Anal. Chem. 2007, 79, 6222-6229
Characterization of Naphthenic Acids from Athabasca Oil Sands Using Electrospray Ionization: The Significant Influence of Solvents John V. Headley* and Kerry M. Peru
Aquatic Ecosystem Protection Research Division, Water Science and Technology Directorate, Environment Canada, 11 Innovation Boulevard, Saskatoon, Saskatchewan, S7N 3H5, Canada Mark P. Barrow and Peter J. Derrick
Institute of Mass Spectrometry and Department of Chemistry, University of Warwick, Coventry, CV4 7AL, United Kingdom
There is a need to develop routine and rugged methods for the characterization of oil sands naphthenic acids present in natural waters and contaminated soils. Mass spectra of naphthenic acids, obtained using a variant of electrospray ionization coupled with a Fourier transform ion cyclotron resonance mass spectrometer, are shown here to vary greatly, reflecting their dependence on solubilities of the acids in organic solvents. The solubilities of components in, for example, 1-octanol (similar solvent to fatty tissue) compared to polar solvents such as methanol or acetonitrile are used here as a surrogate to indicate the more bioavailable or toxic components of naphthenic acids in natural waters. Monocarboxylic compounds (CnH2n+zO2) in the z ) -4, -6, and -12 (2-, 3-, and 6-ring naphthenic acids, respectively) family in the carbon number range of 13-19 were prevalent in all solvent systems. The surrogate method is intended to serve as a guide in the isolation of principle toxic components, which in turn supports efforts to remediate oil sands contaminated soils and groundwater. Naphthenic Acids. There is a need to better characterize naphthenic acids within crude oils and aquatic environments.1,2 Naphthenic acids are defined as carboxylic acids that include one or more saturated ring structures, though the definition has become more loosely used to describe the range of organic acids found within crude oil (Figure 1). The structural formulas for such acids may be described by CnH2n+zO2,3-7 where “z” is referred to as the “hydrogen deficiency” and is a negative, even integer. More * To whom correspondence should be addressed. E-mail: John.Headley@ ec.gc.ca. (1) Barrow, M. P.; McDonnell, L. A.; Feng, X.; Walker, J.; Derrick, P. J. Anal. Chem. 2003, 75, 860-866. (2) Barrow, M. P.; Headley, J. V.; Peru, K. M.; Derrick, P. J. J. Chromatogr., A 2004, 1058, 51-59. (3) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Anal. Chem. 1988, 60, 1318-1323. (4) Fan, T.-P. Energy Fuels 1991, 5, 371-375. (5) Wong, D. C. L.; van Compernolle, R.; Nowlin, J. G.; O’Neal, D. L.; Johnson, G. M. Chemosphere 1996, 32, 1669-1679. (6) St. John, W. P.; Rughani, J.; Green, S. A.; McGinnis, G. D. J. Chromatogr., A 1998, 807, 241-251.
