Energy Fuels 2009, 23, 5544–5549 Published on Web 09/23/2009
: DOI:10.1021/ef900594d
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Characterization of Naphthenic Acid Singly Charged Noncovalent Dimers and Their Dependence on the Accumulation Time within a Hexapole in Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Raffaello Da Campo,*,† Mark P. Barrow,† Andrew G. Shepherd,‡ Malcolm Salisbury,§ and Peter J. Derrick
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† Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, United Kingdom, ‡Shell Global Solutions, Post Office Box 38000, 1030 BN Amsterdam, The Netherlands, §Shell Global Solutions, Post Office Box 1, Chester CH1 3SH, United Kingdom, and Institute of Fundamental Sciences, Massey University, Palmerston North 4442, New Zealand
Received June 10, 2009. Revised Manuscript Received September 4, 2009
Naphthenic acids are believed to be responsible for a number of unwanted phenomena occurring during the processing and transport of crude oil, such as pipeline corrosion and precipitation of calcium salts. In this paper, Fourier transform ion cyclotron resonance mass spectrometry is used to analyze a mixture of naphthenic acids. Naphthenic acids have been shown to form multimers, and the study of multimer association could lead to a better understanding of naphthenic acid phase behavior in crude oil production systems. The dependence of the signal intensity of such aggregates on the accumulation time within the ion source hexapole has been studied, and it has been highlighted that such a dependence suggests a noncovalent interaction as the primary cause for aggregation. This would account for the decrease in signal intensity with accumulation time as a result of the increasing chance of undergoing collisional dissociation. The nature, role and behaviour of naphthenic acid dimers may be better understood by the application of mass spectrometry and this has potential to be applied to samples of importance to the oil industry.
authors. Asphaltenes represent the solubility fraction of crude oils which are insoluble in alkane solvents such as pentane or heptane. It has been demonstrated that, even in diluted solutions of crude oils and in different solvents, asphaltenes give rise to aggregates that have been detected experimentally via mass spectrometry,2,6 absorption and emission fluorescence studies,7-9 nuclear magnetic resonance (NMR),10 and X-ray techniques.11 Goncalves et al.9 have found that asphaltenes form multimers at concentrations as low as 50 mg/L when dissolved in toluene. Recently, Qian et al.12 highlighted the presence of dimers and trimers of acids within a petroleum distillate. In their electrospray ionization-mass spectrometry (ESI-MS) experimental setup, it was found that the cone and
Introduction The oil industry is increasingly interested in the study and characterization of naphthenic acids (NAs). This class of compounds is believed to play a major role in the corrosion of pipelines and other infrastructures that are used to transport, store, and process crude oil. Their corrosivity is dependent upon size and structure,1,2 and their impact on the environment is of great importance.3-5 NAs were initially defined as non-aromatic carboxylic acids that could be found in crude oil. A classification method for different classes of such molecules is based on the pairs of hydrogen atoms that would be needed to form a fully saturated structure. In this way, NAs can be defined by the formula CnH2nþZO2, where Z is a negative, even number and is referred to as hydrogen deficiency. A tendency toward self-aggregation of the heaviest crude oil fractions, such as asphaltenes, has been highlighted by several
(6) Smith, D. F.; Schaub, T. M.; Rahimi, P.; Teclemariam, A.; Rodgers, R. P.; Marshall, A. G. Self-association of organic acids in petroleum and Canadian bitumen characterized by low- and highresolution mass spectrometry. Energy Fuels 2007, 21 (3), 1309–1316. (7) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. Assembly of asphaltene molecular aggregates as studied by near-UV/visible spectroscopy;I. Structure of the absorbance spectrum. J. Pet. Sci. Eng. 2003, 37 (3-4), 135–143. (8) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. Assembly of asphaltene molecular aggregates as studied by near-UV/visible spectroscopy;II. Concentration dependencies of absorptivities. J. Pet. Sci. Eng. 2003, 37 (3-4), 145–152. (9) Goncalves, S.; Castillo, J.; Fernandez, A.; Hung, J. Absorbance and fluorescence spectroscopy on the aggregation behavior of asphaltene-toluene solutions. Fuel 2004, 83 (13), 1823–1828. (10) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. Initial stages of asphaltene aggregation in dilute crude oil solutions: Studies of viscosity and NMR relaxation. Fuel 2003, 82 (7), 817–823. (11) Tanaka, R.; Sato, E.; Hunt, J. E.; Winans, R. E.; Sato, S.; Takanohashi, T. Characterization of asphaltene aggregates using X-ray diffraction and small-angle X-ray scattering. Energy Fuels 2004, 18 (4), 1118–1125. (12) Qian, K.; Edwards, K. E.; Dechert, G. J.; Jaffe, S. B.; Green, L. A.; Olmstead, W. N. Measurement of total acid number (TAN) and TAN boiling point distribution in petroleum products by electrospray ionization mass spectrometry. Anal. Chem. 2008, 80 (3), 849–855.
