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Energy & Fuels 2004, 18, 16-21
Mass Spectrometric Characterization of Organosulfur Compounds Using Palladium(II) as a Sensitivity-Enhancing Reagent Walter E. Rudzinski,* Kai Zhou, and Xuemei Luo Department of Chemistry and Institute for Environmental and Industrial Science, Texas State University, San Marcos, Texas 78666-4616 Received June 17, 2003. Revised Manuscript Received September 8, 2003
Electrospray ionization/ mass spectrometry (ESI/MS) experiments were performed on several polyaromatic sulfur heterocycles (PASH) in the presence of Pd(II) in order to promote charge transfer and radical cation formation. Critical experimental parameters, i.e., Pd(II)/organosulfur mole ratio, the effect of polyaromatic hydrocarbon interferences and sample solution flow rate, and MS2 collision energy, were optimized. PASH compounds in a spiked hydrogenated oil were then identified to determine matrix effects. Selected peaks from a PASH-enriched aromatic fraction obtained from the heavy distillate of an Arabian crude were then analyzed in order to demonstrate the applicability of the method.
Introduction Air pollution is a serious environmental problem, and the combustion of fossil fuels containing organosulfur compounds is a major source of SOx in the atmosphere. In an effort to reduce the amount of SOx generated, the European Community (Directive 98/70/E/C) has mandated a reduction in the sulfur content of gasoline and diesel fuel to 50 ppm by 2005,1 while the U.S. Environmental Protection Agency has mandated a reduction in the sulfur content of diesel to 15 ppm by 2006.2 A number of analytical methods have been used in order to isolate, identify, and quantify the types of organosulfur compounds which may be present.3-9 Most chromatographic approaches for sulfur isolation are difficult to employ since the properties of polyaromatic sulfur heterocycles (PASH) are very similar to those of polyaromatic hydrocarbons (PAH). One approach which has worked exploits the interaction of PASH compounds with PdCl2 immobilized on silica gel.10-15 There are, however, some deficiencies in the * Author to whom correspondence should be addressed. Phone: 512245-3120. Fax: 512-245-2374. E-mail:
[email protected]. (1) Off. J. Eur. Commun. 28 December 1998, L350, 58. (2) EPA Federal Register 2001, 66, 5001. (3) Speight, J. G. The Desulfurization of Heavy Oils and Residua, 2nd ed.; Marcel Dekker: New York, 2000. (4) Altgelt, K. H.; Jewell, D. M.; Latham, D. L.; Selucky, M. L. Chromatography in Petroleum Analysis; Altgelt, K. H., Gouw, T. H., Eds; Chromatogr. Sci. Vol. 11; Marcel Dekker: New York, 1979. (5) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; Marcel Dekker: New York, 1994; pp 203307. (6) Leontaritis, K. J. Proc. Int. Symp. Oilfield Chem. 1997, 421440. (7) Mansfield, C. T.; Barman, B. N.; Thomas, J. V.; Mehrotra, A. K.; McCann, J. M. Anal. Chem. 1999, 71, 81R-107R. (8) Barman, B. N.; Cebolla, V. L.; Membrado, L. Crit. Rev. Anal. Chem. 2000, 30, 75-120. (9) Rudzinski, W. E. Chromatographic Separation and Atmospheric Pressure Ionization/Mass Spectrometric Analysis of Nitrogen, Sulfur and Oxygen Containing Compounds in Crude Oils. In Analytical Advances for Hydrocarbon Research; Hsu, C., Ed.; Kluwer Academic: New York, 2003; Chapter 13, pp 313-336.
