Anal. Chem. 1999, 71, 58-69
Determination of Polycyclic Aromatic Sulfur Heterocycles in Fossil Fuel-Related Samples Stephanie G. Mo 1 ssner† and Stephen A. Wise*
Chemical Science and Technology Laboratory, Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
An analytical method is described for the separation, identification, and quantification of a number of polycyclic aromatic sulfur heterocycles (PASHs) in three fossil fuelrelated samples including two Standard Reference Materials (SRMs), SRM 1597 (coal tar) and SRM 1582 (petroleum crude oil), and a decant oil. The compounds measured include the 3 possible naphtho[b]thiophenes; dibenzothiophene and selected methyl-, ethyl-, dimethyl-, and trimethyl-substituted isomers; the 3 possible benzo[b]naphthothiophenes; and the 30 methylbenzo[b]naphthothiophenes isomers. Because of the occurrence of polycyclic aromatic hydrocarbons and PASHs together with their large number of possible alkyl-substituted isomers, the analytical method described requires a number of prerequisites: effective sample cleanup, selective stationary phases, and selective methods of detection. The sample cleanup involves solid-phase extraction using aminopropylsilane cartridges with different solvent mixtures followed by normal-phase liquid chromatographic isolation of the PASHs based on the number of aromatic carbons. These aromatic ring fractions are then separated by capillary gas chromatography using two stationary phases with different selectivities, 5% phenyl-substituted methylpolysiloxane stationary phase and 50% phenylsubstituted methylpolysiloxane stationary phase, and analyzed with mass-selective detection and atomic emission detection. A liquid crystalline stationary phase was also used to separate the methylbenzo[b]naphthothiophene isomers in the crude oil sample. Advantages and limitations of each chromatographic and detection technique are discussed. Polycyclic aromatic hydrocarbons (PAHs) are highly carcinogenic and mutagenic,1,2 and, as a result they have been one of the most measured groups of chemicals in recent decades.3-5 However, their sulfur analogues, the polycyclic aromatic sulfur * Corresponding author: (e-mail)
[email protected]; (phone) (301) 9753112; (fax) (301) 977-0685. † Present address: Safety and Environmental Technology Group, Chemical Engineering Department, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland. (1) Arcos, J. C.; Argus, M. F. Adv. Cancer Res. 1968, 11, 305-471. (2) Gelboin, H. V.; Ts’o, P. O. P. Polycyclic Hydrocarbons and Cancer; Academic Press: New York, 1978. (3) Lee, M. L.; Novotny, M V.; Bartle, K. D. Analytical Chemistry of Polycyclic Aromatic Compounds; Academic Press: New York, 1981.
58 Analytical Chemistry, Vol. 71, No. 1, January 1, 1999
heterocycles (PASHs), have been neglected for some time, although their occurrence in various fossil fuels6-9 and other environmental samples10-12 as well as their mutagenic and carcinogenic potential has been reported.13-17 PASHs exist in an even greater variety of structures compared to the PAHs due to the presence of the sulfur atom. Therefore, the number of isomers and alkylated isomers can be extremely large, and quantitative determination of individual PASH isomers in these mixtures can be very difficult. Because the concentrations of PASHs are known to equal or even exceed the concentrations of PAHs in some fossil fuels,18,19 it is necessary to increase the focus, both analytically and toxicologically, on these compounds. The identification and quantification of individual PASHs in these complex mixtures require the use of