Isomeric Separation and Structural Characterization of Acids in

May 16, 2015 - Although naphthenic acids are minor constituents in petroleum, their characterization is crucial because they are geochemically importa...
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Isomeric Separation and Structural Characterization of Acids in Petroleum by Ion Mobility Mass Spectrometry Priscila M. Lalli,†,‡ Yuri E. Corilo,†,‡ Steven M. Rowland,†,‡ Alan G. Marshall,†,§ and Ryan P. Rodgers*,†,‡,§ †

National High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States ‡ Florida State University Future Fuels Institute, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States § Department of Chemistry and Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306, United States S Supporting Information *

ABSTRACT: Although naphthenic acids are minor constituents in petroleum, their characterization is crucial because they are geochemically important tracers and play a key role in corrosion processes in refineries. Moreover, different isomers may exhibit different reactivity and may serve as potential biomarkers. However, determination of acid isomers in petroleum/hydrocarbon matrixes remains analytically challenging. Here, we achieve the separation and structural characterization of isomeric homologue series of naphthenic acids in petroleum samples by ion mobility time-of-flight mass spectrometry (IM-TOF MS). IM-TOF MS data processing and molecular formula assignments (for most abundant heteroatom classes), integrated with ion mobility data by PetroOrg software, expose structural differences and patterns among petroleum compounds and facilitate the identification of series of isomers. For example, a family of isomeric acids (CcHhO2) of double bond equivalents (DBE) = 5 and carbon number ranging from C28−C34 appear to correspond to steranoic acids. As another example, CcHhO2 isomers of DBE = 1 likely correspond to linear and isoprenoid acids. Ultra-high-resolution Fourier transform ion cyclotron resonance mass spectra serve to validate molecular formula assignments by IM-TOF and determine whether or not isobaric ions of different mobility are isomers. We conclude that ion mobility mass spectrometry constitutes a valuable new tool for rapid isomeric separation of polar compounds (such as naphthenic acids) in petroleum and other complex organic mixtures.

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INTRODUCTION The complex mixture of carboxylic acids in petroleum is collectively termed “naphthenic acids”. Naphthenic acids have the general chemical formula, CnH2n+ZO2, in which n is the carbon number, and Z is the hydrogen deficiency, which can be zero or a negative even number. Although these compounds are minor constituents in petroleum, their characterization is crucial because they can be responsible for corrosion in refineries,1 they are surface-active,2 and their presence in waste waters is toxic to animals and plants.3,4 Moreover, they can serve as biomarkers for elucidating geochemical correlations and biodegradation mechanisms.5−10 Bulk analytical techniques used to characterize naphthenic acids in petroleum include Fourier transform infrared (FT-IR) spectroscopy11 and 13C nuclear magnetic resonance.12 However, individual characterization of petroleum compounds by these techniques is a challenge due to their high compositional complexity. Although gas chromatography (GC) and gas chromatography coupled to MS (GC−MS)13,14 have been used for the analysis of low boiling point cuts, they are limited by sample volatility and inability to resolve highly complex, polar naphthenic acid mixtures. The advances of hightemperature gas chromatography (HT-GC)15 and comprehensive two-dimensional gas chromatography (GCxGC)16,17 have expanded the range of GC analysis to higher boiling points and increased peak capacity, but the prolonged time for analysis and the need to derivatize polar compounds (such as naphthenic acids) is undesirable. © 2015 American Chemical Society

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been used for characterization of acids in petroleum at the molecular level.18−21 Due to its ultrahigh resolution and mass accuracy, FT-ICR MS resolves and assigns elemental compositions (CcHhNnOoSs) for thousands of compounds in such complex mixtures.22 Knowledge of those compositions helps one to understand and ultimately predict properties/reactivity (with further advances in quantitation) of petroleum crude oil and its products, and has originated the field of petroleomics.23−25 However, like any other MS-based technique, FT-ICR MS lacks the ability to directly distinguish isomers. The reactivity of compounds depends on their structure; therefore, it is essential to distinguish and characterize structural isomers in petroleum to understand industrial process chemistry (downstream) and their potential use as biomarkers (upstream). Ion mobility spectrometry (IMS) separates ions according to their ability to pass through a cell filled with gas under the influence of an electric field. The mobility of an ion depends on its size, shape, mass, and charge. Therefore, ions of the same mass-to-charge ratio, m/z (i.e., isomers), may be separated according to their size and shape.26 Any ionization source available for MS can also be used for IMS; thus, IMS can access compounds spanning a wide range of polarities and volatilities. Received: March 9, 2015 Revised: May 15, 2015 Published: May 16, 2015 3626

