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Data Visualization for the Characterization of Naphthenic Acids within Petroleum Samples Mark P. Barrow,*,† John V. Headley,‡ Kerry M. Peru,‡ and Peter J. Derrick†,§ Department of Chemistry, UniVersity of Warwick, CoVentry, CV4 7AL, United Kingdom, Aquatic Ecosystem Protection Research DiVision, Water Science and Technology Directorate, EnVironment Canada, 11 InnoVation BouleVard, Saskatoon, Saskatchewan, S7N 3H5, Canada, and Institute of Fundamental Sciences, Massey UniVersity, PriVate Bag 11-222, Palmerston North, New Zealand ReceiVed NoVember 11, 2008. ReVised Manuscript ReceiVed February 13, 2009

Fourier transform ion cyclotron resonance mass spectrometry has made a significant contribution to the characterization of naphthenic acids in petroleum samples. The characterization of naphthenic acids is of particular interest due to their believed involvement in corrosion and deposit formation, as well as their toxicity toward aquatic organisms. Analysis of a complex mixture, such as a petroleum sample, can present challenges in terms of data analysis and visualization. A variety of graphical methods for representing the data are evaluated, and the use of a heat map, a method primarily used within molecular biology, is highlighted. An Athabasca oil sands sample was characterized and compounds of the empirical formula CnH2n+zOx, where x ) 2-5, were observed. The range of oxygen content is of particular relevance in light of other research, which has shown that the total acid number of a petroleum sample is not a reliable method for evaluating the acid content, as not all of the acids are monoprotic.

Introduction There is a resurgence of interest in the characterization of the components within petroleum-related complex mixtures. Detailed characterization of naphthenic acids in petroleum samples is of particular significance, both with respect to the oil industry and to the environment. Naphthenic acids1 have been defined as carboxylic acids that include one or more saturated ring structures, although this definition has become more loosely used to describe the range of organic acids found within crude oil. The empirical formulas for the acids may be described by CnH2n+zO2,,2-6 where z is referred to as the “hydrogen deficiency” and is equal to zero or a negative, even integer. More than one isomer will exist for a given z homologue and the carboxylic acid group is usually attached to a side chain, rather than directly to the cycloaliphatic ring.2,3 The empirical formulas differ by CH2 for a given hydrogen deficiency, and they differ by H2 between z series (i.e., for a given carbon content).7 Naphthenic acids can be solubilized to produce metal salts that have industrial applications.5,8-10 Crude oil typically contains naphthenic acids in quantities of up to 4% by weight and characterization of the acids present * To whom correspondence should be addressed. E-mail: M.P.Barrow@ warwick.ac.uk. † University of Warwick. ‡ Environment Canada. § Massey University. (1) Headley, J. V.; Peru, K. M.; Barrow, M. P. Mass Spectrom. ReV. 2008, 28, 121–134. (2) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Anal. Chem. 1988, 60, 1318–1323. (3) Fan, T.-P. Energy Fuels 1991, 5, 371–375. (4) Wong, D. C. L.; van Compernolle, R.; Nowlin, J. G.; O’Neal, D. L.; Johnson, G. M. Chemosphere 1996, 32, 1669–1679. (5) St. John, W. P.; Rughani, J.; Green, S. A.; McGinnis, G. D. J. Chromatogr. A 1998, 807, 241–251. (6) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217–223.

