Energy Fuels 2010, 24, 2300–2311 Published on Web 01/11/2010
: DOI:10.1021/ef900949m
Analysis of Naphthenic Acids and Derivatization Agents Using Two-Dimensional Gas Chromatography and Mass Spectrometry: Impact on Flow Assurance Predictions† Andrew G. Shepherd,*,‡ Valentijn van Mispelaar,‡ Jean Nowlin,§ Wim Genuit,‡ and Mark Grutters‡ ‡
Shell Global Solutions International B.V., Post Office Box 38000, 1030 BN Amsterdam, The Netherlands and §Shell Global Solutions US, Inc., Post Office Box 4327, Houston, Texas 77210 Received August 30, 2009. Revised Manuscript Received December 12, 2009
In this work, a sensitivity study was conducted on naphthenic acid derivatization agents. Four silylation chemistries and one methylation chemical were initially evaluated on 10 model naphthenic acids using gas chromatography. An experimental design procedure was setup to look at a number of effects, including contact time, catalyst presence, and reagent concentration. Overall, the silylation agents resulted in higher derivatization yields compared to the methylation agent. Moreover, the silylation agents did not show major evidence of selective derivatization as a function of the naphthenic acid structure. A silylation agent [(N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA)] was then used to test commercial naphthenic acid mixtures with twodimensional gas chromatography coupled to time-of-flight mass spectrometry using the electron-impact source (GCGC-TOFMS). The results enabled identification of many different naphthenic acid species. To test the derivatization agents on realistic samples, naphthenic acid extracts obtained from two crude oils of flow assurance significance were separated with a liquid-phase extraction procedure. The naphthenic acids were then treated with a silylation agent (BSTFA) and a methylation agent (BF3/methanol). The derivatized naphthenic acids together with the non-derivatized naphthenic acids from both crude oils were further examined using medium-resolution time-of-flight mass spectrometry with an electrospray source (TOFMS). Differences were observed in the TOFMS spectra for the naphthenic acid extracts. Extracts that did not contain ARN naphthenic acid species did not show major differences between non-derivatized and derivatized spectra in the negative mode. Extracts that contained ARN did show differences between derivatized and nonderivatized samples in the negative mode. Use of BSTFA resulted in enhanced signals for ARN, particularly the second ionization. Use of BF3/methanol resulted in a poor ARN response compared to the non-derivatized spectra. ARN species were also observed in the positive mode after treatment with both BSTFA and BF3/ methanol, but signals were very poor. Moreover, use of BF3/methanol resulted in poor solubility of the naphthenic acid extracts from the crude oil containing ARN species. No solubility issues were observed with the use of BSTFA. Overall, the results point to the shortcomings of the application of methylation chemicals as derivatization agents, particularly for naphthenic acids extracted from crude oil samples containing highmolecular-weight acids of flow assurance significance (e.g., ARN species).
assurance studies, naphthenic acids mostly play a role in emulsion stability and soap formation.3,4 Naphthenic acids have been separated from their parent sources for analysis using either liquid- or solid-phase extractions. The literature has various examples of different liquid extraction procedures.2-13 A number of authors have
Introduction In the oilfield industry, naphthenic acids have been traditionally defined as any derivative of cyclopentane and cyclohexane homologues from petroleum containing the carboxylic acid group functionality. However, it has become common practice to combine all carboxylic-acid-containing compounds (including, for instance, “fatty acids”) under the nomenclature naphthenic acids. Until as recently as 1955, only two naphthenic acids with as many as 10 carbon atoms had been identified from hydrocarbon sources.1 The first structures to be studied in detail were aliphatic isoprenoid species.2 Since then, a vast amount of work has been carried out on naphthenic acids with the focus on corrosion, toxicity, geochemistry, and flow assurance on hydrocarbon, aqueous, and sediment samples. In flow
(3) Baugh, T. D.; Wolf, N. O.; Mediaas, H.; Vindstad, J. E.; Grande, K. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 2004, 49 (3), 274–276. (4) Brocart, B.; Hurtevent, C. J. Dispersion Sci. Technol. 2008, 29 (10), 1496–1504. (5) Seifert, W. K. Anal. Chem. 1969, 41 (4), 562–568. (6) Seifert, W. K.; Howells, W. G. Anal. Chem. 1969, 41 (4), 554–562. (7) Seifert, W. K.; Gallegos, E. J.; Teeter, R. J. Am. Chem. Soc. 1972, 94, 5880–5887. (8) Kazanis, J. Isolation and structure determination of naphthenic acids. Ph.D. Thesis, Department of Chemistry, University of Nebraska, Lincoln, NE, 1971. (9) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Anal. Chem. 1988, 60, 1318–1323. (10) Tomczyk, N. A.; Winans, R. E.; Shinn, J. H.; Robinson, R. C. Energy Fuels 2001, 15, 1498–1504. (11) Rudzinski, W. E.; Oehlers, L.; Zhang, Y.; Najera, B. Energy Fuels 2002, 16, 1178–1185. (12) Rogers, V. V.; Liber, K.; MacKinnon, M. D. Chemosphere 2002, 48, 519–527. (13) Holowenko, F. M.; MacKinnon, M. D.; Fedorak, P. M. Water Res. 2002, 36, 2843–2855.
† Presented at the 10th International Conference on Petroleum Phase Behavior and Fouling. *To whom correspondence should be addressed: Nederlandse Aardolie Maatschappij B.V., Schepersmaat 2, 9405 TA, Assen, The Netherlands. E-mail:
[email protected]. (1) Brient, J. A.; Wessner, P. J.; Doyle, M. N. Naphthenic acids. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Ed.; John Wiley and Sons: New York, 1995; Vol. 16, pp 1017-1029. (2) Cason, J.; Graham, D. W. Tetrahedron 1965, 21, 471–483.