effective sample preparation steps, selective stationary phases in high-resolution capillary gas chromatography (GC), and selective and sensitive methods of detection. The compounds identified and quantified in this study were naphtho[1,2-b]thiophene (N12bT), naphtho[2,1-b]thiophene (N21bT), naphtho[2,3-b]thiophene (N23bT), dibenzothiophene (DBT), all 4 methyldibenzothiophenes (MeDBTs), 3 of the 4 possible ethyldibenzothiophenes (EtDBTs), 15 of the 16 possible dimeth(4) Handbook of Polycyclic Aromatic Hydrocarbons; Bjørseth, A., Ed.; Marcel Dekker: New York, 1983. (5) Handbook of Polycyclic Aromatic Hydrocarbons; Bjørseth, A., Ramdahl, T., Eds.; Marcel Dekker: New York, 1985; Vol. 2. (6) Willey, C.; Iwao, M.; Castle, R. N.; Lee, M. L. Anal. Chem. 1981, 53, 400407. (7) Nishioka, M.; Lee, M. L.; Castle, R. N. Fuel 1986, 65, 390-396. (8) Wang, Z.; Fingas, M.; Li, K. J. Chromatogr. Sci. 1994, 32, 367-382. (9) Andersson, J. T.; Schmid, B. J. Chromatogr., A 1995, 693, 325-338. (10) Vassilaros, D. L.; Stoker, P. W.; Booth, G. M.; Lee, M. L. Anal. Chem. 1982, 54, 106-112. (11) Lee, M. L.; Novotny, M.; Bartle, K. D. Anal. Chem. 1976, 48, 1566-1572. (12) Paasivirta, J.; Herzschuh, R.; Lahtipera¨, M.; Pellinen, J.; Sinkkonen, S. Chemosphere 1981, 10, 919-928. (13) Karcher, W.; Nelen, A.; Depaus, R.; van Eijk, J.; Glaude, P.; Jacob, J. In Proceedings of the 5th International Symposium of Polynuclear Aromatic Hydrocarbons: Chemical Analysis and Biological Fate; Cooke, M., Dennis, A., Eds.; Battelle Press: Columbus, OH, 1981; pp 317-327. (14) Pelroy, R. A.; Stewart, D. L.; Tominaga, Y.; Iwao, M.; Castle, R. N.; Lee, M. L. Mutat. Res. 1983, 117, 31-40. (15) McFall, T.; Booth, G. M.; Lee, M. L.; Tominaga, Y.; Pratap, R.; Tedjamulia, M.; Castle, R. N. Mutat. Res. 1984, 135, 97-103. (16) Eastmond, D. A.; Booth, G. M.; Lee, M. L. Arch. Environ. Contam. Toxicol. 1984, 13, 105-111. (17) Jacob, J. Sulfur Analogues of Polycyclic Aromatic Hydrocarbons; Cambridge University Press: Cambridge, UK, 1990. (18) Andersson, J. T. Int. J. Environ. Anal. Chem. 1992, 48, 1-15. (19) Grimmer, G.; Jacob, J.; Naujack, K.-W. Fresenius J. Anal. Chem. 1983, 314, 29-36. 10.1021/ac980664f Not subject to U.S. Copyright. Publ. Am. Chem. Soc.
Published on Web 12/03/1998
13 environmentally important compounds including DBT have been reported.22 A decant oil is the fraction of the fluid catalytic cracking unit product stream with a boiling point range from 360 to 540 °C. Decant oils are used to produce needle cokes for manufacturing graphite electrodes. They are known to have high concentrations of DBT and BNTs and their respective alkylated isomers.23 In contrast to the coal tar sample, both the decant oil and the crude oil require extensive sample cleanup prior to GC analysis. Due to the increasing importance of certified reference materials (CRMs) for use in analytical method validation and as quality control indicators, it was the purpose of this work to further characterize SRM 1582 and SRM 1597 for additional PASH compounds. This task was facilitated by the recent commercial availability of a number of PASH reference standards.24-26 In this paper, we have significantly expanded on the recent work of Schmid and Andersson,26 who used GC/AED to measure several PASHs in three SRMs (i.e., SRM 1597, SRM 1582, and SRM 1580). In addition, results from the use of sulfur-selective atomic emission detection in conjunction with gas chromatography (GC/AED) were compared with results obtained with mass-selective detection (GC/MSD).
Figure 1. Structures and position numbering of the PASHs of interest.
