Anal. Chem. 2002, 74, 3849-3857
Analysis of Aromatic Sulfur Compounds in Gas Oils Using GC with Sulfur Chemiluminescence Detection and High-Resolution MS C. Lo´pez Garcı´a,*,† M. Becchi,‡ M. F. Grenier-Loustalot,‡ O. Paı1sse,‡ and R. Szymanski†
Institut Franc¸ ais du Pe´ trole, CEDI Rene´ Navarre, 69390 Vernaison, France, and Service Central d’Analyse C.N.R.S, 69390 Vernaison, France
The analysis of alkylbenzothiophenes (alkyl-BT) and alkyldibenzothiophenes (alkyl-DBT) in light cycle oil (LCO) and straight run (SR) gas oils is described. A detailed identification and quantitative analysis of alkyl-BT and alkylDBT present in LCO gas oils was carried out using GC-SCD. For the SR gas oils, the simultaneous presence of thiophenic and nonthiophenic compounds does not allow for a selective analysis of thiophenic compounds by GC-SCD. A new method using gas chromatography coupled with high-resolution mass spectrometry (GC-HRMS) is proposed to selectively detect and quantify the alkyl-BT and alkyl-DBT in SR gas oils. The development of the method and comparison of results between GC-SCD and GC-HRMS are presented. The international gas oil regulations are increasingly more severe, especially concerning the sulfur content. Hence, refiners have to face the challenge of deep hydrodesulfurization. To establish the appropriate high-conversion hydrotreating conditions, an accurate analysis of sulfur compounds in gas oils is necessary. The quantitative analysis of thiophenic compounds, alkylbenzothiophenes and alkyldibenzothiophenes (alkyl-BT and alkyl-DBT, Figure 1), is particularly important since these compounds are the most refractory sulfur species in the gas oil cuts.1 Two types of gas oils were studied: straight run gas oils (SR), which are directly obtained from atmospheric distillation of crude oils, and light cycle oil gas oils (LCO), produced in the fluid catalytic cracking units (FCC). In gas oils, sulfur is found in different organic structures: mercaptans, aliphatic sulfides, cyclic sulfides, and thiophenic compounds. The kind of structures found in a gas oil depends strongly on its origin. Indeed, all the families mentioned above are present in SR gas oils. On the other hand, only the presence of thiophenic compounds is observed in LCO gas oils, since sulfides have been cracked in the FCC process. Many alternatives are proposed to carry out a selective detailed analysis of thiophenic compounds in gas oils, both via direct and indirect methods. The direct methods concern typically a gas chromatographic separation with a specific detector. The indirect methods usually consist in a chemical or physical pretreatment to isolate the thiophenic species before analysis with a sulfurspecific detector. †
Institut Francais du Petrole. Service Central d’Analyse. (1) Whitehurst, D. D.; Isoda, T.; Mochida, I. Adv. Catal. 1998, 42, 345-471. ‡
10.1021/ac011190e CCC: $22.00 Published on Web 06/28/2002
© 2002 American Chemical Society
Figure 1. Structures and position numbering of benzothiophene and dibenzothiophene.