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: DOI:10.1021/ef900949m
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employed alkaline solutions for the extraction of naphthenic acids.10-16 A disadvantage of this procedure is the possible formation of stable emulsions that may retain naphthenic acid species of interest. Moreover, if the naphthenic acids are complexed in crude oils in the form of salts, they may not be removed by contact with the alkaline solution. This can result in low recovery yields.16,17 In addition, long contact times may be required for efficient extraction.18 Borgund et al.17 have shown that liquid extraction may also result in selective removal of more nonpolar acidic compounds from crude oil samples. Solid-phase extraction procedures have been tested with a wide range of materials, including zeolites, clays, aluminosilicates, silica gel, granulated activated carbon, and ion-exchange resins.19,20 Different ion-exchange resins reported have included polystyrene cross-linked matrixes with tertiary and quaternary amine functional groups,21 Amberlite A-26,22-25 Amberlyst A-27,26 SAX quaternary amine,27,28 amino-propyl silica,11,29 Sephadex A-25 quaternary amine,18,30 polymethacrylates,31 divinylbenzene,32 and potassium hydroxide/silica gel with or without quaternary alkylamonium chloride and styrenedivinylbenzene.33-35 Although alkaline washes are used in certain crude processing facilities for acid content reduction,1 to our knowledge, the use of solid-phase extractions has not been reported on large commercial scales and remains a subject of academic research. However, the very mode of operation of the ionexchange resins may affect the identification of particular naphthenic acid species from crude oils. Ion exchange is a
function of the specific chemical compositions of a matrix, chemical structure of functional groups/ion-exchange capacity, overall charges/pH, and eluent concentration, among others.32,36,37 The adsorption of naphthenic acids by ionexchange resins appears to be strongly dependent upon the polymeric backbone on the functional group and less on the ion-exchange capacity of the resin.21 Hajos and Nagy37 have shown a number of interesting effects of ion-exchange chromatography under the presence of carboxylic groups, among which the preferential elution of monocarboxylate anions as opposed to bicarboxylate anions. From their work, an important conclusion was that increased solute charges lead to increased capacity charges and, thus, retention behavior depends upon the exact position of the carboxyl group. Furthermore, Meredith et al.27 and Qian et al.29 have reported the potential for contamination and/or loss of sample during ion-exchange separations with cartridge materials. In addition, Jones et al.28 reported different recovery yields for ionexchange resins as a function of the precise naphthenic acid structure. Certainly, these issues need to be considered when optimizing the extraction of naphthenic acids from crude oils and selecting the best method for analysis, in particular, with respect to flow assurance applications (where relevant naphthenic acid species may be in the parts per million range). Naphthenic acid characterization has been reported with a wide range of techniques with various degrees of sophistication, such as elemental analysis,3,10,11,18,19,38 vapor pressure osmmometry,3,19,34,38 differential scanning calorimetry,39 fluorescence emission spectroscopy,7 infrared spectroscopy,2,5-7,10-12,34,39-42 ultraviolet spectroscopy,7,10 1H and 13 C nuclear magnetic resonance,2,3,7-11,18,19,34,43 gas chromatography (GC),2,14,18,28,44-46 and high-performance liquid chromatography (HPLC).40,47 GC and HPLC alone have limited use in naphthenic acid characterization because of the large areas of unresolved matter, which are produced by the various co-eluting peaks.48-50 In addition, certain highmolecular-weight acids cannot usually be fully resolved with GC8 even with high-resolution instruments.9 This is a direct
(14) Cooper, J. E.; Bray, E. E. Geochim. Cosmochim. Acta 1963, 27, 1113–1127. (15) Clemente, J. S.; Prasad, N. G. N.; MacKinnon, M. D.; Fedorak, P. M. Chemosphere 2003, 50, 1265–1274. (16) Barth, T.; Hoiland, S.; Fotland, P.; Askvik, K. M.; Pedersen, B. S.; Borgund, A. E. Org. Geochem. 2004, 35, 1513–1525. (17) Borgund, A. E.; Erstad, K.; Barth, T. Energy Fuels 2007, 21, 2816–2826. (18) Saab, J.; Mokbel, A. C.; Razzouk, A. C.; Ainous, N.; Zydowicz, N.; Jose, J. Energy Fuels 2005, 19, 525–531. (19) Acevedo, S.; Escobar, G.; Ranaudo, M. A.; Khazen, J.; Borges, B.; Pereira, J. C.; Mendez, B. Energy Fuels 1999, 13, 333–335. (20) Wong, D. C.; van Compernolle, R.; Nowlin, J. G.; O’Neal, D. L.; Johnson, G. M. Chemosphere 1996, 32, 1669–1679. (21) Gaikar, V. G.; Maiti, D. React. Funct. Polym. 1996, 31, 155–164. (22) Fan, T. P. Energy Fuels 1991, 5, 371–375. (23) Sartori, G.; Savage, C. W.; Ballinger, B. H.; Dalrymple, D. C. Process for extraction of naphthenic acids from crudes. U.S. Patent 6,281,328 B1, 2001. (24) Goldszal, A.; Hurtevent, C.; Rousseau, G. Presented at the 4th International Oilfield Scale Symposium, Aberdeen, U.K., Jan 30-31, 2002; SPE 74661. (25) Laredo, G. C.; Lopez, C. R.; Alvarez, R. E.; Cano, J. L. Fuel 2004, 83 (11), 1689–1695. (26) Campos, M. C. V.; Oliveira, E. C.; Filho, P. J. S.; Piatnicki, C. M. S.; Camarao, E. B. J. Chromatogr., A 2006, 1105 (1-2), 95–105. (27) Meredith, W.; Kelland, S.-J.; Jones, D. M. Org. Geochem. 2000, 31, 1059–1073. (28) Jones, D. M.; Watson, J. S.; Meredith, W.; Chen, M.; Bennett, B. Anal. Chem. 2001, 73, 703–707. (29) Qian, K.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Energy Fuels 2001, 15, 1505–1511. (30) Mediaas, H.; Grande, K.; Hustad, B. M.; Rasch, A.; Rueslatten, H. G.; Vindstad, J. E. Presented at the 5th International Oilfield Scale Symposium, Aberdeen, U.K., Jan 29-30, 2003; SPE 80404. (31) Tanaka, K.; Chikara, H.; Hu, W.; Hasebe, K. J. Chromatogr., A 1999, 850, 187–196. (32) Headley, J. V.; Peru, K. M.; MacMartin, D. W.; Winkler, M. J. AOAC Int. 2002, 85 (1), 182–187. (33) Koike, L.; Rebouc-as, L. M. C.; Reis, A. M.; Marsaioli, A. J.; Richnow, H. H.; Michaelis, W. Org. Geochem. 1992, 18 (6), 851–860. (34) Ovalles, C.; Garcia, M. D. C.; Lujano, E.; Aular, W.; Bermudez, R.; Cotte, E. Fuel 1998, 77 (3), 121–126. (35) Teixeira, A. M. de F.; Dutra, K. C.; Muniz, C. H.; Teixeira, M. A. G. Prepr.-Am. Chem. Soc., Div. Pet. Chem. 2002, 47 (1), 1–3.