DOI: 10.1021/acs.energyfuels.5b00503 Energy Fuels 2015, 29, 3626−3633

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formate solution, and external lock-mass calibration was also carried out with a solution of leucine enkephalin (m/z 554.2620). The TOF mass analyzer was operated in W mode with an average mass resolving power of m/Δm50% ≈ 40 000 at m/z 500 and ∼5−10 ppm mass error. Instrument control and data acquisition were performed with MassLynx v4.1 (Waters Corporation, Mildford, MA, USA). 9.4 T FT-ICR MS. FT-ICR mass spectra were acquired with a custom-built FT-ICR mass spectrometer51 at an average mass resolving power of m/Δm50% > 600 000 at m/z 500 and 100−400 ppb mass error. This instrument has a 22 cm horizontal room temperature bore 9.4 T magnet and a modular ICR data station for instrument control, data acquisition, and data analysis.52 Negative ions generated at atmospheric pressure enter the skimmer region (∼2 Torr) through a heated metal capillary, accumulate (1−3 s) in a first rfonly octopole, pass through an rf-only quadrupole, and into a second rf-only octopole equipped with tilted wire extraction electrodes.53 Helium gas introduced in the second rf-octopole collisionally cools ions prior to transfer through two rf-only octopoles (total length 119.5 cm) into an open cylindrical Penning ion trap (9.4 cm i.d. × 30 cm long). Octopole ion guides were operated at 2.0 MHz and 240 Vp‑p rf amplitude. Broadband frequency sweep (“chirp”) excitation (70−700 kHz at a sweep rate of 50 Hz/μs and amplitude, Vp‑p = 350 V) accelerated the ions to a cyclotron orbital radius detected as the differential current induced between two opposed electrodes inside the ICR cell.54 The experimental event sequence was controlled by a Predator data station.52 Multiple (50−150) time-domain acquisitions were averaged for each sample, Hanning-apodized, and zero-filled once prior to fast Fourier transform and magnitude calculation. Data Processing. TWIM-TOF and FT-ICR MS data were processed with PetroOrg software.55 Each m/z spectrum was internally calibrated with respect to an abundant homologous alkylation series whose successive members differ in mass by one methylene (CH2, 14.01565 Da). Elemental compositions (CcHhNnOoSs) were then assigned for each m/z value and classified according to the heteroatom class (NnOoSs), type (double bond equivalents, DBE = number of rings plus double bonds involving carbon), and carbon number. Drift times were also attributed to each m/z value (for TWIM-TOF data) and used along with the classifications to generate images such as DBE versus carbon number, drift time versus DBE, and drift time versus carbon number. Drift time versus m/z images were also generated with DriftScope software v2.7 (Waters Corporation, Manchester, U.K.). Experimental collision cross sections (N2 as drift gas) were also determined for O2 compounds with DBE = 1 by calibration of the TWIM cell with polyalanine.56 The relationship between drift time and CCS was obtained with a modified Mason−Schamp equation.42 The calibration curve was generated by the IntelliStart program (Waters Corporation, Mildford, MA) using reported drift tube based CCS values of polyalanine obtained with N2 as drift gas.56

