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Exploring the Complexity of Two Iconic Crude Oil Spills in the Gulf of Mexico (Ixtoc I and Deepwater Horizon) Using Comprehensive Two-Dimensional Gas Chromatography (GC × GC) Robert K. Nelson,*,† Kelsey M. Gosselin,† David J. Hollander,‡ Steven A. Murawski,‡ Adolfo Gracia,§ Christopher M. Reddy,† and Jagoš R. Radović∥

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Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, United States ‡ College of Marine Sciences, University of South Florida, St. Petersburg, Florida 33701, United States § Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México, México City 04510, Mexico ∥ Department of Geoscience, University of Calgary, Calgary, Alberta T2N 1N4, Canada S Supporting Information *

ABSTRACT: Comprehensive two-dimensional gas chromatography (GC × GC) was used to explore and compare the chemical complexity of oil released from the Deepwater Horizon (DWH) disaster in 2010 and the Ixtoc I spill in 1979-1980, both in the Gulf of Mexico (GoM). To provide the most complete inventory of the compounds present in the DWH and Ixtoc I crude oils, we utilized GC × GC systems coupled to a flame ionization detector and a high-resolution time-of-flight mass spectrometric detector. The results of this study demonstrate the significance of valuable environmental forensics information obtained using GC × GC fingerprinting methods. In particular, the high-resolution mass spectrometer enabled an in-depth characterization of the types and families of GC-amenable compounds present in these crude oils including the detection of highly alkylated sulfur-containing species, alkylated carbazoles and benzocarbazoles, and a suite of unusual de-A-sterane biomarkers in the Ixtoc I oil. This type of specificity is essential for differentiating spill sources of similar origin/type, for example, within northern and southern GoM petroleum families and of the molecular transformations that occur during oil-spill weathering processes.

1. INTRODUCTION Large catastrophic oil spills capture the spotlight of the media, scientists, policymakers, and general public, demanding an effective and immediate response. One overarching tenet to oil spill science is that the type of spilled oil and its distinct chemical composition are critical to the response, damage assessment, and restoration efforts of impacted areas. With this understanding, a useful comparison of variable chemistries of large-scale oil spills provides a means to identify the spill source and disentangle the effects of biotic and abiotic weathering processes, local geography, and other critical factors, all of which contribute to our understanding on the fate and effects of accidental oil releases. Two examples of such oil releases are the Ixtoc I and the Deepwater Horizon (DWH) blowouts, both occurring in the Gulf of Mexico (GoM). The Ixtoc I spill lasted from June 3, 1979 to March 23, 1980, releasing an estimated 3 000 000 barrels of crude into the southern GoM (lat: 19° 24′ 30.00″ N, lon: −92° 19′ 30.00″ W). The spilled product traveled along the Mexican coastline and also impacted the Texas coastline.1 Thirty years later on April 20, 2010 and lasting for 87 days, an estimated 5 000 000 barrels flowed from the damaged Macondo well, following the explosion of the DWH drilling rig (lat: 28° 44′ 11.86″ N, 88° 21′ 57.59″ W).2 Contrary to the DWH accident, the impacts of the Ixtoc I spill are far less understood due to limited post-spill research and monitoring © XXXX American Chemical Society

efforts. In order to close this research gap, recent studies have identified areas in the southern GoM, where residues from the Ixtoc I spill continue to persist.3,4 The goal of these studies is to increase our knowledge of the Ixtoc I spill, weathering of Ixtoc I oil released over decadal time scales, and to use that information to predict the possible long-term fate of the crude oil released during the DWH disaster. In addition, in depth studies of historic spill residues require a much more rigorous head-to-head comparison of petroleum components, which often cannot be achieved using traditional one-dimensional gas chromatography (GC) techniques. Petroleum and refined products are comprised of thousands of compounds with widely varying chemical functionalities, and its complexity is further increased by weathering of molecules in the parent oil.5 Thus, traditional one-dimensional GC systems do not have the peak capacity or resolving power needed for a robust identification of the spill source, in particular, when spill residues are affected by complex weathering processes such as photo-oxidation, water-washing, and biodegradation.6−8 Detailed chemical characterization of oil-spill sources is also crucial for predicting/modeling the ultimate fate and toxicity of the most environmentally Received: December 18, 2018 Revised: March 11, 2019