Various sulfur detectors are commercially available, frequently used after gas chromatographic separation: atomic emission detector (GC-AED), flame photometric detector (GC-FPD), and sulfur chemiluminescence detector (GC-SCD). The GC-AED2,3 is a selective and sensitive multielement detector which has been successfully used to quantify sulfur compounds. However, for lowsulfur gas oil samples, the detection is limited because of the carbon matrix. The FPD4,5 has been widely used for sulfur analysis, but the ancient versions of this detector had a quench effect from hydrocarbon matrixes resulting in erroneous results. In addition, the FPD detector has a nonlinear response of sulfur compounds. Only the novel pulsed flame photometric detector6 (PFPD) avoids the hydrocarbon-quenching problem and has been reported as having good quantitative sulfur analysis performances. Nevertheless, it keeps the quadratic response. The GC-SCD is known as an excellent device for sulfur compound analysis; many authors4,7-10 have worked with this detector; it has an excellent selectivity and sensitivity. In comparison to the other detectors, hydrocarbon interferences are negligible and it has an equimolar sulfur response. All these qualities confer the GC-SCD a large utilization not only for research applications but also in industrial laboratories. (2) Andersson, J. T.; Sielex, K. HRC, J. High Resolut. Chromatogr. 1996, 19, 49-53. (3) Depauw, G. A.; Froment, G. F. J. Chromatogr., A 1997, 761, 231-247. (4) Dzidic, I.; Balicki, M. D.; Rhodes, I. A. L.; Hart, H. V. J. Chromatogr. Sci. 1988, 26, 236-240. (5) Berthou, F.; Dreano, Y. HRC & CC, J. High Resolut. Chromatogr. Chromatogr. Commun. 1988, 11, 706-712. (6) Schulz, H.; Bo¨ringer, W.; Ousmamov, F.; Waller, P. Fuel Process. Technol. 1999, 61, 5-41. (7) Shearer, R. L.; Poole, E. B. J. Chromatogr. Sci. 1993, 31, 82-87. (8) Di Sanzo, F. P.; Bray, W.; Chawla, B. HRC, J. High Resolut. Chromatogr. 1994, 17, 255-258. (9) Tuan, H. P.; Janssen, H. M.; Cramers, C. A. HRC, J. High Resolut. Chromatogr. 1995, 18, 333-341. (10) Tuan, H. P.; Janssen, H. M. HRC, J. High Resolut. Chromatogr. 1995, 18, 525-534.
Analytical Chemistry, Vol. 74, No. 15, August 1, 2002 3849
Figure 2. Sulfur compound profile of gas oils by GC-SCD: (a) LCO1 gas oil 1.2% sulfur; (b) SR1 gas oil 3.07% sulfur.
Concerning the indirect methods, physical and chemical methods have been developed and tested for the isolation of thiophenic compounds. The typical physical methods are based on liquid-solid chromatography procedures11 or on ligand exchange chromatography.12-15 The sulfur compounds are selectively adsorbed since they form complexes with an adsorbent phase impregnated with metallic salts. The main disadvantage is that the sulfur compound recovery is not total, so the quantitative analysis is not reliable. The chemical methods are based on selective oxidation of thiophenic compounds16 to produce sulfoxides or sulfones. Unfortunately, the sulfur compounds are not converted at 100% with these kind of methods. The physical and chemical methods are generally complicated and time-consuming, sometimes they require cleanup procedures and the degrees of recovery are not sufficient for a quantitative analysis. Concerning gas chromatographic analysis with a selective sulfur detector, these techniques can only be applied if the gas chromatographic separation is powerful enough to selectively separate the sulfur compounds. This is well illustrated in Figure 2 where the sulfur compounds profiles obtained by GC-SCD analysis were determined for both LCO and SR gas oils. In the case of LCO gas oils (Figure 2a), which contain alkyl-BT and alkylDBT compounds only, GC separation with sulfur detector is a well-adapted analysis technique. However, for SR gas oils (Figure (11) Poirier, M. A.; Smiley, G. T. J. Chromatogr. Sci. 1984, 22, 304-309. (12) Nishioka, M.; Campbell, R. M.; Lee, M. L.; Castle, R. N. Fuel 1986, 65, 270-273. (13) Nishioka, M. Energy Fuels 1988, 2, 214-219. (14) Andersson, J. T.; Schmid, B. J. Chromatogr., A 1995, 693, 325-338. (15) Schmid, B.; Andersson, J. T. Anal. Chem. 1997, 69, 3476-3481. (16) Bundt, J.; Herbel, W.; Steinhart, H. HRC, J. High Resolut. Chromatogr. 1992, 15, 682-685.