Moreover, separation in IMS occurs on a millisecond time scale, leading to very short analysis time. IM-MS has been extensively used for structural studies of biomolecules28−30 and for the characterization of numerous types of isomers, including cis and trans photoisomers,31 carbohydrates,32,33 drug metabolites,34 drugs of abuse,35 supramolecular assemblies,36,37 protomers,38 steroids,39 and polymers.40 Recently, the interest in the use of IM-MS analysis of petroleum has demonstrated the potential of the technique to elucidate structural relationships among crude oil species.41−44 Structural relationships for naphthenic acid model compounds have also been investigated by IM-MS.45 Moreover, Eberlin and co-workers46 have used IM-MS to separate polar compounds according to their heteroatom classes (N1O1, O2, and N1), and Kim and co-workers47 introduced the potential of combining IM-MS and calculated collision cross sections (CCS) to distinguish between putative molecular structures for species in petroleum samples. Gabryelski and Froese48 used high-field asymmetric waveform ion mobility spectrometry (FAIMS) to reveal multiple structures of ions of a given nominal (nearest integer) mass from naphthenic acid mixtures. However, there has been little attention paid to the potential of the technique to differentiate series of isomeric compounds in petroleum samples. Here, we show the use of IM-MS for separation and structural elucidation of an isomeric homologue series of naphthenic acids from Athabasca bitumen. The identification of the isomeric series is facilitated by plots of drift time (or collision cross section) vs carbon number for individual homologue series. We also introduce the use of drift time vs double bond equivalents (DBE = number of rings plus double bonds to carbon) plots to reveal multiple core structures for a heteroatom class.

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EXPERIMENTAL METHODS

Sample Preparation. Athabasca bitumen acids were isolated and separated into discrete molecular weight ranges by modified aminopropyl silica (MAPS) fractionation.49 Low molecular weight acids have higher ionization efficiency and suppress the ionization of high molecular weight acids when a bitumen sample or acid fraction is analyzed by (−) ESI-MS.49 MAPS fractionation avoids ion suppression and enables the detection of higher molecular weight acids by isolation of acids with similar ionization efficiency in each fraction. In this method, after the sample is loaded onto aminopropyl silica (APS) solid phase extraction (SPE) cartridges, nonacid fractions are eluted successively with dichloromethane (DCM) 100%, DCM:MeOH 50:50, MeOH 100%, and MeOH:H2O 70:30. The acid fractions are collected with a series of eluents of varying polarities (MeOH:H2O 70:30, 80:20, 90:10; MeOH 100%; DCM:MeOH 5:95, 20:80, and 50:50) plus 5% formic acid (which displaces naphthenic acids from the solid phase). All acid fractions were dried under N2 gas and dissolved in toluene:methanol 50:50 at 10 μg/mL for MS analysis. Here, we show the results for the first three acid fractions. Traveling Wave Ion Mobility Mass Spectrometry (TWIMMS). TWIM-MS experiments were performed with a Synapt G2-Si HDMS instrument (Waters Corp., Manchester, U.K.). This instrument, described in detail elsewhere,27,50 has a hybrid quadrupole/ion mobility/orthogonal acceleration time-of-flight (oa-TOF) geometry. Typical (−) ESI source conditions were as follows: capillary voltage 2.0 kV, sample cone 50 V, source offset 80 V, source temperature 100 °C, desolvation temperature 150 °C, desolvation N2 gas flow rate 400 L h−1. The TWIM cell was operated at a pressure of 3.0 mbar of N2. Wave velocity and wave height were set at 800 m s−1 and 40 V for all experiments, optimized such that the ion drift times for all acid fractions analyzed were within the drift time window of 0−12 ms. Calibration of the TOF mass analyzer was performed with sodium

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RESULTS AND DISCUSSION

Double Bond Equivalents vs Carbon Number. (−) ESI FT-ICR MS analysis shows that the most abundant heteroatom class in the three MAPS acid fractions from Athabasca bitumen is the O2 class. Figure 1 shows isoabundance-contoured plots of DBE vs carbon number for members of the O2 heteroatom class from the three acid fractions by (−) ESI 9.4 T FT-ICR MS. The species of highest relative abundance have DBE = 3 and 4 (likely carboxylic acids with two and three naphthenic rings), but species of DBE up to ∼15 (probably containing aromatic rings) are also present. The MAPS fractionation isolates naphthenic acids and fractionates them into discrete molecular weight ranges. Thus, the increase in carbon number shown in the plots in Figure 1 also illustrates the molecular weight range comprised by each fraction. Figure S1 (Supporting Information) compares (−) ESI FTICR and (−) ESI IM-TOF mass spectra for the three MAPS acid fractions, and Figure S2 (Supporting Information) shows a 3627

