Orbitrap MS. For Orbitrap MS the samples were prepared as described for CE. The isolated PASH fraction was methylated before introduction into the Orbitrap MS by ESI nanospray and dissolved in acetonitrile. ESI-MS spectra were measured on an LTQ Orbitrap XL (Thermo Scientific) equipped with the static nanospray probe (spray voltage 1.0−1.4 kV, capillary temperature 200 °C, tube lens approximately 50−120 V). The system was operated in the positive ion mode with a scan range from m/z 100 to 900 (Full MS Mode). Spectra were recorded with a resolution setting from 30,000, 60,000 or 100,000. 2.3.4. Plate Number Calculation for CE and GC. The number of theoretical plates was calculated using the standard equation 99

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Figure 3. Overlay of methylated a) DBT to C9-DBT; b) C1- to C7-THDBT; and c) THBNTs/DHPTs extracted ion electropherograms of aqueous CETOF MS of the heavy gas oil PASHs. N = (t m /w1/2)2 × 5.54

derived from the heavy gas oil by fractionated distillation and having a residual sulfur concentration of 822 ppm. Middle Distillate Cut. The hydrotreated middle distillate cut is expected to show a moderate complexity because most of the PASHs have been removed in the hydrodesulfurization process. Alkyldibenzothiophenes are known to be the most difficult class of PASHs to desulfurize, and therefore they should be of primary importance in these samples. The first CE separations were done using an aqueous electrophoresis buffer and revealed high concentrations of alkyl homologues of DBT with S-methylated

with N = the number of theoretical plates, tm is migration time, and w1/2 is the peak width at half height.

3. RESULTS AND DISCUSSION 3.1. Real World Samples. The PASHs of two desulfurized fuels were analyzed using CE coupled to TOF MS, one being a hydrotreated heavy gas oil derived from Athabasca bitumen with a residual sulfur concentration of 2960 ppm and the second one 100

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Figure 4. Overlay of methylated a) DBT to C9-DBT; b) C1- to C7-THDBT; and c) THBNTs/DHPTs extracted ion electropherograms of nonaqueous CE-TOF MS of the heavy gas oil PASHs.

with reference substances. (The small signal at 13.80 min is an artifact and does not belong to any of the isomers.) The electropherogram was more complex for mass 227.09 because of the large number of possible C2-DBTs isomers (Figure 1a). Because of its substitution pattern, 4,6-dimethylDBT is the most recalcitrant compound,13 and its compact molecular shape led to the shortest migration time of these isomers (13.77 min). The pattern for the higher alkylated compounds turned out to be increasingly more complex due to the increasing number of isomers. The overlay of the electropherograms of all alkylated DBTs revealed that some signals cannot be dibenzothiophenes. The peak at 13.49 min had

DBT showing the shortest migration time (Figure 1a). This excludes the possibility that smaller PASHs are present in this material. The degree of alkylation can easily be obtained by extracting the corresponding m/z values. At m/z 213 three signals appear for the C1-DBTs (mass 213.07), the largest one being 4-methyl-DBT at 13.25 min, the most recalcitrant of the C 1 -DBT isomers. It is known to occur in significant concentrations in desulfurized fuels.11,12 The signal at 13.70 min belongs to the comigrating isomers of 1-methyl- and 3methyl-DBT. 2-Methyl-DBT is present in low concentrations (14.06 min). Identification of the signals was done by comparing 101

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Figure 5. a) Aqueous and b) nonaqueous CE-TOF MS of four standard compounds.

expected in significant concentrations in addition to the compounds found in the middle distillate cut. A small peak for DBT was the first signal seen in aqueous CE. In the hydrotreated middle distillate cut the C1-DBTs were among the main constituents but were marginal in the HGO. Higher molecular weight compounds like C3- to C6-DBTs were the main constituents, and C10 to C12-DBTs appeared late in the electropherogram (Figure 3a). The THDBTs found in the middle distillate cut are not major constituents of the HGO (Figure 3b). The signal at 15.28 min (white signal in Figure 3a) was not derived from a dibenzothiophene homologue and showed a mass of 267.1219. It belongs to a homologous series of compounds beginning with mass 239, increasing by 14 for each additional methyl group (253, 267, 281, 295...). Most probably these homologues belong to tetrahydrobenzonaphthothiophenes (THBNT) and/or dihydrophenanthro[4,5-bcd]thiophenes (DHPT) that are known to occur in the distillable aromatic fraction of Athabasca bitumen,15 the origin of the present material. THBNTs have also been found in desulfurized petroleum fuels,16,17 while phenanthro[4,5-bcd]thiophenes were reported in tar18 and can be hydrogenated to the corresponding dihydrophenanthro[4,5-bcd]thiophenes. An additional class of compounds that would fit the mass of these signals are phenylbenzothiophenes, but as these compounds are known only in coal19,20 it is unlikely that they occur in this sample. It is not possible to resolve the question of the parental structure with the techniques used in this work, so the homologous series will be referred to as THBNT/DHPTs.

