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Tandem Mass Spectrometric Characterization of Commercial Naphthenic Acids and a Maya Crude Oil Walter E. Rudzinski,* Leon Oehlers, and Yi Zhang Department of Chemistry and Institute for Environmental and Industrial Science, Southwest Texas State University, San Marcos, Texas 78666
Blanca Najera Facultad de Ciencias Quimicas, Universidad Autonoma de Nuevo Leon, Monterey, Mexico Received February 15, 2002
Elemental analysis, Fourier transform infrared, C13 nuclear magnetic resonance, and electrospray ionization/mass spectrometry (ESI/MS) experiments were performed on two commercial naphthenic acid mixtures in order to determine the acidity, ring type, and carbon number distribution. Critical MS/MS experimental parameters, i.e., isolation width and collision-activated dissociation energy, were optimized and fragmentation patterns elucidated for a series of acid standards and the commercial naphthenic acid mixtures. The MS/MS experiments confirm that P&B and Fluka naphthenic acid mixtures consist primarily of carboxylic acids with dominant hydrogen deficiency values of -6 and 0, respectively, with respect to an alkanoic acid reference. The MS/MS experiments validated the preponderance of carboxylic acid moieties in these samples. The approach was then used to determine the composition of a Maya crude oil extract. Surprisingly, the extract was found to contain a dominant alkylsulfonic acid homologous series.
Introduction Naphthenic acids are minor constituents in petroleum, but they have special significance because of their use as markers in geochemistry and because of their corrosivity to refinery units. Although there have been concerted efforts to understand the actual molecular structure and group type of naphthenic acids, there still remain a number of challenges in characterization.1 One of the more recent innovations applied to the study of naphthenic acids is the use of atmospheric pressure ionization/mass spectrometry. Hsu and co-workers used atmospheric pressure chemical ionization (APCI) to detect naphthenic acids selectively in crude oil, to determine group types, and carbon number distribution.2 Zhan and Fenn recently showed that electrospray ionization (ESI) could selectively ionize polar molecules in various petroleum distillates; however, their mass spectra could not be interpreted due to insufficient mass resolving power.3 Miyabashi et al. applied ESI highresolution Fourier transform-ion cyclotron resonance (FT-ICR) to the analysis of an Arabian mix vacuum residue.4 Most recently, Qian et al. were able to resolve and identify 3000 nitrogen-containing aromatic compounds from a single electrospray ionization FT-ICR * Author to whom correspondence should be addressed. Fax: 512245-2374. E-mail:
[email protected]. (1) Robbins, W. K. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1998, 43, 137-140. (2) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217-223. (3) Zhan, D. L.; Fenn, J. B. Int. J. Mass Spectrom. 2000, 194, 197208. (4) Miya bashi, K.; Suzuki, K.; Teranishi, T.; Naito, Y.; Tsujimoto, K.; Miyake, M. Chem. Lett. 2000, 172-173.
mass spectrum,5 and extended the approach to a naphthenic acid sample.6,7 In contrast to quadrupole mass spectrometers using in-source collision-activated dissociation (CAD), which fragments ions prior to mass analysis, the Finnigan LCQ is a mass spectrometer which allows one to isolate and trap a select intact ion for collision-activated dissociation within the ion trap. One anomaly of an ion trap, however, is the potential for mass shifting from unwanted fragmentation of the parent ion, prior to application of the CAD energy. This effect has recently been described by Murphy and Yost,9 wherein they attribute mass shifts to the fragmentation of “fragile” ions. The resulting MS/MS spectra, properly referenced to the correct parent ion, yield diagnostic fragments and neutral losses useful for compound identification. In an effort to determine the acidity, ring type, and carbon number distribution of naphthenic acids, we performed elemental analyses, FT-IR, 13C NMR, and both APCI and ESI (positive and negative ion mode)/ MS experiments on a series of carboxylic acid standards, commercial naphthenic acid mixtures, and extracts from a Maya crude oil. Tandem mass spectrometry (MS2) (5) Qian, K.; Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G. Energy Fuels 2001, 15, 492-498. (6) Hughey, C. A.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G.; Qian, K.; Robbins, W. K. Proceedings of the 49th Conference on Mass Spectrometry and Allied Topics; Chicago, IL, May 2001. (7) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505-1511. (8) Rudzinski, W. E.; Aminabhavi, T. M.; Spencer, L.; Sassman, S.; Watkins, L. Energy Fuels 2000, 14, 839-844. (9) Murphy, J. P.; Yost, R. A. Rapid Commun. Mass Spectrom. 2000, 14, 270-273.