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threshold was set high enough to limit the display only to ions from the most abundant heteroatom class (O2). In these plots, most of the peaks arise from homologous series of ions differing in elemental composition by integer multiples of 14 Da (i.e., CH2). In addition, each additional double bond or formation of a ring decreases the mass by 2 Da, but also decreases the drift time by a constant increment (see scaleexpanded segment in Figure 2d). Thus, the observed pattern enables graphical separation of the ions according to their DBE and carbon number in the drift time vs m/z image. Automated Elemental Composition Assignments. Due to the complexity of crude oil composition and large number of peaks in the drift time vs m/z images, extracting and comparing information about isomers and structures for series of compounds without automated elemental composition assignments or any type of classification is not feasible. To overcome that limitation, we employed PetroOrg software55 for data processing and visualization. After recalibration, molecular formula assignments, heteroatom classification, and correlation with ion mobility data for the most abundant species in a petroleum mass spectrum, plots involving carbon number, DBE, heteroatom class, drift time, and m/z significantly enhance the interpretation of petroleum ion mobility mass spectrometry data. As described below, we have found that plots such as drift time (or collision cross section (CCS)) vs carbon number greatly facilitate identification of isomeric series of compounds; moreover, drift time vs DBE plots expose structural changes (different core structures) among compounds of a heteroatom class. Drift Time vs Carbon Number. After elemental composition assignments, drift time vs carbon number images were plotted for the O2 class with PetroOrg software. The software allows the user to highlight each homologue series (same DBE, but different degree of alkylation) individually. For members of the same homologous series, ions of a given carbon number typically exhibit only a single drift time for most series (Figure S3, Supporting Information, for O2 class members of DBE = 4 or 8). However, when O2 class members of DBE = 5 are highlighted (Figure 3, in red), two drift times are observed for a single carbon number, indicating ion structures of two different sizes and/or shapes. Note that the species with higher drift times (less compact) stand out from most O2 species. This separation can also be observed in the drift time vs m/z image (Figure 2c); however, elemental compositions can be readily identified in the drift time vs carbon number plot in Figure 3.

Figure 1. Isoabundance-contoured plots of double bond equivalents (DBE) vs carbon number for members of the O2 heteroatom class from the three acid fractions by (−) ESI 9.4 T FT-ICR MS.

zoom inset in a 15 Da window (m/z 466−481) for Fraction 3. The distribution of peaks obtained by both instruments is very similar, showing that they present comparable ionization, and therefore, ultra-high-resolution (−) ESI FT-ICR MS can be used to validate assignments for the highest magnitude peaks in the (−) ESI IM-TOF. Ion Mobility Drift Time vs Mass-to-Charge Ratio. Drift time versus m/z images for the three acid fractions for IM-TOF MS data is shown in Figure 2. Because TOF mass accuracy is not sufficient to yield unique elemental composition assignments for all peaks of magnitude higher than (say) 6σ of baseline noise, the Driftscope algorithm peak detection

Figure 2. Isoabundance-contoured images of drift time vs m/z for the three acid fractions (a, b, and c) obtained with Driftscope software for (−) ESI IM-TOF MS data. (d) Scale-expanded segment of drift time vs m/z image from Fraction 2. Dots represent peaks selected with a magnitude threshold of ∼10%. Black lines and dotted blue lines were added for emphasis.