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2b), the gas chromatographic separation is not sufficient to separate the thiophenic from the nonthiophenic compounds and allow for a quantitative analysis. In the case of SR gas oils, a direct method may represent a gain in time and selectivity since the indirect methods classically remain at selectivities below 100% of sulfur compound recovery. The utilization of gas chromatography with high-resolution mass spectrometry (GC-HRMS) is proposed in this work. This analysis technique combines the excellent separation qualities of capillary gas chromatography and the excellent sensitivity and selectivity of high-resolution mass spectrometry. This principle has already been used for quantitative analysis of dioxins and furans17 and is largely applied in environmental analysis, but as far as we know, GC-HRMS has not been used for quantitative analysis of alkylBT and alkyl-DBT in gas oils. Based on the industrial needs, our work consisted of establishing an identification of refractory sulfur compounds (alkyl-BT and alkyl-DBT) and development of an adapted technique for their quantitative analysis in SR and LCO gas oils. The detailed identification of thiophenic species was carried out by GC-SCD using a LCO gas oil. This technique was also used for quantitative analysis of this kind of cut. Concerning the SR analysis, a method using GC-HRMS was developed; the calibration procedure based on the principle of a labeled internal standard is used here. EXPERIMENTAL SECTION Materials. The reference compounds used for the identification are 13 alkyl-BT and 12 alkyl-DBT. The abbreviations adopted for the alkyl-BT and alkyl-DBT compounds are as follows: M, methyl; DM, dimethyl; TM, trimethyl; TeM, tetramethyl; Et, ethyl; DEt, diethyl; P, propyl. The alkylbenzothiophene reference compounds are as follows: BT, 7-MBT, 2-MBT, 4/6-MBT, 3-MBT, 2,5-DMBT, 2,4/2,6-DMBT, 2,3-DMBT, 2,5,7-TMBT, and 2,3,4/ 2,3,6-TMBT. Alkyldibenzothiophenes purchased are as follows: DBT, 4-MDBT, 2-MDBT, 1/3-MDBT, 4,6-DMDBT, 2,8-DMDBT, 1,4-DMDBT, 1,3-DMDBT, 2,3-DMDBT, 1,2-DMDBT, and 2,4,6TMDBT. All reference compounds were purchased from Chiron (Trondheim, Norway) except for 2,4,6-TMDBT, which was purchased from Astec (Mu¨nster, Germany). Toluene of 99.3% purity (SDS, Villeurbane, France) was used to prepare all standard solutions. The internal standard used for the quantitative GC-SCD analysis was thiophene (99.1% purity) from Sigma Aldrich. Deuterium-labeled dibenzothiophene (C12D8S) was purchased from Cambridge Isotope Laboratories (98% purity) and used as internal standard for the GC-HRMS analysis. Institut Franc¸ ais du Pe´trole (IFP, France) provided three LCO samples and a SR sample. The sulfur content was determined by an X fluorescence technique (Philips PW 2510) according to the ASTM D2622 method. The sulfur content (w/w) of the LCO gas oils was 1.19%, 1.76%, and 0.52%, respectively. They will be referred as LCO1, LCO2, and LCO3. The SR gas oil had 3.07% sulfur. It will be referred to as SR1. GC-SCD Analysis. The gas chromatograph was a HP 5890 Series II coupled to a sulfur chemiluminescence detector Sievers model 355 B. A 60-m DB1 fused-silica column (J & W Scientific, (17) Fraisse, D.; Gonnord, M. F.; Becchi, M. Rapid Commun. Mass Spectrom. 1989, 3 (3), 79-84.