a mass of 217.10 and was identified as 4-methyltetrahydrodibenzothiophene. Tetrahydro-DBTs (THDBTs) are generated through a reaction competitive with the HDS, leading to a hydrogenation of an aromatic ring of the DBTs (Figure S2).13 An overlay of the extracted ion electropherograms of C1- to C4-THDBTs showed that significant amounts of this compound class were present (Figure 1b). Interestingly, the mass for Smethyl-THDBT itself was missing but this can be explained by the faster HDS than hydrogenation of the aromatic ring.13 4Methyl-DBT has enough time for hydrogenation of an aromatic ring due to its recalcitrance, leading to 4-methyl-THDBT. This also explains why only very low concentrations of isomers (with a methyl group in the 3-, 2-, or 1-positions) were found for C1THDBT for which the HDS should be much faster. Nonaqueous CE of the same sample (Figure 2) showed a different migration order of the analytes. DBT migrated together with different C1- to C3-DBTs. 4-Methyl-DBT was the first compound of the DBT homologues to elute, comigrating with 4,6-dimethyl-DBT. The other C1-DBTs were present with intensities according to those found in aqueous CE with 1- and 3methyl-DBT comigrating here, too. The THDBTs appeared in the earlier part of the electropherogram with 4-methyl-THDBT as the fastest migrating major compound in the sample. Heavy Gas Oil. The hydrotreated heavy gas oil (HGO) from which the middle distillate cut was derived by fractionated distillation was the second real world sample measured. This synthetic crude was expected to be considerably more complex and hence more challenging to analyze. It covers a larger boiling range so heavier PASHs such as highly alkylated DBTs were 102

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Figure 6. GC-MS chromatogram of the middle distillate cut PASHs with the extracted ion chromatograms for DBT to C3-DBTs.

data to identify the compounds as discussed above, standard compounds were investigated under identical conditions as the samples. For instance, the separation of a standard mixture of dibenzothiophene (DBT), 4-methyl-DBT, 4,6-dimethyl-DBT, and 4,6-diethyl-DBT as their S-methyldibenzothiophenium ions is presented in Figure 5. The elution order of alkyldibenzothiophenes is linearly related to their size3 in an aqueous buffer. A baseline separation of all four compounds could easily be achieved in 16 min (Figure 5a); the number of theoretical plates ranged from 110 000 (4,6-diethyl-DBT) to 150 000 (DBT; tm = 13.28 min, w1/2 = 0.08 min; N = (tm/w1/2)2 * 5.54, see the Experimental Section). In nonaqueous CE the separation was finished in 9.5 min but a separation by size was not found (Figure 5b), and 4-methyl-DBT comigrated with 4,6-dimethyl-DBT and with 4,6-diethyl-DBT as the last-eluting compound. Despite having the smallest molecular volume, DBT migrated in between the other signals. The extracted ion electropherograms exactly fit the calculated masses of the S-methylated PASHs. Narrow and symmetric peaks were obtained with a very good resolution. The number of theoretical plates ranged from 150,000 to ca. 200,000. Although two of the compounds used in this mixture comigrate, nonaqueous CE comes with the larger number of theoretical plates, and on the whole seems to be the more promising method for the separation of PASHs. The number of theoretical plates achieved by CE is much higher than for reversed phase HPLC with small particles (3.5 μm) that can generate some 20,000 theoretical plates in a 150 mm column. The separation efficiency of CE is somewhat lower than that of capillary GC. In addition, GC comes with the advantage that further increasing the number of theoretical plates can be done by increasing the length of the column.