within a sample has become a topic of great interest, due to the fact that the acids corrode refinery units and therefore present the oil industry with significant additional costs. At a time when the price of crude oil was less than $20 per barrel, it was stated that savings of several dollars per barrel could be made if the corrosiveness of the acids could be defined properly.11 Since that time, the price of crude oil has continued to rise and was greater than $140 in June 2008. As crude oil supplies become more scarce, the naphthenic acid content of crude oils is believed to be on the rise due to the increasing use of “opportunity crude oils”.12 The characterization of the components within petroleum samples is thus of increasing importance, as both the consumption of oil and the price of oil continue to rise. The “total acid number” (TAN), a measure of the corrosiveness of the crude oil, is defined as the mass of potassium hydroxide (in milligrams) required to neutralize one gram of crude oil. As shown by Turnbull et al., the size and structure of naphthenic acids influence their corrosiveness.11 The TAN value alone cannot therefore accurately account for corrosiveness and thus there is a need for detailed characterization of the acid fraction of crude oil. A variety of techniques have been used during the mass spectrometric investigation of the acids, including gas chromatography-mass spectrometry (GC-MS),2,5,13 electron ionization (EI),5 liquid secondary ion mass spectrometry (7) Herman, D. C.; Fedorak, P. M.; Costerton, J. W. Can. J. Microbiol. 1993, 39, 576–580. (8) Davis, J. B. Petroleum Microbiology; Elsevier Publishing Co.: Amsterdam, 1967. (9) Herman, D. C.; Fedorak, P. M.; MacKinnon, M. D.; Costerton, J. W. Can. J. Microbiol. 1994, 40, 467–477. (10) Brient, J. A. Abstr. Am. Chem. Soc. 1998, 215, U119–U119. (11) Turnbull, A.; Slavcheva, E.; Shone, B. Corrosion 1998, 54, 922– 930. (12) Slavcheva, E.; Shone, B.; Turnbull, A. Br. Corros. J. 1999, 34, 125–131. (13) Green, J. B.; Stierwalt, B. K.; Thomson, J. S.; Treese, C. A. Anal. Chem. 1985, 57, 2207–2211.

10.1021/ef800985z CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

Characterization of Naphthenic Acids in Petroleum

(LSIMS),4 fast atom bombardment (FAB),3 chemical ionization (CI),2,3,5,6 atmospheric pressure chemical ionization (APCI),6 and electrospray ionization (ESI).6,14-26 A derivative of ESI, known as nanospray,27 is well-suited for characterization of naphthenic acids, consuming less sample and minimizing risks of contaminating the ion source. With the increasing pressures to find new sources of petroleum, the Athabasca (northern Alberta, Canada) oil sands in Canada have become subject to much interest. In unrefined Athabasca bitumen, the carboxylic fraction is about 2%, of which approximately 90% is comprised of the tricyclic acids that primarily make up the naphthenic acid fraction.28,29 Extraction of crude oil from the bituminous sands requires large quantities of water and leads to the creation of ponds of oil sands processed water (OSPW). In general, naphthenic acids may enter surface water systems through such mechanisms as groundwater mixing and erosion of riverbank oil deposits in oil-producing regions.30 While ambient levels of naphthenic acids in northern Alberta rivers in the Athabasca oil sands are generally found to be below 1 mg/L, OSPW may contain as much as 110 mg/L. Naphthenic acids are known to be toxic to aquatic organisms.4,15,31-35 The acids’ aquatic toxicity is associated with their concentration36 and surfactant characteristics.32,37-39 The relatively low aqueous solubility and moderately strong sorption to soils, however, serve to limit the bioavailability of oil sands naphthenic acids in aquatic environments.40 Recent research41 (14) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505–1511. (15) Headley, J. V.; Peru, K. M.; McMartin, D. W.; Winkler, M. J. AOAC Int. 2002, 85, 182–187. (16) Rudzinski, W. E.; Oehlers, L.; Zhang, Y. Energy Fuels 2002, 16, 1178–1185. (17) Barrow, M. P.; McDonnell, L. A.; Feng, X.; Walker, J.; Derrick, P. J. Anal. Chem. 2003, 75, 860–866. (18) Lo, C. C.; Brownlee, B. G.; Bunce, N. J. Anal. Chem. 2003, 75, 6394–6400. (19) Kim, S.; Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K.; Wenger, L. M.; Mankiewicz, P. Org. Geochem. 2005, 36, 1117–1134. (20) Hemmingsen, P. V.; Kim, S.; Pettersen, H. E.; Rodgers, R. P.; Sjo¨blom, J.; Marshall, A. G. Energy Fuels 2006, 20, 1980–1987. (21) Lo, C. C.; Brownlee, B. G.; Bunce, N. J. Water Res. 2006, 40, 655–664. (22) Headley, J. V.; Peru, K. M.; Barrow, M. P.; Derrick, P. J. Anal. Chem. 2007, 79, 6222–6229. (23) Rostad, C. E.; Hostettler, F. D. EnViron. Forensics 2007, 8, 129– 137. (24) Stanford, L. A.; Kim, S.; Klein, G. C.; Smith, D. F.; Rodgers, R. P.; Marshall, A. G. EnViron. Sci. Technol. 2007, 41, 2696–2702. (25) Teravainen, M. J.; Pakarinen, J. M. H.; Wickstrom, K.; Vainiotalo, P. Energy Fuels 2007, 21, 266–273. (26) Barrow, M. P.; Headley, J. V.; Peru, K. M.; Derrick, P. J. J. Chromatogr. A 2004, 1058, 51–59. (27) Wilm, M.; Mann, M. Anal. Chem. 1996, 68, 1–8. (28) Cyr, T. D.; Strausz, O. P. Org. Geochem. 1984, 7, 127–140. (29) Strausz, O. P. J. Am. Chem. Soc. 1988, 33, 264–268. (30) 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. (31) Lee, L. E. J.; Haberstroh, K.; Dixon, D. G.; Bols, N. C. Proceedings of the 27th Annual Aquatic Toxicity Workshop, St. John’s, Newfoundland, October 1-4, 2000; 91. (32) Rogers, V. V.; Wickstrom, M.; Liber, K.; MacKinnon, M. D. Toxicol. Sci. 2002, 66, 347–355. (33) Leung, S. S.; MacKinnon, M. D.; Smith, R. E. H. Aquat. Toxicol. 2003, 62, 11–26. (34) Clemente, J. S.; Fedorak, P. M. Chemosphere 2005, 60, 585–600. (35) Nero, V.; Farwell, A.; Lee, L. E. J.; Van Meer, T.; MacKinnon, M. D.; Dixon, D. G. Ecotoxicol. EnViron. Safe. 2006, 65, 252–264. (36) Holowenko, F. M.; MacKinnon, M. D.; Fedorak, P. M. Water Res. 2002, 36, 2843–2855. (37) Dokholyan, V. K.; Magomedov, A. K. J. Ichthyol. 1983, 23, 125– 132.