0.25-mm i.d. with 0.25-µm film thickness) was used. The GC separation was performed under the following conditions: helium as carrier gas, column temperature programmed from 60 (0 min) to 120 °C (0 min) at a rate of 10 °C/min, then a rate of 1 °C/min to 240 °C (0 min), and finally to 280 °C (10 min) at a rate of 10 °C/min. Injector in split mode at a temperature of 270 °C (split vent 100 mL/min, column 2 mL/min, purge vent 2 mL/min measured at 35 psig and 60 °C). The FID detector was set to the following flow rates: 350 mL/min air, 31 mL/min hydrogen, ∼213 Torr. Burner: 800 °C, 4 mL/min air, 97 mL/min hydrogen. Reaction cell: ∼8 Torr. Ozone generator: 60 mL/min at 60 psig and 25 °C. The injection volume was 0.5 µL of sample diluted to obtain ∼500 ppm of total sulfur. GC-HRMS Analysis. A HP 5890 gas chromatograph interfaced with a VG 70-SEQ mass spectrometer (VG Analytical, Manchester, U.K.) was used for this work. The same capillary column and chromatographic conditions used in the GC-SCD analysis were applied to the experimental GC-HRMS work, differing only in the injection mode (splitless 0.8 min). Deuteriumlabeled dibenzothiophene (C12D8S) was used as internal standard. The electron ionization source conditions were set to 220 °C, 200 µA, and 70 eV of electron energy. All standard solutions were prepared by weighing and with toluene as solvent. RESULTS AND DISCUSSION GC-SCD Analysis. The LCO1 was used for the sulfur profile characterization. The alkyl-BT and alkyl-DBT listed in the Materials section were used to identify the peaks by retention times. This identification was completed by using correlations developed to predict the retention times of thiophenic compounds and also by comparing with the literature identification on nonpolar phases.3,18,19 Andersson20 indicated that, for benzothiophene, the alkyl substituents on positions 2 or 7 decrease the retention time on nonpolar columns. In fact, this effect is said to be a function of the lower vapor pressure exhibited by the isomers with alkyl substituents next to the sulfur atom. Depauw and Froment3 proposed correlations to predict the retention times of alkyl-BT based on this observation and also on the ortho effect (two adjacent alkyl groups increase the elution time). The linear correlation reported by Depauw et al.3 is
tr ) ta + p2δ2 + p7δ7 + p3δ3 + p4δ4 + pBδB + p2,3δ2,3 + p3,4δ3,4 (1)
where tr is the predicted retention time, ta is a reference retention time for a group of isomers, p are the estimated parameters, and δ are coefficients indicating the presence of a methyl group in the specified position. Three types of neighbor interactions are described: the presence of a methyl substituent in position i (δi), the presence of two methyl substituents in positions i and j (δi,j), and the number of neighbor methyls in the benzenic ring (δB). The coefficients δi and δi,j take the value of 1 if the condition is true; otherwise the value is zero. (18) Chawla, B.; Di Sanzo, F. J. Chromatogr. 1992, 589, 271-279. (19) Mo ¨ssner, S. G.; Wise, S. A. Anal. Chem. 1999, 71, 58-69. (20) Andersson, J. T. J. Chromatogr. 1986, 354, 83-98.
Table 1. Parameter Values of the Retention Time Correlations for Dimethyl-BT and Dimethyl-DBT dimethyl-BT
dimethyl-DBT
param
value (min)
t value
param
value (min)
t value
p2 p7 p3 p4 pB p2,3 p3,4 ta
-0.3898 -1.0011 0.6915 0.5031 2.4499 0.9778 2.4659 26.9895
-6.0 -14.3 7.9 8.5 32.3 9.3 24.3 367.5
p4 p6 pC pD ta
-1.75 -1.62 2.01 3.52 78.36
-14.3 -11.8 17.9 25.6 788.2
The parameters of correlation (1) were estimated based on our experimental retention times for the DMBT isomers. The estimated parameters with their statistical t values are shown in Table 1. All the resulting parameters are significant at the 95% confidence level, since their absolute t values are all higher than 1.96. The correlation coefficient (r2) has a value of 0.999; a good agreement between experimental and calculated retention times was obtained (Figure 3). In Figures 3-8, reference compounds are distinguished with an asterisk. The differences between experimental and predicted retention times varied from 0.01 to 0.07 min. The parameter values were compared with those estimated by Depauw and Froment.3 The same main effect of each parameter is observed; for example, p2 and p7 have a negative value indicating that a substituent in these positions accelerates the elution of the compound. Correlation 1 was then used to estimate the retention times of the remaining DMBT isomers. The results of the identification of BT, C1-BT, and C2-BT are shown in Figure 4. For the identification of the C3-BT family, the number of isomers available as standards was limited to three compounds, even though the number of possible isomers increases rapidly with the number of carbon substituents. There are six possible C1-BT isomers, 21 for C2-BT, 62 for C3-BT, and 174 for C4-BT. As it was possible to predict the experimental retention times of the C2-BT with an excellent accuracy, correlation 1 could be applied with great confidence to identify the C3-BT compounds. Only the coefficient ta was re-estimated from the retention times of the three standards, while the other parameters were kept at the same value. The value estimated for ta was 35.67 min and was used to estimate the retention times of the other isomers. The C3-BT identification was completed by a profile comparison with the data from Depauw and Froment.3 The results for the C3-BT identification are shown in Figure 5. Concerning the C4-BT, no standard compounds were available to calculate ta and compare the results with the literature. Only a profile identification was possible by comparison with one provided by the same author. For the DBT, a similar approach was used. The DBT, C1-DBT, and C2-DBT identification was established with our reference compounds and completed by comparison with literature information.3,18,19 Depauw and Froment3 did not report a retention time correlation for the alkyl-DBT compounds. Concerning the elution order, the remarks made for the alkyl-BT are also valid for the alkyl-DBT. The closest positions to sulfur atom decrease the Analytical Chemistry, Vol. 74, No. 15, August 1, 2002
3851
Figure 3. Parity plot of the retention time C2-BT correlation.