Figure 3. Drift time vs carbon number for O2 species from acid Fraction 3. Species with DBE = 5 are highlighted in red, whereas the remaining O2 species appear in light gray. 3628

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exhibit DBE = 5. Therefore, it appears likely that the isomeric series with longer drift times correspond to steranoic acids. Steranoic Acids? To test that hypothesis, we considered two model compounds of elemental composition, C24H40O2 (DBE = 5): 5-β-cholanic acid and 18-phenyl octadecanoic acid. Figure 5 shows a plot of drift time vs carbon number for the O2

For instance, it is evident from Figure 3 that the species separated from most petroleum compounds by higher drift times belong to the O2 class with DBE = 5 and C28−C34, that is, elemental compositions ranging from C28H48O2 to C34H60O2. Isobars vs Isomers. Mobiligrams extracted for m/z ranges (∼0.01 Da window) corresponding to the series of O2 heteroatom species with DBE = 5 (Figure 4) show a bimodal

Figure 5. Top: Drift time vs carbon number for the O2 class species of DBE = 5 discussed in Figure 4 (average drift times from five replicates, standard error calculated with a 99% confidence interval). Points in red correspond to the more compact isomeric series (ions of lower drift time), whereas points in blue correspond to the less compact isomers (ions of higher drift time). Bottom: Experimental drift times for two standard compounds (both C24): 5-β-cholanic acid and 18-phenyl octadecanoic acid.

Figure 4. Left: Mobiligram segments for O2 class species of DBE = 5 from C28 to C33. Two IM peaks for each m/z suggest the presence of two structures that could be isomers or isobars (unresolved by TOF MS). Right: Mass scale-expanded segments of IM-TOF and FT-ICR mass spectra for ions of nominal m/z 415 and 471. Ultra-highresolution FT-ICR MS shows no significant isobars in the mass range from which IM peaks were extracted, confirming that the species separated by IM are indeed isomers.

ions of DBE = 5 discussed above (blue and red data points) as well as for the two model compounds. Although the model compounds do not fall in the same carbon range of the isomers in the sample (C28−C34), an extrapolation of the best-fit line for the less compact isomeric series (in blue) matches exactly the drift time for 5-β-cholanic acid, reinforcing the putative identification of those compounds as steranoic acids. Drift times for the more compact isomers (in red) are not consistent with 18-phenyl octadecanoic acid: as a phenyl acid with a single long straight alkyl chain is not an accurate representation of native petroleum acids because straight chains are more readily degraded than branched alkyl chains.4 Multiple and branched alkyl chains would result in a more compact structure and shorter drift times, so the more compact isomers might correspond to phenyl acids. Collision Cross Section vs Carbon Number (O 2 isomers of DBE = 1). Figure 6 is a plot of experimental collision cross section (CCS) vs carbon number for O2 ions of DBE = 1 (saturated carboxylic acids) in the three acid fractions. Note that two linear series are present: Fraction 1 (blue) comprises only ions from the upper series (larger CCS), whereas Fraction 3 (red) contains ions only from the bottom

distribution and also illustrate the separation. Ions of the same nominal mass but different size and/or shape, and thus different ion mobility, may correspond to isomers (same elemental composition) or to isobars (same nominal mass, but different elemental composition) that are not resolved by the TOF mass analyzer. Ultra-high-resolution FT-ICR MS, used here to validate IM-TOF elemental composition assignments for O2 ions of DBE = 5, reveals the absence of isobars (compare the IM-TOF and FT-ICR mass spectra for ions of nominal m/z 415 and 471 in Figure 4), therefore, confirming that the series of species separated by ion mobility are indeed isomers. As mentioned above, the isomers separated by ion mobility correspond to O2 species (most likely carboxylic acids based on the MAPS separation) of DBE = 5, and the less compact isomeric series (with larger drift times) is present only for the carbon number range from C28 to C34 (Figure 3). It is interesting to note that steranes (4-ring compounds derived from steroids)57 are present in petroleum in the same carbon range (C27−C35), and their corresponding carboxylic acids also 3629