method: organosulfur compounds elute as PdCl2 complexes, thiophenes and benzothiophenes are not completely recovered, and constituents with a terminal (as opposed to an internal) thiophene ring elute early.14 In addition, the affinity of the PdCl2 varies with the organosulfur ring size. Milenkovic et al.16 and Rudzinski and co-workers17 found that the selectivity decreased in the order 3-ring > 2-ring> 1-ring PASH compounds. The results seem to indicate that the goal of a general chromatographic method for the separation of polyaromatic sulfur heterocycles (PASH) still is elusive. Mass spectrometry (MS) is an alternative approach which may not require preliminary chromatographic separation. MS has been employed for the identification of nitrogen-sulfur-oxygen (NSO) compounds in petroleum. In particular, atmospheric pressure ionization/ mass spectrometry (API/MS)18-20 has been particularly useful for the analysis of nonvolatile and thermally labile compounds. API coupled with Fourier transform/ ion cyclotron resonance/mass spectrometry (FT/ICR/ MS)21-23 offers unparalleled mass resolution and accuracy, while API coupled with an ion trap capable of (10) Later, D. W.; Lee, M. L.; Bartle, K. D.; Kong, R. C.; Vassilaros, D. L. Anal. Chem. 1981, 53, 1612-1620. (11) Wright, B. W.; Peaden, P. A.; Lee, M. L.; Stark, T. J. Chromatogr. 1982, 248, 17-34. (12) Grang, B. Y. Anal. Lett. 1985, 18, 193-202. (13) Nishioka, M.; Campbell, R. M.; Lee, M. L.; Castle, R. N. Fuel 1986, 65, 270-273. (14) Andersson, J. T. Anal. Chem. 1987, 59, 2207-2209. (15) Nishioka, M. Energy Fuels 1988, 2, 214-219. (16) Milenkovic, A.; Schultz, E.; Meille, V.; Loffreda, D.; Forissier, M.; Vrinat, M.; Sautet, P.; Lemaire, M. Energy Fuels 1999, 13, 881887. (17) Rudzinski, W. E.; Aminabhavi, T. M.; Tarbox, T.; Sassman, S.; Whitney, K.; Watkins, L. M. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 2000, 45, 60-63. (18) Zhan, D. L.; Fenn, J. B. Int. J. Mass Spectrom. 2000, 194, 197208. (19) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217-223. (20) Rudzinski, W. E.; Aminabhavi, T. M.; Spencer, L.; Sassman, S.; Watkins, L. Energy Fuels 2000, 14, 839-844.
10.1021/ef030115z CCC: $27.50 © 2004 American Chemical Society Published on Web 10/31/2003
MS Characterization of Organosulfur Compounds
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Figure 1. Structures of organosulfur compounds.
tandem mass spectrometry offers the ability to confirm structure through characteristic fragmentation patterns.24-26 The applications of chromatography and mass spectrometry to petroleum and NSO analysis have recently been reviewed.7-9 As a first step in an effort to improve upon the identification of organosulfur compounds in petroleum analysis, we have used tandem mass spectrometry (MS/ MS) for the identification and confirmation of several organosulfur standards. Critical experimental parameters, i.e., concentration, Pd(II)/organosulfur mole ratio, effect of interferences and sample solution flow rate, and MS2 collision energy were optimized. PASH compounds in a spiked hydrogenated oil were identified to determine matrix effects. Finally, selected peaks from a PASH-enriched aromatic fraction obtained from the heavy distillate of an Arabian crude were analyzed in order to demonstrate the applicability of the method. Experimental Section Reagents and Chemicals. Methanol (CH3OH), hexane (C6H12), dichloromethane (CH2Cl2), acetonitrile (CH3CN), and chloroform (CHCl3 ) were all chromatographic grade and obtained from EM Science (Gibbstown, NJ); toluene, diethyl ether, and alumina were reagent grade and obtained from Sigma-Aldrich (Milwaukee, WI). PdCl2, the polyaromatic sulfur heterocycles (PASH): dibenzothiophene (DBT), thianthrene (TAN), benzonaphthothiophene (BNTP), 4,6-dimethyldibenzothiophene (4,6-DBT), and 2-methyldibenzothiophene (2DBT), and the following polyaromatic hydrocarbons (PAH): phenathrene, pyrene, fluoranthene, chrysene, 2,3-benzanthracene, and benzo[k]fluoranthene were obtained from SigmaAldrich. The structure and formula weight (F.W.) of representative PASH compounds are given in Figure 1. The hydrogenated oil was obtained from Exxon-Mobil and is a clear, viscous liquid with a boiling range of 160 to >350 °C. (21) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492-498. (22) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505-1511. (23) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2002, 74, 4145-4149. (24) Rudzinski, W. E.; Oehlers, L.; Zhang, Y. Energy Fuels 2002, 16, 1178-1185. (25) Rudzinski, W. E.; Zhang, Y. Proceedings of the 50th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, June 6, 2002. (26) Rudzinski, W. E.; Zhang, Y.; Luo, X. Mass Spectrometry of Polyaromatic Sulfur Compounds in the Presence of Palladium(II). J. Mass Spectrom. 2003 38, 167-173.