Figure 4. GC-SCD analysis of LCO1: BT, C1-BT, and C2-BT chromatographic profiles.
Figure 5. GC-SCD analysis of LCO1: C3-BT and C4-BT chromatographic profiles.
retention time; the neighborhood of two methyl substituents also has an influence on the elution time. A correlation based on four interactions was tested:
tr ) ta + p4δ4 + p6δ6 + pCδC + pDδD 3852
Analytical Chemistry, Vol. 74, No. 15, August 1, 2002
(2)
tr is the predicted retention time (min), and δ4 or δ6 takes the value of 1 if a methyl substituent is present in the 4 or 6 position, respectively. The constant δC is 1 when a methyl substituent is present in the 1 or 9 position. The effect of two neighboring methyl substituents is taken into account with δD.
Figure 6. Parity plot of the retention time C2-DBT correlation.
Figure 7. GC-SCD analysis of LCO1: DBT, C1-DBT, and C2-DBT chromatographic profiles.
The parameters of correlation 2 were estimated based on our experimental retention times for the C2-DBT (Table 1). The parity plot of the calculated versus experimental retention time is presented in Figure 6. The retention times are in most of the cases well predicted, and a good correlation coefficient is obtained. The coelution of 3,7/2,7/2,8-DMDBT is correctly predicted, and the differences between calculated and experimental retention times vary from 0.01 to 0.3 min. This precision is nevertheless insufficient. Indeed, using the estimated retention time to identify these compounds would have induced some identification errors. This is the case of the 3,4-DMDBT, which would have been identified as the 1,3-DMDBT. In addition, the calculated retention time for the 1,3-DMDBT indicates that this compound is eluted with the 1,9/1,7-DMDBT; this result does not correspond to the experimental observations. The C2-DBT and C3-DBT identification was therefore completed by comparison with literature data3,19 only. The results of the alkyl-DBT identification are shown in Figures 7 and 8. All the C1-DBT are identified, and 18 from the 20 C2-DBT can be also recognized. The 64 C3-DBT isomers start to elute after the 2,3DMDBT peak; the C2-DBT that has the highest retention time is the 1,2-DMDBT, which has the methyl substituents in the most distant positions from the sulfur atom. The zone containing the
210 C4-DBT isomers was also distinguished, but no standard compounds are commercially available and literature information is also lacking. The quantitative analysis of LCO1 was also carried out. The addition of thiophene as internal standard was used to determine the content of each sulfur compound. The total sulfur content (1.19%) was compared to the quantitative FX sulfur analysis (1.13%) carried out on a Philips PW 2510 according to ASTM method D2622. A relative error of 5% was obtained which corresponds to the repeatability of these analytical techniques. The quantitative analysis results of each alkyl-BT and alkyl-DBT family are presented in Table 2. The ratio alkyl-BT/alkyl-DBT is 0.6, so the dibenzothiophene family is more abundant than the benzothiophene compounds in this gas oil. GC-HRMS Analysis. A GC-HRMS was used for quantitative analysis of SR gas oil. The calibration method is based on the utilization of deuterium-labeled dibenzothiophene as internal standard. Its physicochemical characteristics are very close to those of the compounds to be quantified. For the analysis, high resolution is necessary since polyaromatic sulfur compounds (PASH) have a mass similar to polyaromatic compounds (PAH). Indeed, the mass of C1-BT is very close to that of C5-benzenes. Analogously, C1-DBT and C5-naphthalenes have very similar Analytical Chemistry, Vol. 74, No. 15, August 1, 2002
3853
Figure 8. GC-SCD analysis of LCO1: C3-DBT and C4-DBT chromatographic profiles. Table 2. Sulfur Content Distribution of LCO1 Determined by GC-SCD family
sulfur content (w/w ppm)
BT C1-BT C2-BT C3-BT C4-BT
44 515 1511 1649 727
total alkyl-BT
4446
DBT C1-DBT C2-DBT C3-DBT C4-DBT+
454 1959 2648 1661 744