(36) Farag, Y.; Nairn, J. G. J. Pharm. Sci. 1988, 77 (10), 872–875. (37) Hajos, P.; Nagy, L. J. Chromatogr., B: Anal. Technol. Biomed. Life Sci. 1998, 717, 27–38. (38) Acevedo, S.; Escobar, G.; Gutierrez, L.; Rivas, H. Fuel 1992, 71, 619–623. (39) Bitsh-Larsen, A. Phase behaviour of naphthenic acid in mixture. M.Sc. Thesis, Department of Chemical Engineering, Technical University of Denmark, Copenhagen, Denmark, 2004. (40) Green, J. B.; Stierwalt, B. K.; Thomson, J. S.; Treese, C. A. Anal. Chem. 1985, 57, 2207–2211. (41) Olsen, S. D. Presented at the 215th National Meeting of the Division of Petroleum Chemistry, Inc., American Chemical Society, Dallas, TX, 1998. (42) Yen, T. W.; Marsh, W. P.; MacKinnon, M. D.; Fedorak, P. M. J. Chromatogr., A 2004, 1033, 83–90. (43) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Energy Fuels 2000, 14, 217–233. (44) Schmitter, J. M.; Arpino, P.; Guiochon, G. J. Chromatogr. 1978, 167, 149–158. (45) Barth, T.; Moen, L. K.; Dyrkorn, C. Presented at the 215th National Meeting of the Division of Petroleum Chemistry, Inc., American Chemical Society, Dallas, TX, 1998. (46) Brient, J. A.; Moyer, R. E.; Freeman, M. H.; Jiang, H. Presented at the 31st Annual Meeting of the International Research Group on Wood Preservation, Kona, HI, 2000. (47) Smith, B. E.; Sutton, P. A.; Lewis, A.; Dunsmore, B.; Fowler, G.; Krane, J.; Lutnaes, B. F.; Brandal, O.; Sj€ oblom, J.; Rowland, S. J. J. Sep. Sci. 2007, 30 (3), 375–380. (48) Jaffe, R.; Albrecht, P.; Oudin, J. L. Geochim. Cosmochim. Acta 1988, 52, 2599–2607. (49) Jaffe, R.; Gallardo, M. T. Org. Geochem. 1993, 20 (7), 973–984. (50) St. John, W. P.; Rughani, J.; Green, S. A.; McGinnis, G. D. J. Chromatogr., A 1998, 807, 241–251.
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result of their polarity and low volatility. Thus, GC and HPLC have preferably been reported in conjunction with mass spectrometry (MS): gas chromatography-mass spectrometry (GC-MS),7,8,13-16,26,28,45,46,48-54 comprehensive twodimensional GC with MS,55 and liquid chromatographymass spectrometry (LC-MS).17,40,43 Nevertheless, also in GC-MS, care has to be taken in proper interpretation of results and assignment of signals to chemical structures.54 Additionally, GC has been run in the high-temperature mode (HTGC), and this has been claimed to improve sensitivity of high-molecular-weight naphthenic acids.47 One of the major challenges in the analysis of naphthenic acids with GC-MS is excessive ion fragmentation, which may be an unwanted artifact when using electron ionization. Another problem is the thermal decomposition of naphthenic acids in GC settings. If the sample containing the naphthenic acids has low volatility, as is the case of certain crude oils, it may be possible to prepare a suitable derivative to aid in molecular ion determination. Derivatization may be carried out using a number of chemicals: urea,14 BF3/methanol,3,6,10,18,27,28,44,55 methanol/HCl,8 N,N-dimethylformamide,40 diazomethane,2,6,16,28,33,45-49,52,53 N-methyl-N-(tertbutyldimethylsilyl)trifluoroacetamide,13,15,26,50,51,54 2-nitrophenylhydrazine,42 and tert-butyldimethylsilylchloride.54 A constant feature of particular derivatization chemistries, as reported in the literature, is the poor recovery of the highmolecular-weight naphthenic acid components. In addition, certain reagents also present health and safety risks, i.e., diazomethane. Jones et al.28 reported distinct derivatization efficiencies as a function of the chemical agent and naphthenic acid structure. The electron-impact source is the predominant ionization source used in combination with GC for naphthenic acid analysis.10,50 For mass spectrometry applications, a number of ionization sources have equally been tested. One of the first was through chemical means with a variety of gases.9,43,56 However, this was reported to lead to excessive fragmentation. Fast ion bombardment, electrospray, and atmospheric pressure chemical ionization are soft ionization sources, which produce less fragmentation, and these have been reported for naphthenic acid studies.12,20-22,24,32,41,57,58 Detailed direct comparisons of different ionization sources are still lacking in the literature, in particular, for flow assurance predictions. The formation of unwanted multimers has been reported with particular crude oil naphthenic acids and soft ionization sources.11,22,43,58 Developments in GC-MS for detailed structural analysis and in soft ionization techniques for more generic naphthenic acid studies have
occurred in parallel. This is most likely because geochemists are most interested in specific (biomarker) acid structures, whereas flow assurance engineers may often look for an optimum bulk naphthenic acid characterization for crude oil types. Over the last few years, many papers have reported the use of high-resolution Fourier transform ion cyclotron mass spectrometry (FT-ICR MS) for the identification of naphthenic acids from crude oils. The technique can be used directly on the crude oil and, therefore, does not require prior naphthenic acid separation or derivatization. Strictly speaking, this would also apply to certain medium-resolution instruments. When used in the negative mode, all acidic species, including the naphthenic acids, may be assigned with accuracy. Different ionization efficiencies however make the development of a truly quantitative method an ongoing research challenge. Most studies report the use of the electrospray and/or nano-electrospray source with these highresolution instruments for naphthenic acid analysis.29,59-64 The work presented here had a number of objectives. In the first instance, a series of derivatization agents was used in a sensitivity study with model naphthenic acids. The derivatization yields of these chemistries were evaluated using