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DOI: 10.1021/acs.energyfuels.8b04384 Energy Fuels XXXX, XXX, XXX−XXX

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effluent at atmospheric pressure while in the GC × GC−TOF instrument, the column effluent has to move through a heated transfer line into the ion source. Since the total distance between the detector and secondary oven is different in each instrument, optimization of the chromatographic plane requires slight modifications to the GC × GC methods. 2.5. GC × GC−HRT Method. GC × GC−HRT chromatographic analysis was performed on a Leco Pegasus GC × GC−HRT 4D system consisting of an Agilent 7890B GC configured with a Leco LPAL3 split/splitless auto-injector system and a dual-stage cryogenic modulator (Leco, Saint Joseph, Michigan). Samples were injected in the splitless mode. The cold jet gas was dry N2 chilled with liquid N2. The hot jet temperature offset was 10 °C above the temperature of the main GC oven and the inlet temperature was isothermal at 310 °C. Two capillary GC columns were utilized in this GC × GC experiment. The first-dimension column was a Restek Rxi-5ms, (30 m length, 0.25 mm I.D., 0.25 μm df) and second-dimension separations were performed on a Restek Rxi-17Sil MS (2 m length, 0.25 mm I.D., 0.25 μm df). The temperature program of the main oven was held isothermal at 60 °C (5 min) and was then ramped from 60 to 335 °C at 1.5 °C min−1. The hot jet pulse width was 2 s with a modulation period of 16 s. The second-dimension oven was held isothermal at 65 °C (5 min) and was then ramped from 65 to 340 °C at 1.5 °C min−1. The carrier gas was helium at a flow rate of 1 mL min−1. HR-TOF data were sampled at an acquisition rate of 100 spectra per second (actual data collection rate was 97.2222 spectra per second) in the mass range of 40−600 amu. The ionization method was electron ionization (EI) with an electron energy of −70 V and the extraction frequency was 1.75 kHz. 2.5.1. GC × GC QA/QC. We used National Institute of Standards and Technology (NIST) standard reference materials (SRM) SRM2266 (Petroleum Biomarkers) and SRM-1582 (Petroleum Crude Oil) to calibrate and validate our GC × GC instruments. A five-point calibration using the certified compounds in SRM-2266 was performed to insure a linear response for peak integration data collected on the GC × GC−FID. Linear regression analysis for each of the certified compounds was performed individually for each certified compound in SRM-2266 as well as linear regression analysis of the combined peak areas of all thirty-five peaks from the five point dilution series of SRM-2266. We calculated the mean area/ng, standard deviation, and percent deviation for each certified compound in the five-point calibration of SRM-2266 as well as the overall (all steranes and hopanoids) mean area/ng, standard deviation, and percent deviation for all seven certified compounds in SRM-2266. In order to accurately assess the mean detection limit (MDL) of the GC × GC−FID data, we used all of the certified compounds in the five point calibration of SRM-2266 to calculate an MDL value of 0.04 ng peak−1 with 34 degrees of freedom at a 99% t-distribution confidence level. These data are presented in the Supporting Information section 8. GC × GC−FID chromatograms of SRM-1582, hexane blank, and SRM-2266 (mountain plots and plan view plots) are shown in the Supporting Information. GC × GC performance was monitored on all instruments using SRM-1582. We routinely intersperse SRM-1582 samples with analytical samples and monitor a suite of biomarker ratios in order to confirm that the instruments are stable and operating as expected. A table containing reproducibility data for fifteen biomarker ratios derived from SRM-1582 chromatograms is shown in the Supporting Information. 2.5.2. GC × GC−HRT Calibration. GC × GC−HRT mass spectra were calibrated using a continuous flow of perfluorotributylamine introduced by opening a valve into the EI source in the GC × GC− HRT instrument. GC × GC−HRT data collected throughout each EI run are calibrated with respect to the molecular ion (+1 charge state) of eight perfluorinated compounds (CF3, C2F4, C2F5, C3F5, C4F9, C5F10N, C8F16N, and C9F20N). The mass values for singly charged ions in the mass range of 40−600 amu, with a relative abundance at least ten times the signal to noise ratio of the baseplane were acquired and stored. Additional GC × GC−HRT data processing were performed using a petroleomics mass spectral data analysis application