10.1021/ef020013t CCC: $22.00 © 2002 American Chemical Society Published on Web 08/03/2002
Commercial Naphthenic Acids and a Maya Crude Oil
Figure 1. Representative acid structures.
experiments were also performed on standard samples, naphthenic acid mixtures, and the extracts of a Maya crude oil in order to determine characteristic fragmentation patterns in order to confirm structure. Experimental Section Materials. Acetic acid (99.99%), ammonia, cyclohexane carboxylic acid (98%), cyclohexane acetic acid (98%), cyclohexane butyric acid (98%), benzoic acid, 1-adamantane carboxylic acid (99%), 1-adamantane acetic acid (98%), 1,2,3,4-tetrahydro2-naphthoic acid (98%), 2-naphthylacetic, 5-β-cholanic acid, pyrene carboxylic acid (97%), pyrene acetic acid (97%), pyrene butyric acid (97%), 1-decane-sulfonic acid, sodium salt, deuterated chloroform (CDCl3), and [Cr(acac)3] (97%) and 3-aminopropyl functionalized silica gel were obtained from SigmaAldrich (St. Louis, MO) and used without further purification. Figure 1 depicts a few representative acid structures. The commercial samples of naphthenic acids were purchased from Fluka and Pfaltz and Bauer (P&B) and donated by ExxonMobil. The Maya crude oil was donated by Exxon-Mobil and previously characterized.8 Toluene, methanol (CH3OH), ethanol (C2H5OH), dichloromethane (CH2Cl2), and acetonitrile (CH3CN) were all chromatographic grade and obtained from EM Science (Gibbstown, NJ). Extraction of Acids in Maya Crude Oil. Two distinct extraction methods (liquid/liquid and liquid/solid) were employed in order to extract the acids from a Maya crude oil (TAN ) 2.8). In the liquid/liquid procedure, the Maya crude oil was dissolved in (50:50) acetonitrile:methanol, the black residue discarded, and the golden supernatant filtered. The supernatant was then blown down to dryness, and the residue reconstituted in methanol containing 0.5% ammonia. In the liquid/solid procedure 25 g of the Maya crude oil were diluted with 175 mL of toluene:methanol (70:30), then loaded on to 12.5 g of 3-aminopropyl-functionalized silica gel. After standing overnight, the bulk oil was removed by washing the sorbent
Energy & Fuels, Vol. 16, No. 5, 2002 1179 three times with toluene:methanol (90:10). The sorbent was then Soxhlet extracted with toluene:acetic acid (70:30). The extract was washed with 150 mL of distilled water to remove the residual acetic acid, dried with Na2SO4, then rotovapped to remove the solvent. The residue was re-extracted with hexane, the residue discarded, and the remainder rotovapped to remove the hexane. The hexane-soluble fraction represented 0.91% of the total mass. The total acid number for this extract was 185. The previous method is an adaptation of the method reported by Qian et al.7 Elemental Analysis. Elemental analyses (C, H, N, and S) were performed by Quantitative Technologies Incorporated (Whitehouse, NJ). Infrared Spectroscopy. Fourier transform infrared (FTIR) spectral measurements were taken on a Perkin-Elmer FTIR (model 1600) using a 0.015 mm NaCl solution cell and scanning between 500 and 4000 cm-1. All spectra were obtained in the absorbance mode. The molar absorptivity of the CdO absorption band was determined by averaging the molar absorptivity of cyclohexane carboxylic acid, cyclohexane acetic acid, cyclohexane butyric acid, 1-adamantane carboxylic acid, 1-adamantane acetic acid, 1,2,3,4-tetrahydro-2-naphthoic acid, and 5-β-cholanic acid. Samples of 7-11 mg of each acid were dissolved in 1.0 mL of CH2Cl2 and the absorbance of the solution was measured. The calculated molar absorptivity was then used to calculate the carbonyl or carboxyl concentration in the commercial naphthenic acid samples. The background absorption of CH2Cl2, though minimal, was subtracted from the total absorbance of the band attributed to the carbonyl group. Nuclear Magnetic Resonance Spectroscopy. 13C NMR was performed on a Varian 400 MHz NMR spectrometer equipped with a Sun workstation. 