GC. The best derivatization agent was then used to study two different commercial naphthenic acid mixtures. We employed twodimensional GC for this purpose. To examine the optimum derivatization agents on realistic samples, stock tank crude oil samples (problematic toward flow assurance issues) were first submitted to naphthenic acid extraction procedures. The naphthenic acid extracts were then examined with a softionization MS source (electrospray) without derivatization. The best and worse derivatization agents previously tested on model naphthenic acids were then also used to treat the naphthenic acid extracts obtained from the crude oil samples. When this sequence of experimental steps was conducted, a good understanding of extraction and derivatization yields together with GC and MS response was achieved. Overall, it is hoped that the present work will point to a better understanding of crude oil naphthenic acid detection and identification and ultimately lead to input for more accurate flow assurance models. In other words, the processing of correct naphthenic acid information will aid in the prediction of, for instance, emulsion formation and/or soap precipitation. Experimental Section Naphthenic Acid Derivatization. The reader is referred to the book by Blau and Halket65 for the rudiments of derivatization theory. For the choice of derivatization agents, we selected silylation and methylation chemistries (Fluka). The compounds used were N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA,
(51) Niemi, B.; St. John, W.; Woodward, B.; DeGroot, R.; McGinnis, G. Presented at the 94th Annual Meeting of the American Wood Preservers Association, Scottsdale, AZ, 1998. (52) Nascimento, L. R.; Rebouc-as, L. M. C.; Koike, L.; Reis, F. A. M.; Soldan, A. L.; Cerqueira, J. R.; Marsaioli, A. J. Org. Geochem. 1999, 30, 1175–1191. (53) Rodrigues, D. C.; Koike, L.; Reis, F. A. M.; Alves, H. P.; Chang, H. K.; Trindade, L. A.; Marsaioli, A. J. Org. Geochem. 2000, 31, 1209– 1222. (54) Clemente, J. S.; Fedorak, P. M. J. Chromatogr., A 2004, 1047, 117–128. (55) Hao, C.; Headley, J. V.; Peru, K. M.; Frank, R.; Yang, P.; Solomon, K. R. J. Chromatogr., A 2005, 1067, 277–284. (56) Roussis, S. G.; Lawlor, L. J. Direct determination of acid distributions in crude and crude fractions. International Patent WO 02/48698 A1, 2002. (57) Hsu, C. S.; Fukuda, E.; Roussis, S. G. Presented at the 215th National Meeting of the Division of Petroleum Chemistry, Inc., American Chemical Society, Dallas, TX, 1998. (58) Lo, C. C.; Lee, B. G.; Bunce, N. J. Anal. Chem. 2003, 75, 6394– 6400.
(59) Miyabayashi, K.; Naito, Y.; Miyake, M.; Tsujimoto, K. Eur. J. Mass Spectrom. 2000, 6, 251–258. (60) Barrow, M. P.; Headley, J. V.; Peru, K. M.; Derrick, P. J. J. Chromatogr., A 2004, 1058, 51–59. (61) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K.; Robbins, W. K. Org. Geochem. 2002, 33, 743–759. (62) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K.; Mankiewicz, P. Org. Geochem. 2004, 35, 863–880. (63) Kim, S.; Stanford, L.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K.; Wenger, L. M.; Mankiewicz, P. Org. Geochem. 2005, 36, 1117–1134. (64) Rodgers, R. P.; Rahimi, O.; Messer, B.; Phillips, T.; Marshall, A. G. Presented at the 61st Annual Corrosion NACE Conference and Exposition, San Diego, CA, 2006. (65) Blau, J.; Halket, J. Handbook of Derivatives for Chromatography, 2nd ed.; John Wiley and Sons: New York, 1993.
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C8H18F3NOSi2, g99%), N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA, C6H12F3NOSi, g98.5%), N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide (MTBSTFA, C9H18F3NOSi, g97%), tert-butyl-dimethylchlorosilane (TBDMSCl, C6H15ClSi, g95%), and boron trifluoride/methanol (∼1.3 M BF3 in methanol). The reaction media investigated for derivatization were cyclohexane (C6H12, Merck, g99%), dichloromethane (CH2Cl2, Merck, g99.9%), dichlorethane (C2H4Cl2, Merck, g99.5%), and pyridine (C5H5N, Riedel-de Ha€en, g99.5%). The catalysts (Fluka) selected for this work were chlorotrimethylsilane (TMCS, C3H9ClSi, 98%) for BSTFA, MSTFA, and MTBSTFA and imidazole (C3H4N2, g99.5%) for TBDMSCl. There was no additional catalyst added to the methylation derivatization agent. Derivatization yields in this work were assessed with a HP5890 series II gas chromatograph (Hewlett-Packard, Waldbronn, Germany) with a HP autosampler 7673. Hotsplit injection was set at 300 °C with splitflow at 50 mL/min at 70 kPa with a split ratio of 1:40. The column used was a DB1 dimethylpolysiloxane type, 20 m 0.25 mm 0.5 μm (J&W Scientific, Folsom, CA). The oven temperature program was 40 °C (1 min) and ramp of 10 °C/min until 350 °C (5 min). A flame ionization detector (FID) was operated at 350 °C. Injection volumes varied between 10 and 40 μL. Experimental design procedures allow for the screening of a large number of exploratory tests, where the overall outcome of a reaction or process cannot be well-predicted.66 An experimental design was carried out in this work to investigate optimum conditions for the derivatization of 10 model naphthenic acids. In this work, the focus was on a number of selected variables: contact time, catalyst presence, and reagent concentration. To limit the number of experiments, temperature was not examined in detail. Thus, for silylation agents, temperatures used were not higher than 110 °C (pyridine boiling point of 115 °C). For the methylation agent, temperatures used were not higher than 60 °C (methanol boiling point of 65 °C). The software used to study the conditions was Design Expert V6. Experimental design results were measured according to the average derivatization yields, according to eqs 1 and 2 peak areaNA derivatization NAyield ¼ 100% ð1Þ peak areatotal average yield ¼ ð
X
NAyield Þ=10