persistent components.9,10 Fortunately, instrumental development, such as the advent of comprehensive two-dimensional GC (GC × GC), has significantly increased the amount of chemical information obtainable from an oil sample or residue.6−17 This is achieved by using two chromatographic columns with different selectivities coupled together by a thermal modulator, thus typically adding an order of magnitude higher resolving power compared to traditional one-dimensional GC.18 In this study, we employed the GC × GC technology to characterize the chemical composition of the source oil from the Ixtoc I blowout and compared it to the Macondo well oil released in the DWH disaster, with the overarching objective to create a valuable assessment for future studies of these two iconic spills. Furthermore, the results of this study demonstrate a significant analytical advancement achieved by coupling GC × GC to a high-resolution mass detector, enabling a more detailed characterization of volatile compounds, including heteroatom-bearing and highly alkylated species.

2. EXPERIMENTAL SECTION 2.1. Sample Procurement. Ixtoc I crude was collected on September 16, 1979 during the joint National Oceanic and Atmospheric Administration (NOAA) ship R/V Researcher and the R/V Pierce “Ixtoc I Research Cruise” (Dr. Donald K. Atwood, Chief Scientist). The DWH crude oil was collected directly from the damaged blowout preventer at the Macondo well on June 21, 2010 using a remotely operated vehicle and an isobaric gas-tight sampling device.19 2.2. Sample Preparation. Ixtoc I and DWH crude oil samples were diluted in hexane to a final concentration of 15 mg·mL−1. The hexane-soluble fractions were injected onto a gas chromatograph with a flame ionization detector (GC−FID) and GC × GC systems coupled to a FID (GC × GC−FID) and a high-resolution time-offlight mass spectral detector (GC × GC−HRT). 2.3. GC−FID Method. GC−FID analysis was performed on an Agilent 6890 GC configured with a split/splitless auto-injector (7683B series). The capillary column utilized in this experiment was a Restek Rxi-1ms, (30 m length, 0.25 mm I.D., 0.25 μm df). The temperature program was held isothermal at 70 °C (7 min) and was then ramped from 70 to 320 °C at 6.0 °C min−1. 2.4. GC × GC−FID and GC × GC−TOF Method. GC × GC− FID and −TOF chromatographic analyses were performed on Leco systems consisting of an Agilent 7890A GC configured with a split/ splitless auto-injector (7683B series) and a dual-stage cryogenic modulator (Leco, Saint Joseph, Michigan). Samples were injected in the splitless mode. The cold jet gas was dry N2 chilled with liquid N2. The hot jet temperature offset was 15 °C above the temperature of the main GC oven and the inlet temperature was isothermal at 310 °C. Two capillary GC columns were utilized in this GC × GC experiment. The first-dimension column was a Restek Rxi-1ms, (60 m length, 0.25 mm I.D., 0.25 μm df) and second dimension separations were performed on a 50% phenyl polysilphenylene-siloxane column (SGE BPX50, 1.2 m length, 0.10 mm I.D., 0.1 μm df). The temperature program of the main oven was held isothermal at 50 °C (15 min) and was then ramped from 50 to 335 °C at 1.5 °C min−1. The second dimension oven was isothermal at 60 °C (15 min) and then ramped from 60 to 345 °C at 1.5 °C min-1. The hot jet pulse width was 0.75 s, while the modulation period between stages was 7.50 s with a 3.00 s cooling period between modulations for the FID method, and 10.00 s and a 4.25 s cooling period between modulations for the TOF method. FID data were sampled at an acquisition rate of 100 data points per second, while the TOF data were sampled at an acquisition rate of 50 spectra per second in the mass range of 40−500 atomic mass units (amu). Different modulation periods were used because of differences between GC × GC instruments, for example, the GC × GC−FID combusts the column B

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Energy & Fuels for LECO’s ChromaTOF software (see the Supporting Information for a table of the perfluorinated compounds).

the two oils. Figure 2 is a comparison of the dibenzothiophene suite (dibenzothiophene, C1-dibenzothiophenes, C2-dibenzo-