6222 Analytical Chemistry, Vol. 79, No. 16, August 15, 2007
than one isomer will exist for a given z homologue, and the carboxylic acid group is usually bonded or attached to a side chain, rather than directly to the cycloaliphatic ring.3,4 The molecular weights differ by 14 mass units (CH2) between n series and by two mass units (H2) between z series.8 Naphthenic acids can be solubilized to produce metal salts (e.g., sodium and copper naphthenates) that have industrial applications such as surfactants and fungicides for wood preservatives.6,9,10 Naphthenic Acids: The Oil Industry and Fate in the Environment. Several studies have focused on the environmental fate, transport, degradation, epidemiology, and isolation of specific toxic naphthenic acids.11 In a recent investigation of several commercial naphthenic acid mixtures and those extracted from oil sands, significant differences were established among four commercial mixtures and between the extracts from various oil sands ores and tailings ponds.12 The concentrations and composition of the naphthenic acids were highly varied among commercial sources, oil sands ore, and tailings pond sources.12 The oil industry is concerned with the presence of naphthenic acids in crude oils due to the association with corrosion. Crude oil typically contain naphthenic acids in quantities of up to 4% by weight, and characterization of the acids present within a sample has become a topic of great interest due to the fact that the acids corrode refinery units. Naphthenic acids are known to be weakly biodegradable and therefore well-suited for use in identification of oil source maturation.13,14 Biodegradation of naphthenic acids also occurs within oil reservoirs as the crude oil matures. Koike (7) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217-223. (8) Herman, D. C.; Fedorak, P. M.; Costerton, J. W. Can. J. Microbiol. 1993, 39, 576-580. (9) Davis, J. B. Petroleum Microbiology; Elsevier Publishing Co.: Amsterdam, 1967. (10) Herman, D. C.; Fedorak, P. M.; MacKinnon, M. D.; Costerton, J. W. Can. J. Microbiol. 1994, 40, 467-477. (11) Seifert, W. K.; Teeter, R. M. Anal. Chem. 1970, 42, 750-758. (12) Clemente, J. S.; Prasad, N. G. N.; MacKinnon, M. D.; Fedorak, P. M. Chemosphere 2003, 50, 1265-1274. (13) Meredith, W.; Kelland, S.-J.; Jones, D. M. Org. Geochem. 2000, 31, 10591073. (14) Headley, J. V.; Tanapat, S.; Putz, G.; Peru, K. M. Can. Water Res. J. 2002, 27, 25-42. 10.1021/ac070905w CCC: $37.00
© 2007 American Chemical Society Published on Web 06/30/2007
Figure 1. Representative structures for oil sands naphthenic acids, showing alkylation sites. For z ) -4, for example, a typical range for n is 1-5. The average molecular weight of oil sands naphthenic acids in the Athabasca region is ∼260, with a preponderance of components in the range of 180-300.
et al. have clearly demonstrated biodegradation of the low molecular weight carboxylic acid fraction15 in crude oil. The susceptibility of crude oil to biodegradation has been shown to increase following photolysis.16-18 In unrefined Athabasca bitumen (northern Alberta, Canada), the carboxylic fraction is ∼2%, of which ∼90% is composed of the tricyclic acids that primarily make up the naphthenic acid fraction.19,20 The naphthenic acids may enter surface water systems through such mechanisms as groundwater mixing and erosion of riverbank oil deposits in oil-producing regions such as the Athabasca oil sands.21 The most significant possible environmental receptor is suspected to be water due to direct contact with oil sands material. Ambient levels in northern Alberta rivers in the Athabasca oil sands are generally below 1 mg/L. However, tailings pond waters may contain as much as 110 mg/L. The relatively low aqueous solubility and moderately strong sorption to soils are key factors limiting the bioavailability of oil sands naphthenic acids in aquatic environments.22 Naphthenic acids are, however, known to be toxic to fish.5 The aquatic toxicity is associated with their concentration23 and surfactant characteristics.24-27 There is little information about mammalian toxicity, but the human lethal dosage was reported as 11 g kg-128 and the oral (15) Koike, L.; Rebouc¸ as, L. M. C.; Reis, F. A. M.; Marsaioli, A. J.; Richnow, H. H.; Michaelis, W. Org. Geochem. 1992, 18, 851-860. (16) Green, J. B.; Stierwalt, B. K.; Thomson, J. S.; Treese, C. A. Anal. Chem. 1985, 57, 2207-2211. (17) Dutta, T. K.; Harayama, S. Environ. Sci. Technol. 2000, 34, 1500-1505. (18) Grzechulska, J.; Hamerski, M.; Morawski, A. W. Water Res. 2000, 34, 16381644. (19) Cyr, T. D.; Strausz, O. P. Org. Geochem. 1984, 7, 127-140. (20) Strausz, O. P. J. Am. Chem. Soc. 1988, 33, 264-268. (21) Brient, J. A.; Wessner, P. J.; Doyle, M. N. In Kirk-Othmer Encyclopaedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Ed.; John Wiley and Sons: New York, 1995; pp 1017-1029. (22) Providenti, M. A.; Lee, H.; Trevors, J. T. J. Ind. Microbiol. 1993, 12, 379395. (23) Holowenko, F. M.; MacKinnon, M. D.; Fedorak, P. M. Water Res., 2002, 36, 2843-2855. (24) Dokholyan, V. K.; Magomedov, A. K. J. Ichthyol. 1983, 23, 125-132. (25) MacKinnon, M. D.; Boerger, H. Water Pollut. Res. J. Can. 1986, 21, 496512. (26) Rogers, V. V.; Liber, K.; MacKinnon, M. D. Chemosphere 2002, 48, 519527. (27) Rogers, V. V.; Wickstrom, M.; Liber, K.; MacKinnon, M. D. Toxicol. Sci. 2002, 66, 347-355.