Table 1. Experimental Design Variables experiment
temperature (°C)
contact time (min) catalyst
reagent concentration
1 2 3 4 5 6
100 100 100 100 100 100
Silylation Agents 60 yes 60 yes 15 no 15 yes 37.5 yes 37.5 no
2:1 5:1 2:1 5:1 3.5:1 3.5:1
1 2 3
60 60 60
Methylation Agent 15 yes 37.5 yes 60 yes
5:1 3.5:1 2:1
to recent references for the application of GCGC to systems of interest in petroleum engineering.67,68 However, on the basis of the more recent results by Hao et al.,55 derivatization of naphthenic acids by BF3/methanol followed by GCGC was suggested to be a good analytical alternative for commercial mixtures and oil-sand tailing samples. Moreover, Campos et al.26 showed the potential for analyzing derivatized naphthenic acid extracts from heavy crude oil with GCGC. In the setup used in this work, the derivatized naphthenic acid samples were first injected in a GC column with temperature programming. Fractions are detected with a thermal modulator and refocused to a second capillary column at a prescribed interval (the second GC dimension). Chromatograms are transformed onto a twodimensional grid, to which intensity is added, creating a 3D map, which gives the overall fingerprint, useful when multicomponent mixtures are present. This was performed using software developed in house. The preliminary experiments with GCGC in this work were carried out with a modified HP6890N series gas chromatograph (Hewlett-Packard, Wilmington, DE) with an autosampler 7683B series (Agilent Technologies, Santa Clara, CA). This gas chromatograph was retrofitted with a KT2004 thermal modulator (Zoex Corp., Pasadena, TX) equipped with a secondary oven. The programmed temperature vaporization for the injector was set at 40 °C and then increased at 12 °C/s up to 250 °C. The splitflow was 100 mL/min at 250 kPa. The column used for the firstdimension separation was a DB1 dimethylpolysiloxane type, 10 m 0.25 mm 0.25 μm (J&W Scientific, Folsom, CA). For the second dimension, the column used was a DPTMDS module with 2 m 100 μm fused silica and a 2 m 0.1 mm 0.07 μm BPX50 column (SGE Analytical Science, Ringwood, Victoria, Australia). The oven temperature program was, for the first dimension, 40 °C (5 min) and ramp of 2.5 °C/min up to 300 °C (20 min). For the second dimension, offsets of 50 and 5 °C were used. The FID was operated at 350 °C. The modulation period was 7.5 s, with a hot-pulse duration of 400 ms. The temperature of the hot pulse was running at an offset of 50 °C. The derivatization of naphthenic acids discussed earlier also has the potential to offer unique mass fragments, which can be explored with MS. For instance, the BSTFA naphthenic acid derivatives produce stable fragments corresponding to the [M þ 57]þ ion.26 Therefore, we applied GCGC separation combined with a time-of-flight (TOF) mass spectrometer, using the heartcut method, which is preferred for complex mixtures. The instrument used for this study was equipped with an electronimpact ionization source. The TOF is a medium-resolution mass analyzer, which can generate up to 500 spectra/s. For this work, a GCGC module system comprising a 6890N series gas chromatograph (Agilent Technologies, Santa Clara, CA) coupled to a LECO Pegasus III TOF mass spectrometer (LECO Corp., St. Joseph, MI) was used. Commercial naphthenic acid mixtures
ð2Þ
where NAyield is the percentage of derivatized naphthenic acids, peak areatotal is equal to the sum of areas of non-derivatized and derivatized naphthenic acids, and average yield is the average overall derivatization yield considering 10 different model naphthenic acid compounds used. The preliminary experimental design variables are presented in Table 1 after selection of the optimum reaction media. For silylation agents, six experiments per agent were carried out, as opposed to three experiments per agent for the methylation agent. The selection of model naphthenic acid systems for this work focused on species with different structures, most of which are found in crude oil systems, such as aliphatic, cycloaliphatic, and mono- and bicarboxylic functionalities. In addition, compounds with the carboxylic acid group attached to an alkyl side chain or directly to the naphthenic ring were short-listed. Table 2 presents the list of model compounds selected for use. GCGC and GCGC-TOFMS. Previous work in our laboratories with GCGC67 had made us suspect that crude oil naphthenic acids might be too polar to be analyzed by this technique. In addition, GCGC is preferred for less polar crude oil components, because these do not have the tendency to adsorp onto the stationary phase columns. The reader is referred (66) Montgomery, D. C. Design and Analysis of Experiments, 5th ed.; John Wiley and Sons: New York, 2001. (67) van Mispelaar, V. G.; Smilde, A. K.; de Noord, O. E.; Blomberg, J.; Schoenmakers, P. J. J. Chromatogr., A 2005, 1096, 156–164.
(68) Reddy, C. M.; Nelson, R. K.; Sylva, S. P.; Xu, L.; Peacock, E. A.; Raghuraman, B.; Mullins, O. C. J. Chromatogr., A 2007, 1148, 100–107.
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Table 2. Selected Model Naphthenic Acid Compounds for Derivatization Worka number 1 2 3 4 5 6 7 8 9 10 a
name
molecular mass
basic structural description
hexanoic acid (Sigma, 98%) cyclohexanecarboxylic acid (Fluka, 95%) 4-tert-butylbenzoic acid (Sigma, 98%) cyclohexane pentanoic acid (Sigma, 98%) 5-phenyl valeric acid (Sigma, 99%) 1-naphthoic acid (Fluka, 90%) 1,2,4,5-cyclohexane-tetracarboxylic acid (Sigma, 95%) stearic acid (Sigma, 98%) 5β-cholanic acid (Sigma, 99%) melissic acid (Sigma, 98%)
116.16 128.17 178.23 184.28 178.23 172.18 260.20 284.48 360.57 452.80
aliphatic monocyclic monoaromatic monocyclic monoaromatic biaromatic monocyclic and tetracarboxylic aliphatic tetracyclic aliphatic
All species are monocarboxylic unless stated otherwise.