3. RESULTS AND DISCUSSION The GC × GC−FID contour plots in Figure 1 illustrate the higher peak capacity of two-dimensional chromatography in

Figure 2. GC × GC−HRT selected ion mountain plot chromatograms (m/z 184.034, 198.049, 212.065, and 226.081) highlighting the molecular ions of the suite of sulfur containing molecules known as dibenzothiophenes in (a) Ixtoc I and (b) DWH crudes. Both samples were prepared as 15 mg mL−1 hexane solutions, and 1 μL of each was injected on the GC × GC−HRT so that a visual comparison of dibenzothiophenes was apparent. Both chromatograms are scaled identically.

thiophenes, and C3-dibenzothiophenes) produced with the molecular ions for each (m/z 184.034, 198.049, 212.065, and 226.081, respectively) using the GC × GC−HRT. The most evident difference was the variation in dibenzothiophene and alkylated dibenzothiophenes. Ixtoc I crude contains a higher abundance of dibenzothiophenes than the DWH oil. Inspection of both GC × GC chromatograms shows that the dibenzothiophene suite of compounds are a robust discriminator molecules between Ixtoc I and DWH crudes. A comparison of dibenzothiophene and phenanthrene compounds is presented in Table 1. Ratios such as dibenzothiophene/phenanthrene and pristane/phytane have traditionally been used to compare depositional environments of petroleum source rocks and as such, these ratios are useful petroleum fingerprinting molecules.34 This information is particularly useful for researchers studying samples of unknown origin in the GoM because (a) polycyclic aromatic sulfur-containing heterocycles (PASHs) have been proven as valuable forensic tools useful for fingerprinting crude and refined petroleum products,22 and thus may be used to confirm whether Ixtoc I or the DWH is the source of a given spill residue, and (b) dibenzothiophenes are easily identified using one-dimensional bench top GC−MS systems, and so, researchers with these instruments can utilize this suite of compounds in their own analytical procedures.23 In order to further explore the abundant sulfur-containing compounds in Ixtoc I, we utilized a petroleomics application in LECO’s ChromaTOF software tailored for high-resolution multidimensional GC × GC data (Figure 3). Accurate mass measurements can be used to determine the elemental

Figure 1. Full GC × GC−FID plan view chromatograms of (a) Ixtoc I and (b) DWH crudes. In this figure the x-axis is the n-alkane carbon number retention index21 and the y-axis is the second-dimension retention time in seconds. Elution fairways where examples of compound families, and elution positions in two-dimensional space are identified in panel (a).

capturing the compositional complexity of the investigated oils, afforded by the orthogonal column separation mechanism, which resolves compounds based both on their volatility (1st dimension) and polarity (2nd dimension) relative to the GC− FID chromatograms (Figure S1). Thus, a whole GC × GC chromatographic separation serves as a topographic map (or inventory) of the GC-amenable chemical compounds resolved and highlights where each compound elutes on the twodimensional chromatographic plane, especially for petroleum hydrocarbons that have similar response factors. A closer inspection of these figures indicates hundreds of compounds that would elute as the same peak or are partially resolved in one-dimensional GC (i.e., would be included within the unresolved complex mixture “UCM”20 but are resolved using a second chromatographic phase with different chemical or physical selectivity in GC × GC) (Figure S1). For a detailed comparison of Ixtoc I and DWH crude oils, we focused on compounds and compound classes/families showing the most illustrative/prominent differences between C

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Energy & Fuels Table 1. Diagnostic Ratios of Dibenzothiophenes and Phenanthrenesa (a) (b) (c) (d) (e) (f) (g)

ratio

Ixtoc I

DWH

dibenzothiophene/phenanthrene C1-dibezothiophenes/C1-phenanthrenes C2-dibezothiophenes/C2-phenanthrenes 2&3-methyldibenzothiophene/4-methyldibenzothiophene 1-methyldibenzothiophene/4-methyldibenzothiophene dibenzothiophene/17α(H),21β(H)-hopane phenanthrene/17α(H),21β(H)-hopane

1.4 1.4 2.2 0.6 0.3 1.8 1.3

0.1 0.2 0.3 0.3 0.3 0.4 3.6

a

In particular, note the differences in sulfur-containing molecules (dibenzothiophenes) between Ixtoc I and DWH crude oils.