LD50 for rats is between 3.0 and 5.2 g kg-1 with death caused by gastrointestinal disturbances.29 Instrumental Techniques. Conventionally employed chromatographic methods are not effective for a complete separation of the naphthenic acid species prior to mass spectrometric detection. However, a variety of mass spectrometric techniques have been used for the structural elucidation of naphthenic acids, including useful approaches such as gas chromatography-mass spectrometry,3,6,16 liquid chromatography-mass spectrometry,30 atmospheric pressure chemical ionization (APCI),7 and electrospray ionization (ESI).31-33 High mass accuracy and high resolution are a prerequisite for full characterization of different ions of very similar mass, but same class in complex mixtures. Although magnetic sector and FTICR34-36 instruments have been most commonly applied to such investigations, time-of-flight mass spectrometry has also been used to analyze crude oil samples.37 Techniques such as APCI in negative-ion mode can produce very clean spectra with good sensitivity compared to other techniques.7 Electrospray ionization, however, is becoming the ionization technique of choice for the mass spectrometry of naphthenic acids in crude oil samples. Electrospray ionization38-42 has the advantage of being an ionization technique that imparts little energy to the nascent ions and (28) Rockhold, W. AMA Arch. Ind. Health 1955, 12, 477-482. (29) Lai, J. W. S.; Pinto, L. J.; Kiehlmann, E.; Bendell-Young, L. I.; Moore, M. M. Environ. Toxicol. Chem. 1996, 15, 1482-1491. (30) Hsu, C. S.; McLean, M. A.; Qian, K.; Aczel, T.; Blum, S. C.; Olmstead, W. N.; Kaplan, L. H.; Robbins, W. K.; Schulz, W. W. Energy Fuels 1991, 5, 395-398. (31) Miyabayashi, K.; Suzuki, K.; Teranishi, T.; Naito, Y.; Tsujimoto, K.; Miyake, M. Chem. Lett. 2000, 172-173. (32) Miyabayashi, K.; Yasuhide, N.; Miyake, M.; Tsujimoto, K. Eur. J. Mass Spectrom. 2000, 6, 251-258. (33) Zhan, D.; Fenn, J. B. Int. J. Mass Spectrom. 2000, 194, 197-208. (34) Amster, I. J. J. Mass Spectrom. 1996, 31, 1325-1337. (35) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Mass Spectrom. Rev. 1998, 17, 1-35. (36) Barrow, M. P.; Burkitt, W. I.; Derrick, P. J. Analyst 2005, 130, 18-28. (37) Dutta, T. K.; Harayama, S. Anal. Chem. 2001, 73, 864-869. (38) Dole, M.; Mack, L. L.; Hines, R. L.; Mobley, R. C.; Ferguson, L. D.; Alice, M. B. J. Chem. Phys. 1968, 49, 2240-2249. (39) Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4451-4459. (40) Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4671-4675. (41) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64-71.