yldibenzothiophenes (DiMeDBTs), and 6 of 28 possible trimethyldibenzothiophenes (TriMeDBTs). In addition, the three benzo[b]naphthothiophenes (BNTs) were determined: benzo[b]naphtho[1,2-d]thiophene (BN12T), benzo[b]naphtho[2,1-d]thiophene (BN21T), and benzo[b]naphtho[2,3-d]thiophene (BN23T). The 30 possible methyl-BNTs (MeBNTs) were also identified and quantified. Total values of C1- to C5-DBTs and C1- to C5-BNTs were determined. The structures and the substitution position numbering for these PASHs are illustrated in Figure 1. Three fossil fuelrelated samples, including two Standard Reference Materials (SRMs), SRM 1597 (coal tar), and SRM 1582 (petroleum crude oil), and a decant oil were analyzed. SRM 1597, Complex Mixture of Polycyclic Aromatic Hydrocarbons from Coal Tar, is a natural, combustion-related mixture of PAHs and PASHs isolated from a crude coke oven tar and is dissolved in toluene. This mixture is suitable for direct analysis by GC without prior sample cleanup. Certified concentrations (based on the agreement of the results from two or more independent analytical methods) are reported for SRM 1597 for 12 PAHs; however, noncertified concentrations (based on measurements by only one analytical method) are given for only two PASHs, benzothiophene and dibenzothiophene.20,21 The petroleum crude oil (SRM 1582) was obtained originally from the U.S. Environmental Protection Agency repository at the Oak Ridge National Laboratory (Oak Ridge, TN) and is known as a Wilmington crude oil. Certified concentrations in SRM 1582 for (20) Certificate of Analysis, Standard Reference Material 1597, Complex Mixture of Polycyclic Aromatic Hydrocarbons from Coal Tar; National Institute of Standards and Technology: Gaithersburg, MD, originally issued in 1987, certificate revised in 1992. (21) Wise, S. A.; Benner, B. A.; Byrd, G. D.; Chesler, S. N.; Rebbert, R. E.; Schantz, M. M. Anal. Chem. 1988, 60, 887-894.
EXPERIMENTAL SECTION Sample Preparation/Cleanup. SRM 1597 and SRM 1582 were both available from the Standard Reference Material Program at the National Institute of Standards and Technology (Gaithersburg, MD). The decant oil sample was provided by Energy BioSystems Corp. (The Woodlands, TX). All three samples (1-3 g) were diluted in dichloromethane (DCM), perdeuterated internal standard solutions were added, and the resulting mixtures were sonicated. The first cleanup procedure for the decant oil and SRM 1582 (this step was not necessary for SRM 1597) was a threestep procedure using aminopropylsilane solid-phase extraction (SPE) cartridges (Waters Corp., Milford, MA) with sequentially changing mobile phases: 100% DCM, 50% DCM/50% hexane, and finally 5% DCM/95% hexane (100 mL for each solvent). For this cleanup step, five SPE cartridges were coupled together; the aliquot of the diluted sample (approximately 30-40%) was added to the top of the first cartridge and eluted with the first solvent. The eluant was concentrated, placed on the second set of five cartridges, and eluted with the next solvent mixture. The process was repeated a third time with the final solvent mixture. The threestep SPE cleanup procedure was necessary to remove the polar components in the oil and to exchange the 100% DCM, in which the oil samples were completely soluble, to the relatively nonpolar composition of the mobile phase required for the NPLC step described below. The next sample preparation step, which also included SRM 1597, was the fractionation of the eluant from the 5% DCM/95% hexane SPE cleanup step into subfractions by NPLC on a semipreparative (7.8 mm × 300 cm) aminopropylsilane column (µBondapak NH2, 10-µm particle size, Waters Corp.) using 2% DCM in hexane as the mobile phase at 5 mL/min. In this (22) Certificate of Analysis, Standard Reference Material 1582, Petroleum Crude Oil; National Bureau of Standards: Washington, DC 20234, 1984. (23) Filley, R. M.; Eser, S. Energy Fuel 1997, 11, 623-630. (24) Andersson, J. T.; Sielex, K. J. High Resolut. Chromatogr. 1996, 19, 49-53. (25) Polycyclic Aromatic Sulfur Heterocycles. Reference Solutions. Astec, Nottulner Landweg 90, D-48161 Mu ¨ nster, Germany. (26) Schmid, B.; Andersson, J. T. Anal. Chem. 1997, 69, 3476-3481.