total alkyl-DBT total sulfur
7466 11911
masses. A resolution of 10 000 (5% valley definition) has been proven to be theoretically and practically sufficient to selectively detect thiophenic compounds. Therefore, the analyses were carried out at this resolution. Selected-ion monitoring (SIM) was used as the signal acquisition mode. Two groups of ions consisting of alkyl-BT and alkylDBT molecular ions were used for signal acquisition (dwell time: 80 ms for each ion in each group). The choice of molecular ions is justified since this ion always had a high intensity in the mass spectra of reference compounds, including the alkylated BT and DBT (C1-C4). The first group constituted of alkyl-BT was detected from 11 to 49 min and the second group (alkyl-DBT) from 49.03 to 90 min. For each group, perfluorokerosene (PFK) was monitored as lock-mass and for lock-mass stability. Ions of perfluorokerosene chosen were m/z 149.9 for the first group and m/z 204.9 for the second group. For the GC-HRMS analysis, the temperature programming and the capillary column are the same as for the GC-SCD analysis, so the identification of the thiophenic families remains valid. The ion chromatogram of alkyl-BT compounds of SR1 gas oil is illustrated in Figure 9. As expected, the superposition of all ion traces clearly reconstitutes the profile of this thiophenic family. The alkyl-DBT compounds shown in Figure 10 are also well recognized in the different ion traces. DBTD8 3854
Analytical Chemistry, Vol. 74, No. 15, August 1, 2002
(not shown) has a slightly lower retention time compared to the native molecule. All the alkyl-DBT peaks are well identified. Both Figures 9 and 10 show the thiophenic compound profiles in a SR gas oil detected by GC-HRMS. The source conditions mentioned in the Experimental Section were optimized in order to ensure a repeatability of the calibration method of ∼15%. This value had been determined in this device for other quantitative SIM methods.17 As stated before, the quantitative method developed is based on internal standard calibration. LCO1 was used as reference standard to determine the response coefficients of each thiophenic compound. This choice has the advantage of allowing the internal calibration of the naturally most abundant alkyl-BT and alkyl-DBT present in gas oils. Indeed, the availability of pure standard compounds was very limited. The GC-SCD quantification of each peak of LCO1 was used as data to determine the calibration curves. The amount of each compound was obtained by correcting the GC-SCD data by the molecular weight of the molecule and atomic weight of sulfur. Six solutions of diluted LCO1 gas oil were used in the following weight percent concentrations: 0.30%, 0.25%, 0.17%, 0.12%, 0.06%, and 0.04%. The selected upper range was established based on the saturation of the most abundant ion. The lower concentration solution corresponds to the quantitative limit of the method. A constant quantity of DBTD8 solution was added to each standard gas oil solution. The DBTD8 concentration was enclosed in the DBT calibration range. Calibration coefficients and correlation coefficients of the calibration curves are reported in Table 3 for some identified peaks of each family. It can be noticed that the calibration coefficients are of the same order of magnitude for both thiophenic families. To confirm the calibration coefficients determined with LCO1, analysis of other LCO gas oils (LCO2, LCO3) was carried out. The detailed composition of LCO2 and LCO3 was also determined by GC-SCD. The quantitative results obtained with both devices are compared in Table 4. Relative difference was calculated with GC-SCD as reference. These results indicate that alkyl-BT quantitative analysis is less accurate than alkyl-DBT determination. For LCO3, the results are generally better than those obtained for LCO2. Compounds that are closer to the internal standard in
Figure 9. Ion chromatogram of alkyl-BT compounds of a SR gas oil.