Figure 1. GC chromatogram for non-derivatized model naphthenic acids. Numbers represent different model naphthenic acid species (Table 2).
dcm solutions were subsequently diluted 30 times in methanol and another 10 times in methanol/ammonia (pH 10) to obtain total acid concentrations of about 33 ppm in the samples infused. The solvent was pumped at a rate of 0.1 mL/min into the electrospray chamber. A sample volume of 10 μL was injected into the solvent stream. Mass spectra were recorded at a rate of 0.9 scans/s, in a mass range of 10-3200 atomic mass units. The total cycle time for analysis was 3 min per sample. For the analysis of the derivatized naphthenic acid extracts in the negative mode, methanol/ammonia was used to obtain total acid concentrations of 1000 ppm in the samples infused. For the analysis of derivatized naphthenic acids in the positive mode, methanol/formic acid (pH 4) was used, with a total acid concentration of 1000 ppm.
Table 3. Naphthenic Acid Data from Liquid-Phase Extraction for Crude Oil Samples crude sample
intake (g)
extract (mg)
acid content (wt %)
KOH equivalent TAN
field 1 field 2
10.72 24.80
200.70 80.61
1.87 0.32
2.2 0.1
from Kodak and Sigma were used in the GCGC-TOFMS experiments. For these tests, we used between 4.0 ( 0.1 mmol of acid/g of derivatization agent. These tests employed BSTFA. Naphthenic acid extracts obtained using liquid extraction procedures applied on crude oils were also tested with BSTFA and BF3/methanol derivatization agents. The main justification for including BF3/methanol was to allow for a comparison because this agent is reported in many studies in the literature. The extracts were obtained using 50 g of crude oil placed in contact with 50 mL of alkaline solution (1% NaOH in 70% ethanol) and shaken vigorously for 1 min. After separation of the alkaline phase, the crude sample is further washed with fresh alkali. After separation of the alkaline phase, a pH adjustment to 2 is carried out with HCl. The naphthenic acids are then back-extracted with cyclohexane, which is evaporated subsequently under flow of dry nitrogen. Our experience with this procedure is that high (e.g., >95%) yields may be obtained, provided that the crude oil sample has not been heavily contaminated with brine. Crude oils used were from fields 1 and 2, both from the North Sea. Both are also problematic toward flow assurance issues. Table 3 contains further details of the extraction procedures applied on both crude oil samples. Medium-resolution TOFMS was employed to examine the neat and derivatized naphthenic acid extracts from the crude oils from both fields. For this purpose, an Agilent G1969A series TOF (Agilent Technologies, Santa Clara, CA) was used, equipped with an electrospray source. For analysis of the neat naphthenic acid extracts in the negative mode, the samples were diluted 100 times to 1.0 wt % in dichloromethane (dcm). The
Results and Discussion Of the four different derivatization reaction media selected, cyclohexane, dichloromethane, and dichloroethane were not suitable. This was due to the poor solubility of the extracts in these media. Therefore, pyridine was selected for the remaining sensitivity experiments. Figures 1 and 2 present the GC chromatogram of the non-derivatized and derivatized model naphthenic acid species described in Table 2. It can be observed from Figures 1 and 2 that the response factors (peak heights) for the derivatized model naphthenic acids are greater than for the non-derivatized model naphthenic acids, even for the worse derivatization agent. The chromatogram in Figure 1 shows significant tailing. This is most likely due to the high polarity of the carboxylic acid functionalities. The use of silylation agents results in the GC signals eluting later than non-derivatized naphthenic acids. The use of the methylation agents resulted in GC signals eluting earlier than non-derivatized naphthenic acids. 2304
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Figure 2. GC chromatogram for derivatized model naphthenic acids. Captions represent the different derivatization agents used. Numbers represent different model naphthenic acid species (Table 2). Table 4. Average Yield Percent for Derivatization Agents Applied To Model Naphthenic Acidsa experiment 1 2 3 4 5 6 a
BSTFA (average yield percent)
MSTFA (average yield percent)
MTBSTFA (average yield percent)
TBDMSCl (average yield percent)
BF3/MeOH (average yield percent)
11.32 99.89 99.87 99.51 87.80 99.86
11.29 88.00 12.21 8.01 8.98 59.09
20.39 99.84 79.12 99.87 84.11 88.63
14.85 69.39 14.33 86.71 23.16 84.68
2.58 1.97 2.20 na na na
na = not applicable. Refer for Table 1 for experimental details.
active H atom. This group has 14.03 atomic mass units. The addition of groups with larger atomic mass units may be the reason why derivatization yields for the silyl groups are vastly improved. However, some publications report good yields when diazomethane is used as a derivatization agent, which also adds methyl to the naphthenic acids.25 Therefore, this effect could also be related to solubility. Overall, in the experiments discussed here, BSTFA and MTBSTFA showed the best results. Further fine tuning of the derivatization conditions for these chemistries is shown in Table 5. No yield gains were obtained from the use of catalysts, temperature, contact time, or reagent concentration. We thus selected 60 °C for both agents to minimize safety risks (as opposed to higher temperatures, e.g., 100 °C). Figure 3 presents the GCGC chromatogram of the derivatized model naphthenic acid compounds used in this work analyzed under the conditions described in Table 5. Clearly, the compounds present different retention properties using the setup described earlier (as a function of the structure), which highlights the potential of the technique and procedure. We therefore decided to apply this technique to more complex naphthenic acid mixtures.