Analytical Chemistry, Vol. 71, No. 1, January 1, 1999
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Table 1. Summary of the Results (µg/g) for the Determination of PASHs in SRM 1597 Coal Tar
compoundsa
mol
wtb
GC/MSD [4]c DB-17MSd
this work GC/MSD [4]c GC/AED [2]c DB-5MSd DB-17MSd
LC2 fraction DBTf N12bTh N21bTh N23bTh
184 184 184 184
18.1(0.4) 8.7(2.2) 6.22(0.11)i 3.35(0.06)i
25.7(1.6)g g 5.87(0.01)i 2.38(0.04)i
4-MeDBTf 2-MeDBTf 3-MeDBTh 1-MeDBTf
198 198 198 198
1.40(0.06) 1.23(0.10) 1.02(0.09) 0.27(0.01)
1.43(0.11) 1.97(0.11)k k 0.25(0.02)
1.32(0.01) 1.24(0.01) 0.85(0.08) 0.31(0.03)
LC4 fraction BN21Tf BN12Tf BN23Tf
234 234 234
9.88(0.23) 2.28(0.04) 3.66(0.14)
9.76(0.07) 2.26(0.04) 4.26(0.77)
10.45(0.25) 2.51(0.05) 3.48(0.09)
18.0(0.8) 7.3(2.0) j j
GC/AED [2]c DB-5MSd 24.4(2.6)g g j j 1.31(0.03) 1.85(0.06)k k 0.29(0.06) 10.30(0.20) 2.58(0.26) 3.30(0.04)
other work GC/AED SP 2331e 18.2 8.8 5.9 3.1 1.1 0.7 0.7 ndl 9.7 ndl ndl
a Compounds are listed in order of GC elution on DB-17MS. b Molecular weight assignment based on the highest mass ion of significant relative abundance observed in the electron impact mass spectrum. c Value in brackets indicates number of samples processed; each sample analyzed in duplicate by GC/MSD and GC/AED. d GC values quantified using DBT-d8 as internal standard for the LC2 fraction and BN21T-d10 as internal standard for the LC4 fraction. Concentrations are presented as the mean, followed by one standard deviation of a single measurement given in parentheses. e GC/AED values from Schmid and Andersson.26 f Quantification based on response factors determined using reference standard of known purity. g Value includes both DBT and N12bT. h Quantification based on response factors determined using the mean value of response factors of available reference standards of the same degree of alkylation. i Value is the mean of the two samples analyzed by direct analysis without prior fractionation. j No value determined because N21bT and N23bT eluted in the LC3 fraction. k Value includes both 2- and 3-MeDBT. l Value not determined by Schmid and Andersson.26
Figure 2. NPLC fractionation of the three fossil fuel samples with UV detection (290 nm).
NPLC procedure, the separation is based on the number of aromatic carbons in the molecule regardless of the degree of alkylation.27-30 The fractionation was monitored using UV-absorbance detection at 290 nm. Cut points for the subfractions were (27) Wise, S. A.; Chesler, S. N.; Hertz, H. S.; Hilpert, L. R.; May, W. E. Anal. Chem. 1977, 49, 2306-2310. (28) May, W. E.; Wise, S. A. Anal. Chem. 1984, 56, 225-232. (29) Kline, W. F.; Wise, S. A.; May, W. E. J. Liq. Chromatogr. 1985, 8, 223237.
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based on retention information for standard compounds known to be present in the samples. The two subfractions investigated in this paper, LC2 (DBTs) and LC4 (BNTs), were collected, concentrated, and analyzed by GC. Gas Chromatographic Analyses. The GC analyses were performed on two columns with different selectivities for the separation of PASHs: a 5% phenyl/95% (mole fraction) methylpolysiloxane stationary phase (DB-5MS) and a 50% phenyl/50% methylpolysiloxane stationary phase (DB-17MS) both 60 m × 0.25 mm i.d. with a film thickness of 0.25 µm (J&W Scientific, Folsom, CA). Samples were introduced by on-column injection, and helium was used as carrier gas. The AED also required hydrogen and oxygen as makeup gases. GC/MSD analyses were performed in the single ion monitoring (SIM) mode monitoring molecular ions, and the GC/AED analyses were performed in the carbon- and sulfur-selective modes. The temperature program for both columns consisted of an initial isothermal period of 1 min at 60 °C, then programming at 45 °C/min to 150 °C, followed by an isothermal period of 1 min at 150 °C, and then programming at 2 °C/min to 320 °C with an isothermal period of 10 min at 320 °C. Due to the complexity of the chromatograms when separating the 30 MeBNTs, a third stationary phase was used in conjunction with GC/MSD to separate and quantify individual MeBNT isomers. As reported in previous papers,31-36 the liquid crystalline (30) Wise, S. A.; Campbell, R. M.; May, W. E.; Lee, M. L.; Castle, R. N. In Proceedings of the 5th International Symposium of Polynuclear Aromatic Hydrocarbons: Formation, Metabolism, and Measurement; Cooke, M., Dennis, A., Eds.; Battelle Press: Columbus, OH, 1983; pp 1247-1266. (31) Mo ¨ssner, S. G.; Lopez de Alda, M. J.; Sander, L. C.; Lee, M. L.; Wise, S. A., submitted to J. Chromatogr., A. (32) Nishioka, M.; Jones, B. A.; Tarbert, B. J.; Bradshaw, J. S.; Lee, M. L. J. Chromatogr. 1986, 357, 79-91. (33) Kong, R. C.; Lee, M. L.; Tominaga, Y.; Pratap, R.; Iwao, M.; Castle, R. N. Anal. Chem. 1982, 54, 1802-1806. (34) Kong, R. C.; Lee, M. L.; Tominaga, Y.; Pratap, R.; Iwao, M.; Castle, R. N.; Wise, S. A. J. Chromatogr. Sci. 1982, 20, 502-510.