Figure 10. Ion chromatogram of alkyl-DBT compounds of a SR gas oil.
retention time have in both cases satisfactory quantitative results (DBT, C1-DBT). Alkyl-BT ions are not detected in the same internal standard ion group, so the inherent differences in source conditions for these compounds cannot be taken into account. The addition of other internal standards is therefore considered necessary. Unfortunately, other labeled alkyl-BT or alkyl-DBT (deuterium or 13C) are not commercially available. For an overall alkyl-BT and alkyl-DBT quantitative analysis, average response coefficients of each thiophenic family (K h j) were calculated since the individual response coefficients (KiS) have very close values within the same alkylated family. The overall sulfur content obtained for each alkyl-BT and alkylDBT family using average response coefficients is presented in Table 5. Individual BT contents are not correctly determined. DBT and C1-DBT are well-determined; the C4-DBT family is calculated with the K h j of the C3-DBT family since no sufficient data were available to calculate a response coefficient for this family. Concerning LCO3, acceptable results are obtained for the different
Table 3. Calibration Results for Alkyl-BT and Alkyl-DBT Compounds
a
compound
Ka
corr coeff (r2)
BT 7-MBT 2,7-DMBT 2,5,7-TMBT 2,3,5,7-TeMBT DBT 4-MDBT 2,4-DMDBT 2,4,6-TMDBT
0.0137 0.0184 0.0172 0.0229 0.0268 0.0145 0.0174 0.0213 0.0210
0.95 0.99 0.93 0.99 0.98 0.93 0.99 0.94 0.88
K, response coefficient for the molecule.
alkyl-BT families. The relative errors for alkyl-DBT vary between 2% and 17%; no C4-DBT were found in this gas oil. These results show that alkyl-DBT compounds in gas oils are quantified correctly, leading to a good order of magnitude by GC-HRMS. Analytical Chemistry, Vol. 74, No. 15, August 1, 2002
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Table 4. Quantitative Analysis Results for Some Alkyl-BT and Alkyl-DBT Compounds in LCO Gas Oils LCO2
LCO3
compound
Ci (ppm) GC-HRMS
Ci (ppm) GC-SCD
rel diff (%)
Ci (ppm) GC-HRMS
Ci (ppm) GC-SCD
rel diff (%)
BT 7-MBT 2,7-DMBT 2,5,7-TMBT 2,3,5,7-TeMBT DBT 4-MDBT 2,4-DMDBT 2,4,6-TMDBT
2478 2772 1928 1015 650 2419 2688 1083 498
1470 1853 2214 1500 1054 2488 2759 1297 671
68.5 49.6 -12.9 -32.3 -38.3 -2.8 -2.6 -16.5 -25.7
796 866 696 413 75 972 1173 487 216
634 677 669 414 297 946 1284 576 234
25.7 27.9 4.2 0.3 -74.7 2.7 -8.7 -15.6 -7.5
Table 5. Quantitative Analysis for Alkyl-BT and Alkyl-DBT Families in LCO Gas Oilsa LCO2
LCO3
family
K hj
CSj (ppm) GC-HRMS
CSj (ppm) GC-SCD
rel error (%)
CSj (ppm) GC-HRMS
CSj (ppm) GC-SCD
rel error (%)
BT ∑C1-BT ∑C2-BT ∑C3-BT ∑C4-BT
0.0137 0.0217 0.0222 0.0457 0.0416
592 3125 3204 3404 873
351 2531 4556 3645 1346
68.7 23.5 -29.7 -6.6 -35.1
188 906 1282 1124 390
152 703 1249 931 468
24.4 28.8 2.6 20.8 -16.6
11198
12429
3890
3503
421 1161 1208 618 105
433 1162 1940 941 110
168 467 506 189 0
165 562 565 218 0
3513 14711
4586 17015
1330 5220
1510 5013
total BT DBT ∑C1-DBT ∑C2-DBT ∑C3-DBT ∑C4-DBT total DBT total a
0.0145 0.0197 0.0222 0.0320 0.0320
-2.7 -0.1 -37.8 -34.3 -4.4
1.8 -16.9 -10.4 -13.2 0
CSj, sulfur content of the j thiophenic family. K Bj, average response coefficient of the j thiophenic family.