13C NMR spectra were obtained at an absorption frequency of 100 MHz. CDCl3 was used as the solvent for making 30% w/v solutions and also provided the internal field-frequency lock signal. To obtain quantitative 13C NMR data, the relaxation agent [Cr(acac)3]+3 was added at a concentration of 25 mg/mL of CDCl3. The Nuclear Overhauser Enhancement was suppressed by operating the spectrometer in the inverse-gated, decoupling mode in which the protons are irradiated only during the processing of the free induction decay. A relaxation delay of 4 s was used for all measurements. The signal-to-noise ratio was maximized by taking a total of 1024 scans. Cyclohexane carboxylic acid, cyclohexane acetic acid, cyclohexane butyric acid, 1-adamantane carboxylic acid, 1-adamantane acetic acid, 1,2,3,4-tetrahydro-2-naphthoic acid, 2-naphthylacetic, and pyrene butyric acid were used to calibrate the spectrometer pulse settings, and determine the chemical shift range in order to obtain the correct carbon mole ratio. Atmospheric Pressure Ionization/Mass Spectrometry. The mass spectra were obtained on a Finnigan LCQ ion-trap mass spectrometer equipped with either an atmospheric pressure chemical ionization (APCI) or electrospray ionization (ESI) source (Finnigan MAT, San Jose, CA). The solutions were infused into the mass spectrometer source using the instrument syringe pump and a Unimetrics 250 µL syringe. A Hometown Computing PC (Hometown Computing, San Marcos, TX) was used for control of the Finnigan LCQ while Excalibur software, revision 1.2 (Finnigan Corp., 2000), was used for data acquisition and plotting. APCI Mode Experiments. Mass spectra for a number of standards: cyclohexane carboxylic acid, 1-adamantane carboxylic acid, 1,2,3,4-tetrahydro-2-naphthoic acid, 5-β-cholanic acid, and pyrene butyric acid were obtained in order to compare the relative efficacy of APCI and ESI in both the positive and negative ion modes. Acid standards used in the APCI mode (both positive and negative) were prepared by dissolving 5-20 mg of each acid in 2 mL of (50:50) CH3OH: CH3CN. The Fluka and P&B naphthenic acids were dissolved in CH3OH or CH3CN to a final concentration of 5 mg/mL.
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Table 1. ESI MS/MS Analysis of Acid Standards
a
compound
parent m/z [M-H]-
daughter (m/z)a
mass loss
neutral loss fragment
cyclohexylacetic acid cyclohexanebutyric acid adamantaneacetic acid 5β-cholanic acid 2-naphthylacetic acid 1,2,3,4-tetrahydronaphthoic acid benzoic acid 1-pyrenecarboxylic acid 1-pyreneacetic acid 1-pyrenebutyric acid
141 169 193 359 185 175 121 245 259 287
97 125, 151 149, 175, 165 341, 331, 315 141, 157 131, 147, 157 77 201 215 215
44 44, 18 44, 18, 28 18, 28, 44 44, 28 44, 28, 18 44 44 44 72
CO2 CO2, H2O CO2, H2O, CO H2O, CO, CO2 CO2, CO CO2, CO, H2O CO2 CO2 CO2 CO2 + C2H2
Daughter peaks listed in order of relative abundance.
The APCI source parameters used were the following: corona discharge current, 5 A; vaporizer temperature, 450 °C; nitrogen sheath gas pressure, 60 psi; heated capillary temperature, 150 °C; and capillary voltage, 10 V. Sample was introduced at a flow rate of 100µL/min. ESI Negative Ion Mode Experiments. Acid standards used in the ESI negative ion mode, ESI(-), experiments were prepared by dissolving 20 mg of each acid in 2 mL of (50:50) CH3OH:CH3CN containing 0.5% ammonia (NH3). The addition of NH3 to the solutions dramatically improved the ionization efficiency and instrument response. The Fluka and P&B naphthenic acids and the Maya crude extract were dissolved in CH3OH containing 0.5% NH3 to a final concentration of 70 mg/mL. The ESI(-) source parameters were the following: capillary voltage, 4.5 kV; nitrogen sheath gas pressure, 80 psi; nitrogen sheath gas flow rate, 40 (arbitrary units); heated capillary temperature, 200 °C; capillary voltage, -19 V; tube lens offset voltage, 10 V; 3 microscans at 200 ms ion injection time with automatic gain control enabled. Sample was introduced at a flow rate of 3µL/min. Averaged scans were obtained over the m/z range 50-500. ESI MS/MS Experiments. Prior to MS/MS scans the tube lens offset voltage and octapole ion optics were optimized using the cyclohexanebutyric acid [M-H]- peak at m/z 169. ESIMS/MS experiments were then performed on the remaining acid standards in order to ascertain characteristic fragmentation patterns. Mass shift effects for the fragmentation of naphthenic acids could be avoided by careful selection of the isolation width. The isolation width was increased until a strong parent ion signal of correct m/z ratio was detected in the MS/MS scan. Following selection of the parent ion, the CAD energy was increased until efficient fragmentation was observed. For most ions, the CAD energy could be adjusted such that both the parent and daughter ion peaks could be detected in the MS/MS scan. It was found that isolation widths of 3-5 amu were optimal, whereas narrower widths (