*To whom correspondence should be addressed. E-mail: r.da-campo@ warwick.ac.uk. (1) Qu, D. R.; Zheng, Y. G.; Jang, X.; Ke, W. Correlation between the corrosivity of naphthenic acids and their chemical structures. AntiCorros. Methods Mater. 2007, 54 (4), 211–218. (2) Hsu, C. S.; Dechert, G. J.; Robbins, W. K.; Fukuda, E. K. Naphthenic acids in crude oils characterized by mass spectrometry. Energy Fuels 2000, 14 (1), 217–223. (3) Barrow, M. P.; Headley, J. V.; Peru, K. M.; Derrick, P. J. Fourier transform ion cyclotron resonance mass spectrometry of principal components in oilsands naphthenic acids. J. Chromatogr., A 2004, 1058 (1-2), 51–59. (4) Bataineh, M.; Scott, A. C.; Fedorak, P. M.; Martin, J. W. Capillary HPLC/QTOF-MS for characterizing complex naphthenic acid mixtures and their microbial transformation. Anal. Chem. 2006, 78 (24), 8354–8361. (5) McMartin, D. W.; Headley, J. V.; Friesen, D. A.; Peru, K. M.; Gillies, J. A. Photolysis of naphthenic acids in natural surface water. J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng. 2004, 39 (6), 1361–1383. r 2009 American Chemical Society
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extraction voltages were inversely proportional to the intensity of the multimer signals. This has been explained by invoking the occurrence of collision-induced dissociation (CID). In fact, for higher voltages, the chances of undergoing CID are enhanced because of a higher average kinetic energy of the ions; this phenomenon is also known as in-source CID. In recent years, one of the main techniques used for studying the complex nature of crude oils has been Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), because of its ultra-high resolving power and mass accuracy.3,13-16 This paper focuses on NAs that are soluble in methanol. FT-ICR MS studies highlighted the presence of species fitting the general formula CnH2nþZO4, where Z represents the sum of the hydrogen deficiencies of the monomers that constitute a specific dimer. The study of multimer association would shed further light on the general properties of such compounds and ultimately lead to a better understanding of NA phase behavior in crude oil production systems. Noncovalent interactions are believed to be the cause of the presence of the CnH2nþZO4 species. The general CnH2nþZO4 formula can describe noncovalent dimers formed by the aggregation of monocarboxylic NAs. The nature of the noncovalent interactions, which are essential to the existence of these clusters, is such that their lifetime is affected by the accumulation time within the hexapole ion trap, where ions must be collected within the ion source, prior to excitation and detection within the FT-ICR cell. This behavior has been attributed to the fragmentation of CnH2nþZO4 species. A longer residence time inside the hexapole would cause an increase in the number of collisions that any ion undergoes while moving inside the trap. Given the weak nature of the bond between noncovalent aggregates, this would result in a fragmentation process that would cause the aggregates to dissociate into their monomeric components. Additionally, Scott et al.17 produced evidence that NA dimers can exist in aqueous solutions via Fourier transform infrared spectroscopy studies; in their paper, they highlight perturbation that causes a shift of the CO stretch resonant frequency for dimers, indicating a direct involvement of the carboxylic group in the formation of such species. Such an observation would suggest a qualitative agreement with the findings presented here.