Mass 239.0913 was the lowest mass of this homologous series and the electropherogram showed three small signals (Figure 3c). Because the mass of S-methylated tetrahydrobenzonaphthothiophene itself is 253, these signals most likely belong to other parent systems and may be any of the isomeric C1derivatives of dihydrophenanthro[4,5-bcd]thiophene, cyclopentenodibenzothiophene, or fluorenothiophene. These compounds all possess a molar mass of 239 (S-methylated) and the ability to form several isomers. The former have never been reported in hydrogenated materials, but the two groups with fivemembered rings were suggested by GC-MS to occur in Siberian crudes, albeit without a stringent identification.21,22 The results of the desulfurized crude oil sample with nonaqueous CE-TOF MS agreed well with those from the aqueous CE. Alkylated DBTs with 12 and more side-chain carbons can be detected (Figure 4a). THDBTs eluted in the first part of the electropherogram with 4-methyl-THDBT as the fastest migrating species of this compound class (Figure 4b). The homologous series of DHPT/THBNT was present in high concentrations. An overlay of the extracted ions of these fourring compounds is presented in Figure 4c. Although aqueous and nonaqueous CE provide similar results with respect to the compound classes and the distribution of isomeric groups, nonaqueous CE offers some advantages such as a higher separation efficiency and shorter migration times. Although this makes the nonaqueous technique the preferred one when the highest resolution is required, this advantage may at times be outweighed by the fact that a selectivity correlated with the molecular size is obtained in the aqueous system, and this may be exploited in other circumstances. 3.2. Aqueous and Nonaqueous CE-TOF MS of Standard Compounds. To learn more about this separation and to gain 103

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Figure 7. GC-MS chromatogram of the middle distillate cut PASHs with the extracted ion chromatograms for C1-THDBTs to C4-THDBTs.

3.3. GC-MS. Since GC is traditionally used for the analysis of petroleum products, a comparison of CE-MS with GC-MS to compare the usefulness of CE with that of traditional analytical techniques. The identification of the PASHs in GC-MS of the middle distillate cut was performed by identifying the M+ ions and the fragmentation pattern. The chromatogram (Figure 6) showed significant concentrations of C1-DBTs with 4-methylDBT (27.86 min) as the most abundant one. 2-Methyl- and 3methyl-DBT coeluted8 (28.36 min), while the signal for 1methyl-DBT occurred at 28.78 min. (The other peaks visible were derived from fragmentation of larger compounds.) C2DBTs were present in high concentrations, 4,6-dimethyl-DBT (29.88 min) being the most abundant one. 4-Ethyl-DBT is also readily seen (30.79 min). THDBTs detected by the CE-TOF MS method are indicated by the peak of 4-methyl-THDBT at 27.46 min (Figure 7). The mass spectrum shows the M+ (m/z = 202) and a loss of 28 mass units from the loss of a C2H4 fragment of the hydrogenated ring in the molecule. This reverse Diels−Alder reaction is known to be prominent in tetrahydroaromatic compounds.23 The HGO was again expected to contain all the constituents detected in the previous sample. DBT homologues with up to 12 carbon atoms in the side chains were recorded in the sample. The concentration of THDBTs in the parental sample was low: however, the expected signals for C1-THDBT and higher homologues can be observed in agreement with the results from CE-MS. 3.4. Orbitrap MS. Ultrahigh resolution MS methods like FTICR MS and Orbitrap MS gain more and more prominence in the analysis of fossil fuels.24−27 Due to the extremely high

resolution (>100 000 for Orbitrap MS) it is possible to obtain much more information about highly complex samples than with other methods. PASHs are typically analyzed by this method after isolation by means of LEC followed by the same derivatization reaction as for the CE measurements28 but without any further separation. Here Orbitrap MS was used to demonstrate that the results obtained by CE can be verified by high resolution MS results. From CE and GC it is known that the main PASH constituents of the middle distillate cut are alkylated DBTs, and this was confirmed by Orbitrap MS with a mass accuracy better than 5 ppm for all of the identified compounds (accurate to the third digit). DBTs with up to twelve carbon atoms in the side chains were present with C2- to C4-DBTs being the most abundant. Signals for DBTs having lost the S-methyl group were also recorded, starting at m/z 184.03. This demethylation, that leaves the charge in the aromatic system, has been described previously.4,29 The hydrotreated HGO contains all the compound classes expected as shown in the mass spectrum in Figure 8. DBTs with up to 25 carbon atoms in the side chains are detected as well as THDBTs with more than 15 side chain carbon atoms. THBNTs/DHPTs are the other main constituents in the sample with the mass 323.18 being the most abundant one (C5THBNT/C7-DHPT). The degree of alkylation of the compound classes found in the Orbitrap measurements seems to be higher than the ones obtained by CE-MS or GC-MS, but this can be explained by the fact that the very large number of isomers are separated in CE or 104