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has demonstrated the presence not only of CnH2n+zO2 species but also further oxidized species (CnH2n+zO3 and CnH2n+zO4) in a sample taken from an Athabasca tailings pond. For the study of complex mixtures, high mass accuracy and high resolution are a prerequisite for full characterization. Increasingly, Fourier transform ion cyclotron resonance (FT-ICR)42-44 mass spectrometers have been applied to such investigations. The ultrahigh mass accuracy and ultrahigh resolution associated with FT-ICR mass spectrometry14,17,25,26,45,46 result in the technique being strongly suited for the application to analysis of complex mixtures, as the many components can be resolved and assignments can be made with a high degree of confidence. By comparison, it is known that mass spectra obtained using low-resolution instrumentation are not sufficient to avoid false positives and misclassification.1,41 Han et al.47 examined complex mixtures within the context of metabolomics, comparing direct infusion of samples prior to analysis by highfield FT-ICR mass spectrometry with LC-MS methods. The authors stated that LC-MS can enhance sensitivity through reduction of suppression effects when using concentrated sample solutions, but they cited disadvantages due to variation between runs and that both the time taken and sample consumed are significantly increased when compared to direct infusion using a high-field FT-ICR mass spectrometer. When coupled with the advantages of electrospray ionization and operated in the negative-ion mode, FT-ICR mass spectrometry is an attractive method for the characterization of naphthenic acids within a petroleum-related sample. Experimental Section The naphthenic acid sample was obtained from a site in the oil sands region of the Athabasca River Basin, Alberta, Canada. The sample concentrate was obtained by extraction of OSPW following procedures described by Rogers et al.39 The extracted naphthenic acid concentrate was made up in acetonitrile to a measured concentration of 8500 mg/L, as determined using methods based upon those employed by Rogers et al.39 and Amrstrong et al.48 This was then used to prepare soluble fractions in different solvents using a ratio of 1 mg of extract to 1 mL of solvent, where it is known that the choice of solvents can influence the range of species observed within a mass spectrum of a complex mixture.22 The solvent systems tested included Milli-Q water/acetonitrile, dichloromethane/acetonitrile, and 1-octanol/acetonitrile. An aliquot of 1% (by volume) ammonia solution (35% by volume in water) was added to each of the solutions to assist deprotonation of the naphthenic acid species. Mass spectra were obtained using a 9.4 T Bruker BioApex II (Bruker Daltonics, Billerica, MA) Fourier transform ion cyclotron (38) MacKinnon, M. D.; Boerger, H. Water Pollut. Res. J. Can. 1986, 21, 496–512. (39) Rogers, V. V.; Liber, K.; MacKinnon, M. D. Chemosphere 2002, 48, 519–527. (40) Providenti, M. A.; Lee, H.; Trevors, J. T. J. Ind. Microbiol. 1993, 12, 379–395. (41) Bataineh, M.; Scott, A. C.; Fedorak, P. M.; Martin, J. W. Anal. Chem. 2006, 78, 8354–8361. (42) Amster, I. J. J. Mass Spectrom. 1996, 31, 1325–1337. (43) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Mass Spectrom. ReV. 1998, 17, 1–35. (44) Barrow, M. P.; Burkitt, W. I.; Derrick, P. J. Analyst 2005, 130, 18–28. (45) Wu, Z. G.; Jernstrom, S.; Hughey, C. A.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2003, 17, 946–953. (46) Schaub, T. M.; Hendrickson, C. L.; Horning, S.; Quinn, J. P.; Senko, M. W.; Marshall, A. G. Anal. Chem. 2008, 80, 3985–3990. (47) Han, J.; Danell, R. M.; Patel, J. R.; Gumerov, D. R.; Scarlett, C. O.; Speir, J. P.; Parker, C. E.; Rusyn, I.; Zeisel, S.; Borchers, C. H. Metabolomics 2008, 4, 128–140. (48) Armstrong, S. A.; Headley, J. V.; Peru, K. M.; Germinda, J. J. J. EnViron. Sci. Health A 2008, 43, 36–42.