a nitrogen flow atmosphere. The number of scans was 200 for both sample and baseline acquisitions. Mass spectra were acquired with a BioApex II (Bruker Daltonics, Billerica, MA) 9.4 T FT-ICR mass spectrometer equipped with an Infinity Cell.18 Ions were generated with a nanospray ion source based on the existing electrospray ion source by Analytica of Branford (Branford, CT). Metal-coated nanospray needles (Proxeon, Denmark) were used to spray sample solutions (typically 10 μL). The chosen propelling gas was CO2, and the backing pressure was kept at approximately 12 psi. The needles were loaded with the sample, and then the propelling gas backing line was connected and raised to the required pressure; subsequently, the needles were also earthed. The needle tip was then opened by tapping it against the end cap covering the Pyrex capillary to allow the sample solution to flow. At this stage, a microscope was employed to ensure that the sample solution was flowing properly and that the needle was correctly positioned with respect to the capillary front end. A potential of typically 300 V was then applied to the capillary front end to start the nanospray ions formation, and the back end potential for the capillary was circa -102 V. Once generated, the ions were transferred through the Pyrex capillary and a skimmer (-4.0 V) to the hexapole; here, they were accumulated for a variable period of time (D1). After the accumulation time, ions were extracted and conveyed to the ICR cell for a period of 2500 μs by employing electrostatic ion optics. The ICR cell-trapping plates were each maintained at a potential of -1.5 V. The excitation and detection range was between m/z 130 and 1000 for all of the acquired spectra. The Kodak NA mix was diluted in MS-grade methanol (Sigma-Aldrich, St. Louis, MO); such a solvent was chosen for its capability of dissolving the NA mix and for its high suitability in a nanospray source. The solution was treated with a solution of ammonia (35%), obtaining a final concentration of 1% of ammonia by volume. Such a procedure assists the deprotonation of the acidic species, facilitating the ionization in negative-mode nanospray. A similar preparation method was adopted for solid stearic acid and β-cholanic acid samples (both from SigmaAldrich, St. Louis, MO). They were dissolved in methanol, and the solution was treated as above. The instrument control, data acquisition, and processing were performed with a Silicon Graphics Indy workstation running XMASS 5.0.6 (Bruker Daltonics, Billerica, MA) under IRIX 5.3. Data files consisted of 512K (524288) data points and represent typically 64 scans. The peak list was generated by exporting the list of all peaks picked within the mass spectrum, to create a file that was then analyzed with a custom-designed routine to assign the species of interest. For clarity, isotopomers, such as contributions from 13C isotopes, are not shown in the plots. Nevertheless, such species were taken into account during data analysis, and the spacing between isotopomers was indicative of the charge state of each species detected. All of the broad-band acquired spectra were internally calibrated using peaks associated with abundant NAs spread over the m/z range of interest. Data were further processed with Origin 7.1 (by OriginLab), and the same software was used to plot the results in graphical form.
Experimental Section Infrared spectroscopy was performed using a JASCO FT/ IR-470 plus. A Kodak NA mix (The Eastman Kodak Company, Rochester, NY) was analyzed directly, without dilution in organic solvents. The sample Fourier transform infrared (FTIR) spectrum was acquired after having corrected for the baseline in (13) Amster, I. J. Fourier transform mass spectrometry. J. Mass Spectrom. 1996, 31 (12), 1325–1337. (14) Barrow, M. P.; McDonnell, L. A.; Feng, X. D.; Walker, J.; Derrick, P. J. Determination of the nature of naphthenic acids present in crude oils using nanospray Fourier transform ion cyclotron resonance mass spectrometry: The continued battle against corrosion. Anal. Chem. 2003, 75 (4), 860–866. (15) Brandal, O.; Hanneseth, A. M.; Hemmingsen, P. V.; Sjoblom, J.; Kim, S.; Rodgers, R. P.; Marshall, A. G. Isolation and characterization of naphthenic acids from a metal naphthenate deposit: Molecular properties at oil-water and air-water interfaces. J. Dispersion Sci. Technol. 2006, 27 (3), 295–305. (16) Marshall, A. G.; Hendrickson, C. L.; Jackson, G. S. Fourier transform ion cyclotron resonance mass spectrometry: A primer. Mass Spectrom. Rev. 1998, 17 (1), 1–35. (17) Scott, A. C.; Young, R. F.; Fedorak, P. M. Comparison of GC-MS and FTIR methods for quantifying naphthenic acids in water samples. Chemosphere 2008, 73 (8), 1258–1264.