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left). As a result, the single drift time (or CCS) reported for ions of each m/z represents an average of the drift times for both structures. Nevertheless, the two isomeric structures are clearly resolved if different isomers are dominant in different acid fractions (Figure S2, Supporting Information, right). Future work will focus on strategies to improve the isomeric separation, including the use of more polarizable drift gases such as CO2.58,36,33 Drift Time vs Double Bond Equivalents. Figure 7 is a plot of average drift time vs DBE for O2 ions of three carbon numbers from each of Fractions 1 and 3, spanning the entire carbon range for the fractions studied here (C16−C40). This type of plot exposes structural differences among members of a heteroatom class, because ions with equivalent structures follow a trend line. For a given carbon number, drift time generally decreases as DBE increases, because ions become more compact on formation of cycloalkanes or addition of double bonds. However, at DBE = 6, drift time is approximately the same or greater than the drift time for ions of DBE = 5 consistently for all carbon numbers, suggesting a change in core structure. To understand the nature of that change, we need to consider possible core structures for these species. O2 species of DBE = 1 correspond to saturated carboxylic acids, and the increase in DBE up to DBE = 5 most likely results from the addition of cycloalkanes rather than addition of double bonds because of the higher reactivity of unsaturated compounds. The high reactivity makes unsaturated compounds unlikely to survive the geochemical environment during petroleum formation and evolution. On the other hand, carboxylic acids of DBE = 5 and 6 may contain one aromatic ring, which is impossible for acids of DBE < 5. As a result, the formation of an aromatic ring instead of multiple naphthenic rings for species of DBE = 5 and 6 could account for the observed change in core structure. It is at first surprising that aromatization appears to result in larger CCS than corresponding isomers with multiple naphthenic rings, because an aromatic ring is more compact than multiple naphthenic rings. However, the contribution to CCS by the alkyl chains attached to the core must also be considered. An aromatic ring core contains fewer carbon atoms than multiple naphthenic rings and thus must exhibit a higher extent of alkylation for compounds with the same carbon

Figure 6. Experimental collision cross section (CCS) vs carbon number for O2 class species of DBE = 1 in the three acid fractions. CCS values were determined by calibration with polyalanine. (Standard error calculated from five replicates with a 99% confidence interval.)

series (smaller CCS) and Fraction 2 (green) contains a mixture of both series. The two series are isomeric because they present different CCS (different size and/or shape) for a single carbon number, and the absence of isobars was confirmed by FT-ICR MS. Isomeric O2 species of DBE = 1 likely correspond to linear and isoprenoid carboxylic acids. We believe that the isoprenoid carboxylic acids correspond to the bottom series, because the alkyl chain branching will likely render the compounds more compact than straight chain acids. Figure 6 also reveals how these compounds behave in the MAPS fractionation. The first fraction (blue) contains only linear acids up to C26. The linear acids of C > 26 elute in the second fraction (green) along with the isoprenoid acids of low carbon number (C < 24). The third fraction (red) contains the isoprenoid acids of high carbon number (C > 24). The two isomeric series are not fully resolved by ion mobility; that is, an asymmetrical broad single mobility peak is observed for ions of each m/z (rather than two mobility peaks) when both series coexist in a sample or fraction, such as C24− C28 species in Fraction 2 (Figure S4, Supporting Information,

Figure 7. Drift time vs DBE for O2 class species of three different carbon numbers in Fractions 1 and 3 (average drift times from five replicates, standard error with a 99% confidence interval). 3630