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Figure 1. Plot resulting from principal component analysis of repeat mass spectra, obtained using the following solvent systems: Milli-Q water/ acetonitrile, dichloromethane/acetonitrile, and 1-octanol/acetonitrile. The clustering of data points illustrates the repeatability of the experiments and the significance of the effects upon the mass spectra due to the change in choice of solvents. Note the ellipses serve as a guide and do not describe confidence limits.

resonance mass spectrometer,49 which incorporated an Infinity Cell.50 The instrument was operated in the negative-ion mode and the experimental parameters have been described elsewhere.22 The ionization method chosen was nanospray, a variant of electrospray ionization. This method of ionization offers the main advantage of electrospray ionization, whereby fragmentation is minimized. Furthermore, sample consumption and risk of contamination of the ion source are minimized. For the purposes of tuning and externally calibrating the instrument, “ESI Tuning Mix” (Agilent, Palo Alto, CA) and a Kodak naphthenic acid mixture (Kodak Chemicals, Rochester, NY) were used. External calibrations that were based upon these two mixtures were compared and found to be virtually indistinguishable once applied to the data. Once the mass spectra were obtained, the data were processed using XMASS. The relevant signals were categorized according a number of parameters, including mass-tocharge ratio, carbon content, hydrogen deficiency (z), double bond equivalents, Kendrick mass defect, and H/C and O/C ratios. The sorted data were transferred to Aabel 2.4 (Gigawiz Ltd. Co., Tulsa, OK) prior to the creation of a variety of plots for the purposes of data visualization.