Figure 3. Plot of the RDBE vs carbon number for compounds containing sulfur hetero atoms detected in Ixtoc I crude oil, produced using a petroleomics application for LECO’s ChromaTOF software tailored for high resolution multidimensional GC × GC data. In this case, the PASHs were identified for compounds with RDBE values of 6, 9, 11, 12, and 14 (corresponding to benzothiophenes, dibenzothiophenes, phenanthrothiophenes, benzonaphthothiophenes, and chrysenothiophenes).

composition of an analyte molecule as well as the composition of fragment ions produced by EI or chemical ionization (CI; not employed in this study). After signal processing and deconvolution of the GC × GC−HRT data, a plot of the ring double-bond equivalents (RDBE) versus carbon number was produced. Plots analogous to Figure 3 are already extensively used in petroleomics studies for non-GC amenable compounds using ultrahigh resolution Fourier-transform ion cyclotron mass spectrometry (FTICR-MS) methods (so-called modified Kendrick plots).25−27 Here, we have applied a similar mass spectral analysis application to GC × GC−HRT data. Utilizing this approach, we were able to tentatively identify three additional families of sulfur-containing compounds in addition to benzothiophenes and dibenzothiophenes: phenanthrothiophenes, benzonaphthothiophenes, and chrysenothiophenes. Structures, molecular formulae, monoisotopic mass of molecular ions, RDBE, and mountain plots for all of PASH species are provided in Figures S2−S4 and Tables S1−S5. Mass spectra for representative molecules of the phenanthrothiophene, the benzonaphthothiophene, and the chrysenothiophene suites of compounds are provided in Figures S5−S11, S12−S17, and S18−S20, respectively. Particularly notable is the capability of the HRT accurate mass data for

detailed characterization of alkylated homologs of sulfur heteroaromatics. We expected to find these alkylated components in the HRT data matrix based upon our analysis of GC × GC−TOF mass spectral data, although we had not anticipated the alkylation series to extend out to 22 carbons in the benzothiophene series (Figure 3). Uncovering an alkylated phenanthrothiophene series to C5, an alkylated benzonaphthothiophene series to C6, and an alkylated chrysenothiophene series to C3 while mining the HRT data highlights the power of GC × GC coupled with a high mass resolution detector and petroleomics-based mass spectral data analysis tools. Similar species of highly alkylated benzothiophenes, dibenzothiophenes, and their naphthenoand higher benzo-analogs have been previously observed in heavy biodegraded oils, that is, oil sands bitumen, analyzed by field ionization mass spectrometry.24 Thus, RDBE versus carbon number plots for specific heteroatoms are a rapid and potent data reduction tool to quickly identify families of petroleum fingerprinting compounds that can be used as powerful source discriminators. These findings greatly expand the inventory of previously characterized sulfur heteroaromatic species in Ixtoc I crude oil, which, for example, D

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Figure 4. Plot of the RDBE vs carbon number for compounds containing nitrogen hetero atoms detected in Ixtoc I crude oil, produced using a petroleomics mass spectral data analysis application for LECO’s ChromaTOF software. In this case, nitrogen containing compounds were identified as condensed, polycyclic pyrrolic systems, such as carbazoles, RDBE 9, and benzocarbazoles, RDBE 12. All of the ions identified within the black dotted line oval are derived from carbazoles with an RDBE value of 9 (RDBE 10 & 11 data points are produced from fragment ions of RDBE 9 carbazole compounds).