Analytical Chemistry, Vol. 79, No. 16, August 15, 2007
6223
results in the vaporization and ionization of a sample while minimizing fragmentation. The combination of the ultrahigh mass accuracy, ultrahigh resolution, and selective observation of the deprotonated naphthenic acids make negative-ion mode FT-ICR mass spectrometry an attractive technique for the characterization of the naphthenic acids within a crude oil.1,43-45 The ability to determine the empirical formulas of the acidic species contained within a given crude oil is of relevance to the continued fight against corrosion within the petroleum industry and environmental concerns.2 Fourier transform ion cyclotron resonance mass spectra of naphthenic acids are shown here to be strongly dependent on the choice of solvents. The differences among the FT-ICR mass spectra for different solvents are assumed to be primarily a result of the selective solubilities of the respective components in a given solvent. The electrospray process involves formation of colloidal droplets, which shrink through evaporation of volatile solvents.46 The more volatile the solvent, the more readily desolvation of droplets occurs in the electrospray ionization process. The consequence is effectively to concentrate the solution and to make relevant the relative solubilities of the naphthenic acids in the particular solvent. The solubilities of components in, for example, 1-octanol (similar solvent to fatty tissue) compared to polar solvents such as methanol or acetonitrile are used here as a surrogate to indicate the more bioavailable or toxic components of naphthenic acids in natural waters. This approach is a simplification of the complex processes occurring in real world environments, as depending on the organism, the mode of living and exposure (ingestion of benthos, dermal, across gills, etc.); the less lipophilic, more polar fractions may also be important. However, the surrogate method is intended to serve as a tool to guide researchers in the isolation of principle toxic components, which in turn supports efforts to remediate oil sands contaminated soils and groundwater. EXPERIMENTAL SECTION The surrogate procedure for isolating potentially toxic and bioavailable components of oil sands naphthenic acids in natural waters partially depends upon the repeatability of the FT-ICR mass spectra. Due care was given to experimental procedures to ensure conditions were comparable from run to run. Concentration is known to be an important factor, as there is mass spectrometric evidence for naphthenic acids being prone to multimer formation.47 The concentration at which multimer formation occurs is, however, strongly instrument dependent. In particular, adjustment of parameters such as cone voltage and curtain gas can be used to promote or eliminate multimers at a given concentration. Care was therefore given to ensure that the same concentration and (42) Mann, M., Ed. Electrospray Mass Spectrometry; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992. (43) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505-1511. (44) Brandal, O.; Hanneseth, A. M.; Hemmingsen, P. V.; Sjoblom, J.; Kim, S.; Rodgers, R. P.; Marshall, A. G. J. Dispersion Sci. Technol. 2006, 27, 295305. (45) Hemmingsen, P. V.; Kim, S.; Pettersen, H. E.; Rodgers, R. P.; Sjoblom, J.; Marshall, A. G. Energy Fuels 2006, 20, 1980-1987. (46) Peschke, M.; Verkerk, U. H.; Kebarle, P. J. Am. Soc. Mass Spectrom. 2004, 15, 1424-1434. (47) Headley, J. V.; Peru, K. M.; McMartin, D. W.; Winkler, M. J. AOAC Int. 2002, 85, 182-187.
6224
Analytical Chemistry, Vol. 79, No. 16, August 15, 2007
instrument conditions were used in comparisons of respective mass spectra. Changes in the mass spectra may also arise from possible degradation of standards with time. To achieve the optimum results, experiments were performed under the same experimental conditions, as the temperature of the ions and fields within the ion source are known to influence significantly the mass spectrum.48 Additionally, the same extract was used and the samples were analyzed on the same day. The repeatability of the FT-ICR mass spectra was determined using repeat, consecutive runs (typically six) for the same solvent system under the same instrumental conditions. For the octanol-acetonitrile solvent system, runs with poor stability of the spray were discarded in preference for runs acquired with a more stable spray. The selection in this case was thus based upon the experience of the instrument operator. Instrumentation. Experiments were performed using a 9.4-T Bruker BioApex II (Bruker Daltonics, Billerica, MA) Fourier transform