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Figure 8. Orbitrap MS spectrum of the hydrotreated HGO PASHs with DBTs marked in red, THDBTs marked in blue, and THBNTs/DHPTs marked in green.

Table 1. Degree of Alkylation Found in the Middle Distillate Cut and the HGO with CE-TOF MS and Orbitrap

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CONCLUSION Capillary electrophoresis is not generally thought of as a typical technique for the separation of PASHs, but the results shown here demonstrate that CE-MS is worth being added to the list of applicable methods. The isolation by LEC and ionization through methylation leads to a unique selectivity for sulfur aromatic compounds in the crude oil matrix. Further simplification of the sample is obtained by the electrophoretic separation of the analytes in the electric field. During the detection process, a further dimension of separation is added by using the TOF mass spectrometer. The combination of all these

GC but bunched together in the Orbitrap measurement since they have the same m/z value. Hence, with a separation, the signals for the isomers are spread out over a migration time window so that the signal for each of the isomers can be very small and lost in the background. The signal in Orbitrap-MS is generated by the superimposition of all (isobaric) isomers so that groups of isomers with a high degree of alkylation give detectable signals. Table 1 compares the degree of alkylation for each of the compound classes that can be visualized using the two techniques CE-TOF MS and Orbitrap-MS. 105

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Energy & Fuels methods leads to a promising way to analyze this class of compounds. The main drawback of the CE method is that electrophoretic mobility has to be imparted to the analytes via a derivatization which introduces one more sample preparation step. However, this disadvantage is balanced by the high resolution and lack of volatility constraints as typical for GC. Furthermore, recent studies indicate that the derivatization reaction is quantitative after only a few minutes, and therefore the 24 h preparation time originally supposed to be necessary can be drastically shortened. If this rapid derivatization is combined with a fast separation on short capillaries (or even microchips), very short analysis times should result and hence increase the applicability in PASH analysis. The analysis of high-boiling samples is still problematic because the number of isomers may be extremely large and a resolution of all the isomers is impossible. However, this objection applies equally to all separation methods, also GC. The lack of volatility restrictions in CE-TOF MS should make it a serious alternative to GC for the analysis of this kind of samples. A further major advantage of the CE-MS method over GC-MS arises through the lack of fragmentation observed for CE-MS. The ions are produced before the analytes are introduced into the mass spectrometer, and the soft ionization of ESI can be used so that no fragmentation is obtained. The electropherogram hence presents the analytes in their true ratio. GC-MS chromatograms usually do not give a correct picture of the ratio of the analytes when single ion monitoring is used because the M+-signal originates from only one out of several ions produced through ionization/fragmentation. Fragmentation is enormously helpful for structural elucidation of analytes but makes studies of complex mixtures, such as those used here, difficult and impedes quantification by not giving the true analyte ratios. If the S-methylation is quantitative for all compounds, in principle a quantification using one standard compound should be possible so that the determination of individual response factors can be dispensed with. So far no attempts at quantification were done. Several steps need to be further studied to achieve this, beginning with establishing whether the methylation yield is independent of the parent structure and the alkylation pattern. An advantage of the methylation method is that the ionization efficiency in MS is not an issue anymore since only preformed ions are introduced into the instrument. It can hardly be expected that many individual compounds can be quantified since even the combination CEMS does not resolve enough individual compounds of isobaric structures, as is true for GC-MS also. As in GC-MS, compounds that are either resolved in the separation or have a mass separate from coeluting compounds can be analyzed individually, and an internal standard with a unique molar mass can be used even if it is not resolved from the sample components.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by Syncrude Canada Ltd. We are grateful to Richard Paproski for providing the fuel samples and to Heinrich Luftmann for the Orbitrap measurements. T.P. and C.H. thank the Helmholtz Initiative and Networking Fund for financial support.

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