ment, and distinguishing of different source oil families in the GoM (and other) spill scenarios. In addition to heteroatom species, another distinction between the Ixtoc I and DWH crude oils has been achieved by analyzing the distribution of biomarker molecules (Table 1). Biomarkers are molecular fossils that provide insights into the depositional environment and in some cases, the biological source of petroleum-forming organic material; in addition, the degree of epimerization of some of the biomarkers is an indicator of the thermal history (thermal maturity) of a given petroleum.30 Because the compounds present in this region of the GC × GC chromatogram have high boiling points, are nonpolar, and are somewhat resistant to biodegradation, they are among the most useful compounds for fingerprinting petroleum.17 A pair of plan view chromatograms focusing on the biomarker region of Ixtoc I and DWH crude oils is presented in Figure 5; even by simple visual inspection, it is clear that these two oils can be easily differentiated from one another using biomarker ratios. The most obvious difference between these crudes is the presence or absence of de-Asteranes (de-A-cholestane, de-A-methylcholestane, and de-Aethylcholestane are present in Ixtoc I crude and absent in the DWH crude). The de-A-sterane peaks represent a series of compounds that are tricyclic in nature containing a mass spectral base ion at m/z 219.2107 amu. High-resolution mass spectra of the de-A-sterane series of compounds is characterized by molecular ions at theoretical monoisotopic masses of m/z 374.3907, 388.4064, and 402.4220 for de-A-cholestane, de-A-methylcholestane, and de-A-ethylcholestane, respectively. Interestingly, de-A-sterane compounds have been reported in heavily biodegraded crude oils from the Zhungeer Basin in northwest China, but the Ixtoc I crude does not appear to show other indications of the same level of biodegradation.30,31

extended only up to C6 alkyl benzothiophenes and C3 alkyl dibenzothiophenes.28 Another heteroatom-bearing compound class that was detected utilizing petroleomics spectral analysis of the HRT data from Ixtoc I crude oil are the homologous series of alkylated carbazoles and benzocarbazoles, Figure 4. These nitrogen containing compounds are barely detectable in the DWH crude. We were able to identify an abundant alkyl homolog series of carbazoles (RDBE 9), up to C6, thus slightly extending previously reported C2−C5 carbazole suite in Ixtoc I crude oil, identified using GC−MS.28 We also note an alkylated series, up to C4, of nitrogen species with RDBE values of 10 and 11. Examination of accurate mass ion chromatograms and mass spectra indicates that the ions identified as RDBE 10 and 11 compounds are, in fact, fragment ions produced in the EI source of the GC × GC−HRT of the carbazole (RDBE 9) compounds. The mass spectra of the Ixtoc I crude suite of carbazole compounds all contain ions identified with the RDBE spectral analysis software.29 An important caveat is that RDBE spectral analysis of compounds that have been ionized via an EI source contain numerous fragment ions that present themselves in the RDBE plot; thus, careful analysis of the mass spectra and GC × GC elution positions of all of the ions identified via RDBE spectral analysis need to be verified, especially when the data have been acquired in the EI mode. Further information on the elution position and ions identified via RDBE plots of nitrogen containing molecules can be found in Figures S21−S23. In addition, we also detected a similar alkylation series, extending to the C5 member, within the benzocarbazole suite (RDBE 12). As in the case of sulfur species, this highly specific characterization of nitrogen containing compounds can be very useful for source apportionE