Figure 2 presents evidence of selective derivatization as a function of different naphthenic acid structures used. The use of BF3/methanol as a derivatization agent resulted in a chromatogram with less signals for the more polar melissic acid (10) and 5β-cholanic acid (9) as well as 1-naphthoic acid (6). Note that trace evidence of only the tetracarboxylic acid (7) could be observed in the chromatogram for the derivatized samples with BF3/methanol. The chromatograms obtained with the use of the other agents do not appear to show much evidence of selectivity. The combined results of the experimental design in terms of average yield percent are shown in Table 4. It can be observed from Table 4 that the four silylation agents are much more effective than the BF3/methanol derivatization agent, because of the higher derivatization yields. However, lower yields are observed when a lower reagent concentration is used (experiment 1). With BSTFA and MSTFA, active H atoms in the naphthenic acids, which are derivatized, are replaced by a trimethylsilyl group with 72.18 atomic mass units. With MTBSTFA and TBDMSCl, the active H atoms in the naphthenic acids are replaced by a tert-butyldimethylsilyl group with 114.26 atomic mass units. With the use of the BF3/methanol, a methyl group replaces the 2305
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The next figures discuss the results for the GCGCTOFMS of the derivatized commercial naphthenic acid mixtures with BSTFA. BTSFA was chosen over MTBSTFA because it forms “lighter” derivatives, which elute earlier in
the GCGC chromatogram, as shown in Figure 3. Figures 4 and 5 present the result for the Sigma commercial naphthenic acid mixture treated with BTSFA. The color schemes for the figures are blue as the base color through green and yellow to red. The sample mainly contains aliphatic acids with small traces of cyclics. Note that in Figure 4 a strong overlap between the aliphatic and monocyclic species can be observed. This is particularly noticeable for naphthenic acids with carbon numbers between 9 and 12. Nevertheless, a better separation was possible without the 50 °C offset, and this is presented in Figure 5.
Table 5. Optimum Derivatization Conditions for BSTFA and MTBSTFA Agents agent
temperature (°C)
contact time (min)
reagent media
average yield percent
BSTFA MTBSTFA
60 60
15 15
2:1 5:1
99.9 99.9
Figure 3. GCGC chromatogram for derivatized model naphthenic acids. Overlays of BSTFA (darker signals) and MTBSTFA (green).
Figure 4. GCGC-TOFMS chromatogram for the Sigma commercial naphthenic acid mixture derivatized with BTSFA, with 50 °C offset.
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Figure 5. GCGC-TOFMS chromatogram for the Sigma commercial naphthenic acid mixture derivatized with BTSFA, without 50 °C offset.
Figure 6. GCGC-TOFMS chromatogram for the Kodak commercial naphthenic acid mixture derivatized with BTSFA, without 50 °C offset.
though BF3/methanol presented the worst results for our model naphthenic acid work, we included this agent in our tests because it is one of the most common chemistries employed in the literature for derivatization. Moreover, we aimed for a comparison of two derivatization chemistries with the same crude oil naphthenic acids. First, we applied the BSTFA derivatization agents directly on aliquots of crude oils from fields 1 and 2. The results of these tests were not satisfactory because of poor chromatogram signals and are thus not presented here. This motivated work on the naphthenic acid extracts obtained from the parent crude oil samples, as per the procedure described previously. Figure 7 presents our first attempt at GCGCTOFMS for a derivatized naphthenic acid extract from field 1
Figure 6 presents the GCGC-TOFMS for the Kodak commercial naphthenic acid mixture. The composition of this sample appears to be very different from that of the Sigma sample. It can be observed that there is a lot of overlap between the aliphatic and monocyclic components as well as between the aromatic and bicyclic components. In principle, however, the technique described in this paper could be used for more complex systems, but identification of particular species may be difficult. This would require more work on greater number model systems. We extended the experimental derivatization and analysis procedures from commercial naphthenic acid systems to naphthenic acid samples of interest to flow assurance. For this work, we used BSTFA as well as BF3/methanol. Even 2307
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Figure 7. GCGC-TOFMS chromatogram for naphthenic acid extract from field 1 crude oil, BSTFA derivatized, with 50 °C offset.
Figure 8. GCGC-TOFMS chromatogram for naphthenic acid extract from field 1 crude oil, BSTFA derivatized (two combined naphthenic acid extracts), with 50 °C offset.
crude oil using BSTFA with 50 °C offset. The naphthenic acid extract, obtained with liquid extraction, was fully dissolved in the pyridine medium. Although traces of derivatized material can clearly be observed, further optimization was necessary because the overall concentrations of naphthenic acids was low, which resulted in a poor signal. The extracted ion of the silyl fragment for a more concentrated naphthenic acid extract from the same field 1 crude oil is presented in Figure 8. Two naphthenic acid extract duplicates were combined for this experiment and used throughout subsequent testing. It can be observed that identification of aliphatics, monocyclics, and bicyclics was much improved. In Figure 8, a few naphthenic acid species are annotated (e.g., C16 and C18). There were however a number of peaks in the light part of the sample, which were difficult to identify. The GCGC-TOFMS for the derivatized naphthenic acid extracts for field 2 crude oil samples are discussed next. Prior
to analysis, the most striking observation was with regard to the impact of solubility of the methylated naphthenic acid material for the different crude oils. This is shown in Figure 9. After methylation, water and cyclohexane are added to deactivate the BF3 and dissolve the methylated acids. For field 2 crude oil, even after heating the sample to 75 °C, not all of the methylated material was dissolved. In contrast, the methylated material for field 1 crude oil was easy to dissolve. Moreover, the same derivatized naphthenic acid extracts with pyridine/BSTFA were very easy to dissolve for both crude oils from fields 1 and 2. The results suggest that the combination of BF3/methanol derivatization agents might not be suitable for the use of the particular crude oil naphthenic acid analysis. We speculate that the reasons behind the solubility effects are the exact composition of the naphthenic acids present. Note that the two crude oils used in the present work are problematic with regard to flow assurance issues. Any data 2308
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generated by the GCGC-TOFMS and used as input for prediction models would necessarily be inaccurate, given the solubility issues discussed above with BF3/methanol. Figures 10 and 11 present the GCGC-TOFMS for the two derivatization products from the naphthenic acid extracts
from field 2 crude oil. The chromatogram generated by BSTFA is much more complex than that of the BF3/methanol product. This could suggest poor derivatization yields for the BF3/methanol agent toward the naphthenic acid extract. However, speciation of the naphthenic acids is difficult because the internal libraries do not support many of the detected components. Hence, we have not attempted to assign many components for these chromatograms. The next discussion relates to the analysis of the naphthenic acid extracts from the crude oil samples from fields 1 and 2 using medium-resolution TOFMS with an electrospray source in the negative mode. Figure 12 presents the spectra for field 1 naphthenic acid extracts, derivatized with BSTFA and BF3/methanol and compared to the neat non-derivatized sample. It can be observed that there is an increase in the naphthenic acid signals with derivatization. The spectra however look quite similar.