During the course of data acquisition, repeat measurements were made for each set of solvent conditions. Principal component analysis (PCA)51,52 was used to examine the reproducibility of the mass spectra, utilizing data that correlated

signal intensities with a range of naphthenic acid species. The results of the principal component analysis are shown in Figure 1, where each data point represents a mass spectrum that was acquired. The data points lie within distinct clusters, according to the solvent system used. This pattern demonstrates that the experiments were repeatable and that the solvents did influence the appearances of the mass spectra. As previously reported by Headley et al.,22 water/acetonitrile was one of the more appropriate solvent systems of a number tested. This solvent combination was chosen for the following work, as it is suitable for a range of oxygen-containing species. Figure 2 shows a typical negative-ion mode broadband mass spectrum of the naphthenic acid mixture. The signals observed correlate with deprotonated species (i.e., negative ions), originating from neutral compounds of the general formula CnH2n+zOx. An alternative mass scale was introduced by Kendrick in 196353 and has been reintroduced in recent years,54 in 1992. More recently, Marshall and co-workers have made use of graphical methods55,56 that utilize the Kendrick mass scale, in order to easily classify families of compound within complex mixtures. The IUPAC mass scale defines 12C as having a mass of exactly 12 Da. Essentially, the Kendrick mass scale uses a mass scale where CH2 is defined as having a mass of 14.000 00 Da, instead of the IUPAC value of 14.015 65 Da. In order to convert an IUPAC mass to the Kendrick mass scale, the following equations are used:

(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. (51) Pearson, K. Philos. Mag. 1901, 2, 559–572. (52) Hotelling, H. J. Educ. Psych. 1933, 24, 417–520.

(53) Kendrick, E. Anal. Chem. 1963, 35, 2146–2154. (54) Hsu, C. S.; Qian, K. N.; Chen, Y. N. C. Anal. Chim. Acta 1992, 264, 79–89. (55) Hughey, C. A.; Hendrickson, C. L.; Rodgers, R. P.; Marshall, A. G.; Qian, K. N. Anal. Chem. 2001, 73, 4676–4681. (56) Wu, Z. G.; Rodgers, R. P.; Marshall, A. G. Anal. Chem. 2004, 76, 2511–2516.

Results and Discussion



Kendrick mass ) CH2 value on Kendrick scale (1) IUPAC mass × CH2 value on IUPAC scale Kendrick mass ) IUPAC mass ×

14.000 00 14.015 65

(2)

The Kendrick mass defect is defined by Kendrick mass defect ) nominal Kendrick mass - exact Kendrick mass (3) where the exact Kendrick mass is the value obtained as outlined above (“Kendrick mass”) and the nominal Kendrick mass is the Kendrick mass of the compound rounded to the nearest integer. For example, an ion of composition C23H43O2- (i.e., a negatively charged C23H44O2 species, with a hydrogen deficiency of z ) -2) would have an IUPAC mass of 351.326 85. Using the Kendrick mass scale, the ion would have a mass of 350.93456 (i.e., the exact Kendrick mass). The nominal Kendrick mass would be 351, and therefore, the Kendrick mass defect is 351 - 350.934 56 ) 0.065 44. Hydrocarbon ions of the same hydrogen deficiency and heteroatom content will have an identical Kendrick mass defect, although their Kendrick masses will be different. If the identity of a data point in a series (i.e., a horizontal, straight line) is known, then adjacent neighbors (i.e., assuming there are no gaps in the series) represent the species with the same hydrogen deficiency and heteroatom content, albeit with one fewer or

additional CH2 group. A plot of this type is therefore good for indicating the range of classes of compounds that are present within a complex mixture as well as providing an indication of the mass range of the ions observed. A plot of Kendrick mass defect versus Kendrick nominal mass for the naphthenic acid sample is presented in Figure 3. A range of CnH2n+zO2, CnH2n+zO3, CnH2n+zO4, and CnH2n+zO5 species was observed. CnH2n+zO and CnH2n+zO6 species were considered in the data analysis but were not detected. The plot also highlights that the majority of the ions were observed between approximately m/z 200 and 380. Usage of the Kendrick mass scale is useful for classification of signals during data analysis and for producing plots of Kendrick mass defect versus nominal Kendrick mass, but disadvantages of such plots, however, are that the relative intensities of the species are not typically indicated and the empirical formula for each data point is not immediately clear. In 1950, van Krevelen introduced a graphical method57 for the classification of potential source rocks according to their products (oil or gas). The usage of such plots was reintroduced and extended by Kim et al.58 for analysis of natural organic matter (NOM) and applications in three-dimensional form.56 During data analysis, once the elemental composition of a compound has been assigned, the molar ratio of the hydrogen content to carbon content (H/C) can be calculated, as can the molar ratio of the oxygen content to carbon content (O/C). A plot of H/C versus O/C is often referred to as a “van Krevelen plot”, and families of compounds can be observed to lie along straight lines, distinguished by hydrogen deficiency and het-