Finally, SR1 gas oil was analyzed by GC-HRMS. As SR gas oils are directly produced by atmospheric distillation, the alkyl chains in benzothiophene and dibenzothiophene families are longer than those found in LCO gas oils. Since C5, C6, C7, and C8 carbon number alkyl chains cannot be calibrated with LCO samples and since no pure standard compounds are available to calibrate these families, the average coefficients from the last calibrated family were applied for the C5+ alkyl-BT and alkylDBT species. Another group of ion acquisition was used for the C5+ compound recording. Molecular ions of each family (DBTD8 and PFK lock-mass ions included) were monitored in one group from 11 to 130 min with the same chromatographic conditions. SR1 gas oil was quantified by GC-HRMS. This sample contains 3.07% (w/w) of sulfur. Quantitative results are listed in Table 6. Calculations were made using average response coefficients. As shown in Table 6, alkyl-BT compounds are more abundant than alkyl-DBT for SR1 gas oil. A ratio of 3.4 between these two families is found. The major alkyl-BT family was the C3-BT family. Only 22% of the alkyl-BT compounds comprised the C5+ families. For the alkyl-DBT compounds, the most abundant families were the C1, C2, and C3-DBT; minor sulfur content contributions were obtained from C5+ dibenzothiophenes (2% of total thiophenic sulfur content). The sulfur from nonthiophenic compounds was calculated by difference between total sulfur and thiophenic sulfur. The results, shown in Table 7, indicate that, for SR1 gas oil, an important sulfur 3856 Analytical Chemistry, Vol. 74, No. 15, August 1, 2002
Table 6. Quantitative Analysis for Alkyl-BT and Alkyl-DBT Families in SR1 Gas Oil by GC-HRMSa Cj (ppm)
CSj (ppm)
BT ∑C1-BT ∑C2-BT ∑C3-BT ∑C4-BT ∑C5-BT ∑C6-BT ∑C7-BT ∑C8-BT
family
31 488 1856 4573 2706 1903 782 337 282
7 106 367 832 456 299 115 47 37
total BT
12959
2264
DBT ∑C1-DBT ∑C2-DBT ∑C3-DBT ∑C4-DBT ∑C5-DBT ∑C6-DBT ∑C7-DBT ∑C8-DBT
305 936 1162 1160 417 237 136 90 47
53 151 176 164 56 30 16 10 5
total DBT total
4491 17449
661 2926
a C , content of the j thiophenic family. C , sulfur content of the j j Sj thiophenic family.
fraction has a nonthiophenic origin (90% from total sulfur). The data in Tables 6 and 7 are the first available information allowing
Table 7. Sulfur Distribution of SR1 Gasoil sulfur-type family
sulfur content (ppm)
concn (%)
thiophenic nonthiophenic
2926 27774
10 90
total
30700
100
the estimation of alkyl-BT and alkyl-DBT distribution in a SR gas oil. CONCLUSIONS GC-SCD is a powerful technique for sulfur profile analysis of LCO gas oils. A detailed identification and quantitative compound analysis was successfully carried out with this device for this type of gas oil. Concerning SR cuts, the basis of a new analysis method for thiophenic compounds using GC-HRMS was established. This method allows one to estimate the content of alkyl-BT and alkylDBT families with a good order of magnitude. As indicated by the LCO2 and LCO3 quantification results, this method still needs
to be improved, especially regarding the alkyl-BT quantification. The addition of more labeled internal standards (alkyl-BT and alkyl-DBT) can be a solution in order to improve the accuracy of the results. The method was applied to determine the distribution between thiophenic and nonthiophenic sulfur in a SR gas oil sample without pretreatment or cleanup. The distribution of alkylDBT families in this gas oil could be therefore estimated. The information obtained from this analysis will be very useful for industrial hydrodesulfurization needs. ACKNOWLEDGMENT We acknowledge Pierre Beccat (IFP), Anne Bre´ (IFP), Daniel Fraisse (CARSO, France), and Jan Verstraete (IFP) for helpful cooperation and discussions. We are also grateful to CONACyT (Mexico) for the financial support of C.L.G.
Received for review November 19, 2001. Accepted April 9, 2002. AC011190E
Analytical Chemistry, Vol. 74, No. 15, August 1, 2002
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