ion cyclotron resonance mass spectrometer.49 The superconducting magnet (Magnex Ltd.) was maintained at the operating temperature using liquid helium and liquid nitrogen, where the entirety of the magnet was enclosed by passive shielding. The heart of the instrument was the Infinity Cell,50 which was cylindrical in geometry, rather than the traditional cubic, and consisted of six plates: two trapping plates, two detection plates, and two excitation plates. Segmentation of the trapping plates, and the application of differing radio frequency (rf) potentials to these segments, lead to homogeneous electrostatic fields from the excitation plates that simulates an infinitely long cell. The advantage of the cell design was the minimization of ion loss during excitation, which can arise from the effect of radial components of the excitation field; such ion loss can lead to discrimination effects during the excitation stage of an experiment. During the course of these experiments, a trapping potential (PV1 and PV2) of -1.5 V was maintained to constrain the ions’ movement within the cell and detection was performed over a range of m/z 144-3000, with the excitation range being slightly wider to compensate for possible inhomogeneities at the edges of the rf chirp used for excitation. The flight tube was pumped by three cryogenic pumps, which maintained a base pressure of the order of 10-10 mbar within the cell region during experiments. A base pressure of the order of 10-7 mbar was maintained within the interface between an external ion source and the flight tube. Ions were accumulated within the hexapole ion trap for a period of 2 s (D1 ) 2 s) prior to extraction. For a user-defined period of time, ions were extracted from the hexapole, traversed the ion optics, and entered the FT-ICR cell. The period of extraction of ions from the hexapole, referred to as P2, was set to 2400 µs for the course of these experiments. The SideKick mechanism was employed to deflect ions off of the ion optical axis as they enter the cell and increase trapping efficiency (EV1 ) 1.09 V, EV2 ) 1.60 V; where EV values corresponded to ICR cell extraction plates), DEV2 (ICR cell delta extraction plate 2) ) -4.13 V). Dipolar excitation was used to excite the ions to (48) Hunt, S. M.; Sheil, M. M.; Belov, M.; Derrick, P. J. Anal. Chem. 1998, 70, 1812-1822. (49) Palmblad, M.; Hakansson, K.; Hakansson, P.; Feng, X.; Cooper, H. J.; Giannakopulos, A. E.; Green, P. S.; Derrick, P. J. Eur. J. Mass Spectrom. 2000, 6, 267-275. (50) Caravatti, P.; Allemann, M. Org. Mass Spectrom. 1991, 26, 514-518.
Figure 2. Representative mass accuracies obtained for oil sands naphthenic acids, illustrated using data for the Milli-Q water/acetonitrile solvent system. The mass error was typically less than 0.5 ppm for most assignments (over 70%).
a detectable cyclotron orbit prior to the detection stage. The duration for the excitation steps, P3, was set to 12 µs, the rf attenuation for the excitation, PL3, was set to 10.15, and XBB, which determines the excitation step size, was maintained at a value of 200. Data acquisition in broadband mode used a 12-bit, 10-MHz digitizer. This maximum sampling rate can be translated to a minimum mass-to-charge ratio of ∼29 on the basis of the Nyquist criterion, which states that the sampling frequency must be at least equal to twice the frequency being sampled. The instrument was controlled using a Silicon Graphics Indy workstation running XMASS 5.0.10 (Bruker Daltonics, Billerica, MA) under IRIX. 5.3. Data files consisted of 512 K (524 288) data points. The raw data were electronically converted from the time domain to the frequency domain via a fast Fourier transform. Oil Sands Naphthenic Acids Samples. The samples of interest were obtained from four sites at an industrial location in the oil sands region of the Athabasca River Basin, Alberta, Canada. The naphthenic acid concentrate was obtained by extraction of oil sands process water following procedures described by Rogers et al.26 The extracted naphthenic acid concentrate was made up in acetonitrile to a measured concentration of 8500 mg/L. This naphthenic acid extract was used to prepare soluble fractions in five separate solvents using a ratio of 1 mg of extract to 1 mL of solvent. Visible differences in solubility were evident particularly for the 1-octanol/acetonitrile solvent system. The resulting solvent systems, with the polarity indices given in parentheses for the solvents, were as follows: Milli-Q water (9.0)/ acetonitrile (5.8); acetonitrile (5.8); methanol (5.1)/acetonitrile (5.8); dichloromethane (3.1)/acetonitrile (5.8); and 1-octanol (