Figure 2. Negative-ion mode, broadband mass spectrum of an Athabasca oil sands sample (dissolved in Milli-Q water/acetonitrile), acquired using an FT-ICR mass spectrometer.

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Figure 3. Plot of Kendrick mass defect versus nominal Kendrick mass for the Athabasca oil sands sample. Species of the same Kendrick mass defect share the same hydrogen deficiency and oxygen content and lie along horizontal lines.

Figure 4. van Krevelen plot for the Athabasca oil sands sample, where the ratios of hydrogen content to carbon content and oxygen content to carbon content are plotted against each other. Species of the same oxygen content and hydrogen deficiency lie along diagonal lines.

eroatom content. Figure 4 is a van Krevelen plot for the naphthenic acid sample. In similarity with the plots based upon the Kendrick mass scale, species of particular hydrogen deficiency and oxygen content are grouped along diagonal lines of differing gradients. The spacing between the data points in a series changes, due to the usage of ratios, rather than fixed step sizes. The advantage of a van Krevelen plot is that classes of compounds lie on straight lines, which can highlight trends and aid compound identification. Most van Krevelen plots do not, however, provide information about the relative intensity (57) van Krevelen, D. W. Fuel 1950, 29, 269–284. (58) Kim, S.; Kramer, R. W.; Hatcher, P. G. Anal. Chem. 2003, 75, 5336–5344.

of the peak associated with each species, although Kim et al.58 also demonstrated the usage of color-coding and 3D plots to address this. Figure 5 shows a series of plots of relative intensity versus carbon content for the peaks correlated with different CnH2n+zOx species detected in the naphthenic acid sample. Usage of these separate graphs, separated according to oxygen content, can be less convenient than use of a single plot and does not convey an immediate overview of the number of species observed as provided by a van Krevelen plot or a plot based upon the Kendrick mass. The line plots, however, do have the advantages of enabling the viewer to correlate an empirical formula with a particular data point and to also identify the relative intensity associated with a given species. The plots show that hydrogen



Figure 5. Plots of relative intensity versus carbon content (each including the range of hydrogen deficiencies) for the Athabasca oil sands sample, separated according to oxygen content. The CnH2n+zO2 and CnH2n+zO4 families are among the most intense species within the Athabasca oil sands sample.

Figure 6. Plot of double bond equivalents versus carbon number for the CnH2n+zO2, CnH2n+zO3, CnH2n+zO4, and CnH2n+zO5 species present within the Athabasca oil sands sample. Where overlap occurs between species of differing oxygen content, a particular DBE, and carbon number, the relative intensities of the species have been summed; the values have also been normalized.

deficiencies of z ) -4, -6, and -12 are the most intense for the O2 family (with maxima at carbon contents of 14, 15, and 17, respectively), z ) -6 and -8 are the most intense for the

O3 family (with maxima at a carbon content of 15 for both), z ) -6 and -8 are the most intense for the O4 family (with maxima at carbon contents of 15 and 16, respectively), and z

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Figure 7. Heat map for the Athabasca oil sands sample. The empirical formulas (categorized by hydrogen content and oxygen content) are plotted against carbon content, and the (nonzero) relative intensity of the peak associated with each species is denoted by color coding. A value of zero is represented by using a white cell.