Results and Discussion The Kodak NA mix was analyzed using FTIR spectroscopy, without prior dilution in organic solvents. It is known that monomeric NAs can be identified by absorption within the region of 1740-1750 cm-1 and that the dimeric form can be identified by absorption at approximately (18) Caravatti, P.; Allemann, M. The Infinity Cell;A new trapped ion cell with radiofrequency covered trapping electrodes for Fourier transform ion cyclotron resonance mass spectrometry. Org. Mass Spectrom. 1991, 26 (5), 514–518.
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Figure 1. Enlarged region of an FTIR spectrum of the Kodak NA mixture. The region shows the bands associated with stretching of the carbonyl group, where the peak at 1703 cm-1 is associated with the presence of the dimeric species and the peak at 1742 cm-1 is associated with the monomeric species.
Figure 3. FT-ICR mass spectra of a 0.25 mg/mL NA Kodak mix in methanol at different accumulation times.
Figure 2. FT-ICR mass spectrum of a 0.25 mg/mL NA Kodak mix in methanol at an accumulation time of 0.14 s.
1700-1715 cm-1.19,20 The relevant infrared spectrum is shown in Figure 1, and it can be seen that the predominant peak correlated with the presence of NA dimers, while a significantly less intense peak corresponded with the monomeric NA structures. This clearly illustrates that the compounds within the Kodak mixture formed dimers in the solution phase at a sufficiently high concentration. In addition to the spectroscopic investigations of NAs in the presence of organic solvents by Rogers et al. and Yen et al., work by Saab et al.21 has shown intense absorption at the (19) Rogers, V. V.; Liber, K.; MacKinnon, M. D. Isolation and characterization of naphthenic acids from Athabasca oil sands tailings pond water. Chemosphere 2002, 48 (5), 519–527. (20) Yen, T. W.; Marsh, W. P.; MacKinnon, M. D.; Fedorak, P. M. Measuring naphthenic acids concentrations in aqueous environmental samples by liquid chromatography. J. Chromatogr., A 2004, 1033 (1), 83–90. (21) Saab, J.; Mokbel, I.; Razzouk, A. C.; Ainous, N.; Zydowicz, N.; Jose, J. Quantitative extraction procedure of naphthenic acids contained in crude oils. Characterization with different spectroscopic methods. Energy Fuels 2005, 19 (2), 525–531.
Figure 4. FT-ICR mass spectra of (a) stearic acid, (b) β-cholanic acid, and (c) a mixture of stearic and β-cholanic acids. Different dimers are labeled for ease of comparison.
relevant wavenumber for NA dimers in the acidic fraction of a crude oil. This would indicate that NA dimers, similar to those studied here, may be expected to be present natively in crude oils and/or related fractions. 5546
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Figure 6. Kendrick plots produced from the spectra acquired with an accumulation time of (a) 0.14 s and (b) 0.20 s. Scheme 3. Possible Structure for the Singly Charged NA Aggregates That Involve the Sharing of a Proton Figure 5. Relative intensity of singly charged heterodimer (stearic acid in combination with formic acid). The dimer signal intensity is inversely proportional to the accumulation time. Scheme 1. Structure for Stearic Acid
the Experimental Section). It has been found that an increase of the accumulation time leads to a decrease of the relative intensities of such species, as shown in Figure 3. This behavior can be explained by assuming that such molecular species are dimeric noncovalent aggregates of NAs. In this case, a longer accumulation time inside the hexapole ion trap enhances the chances of undergoing collisions that cause the fragmentation of these aggregates.22,23 The formation of dimers for organic acids has been investigated also by running samples of stearic acid (shown in Scheme 1), β-cholanic acid (shown in Scheme 2), and a mixture of the two. Figure 4a shows a mass spectrum for stearic acid, corresponding to the most intense signal, where the singly charged dimer has been highlighted. It is interesting to note that an additional peak is observed. This peak has been assigned as the singly charged heterodimer of stearic acid in combination with formic acid. Figure 4b shows the same type of spectrum as Figure 4a for β-cholanic acid, where both β-cholanic acid and the singly charged heterodimer (a combination of formic acid and β-cholanic acid) are highlighted. Despite their very different molecular structures, it is evident in these spectra that both of these compounds are able to form singly charged hetero- and homodimers. Figure 4c introduces a further degree of complexity by mixing both solutions of stearic and β-cholanic acids. The new mixture allows for the
Scheme 2. Structure for β-Cholanic Acid