Figure 2. Scheme for the fractionation of an Arabian crude oil. The halogenated oil is completely saturated with no trace of aromatics, as confirmed by 13C NMR. Preparation of Samples. PASH stock solutions were prepared as either 10 mM or 1 mM (TAN) solutions in either CH3OH/CH3CN (50:50) or CH2Cl2. A 20 mM PdCl2 stock solution was prepared either in CH3OH/CH3CN (50:50) or CH3CN. The hydrogenated oil was dissolved in hexane to a final concentration of 100 mg/mL. PASH mixture #1 was prepared by taking an aliquot of the appropriate PASH compound in CH3OH/CH3CN (50:50), then diluting it to 1 mL in CH3OH/CH3CN (50:50) to the following concentrations: 1.5 × 10-4 M TAN, 1 × 10-3 M DBT, 1 × 10-3 M 2-DBT, 1 × 10-3 M 4,6-DBT, 2 × 10-3 M BNTP, and 2-4 mM PdCl2. PASH mixture #2 was prepared by taking the appropriate PASH in CH2Cl2, then diluting to 1 mL in CH3OH/CH2Cl2 (50: 50) to the following concentrations: 1 × 10-6 M TAN, 5 × 10-4 M DBT, 1 × 10-4 M 2-DBT, 2 × 10-5 M 4,6-DBT, 5 × 10-5 M BNTP, and 0.5-1 mM PdCl2. PASH mixture #3 was prepared by taking the appropriate PASH in CH2Cl2, then diluting to 1 mL in CH3OH/CH2Cl2 (50: 50) to the following concentrations: 1 × 10-6 M TAN, 5 × 10-4 M DBT, 1 × 10-4 M 2-DBT, 2 × 10-5 M 4,6-DBT, 5 × 10-5 M BNTP, 0.5 mM PdCl2 then introducing the following PAH compounds: phenanthrene, pyrene, fluoranthene, chrysene,
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Figure 3. Effect of Pd(II) concentration on the MS full scan of PASH standard mixture #1: (A) 0 mM Pd(II), (B) 0.5 mM Pd(II), (C) 2.0 mM Pd(II). Source fragmentation voltage, 25 V. Ion signal intensities listed in the upper right corner. 2,3-benzanthracene, and benzo[k]fluoranthene to a final concentration of 0.5 mM. PASH mixture #4 in hydrogenated oil was prepared by taking an appropriate aliquot of PASH in CH2Cl2, PdCl2 in CH3CN, and hydrogenated oil in hexane, then diluting to 1 mL in CH3OH/CH2Cl2 (50:50) to the following concentrations: 1 × 10-6 M TAN, 5 × 10-4 M DBT, 1 × 10-4 M 2-DBT, 2 × 10-5 M 4,6-DBT, 5 × 10-5 M BNTP, 0.5 mM PdCl2, and 10 mg/mL hydrogenated oil. Fractionation of Arabian Crude Oil. Figure 2 depicts the scheme used for the fractionation of the Arabian crude oil. A 230 mL sample of Arabian crude oil was separated by a series of distillations into 55 mL of light (