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(8) Kim, S.; Stanford, L. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K.; Wenger, L. M.; Mankiewicz, P. Microbial alteration of the acidic and neutral polar NSO compounds revealed by Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2005, 36 (8), 1117−1134. (9) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Qian, K. N.; Robbins, W. K. Identification of acidic NSO compounds in crude oils of different geochemical origins by negative ion electrospray Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2002, 33 (7), 743−759. (10) Hughey, C. A.; Rodgers, R. P.; Marshall, A. G.; Walters, C. C.; Qian, K. N.; Mankiewicz, P. Acidic and neutral polar NSO compounds in Smackover oils of different thermal maturity revealed by electrospray high field Fourier transform ion cyclotron resonance mass spectrometry. Org. Geochem. 2004, 35 (7), 863−880. (11) Holowenko, F. M.; MacKinnon, M. D.; Fedorak, P. M. Naphthenic acids and surrogate naphthenic acids in methanogenic microcosms. Water Res. 2001, 35 (11), 2595−2606. (12) Tomczyk, N. A.; Winans, R. E.; Shinn, J. H.; Robinson, R. C. On the nature and origin of acidic species in petroleum. 1. Detailed acid type distribution in a California crude oil. Energy Fuels 2001, 15 (6), 1498−1504. (13) Holowenko, F. M.; MacKinnon, M. D.; Fedorak, P. M. Characterization of naphthenic acids in oil sands wastewaters by gas chromatography-mass spectrometry. Water Res. 2002, 36 (11), 2843− 2855. (14) Lima, S. G.; Steffen, R. A.; Reis, F. D. M.; Koike, L.; Neto, E. V. S.; Cerqueira, J. R.; Lopes, J. A. D. Propyl ergostanoic acids: Possible new indicator for oil biodegradation. Org. Geochem. 2010, 41 (4), 325−339. (15) Sessions, A. L.; Zhang, L. C.; Welander, P. V.; Doughty, D.; Summons, R. E.; Newman, D. K. Identification and quantification of polyfunctionalized hopanoids by high temperature gas chromatography-mass spectrometry. Org. Geochem. 2013, 56, 120−130. (16) Rowland, S. J.; West, C. E.; Scarlett, A. G.; Jones, D.; Boberek, M.; Pan, L.; Ng, M.; Kwong, L.; Tonkin, A. Monocyclic and monoaromatic naphthenic acids: Synthesis and characterisation. Environ. Chem. Lett. 2011, 9 (4), 525−533. (17) Ventura, G. T.; Raghuraman, B.; Nelson, R. K.; Mullins, O. C.; Reddy, C. M. Compound class oil fingerprinting techniques using comprehensive two-dimensional gas chromatography (GC x GC). Org. Geochem. 2010, 41 (9), 1026−1035. (18) Qian, K. N.; Robbins, W. K.; Hughey, C. A.; Cooper, H. J.; Rodgers, R. P.; Marshall, A. G. Resolution and identification of elemental compositions for more than 3000 crude acids in heavy petroleum by negative-ion microelectrospray high-field Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2001, 15 (6), 1505−1511. (19) Wu, Z. G.; Rodgers, R. P.; Marshall, A. G. Compositional determination of acidic species in Illinois No. 6 coal extracts by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2004, 18 (5), 1424−1428. (20) Smith, D. F.; Schaub, T. M.; Kim, S.; Rodgers, R. P.; Rahimi, P.; Teclemariam, A.; Marshall, A. G. Characterization of acidic species in Athabasca bitumen and bitumen heavy vacuum gas oil by negative-ion ESI FT-ICR MS with and without acid-ion exchange resin prefractionation. Energy Fuels 2008, 22 (4), 2372−2378. (21) Smith, D. F.; Rodgers, R. P.; Rahimi, P.; Teclemariam, A.; Marshall, A. G. Effect of thermal treatment on acidic organic species from Athabasca bitumen heavy vacuum gas oil, analyzed by negativeion electrospray Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Energy Fuels 2009, 23 (1), 314−319. (22) Rodgers, R. P.; Schaub, T. M.; Marshall, A. G. Petroleomics: MS returns to its roots. Anal. Chem. 2005, 77 (1), 20A−27A. (23) Marshall, A. G.; Rodgers, R. P. Petroleomics: The next grand challenge for chemical analysis. Acc. Chem. Res. 2004, 37 (1), 53−59. (24) Rodgers, R. P.; Marshall, A. G., Petroleomics: Advanced Characterization of Petroleum Derived Materials by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS). In

number (i.e., isomers), and thereby could result in a larger CCS. Finally, it is worth noting that the data presented in Figures 5−7 are plotted as the average from five replicate injections, with error bars showing the standard error with a 99% confidence interval. Relative standard deviations (%RSD) for drift times vary between different measurements by less than 2%. Therefore, the drift time measurements are very robust and even though some of the presently reported drift time differences or deviations are small, they represent statistically significant and repeatable differences. In future work, we shall pursue strategies to improve the isomeric separation. We shall also extend the method to polar species from other heteroatom classes as well as compounds of higher molecular weight.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b00503.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1 850 644 2398. Fax: +1 850 644 1366. E-mail: [email protected] (R.P.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF Division of Materials Research through DMR-11-57490, the Florida State University Future Fuels Institute, and the State of Florida. The authors thank Waters Corporation for their continued support and helpful discussions.

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REFERENCES

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