MS can be used to provide further insight into the formation of NA dimers. The NA mixture must first be diluted (by orders of magnitude) in organic solvents, and therefore, the degree of dimerization would be expected to be reduced when compared to the FTIR data for the more concentrated sample. However, MS can provide useful structural information. Figure 2 shows a FT-ICR mass spectrum for a 0.25 mg/ mL solution in methanol of a commercially available mix of NAs by Kodak. It can be observed that the spectrum consists of two distinct distributions, one from roughly m/z 190 to 370 and the other from approximately m/z 370 to 610. The second distribution has been characterized, and its peaks have been found to fit the general empirical formula CnH2nþZO4. Such a formula describes both several dicarboxylic acids and also noncovalent dimers composed of monocarboxylic NAs. It has also been observed that the signal intensity of such species depends upon the accumulation time within the hexapole (see
(22) Hakansson, K.; Axelsson, J.; Palmblad, M.; Hakansson, P. Mechanistic studies of multipole storage assisted dissociation. J. Am. Soc. Mass Spectrom. 2000, 11 (3), 210–217. (23) Sannes-Lowery, K.; Griffey, R. H.; Kruppa, G. H.; Speir, J. P.; Hofstadler, S. A. Multipole storage assisted dissociation, a novel insource dissociation technique for electrospray ionization generated ions. Rapid Commun. Mass Spectrom. 1998, 12 (23), 1957–1961.
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Figure 7. Relative intensity versus carbon number for the CnH2nþZO2 (top) and CnH2nþZO4 (bottom) species. The accumulation time is 0.14 s for the left column and 0.20 s for the right column.
formation of singly charged aggregates of the β-cholanic and stearic acids. The peaks observed in Figure 4 (also relevant to Figure 5) can be summarized as: stearic acid (m/z 283.264189), a heterodimer of stearic acid and formic acid (m/z 329.269829), β-cholanic acid (359.295639), a heterodimer of β-cholanic acid and formic acid (405.301086), a stearic acid homodimer (567.535646), a heterodimer of stearic acid and β-cholanic acid (643.566833), and a β-cholanic acid homodimer (719.598303). It is worth noticing that no doubly charged homo- and heterodimers were detected. This might indicate that the carboxylic groups are involved in the formation of the dimer, not allowing both protons to be detached. If this is the case, a likely general structure for the dimers is the one shown in Scheme 3, where a proton from the carboxylic group is shared between two NA molecules. In a very similar fashion to what happens in complex NA mixtures, the signal intensities of the dimers decreases with increasing residence time in the hexapole, as shown in Figure 5; this seems to confirm the general behavior of such aggregates. A useful visual aid to highlight the features and behavior of the species of interest is represented by Kendrick plots.24,25 Such a type of graph can be produced by normalizing the International Union of Pure and Applied Chemistry (IUPAC) mass of a molecule, so that CH2 = 14.0000 Da (Kendrick
mass). This procedure leads to having a homologous series (i.e., differing only in CH2 units) with the same decimal figures for their masses. The difference between the Kendrick mass and its nearest integer is called the Kendrick mass defect (KMD), and it is typical of a series of compounds that differ only in CH2 units. When the nearest integer mass (nominal Kendrick mass) is plotted against the KMD, members of the same family of compounds are bound to lie on a horizontal straight line. Even if two-dimensional Kendrick plots do not show how the relative intensities of the detected species vary, they are informative in terms of the general composition of a complex mixture and give an indication regarding the degree of unsaturation of the sample that is being examined. In panels a and b of Figure 6, the Kendrick plots for the same sample spectra recorded with 0.14 and 0.20 s accumulation time, respectively, are reported. It can be seen that the CnH2nþZO4 species are more numerous for the shorter residence time in the hexapole than for the longer one. The lower m/z portion (from about m/z 170 to 370) of the CnH2nþZO4 sample species is clearly more strongly affected by the residence time than the higher portion that goes from approximately m/z 370 to roughly 630. It can be seen that, with an accumulation time of 0.20 s, the number of CnH2nþZO4 species (shown in red) is lower than 0.14 s. The change in the signal of the CnH2nþZO4 species can be better quantified by plotting the signal intensity against the carbon number of each family (same hydrogen deficiency). The left column in Figure 7 shows the distribution of the relative intensity against the carbon number for the CnH2nþZO2 and CnH2nþZO4 species for an accumulation time
(24) Kim, S.; Kramer, R. W.; Hatcher, P. G. Graphical method for analysis of ultrahigh-resolution broadband mass spectra of natural organic matter, the van Krevelen diagram. Anal. Chem. 2003, 75 (20), 5336–5344. (25) Marshall, A. G.; Rodgers, R. P. Petroleomics: The next grand challenge for chemical analysis. Acc. Chem. Res. 2004, 37 (1), 53–59.