Figure 3. GC(DB-5MS)/AED analysis in the sulfur-selective mode (S 181) of the LC2 fraction (DBT, C1- to C3-DBTs) for crude oil (SRM 1582), decant oil, and coal tar (SRM 1597). Peak identification: numbers refer to the position of methyl, dimethyl, and trimethyl substitution for the C1-DBTs, C2-DBTs, and C3-DBTs; Et ) ethyl.
polysiloxane phase provides a unique, shape-selective separation mechanism, and therefore it is a very powerful column for the separation of complex isomeric PASH mixtures. The liquid crystalline stationary phase used was a SB-Smectic column (25 m, 0.20 mm i.d., 0.15 µm film thickness, Dionex, Salt Lake City, UT). For the SB-Smectic column the temperature program had an initial isothermal period of 1 min at 60 °C, then programming at 45 °C/min to 150 °C, followed by an isothermal period of 1 min at 150 °C, and then programming at 1 °C/min to 270 °C. Reference Compounds. The following compounds were purchased in high purity (>98%) for quantification purposes: DBT (Acros Organics, Springfield, NJ); 1-, 2-, and 4-MeDBT, 2,8DiMeDBT, 1,4,7-, 3,4,7-, 2,4,7-, 2,4,8-, 2,4,6-, and 1,3,7-TriMeDBT (all from Astec, Mu¨nster, Germany); and BN12T, BN21T, and (35) Jones, B. A.; Bradshaw, J. S.; Nishioka, M.; Lee, M. L. J. Org. Chem. 1984, 49, 4947-4951. (36) Budzinski, H.; Garrigues, P.; Radke, M.; Connan, J. Org. Geochem. 1993, 20, 917-926.
BN23T (all three from BCR, Brussels, Belgium). DBT-d8, benzo[b]naphtho[2,1-d]thiophene-d10 (BN21T-d10), fluorene-d10 (all three obtained from Cambridge Isotope Laboratories, Andover, MA), pyrene-d10, and fluoranthene-d10 (both obtained from MSD Isotopes, Merck & Co., Rahway, NJ) were dissolved in isooctane and used as internal standards. The remaining alkyl-substituted DBTs and methyl-substituted BNTs were synthesized and kindly provided by M. L. Lee at Brigham Young University (Provo, UT) and were used for identification purposes. Quantification Procedure. The DBT-d8 and fluorene-d10 were used for the quantification of the LC2 fraction, whereas the BN21Td10, pyrene-d10, and fluoranthene-d10 were used for the quantification of the LC4 fraction. Regardless of the internal standard used, the quantification results were in good agreement. In general, the concentrations that are reported in this paper were calculated by using the respective perdeuterated PASH, namely DBT-d8 (LC2) and BN21T-d10 (LC4). However, if coelution with one of the internal PASH standards occurred, the perdeuterated PAHs were Analytical Chemistry, Vol. 71, No. 1, January 1, 1999
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Figure 4. GC/AED analysis of the LC2 fraction (DBT/N12bT) of coal tar (SRM 1597) with two different stationary phases.
used to quantify the respective fraction. Calibration solutions with three different concentrations were used to determine the relative response factors for all PASH standards mentioned above on the GC/MSD and the GC/AED. The response factors of PASH compounds that were not available in high purity were calculated as a mean value from the response factors of available compounds with the same or similar degree of alkylation. Response factors on the AED were always close to unity regardless of the alkylation degree with a standard deviation less than 5%. However, this was not true for the MSD, where the response factors were constant for compounds within the same degree of alkylation (