) -6 and -8 was the most intense for the O5 family (with maxima at a carbon content of 15 for both). While the O2 and O4 families (i.e., of even-numbered oxygen content) were of similar relative intensities, the O3 and O5 families were comparatively less intense. Useful visualization methods may also categorize the data in terms of double bond equivalents (DBE),24,59-62 instead of empirical formulas, as calculated from eq 4 for CcHhNnOoSs: DBE ) c -

n h + +1 2 2

(4)

An advantage of using DBE instead of empirical formulas is that the graphical representation of the sample contents is more convenient for highly complex samples, compared to using multiple plots for families of differing heteroatom contents. A (59) Fu, J.; Klein, G. C.; Smith, D. F.; Kim, S.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Energy Fuels 2006, 20, 1235–1241. (60) Stanford, L. A.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2006, 20, 1664–1673. (61) Panda, S. K.; Andersson, J. T.; Schrader, W. Anal. Bioanal. Chem. 2007, 389, 1329–1339. (62) Purcell, J. M.; Juyal, P.; Kim, D. G.; Rodgers, R. P.; Hendrickson, C. L.; Marshall, A. G. Energy Fuels 2007, 21, 2869–2874.

disadvantage, however, is that information may be lost where different families of compounds overlap in terms of DBE; note that divalent atoms, such as sulfur and oxygen, do not influence the DBE value, for example. The relative intensities of overlapping species may therefore be summed in such instances, and this situation is particularly relevant in the present study of oxygenated species originating from the Athabasca oil sands. An example of a plot of DBE versus carbon number is shown in Figure 6. There is thus a continuing need for new graphical methods for visualizing data of complex data sets, such as methods where empirical formulas and relative intensities can be readily identified, while also clearly comparing trends within families of compounds. One method which potentially offers such progress is the use of “heat maps”. A heat map is a plot where the variables along the two axes form a grid, and the values at each square within the grid are represented through color-coding. Although originally used in fields such as molecular biology, this variety of graph has been adapted here for the display of the different species contained within a complex mixture of petroleum-based origin. Figure 7 is a heat map for the Athabasca oil sands sample, where the vertical axis lists the empirical


formulas of the species (i.e., a combination of the hydrogen deficiency and oxygen content) and the horizontal axis lists the numeric value of the carbon content. In the form shown in Figure 7, a heat map affords the possibility of graphically representing the constitution of a complex mixture, for a number of classes of compounds. For a given data point, an empirical formula can be identified and the relative intensity is indicated by the color coding of the cell. Heat maps may be particularly useful for comparing similar classes of compounds within a particular sample, as trends (the degree of oxidation, for example) could emerge. For the samples investigated, there was a range of species detected that varied according to oxygen content as well as hydrogen deficiency. For example, not only were classical naphthenic acids of the empirical formula CnH2n+zO2, species detected, but other acids were also observed with empirical formula CnH2n+zOx, where x ranged from 2 to 5. Baugh et al. have previously stated that usage of a TAN value, to estimate the molecular weight distribution of naphthenic acids within a petroleum sample, relies upon the assumption that the acids include only one acid group.63 The authors described their discovery of a four-protic acid, when characterizing the “ARN” acid family, which in turn explained why the average molecular weight of the acids was 3-4 times higher than predicted by (63) Baugh, T. D.; Wolf, N. O.; Mediaas, H.; Vindstad, J. E.; Grande, K. Prepr. Am. Chem. Soc., DiV. Pet. Chem. 2004, 49, 274–276.


the TAN alone. Increasingly, it can be seen that the TAN is not sufficient for characterizing the acid content of a petroleum sample. Conclusion A variety of visualization methods are available for data reduction of oil sands acids. No one method is ideal for all circumstances, as each method for presenting the data has associated advantages and disadvantages. The heat map, previously used within fields such as molecular biology, has features well-suited for displaying information about the empirical formulas and relative intensities for different families of compounds within a given sample. Characterization of the acid content for the oil OSPW extracts investigated has revealed the presence of compounds of the empirical formula CnH2n+zOx, where x ) 2-5. The range of oxygen content is consistent with the recent discovery of acid species with oxygen contents higher than 2 and which are not monoprotic. In turn, the presence of such species can partly explain why the total acid number of a petroleum sample is not a reliable method for evaluating its acid content. Acknowledgment. This work was funded in part by the Program of Energy Research and Development. EF800985Z

Data Visualization for the Characterization of Naphthenic Acids within

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