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aggregates formed, and the results show that the Z = -8 dimers are indeed the most intense. Consequently, the heterodimer of the two most abundant CnH2nþZO2 peaks, Z=-4 and -2, should be the second most intense; the results show that the Z=-6 dimers are indeed the second most abundant. The Z=-10 CnH2nþZO4 species (third most intense) can result from either aggregation of the Z=-4 and -6, Z=-2 and -8, or Z=0 and -10 CnH2nþZO2 species. Thus, the CnH2nþZO4 species consist of two groups: (1) individual CnH2nþZO4 molecules of lower carbon number and (2) noncovalently bound dimers of CnH2nþZO2 molecules (at higher carbon numbers), where the hydrogen deficiencies of the dimers reflect the most abundant CnH2nþZO2 series. Such observation seems to lead to the explanation of the presence of dimers as a combination of monomers, where the distribution of dimers ultimately mirrors that of monomers, at least for the most abundant species.
of 0.14 s; the CnH2nþZO2 species profiles are typically bellshaped distributions. The diagram for the CnH2nþZO4 species presents a more complex behavior, in which two distributions are present, one roughly between carbon numbers 12 and 22 and the other from carbon numbers 22 to 40. The two righthand graphs show the relative intensity plotted against the carbon number of CnH2nþZO2 (top) and CnH2nþZO4 (bottom), respectively, for an accumulation time of 0.20 s. It is worth noticing that, as the accumulation time is increased, a shoulder develops on the distribution of CnH2nþZO2 species. Nevertheless, the carbon number for the most intense peaks for all of the species is the same. The most dramatic change, however, concerns the CnH2nþZO4 species, in which relative intensities are remarkably reduced, as can be seen in the bottom right graph in Figure 7. Most of the signals relative to the CnH2nþZO4 species detected with an accumulation time of 0.14 s are very weak or even missing for an accumulation time of 0.20 s. For such a reason, it is difficult to say whether the relative intensity distributions of each family (same Z) are shifted or have simply decreased below the noise level. However, it is evident that the population of the CnH2nþZO4 compounds is dramatically affected by the time spent within the hexapole ion trap. This change in signal intensities is believed to be due to collisions occurring within the hexapole that cause the dissociation of the singly charged CnH2nþZO4. The CnH2nþZO4 compounds for n = 12-22 would appear to mirror the hydrogen deficiencies of the CnH2nþZO2 species, with the predominance of Z=-4, -2, and -6, respectively. In contrast, for CnH2nþZO4 compounds of higher carbon number, it is clear that there is a tendency toward higher hydrogen deficiency instead and they no longer mirror the CnH2nþZO2 species. In a parallel manner to the study of the dimerization stearic and β-cholanic acids, it is possible to examine the formation of homo- and heterodimers (according to hydrogen deficiency) of the Kodak NA mixture. Because the Z = -4 CnH2nþZO2 species are the most abundant, homodimers of these species may be expected to be among the most abundant
Conclusions The tendency of NAs to form dimers in solution has been studied by FT-ICR MS. The dependence of such species signal intensities on the accumulation time within the hexapole ion trap is seen as an indication of the nature of the weak intermolecular bond. In fact, a longer residence time inside the hexapole results in a higher number of collisions, enhancing the chances for the aggregates to dissociate. The dimerization of NAs using a commercial mixture may be expected to parallel the dimerization process that occurs in crude oils and related fractions. Through characterization of the hydrogen deficiency of the CnH2nþZO4 species present and when the dissociation of these species within the mass spectrometer is monitored, it is possible to discriminate between dicarboxylic acids and noncovalently bound NA dimers. Acknowledgment. The authors thank Shell Global Solutions for providing funding and Dr. Andrew Beevers (University of Warwick) for assistance with the infrared spectroscopy.
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