Figure 9. Naphthenic acid extracts from field 1 crude oil (a, pyridine/BSTFA-derivatized sample; b, BF3/methanol-derivatized sample) and field 2 crude oil (c, pyridine/BSTFA-derivatized sample; d, BF3/methanol-derivatized sample).
Figure 10. GCGC-TOFMS chromatogram for naphthenic acid extract from field 2 crude oil, BSTFA derivatized, with 50 °C offset.
Figure 11. GCGC-TOFMS chromatogram for naphthenic acid extract from field 2 crude oil, BF3/methanol derivatized, with 50 °C offset.
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Figure 15. Negative-mode electrospray TOFMS mass spectrum for naphthenic acid extract from field 2 crude oil, BF3/methanol derivatized.
Figure 12. Negative-mode electrospray TOFMS mass spectra for naphthenic acid extract from field 1 crude oil. A comparison of a neat non-derivatized sample with BSTFA- and BF3/methanolderivatized samples.
Figure 16. Positive-mode electrospray TOFMS mass spectrum for naphthenic acid extract from field 2 crude oil, BSTFA derivatized. Figure 13. Negative-mode electrospray TOFMS mass spectrum for naphthenic acid extract from field 2 crude oil, non-derivatized.
Figure 17. Positive-mode electrospray TOFMS mass spectrum for naphthenic acid extract from field 2 crude oil, BF3/methanol derivatized. Figure 14. Negative-mode electrospray TOFMS mass spectrum for naphthenic acid extract from field 2 crude oil, BSTFA derivatized.
tion. In the methylated sample (Figure 15), a singly charged, triply methylated ion at m/z 1276 and doubly charged, doubly methylated ion at m/z 631 are observed. This suggests that the ARN acids have also undergone derivatization. Thus, the presence of ARN in the naphthenic acid extract is leading to different derivatization yields. Note that even though ARN is being derivatized with the methyl agent, solubility issues showed in Figure 9 would discourage the use of this procedure for analysis. Because the presence of ARN in crude oils is used as an input in flow assurance models, data generated from Figure 15 must be viewed with reservations. Figures 16 and 17 present the positive-mode spectra of the silylated and the methylated derivatized naphthenic acids, respectively, for the field 2 crude oil sample. In the silylated sample in positive mode, we see some prominent ARN-related peaks at m/z 1256 and 1290. The first mass may correspond to the sodiated ARN acid; the m/z 1290 peak is unknown. Silylated ARN however is not observed. If all of the polar acid groups are shielded by silyl groups, the molecule may not be amenable to either positive or negative electrospray ionization. In the methylated sample, a base peak at m/z 1346 is
Figure 13 presents the spectrum for the neat non-derivatized naphthenic acid extracts from field 2 crude oil. In this spectrum, singly and doubly charged ions at m/z 1234 and 617, respectively, are observed. These have been assigned to tetracarboxylic ARN naphthenic acid species associated with flow assurance soap deposition problems.3 Hence, these represent the first and second ionization of the ARN species. Note that we are representing the m/z data in terms of their normalized signal values (relative percent). Figures 14 and 15 present the negative-mode spectra of the silylated and methylated derivatized naphthenic acids from field 2 crude, respectively, using TOFMS. Derivatization of the naphthenic acid extracts resulted in only a slight improvement of the overall signal of ARN species in the negative mode for the first ionization. However, use of BSTFA improved detection of the second ionization of the ARN species in the negative mode. Results for the BSTFA sample suggest that at least some of the ARN acids have not undergone derivatiza2310
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that selective derivatization occurred with this agent as a function of the naphthenic acid structure. When naphthenic acid extracts were examined by TOFMS, it was shown that the results were crude-oil-specific. Field 1 crude oil results did not show major differences in mass spectra in the negative mode for the derivatized and nonderivatized naphthenic acid extracts. With field 2 crude oil naphthenic acid extracts, differences were noted between derivatized and non-derivatized samples. Use of BSTFA resulted in an improved signal for the second ionization of ARN naphthenic acid species in the negative mode. Use of BF3/methanol resulted in poor signals for ARN in the negative mode. Solubility issues identified with the use of BF3/ methanol discourage the use of this derivatization agent for naphthenic acid extracts containing ARN. Identification of ARN was also possible in the positive mode for both BSTFA and BF3/methanol, but the signals were poor. Naphthenic acid distributions obtained from BF3/methanol treatment should therefore be used with caution when attempting to predict/model flow assurance processes. Future work should examine the quantification of naphthenic acids in crude oil samples given the procedures and limitations described in this paper.
observed. Some of the peaks observed in the range of 900-2000 atomic mass units are due to siloxane contamination, but those around m/z 1310, 1318, 1332, 1346, and 1360 could be assigned to ARN. Methylation of all four ARN acid groups should have resulted in a total molar mass of approximately 1292 atomic mass units. At this moment, therefore, it can only be said that ARN-related compounds have followed some other derivatization pathway, resulting in a good response in positive electrospray. Note, however, that according to Figure 9, not all of the methylated naphthenic acid extract was soluble prior to the MS work. Thus, the results in Figure 17 must be regarded with reservations. Conclusions In this work, we presented the results of derivatization studies with model naphthenic acids, commercial naphthenic acid mixtures, and naphthenic acids extracted from crude oils of flow assurance significance. Overall, the results in this work, given by GCGC and GCGC-TOFMS suggested that the best derivatization agents for naphthenic acids were the silylation chemicals, namely, BSTFA and MTBSTFA. Derivatization yields for the BF3/methanol agent were low. Moreover, it was observed
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