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6, illustrating once more their usefulness for identification and differentiating these crudes in marine spill scenarios. An additional powerful tool for interpreting complex GC × GC data are “difference chromatograms”, which can easily capture the overall compositional variability between any two samples, stemming from genetic differences of investigated crude oils, and/or weathering processes such as evaporation, water washing, photo-oxidation, and biodegradation.7,32 Figure 7 is an example of a difference chromatogram produced by subtracting the composition of DWH crude from that of Ixtoc I crude. In this chromatogram, the base plane appears white, compounds that are more abundant in the DWH crude appear red (negative values), and compounds that are more abundant in the Ixtoc I crude are blue (positive values). These chromatograms were normalized to mass injected on column, and numerous distinct differences between these two crude oils from the GoM are readily apparent. The most notable difference between these two crude oils is that the light-end compounds (n-C8 to ∼n-C14) are far more abundant in the DWH crude. This result stems from how these two samples were collected and/or stored, and may not accurately reflect the composition of the light-end compounds in Ixtoc I crude. The DWH crude oil was collected directly from the pipe inside the damaged blow-out preventer on the seafloor as reservoir hydrocarbon fluids were escaping into the environment on June 21, 2010 and the Ixtoc I crude was collected as floating oil on the sea surface near the head of the surface oil plume on September 16, 1979 during a joint NOAA ship R/V Researcher and R/V Pierce Ixtoc I Research cruise (Dr. Donald K. Atwood, Chief Scientist). Evaporative losses of the light hydrocarbons from the Ixtoc I crude sea-surface residence time versus intact reservoir hydrocarbons in the DWH crude are the primary reason that the light-end compounds in the DWH crude appear to be more abundant. Although there is a sample bias between Ixtoc I and DWH (light-end bias toward DWH and mid- to heavy-end bias toward Ixtoc I) on a per mass basis due to evaporation of light-end compounds in the Ixtoc I crude sample, the diasteranes in the DWH crude are more abundant (appear red). This is an indication that the DWH crude oil is more thermally mature than Ixtoc I as diasteranes are thermal rearrangement products of steranes and good indicators of petroleum source rock thermal history.30 This observation is supported by other thermal maturity indicators, specifically, the Ts/Tm ratio (Ixtoc I = 0.7 and DWH = 1.3, Table S1). In addition to the diasteranes, the DWH crude also shows more abundant isoprenoid alkanes in the n-C24−n-C34 carbon range, but the isoprenoid alkane biphytane (C40H82) is more abundant in the Ixtoc I crude (isoprenoid alkanes including biphytane believed to be derived from thermophilic and/or planktonic archaea are labeled in Figure 7).30 Lastly, although the subtraction chromatogram is biased toward midto heavy-end compounds in the Ixtoc I crude, the normal alkanes in the n-C33−n-C38 carbon range appear to be more abundant in the DWH crude. As shown in Figure 3, Ixtoc I crude oil contains numerous PASH compounds. Figure 2 highlighted just the region encompassing dibenzothiophene along with the C1, C2, and C3 dibenzothiophenes; in the Figure 7 difference chromatogram, all of the benzothiophenes and dibenzothiophenes are present and the most abundant sulfur containing compounds are circled (black dotted lines). In summary, the difference chromatogram between Ixtoc I and DWH support or reinforce visual inspections of each individual GC × GC chromatogram. While generally similar, the major

Figure 5. GC × GC−FID plan view plot comparison of the diasterane/sterane and hopane biomarker region of (a) Ixtoc I and (b) DWH crude oils. This figure provides a visual comparison of the sterane and hopanoid biomarker molecules present in each sample. Here, the differences between both crude oils are most apparent. For example, the ratio of the DiaC27βα-20S diasterane peak and the peak labeled H (17α(H),21β(H)-hopane) is dramatically different in each crude oil. The NH (17α(H),21β(H)-30-norhopane ) to H (17α(H),21β(H)-hopane) ratio between the Ixtoc I and DWH samples is another easily visualized biomarker pair that can be used to distinguish these oils from one another. Lastly, note the compounds labeled de-A-cholestane, de-A-methylcholestane, and de-A-ethylcholestane in the Ixtoc I sample (blue print) are steranes in which the A ring of the sterane molecules have been cleaved open (presumably via biodegradation) and absence of these compounds in the DWH sample (mass spectra of the de-A-steranes are present in Figures S25−S27).

Mass spectra of the de-A series of steranes are provided in Figures S25−S27. Another feature of the Ixtoc I crude, compared to the DWH product, is that the 17α(H),21β(H)-30-norhopane; the NH peak is more abundant than the peak labeled 17α(H),21β(H)hopane; H in the Ixtoc I crude, this is a feature characteristic of oils sourced from organic rich carbonates30,35 In addition, another notable difference, useful for discriminating between Ixtoc I and DWH is that the diasterane DiaC27βα-20S is far more abundant than 17α(H),21β(H)-hopane (H) in the DWH crude than in Ixtoc I crude. The relatively high abundance of the diasteranes in DWH crude is an indication that this petroleum is thermally mature as a high degree of thermally induced rearrangements from steranes to diasteranes has occurred.30 Biomarker ratios for Ixtoc I and DWH crude oils are graphically summarized as a spider web plot in Figure F

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Figure 6. Graphical representation of potentially useful biomarker ratios for differentiating between Ixtoc I and DWH crude oils. A more extensive table of ratios can be found in Tables S9−S11.

Figure 7. Difference chromatogram produced with GC × GC−FID data that represent a comparison of two crude oil spills from the GoM. The base plane in this chromatogram appears white, compounds that are more abundant in the Ixtoc I crude appear blue, and compounds that are more abundant in the DWH crude appear red. This subtraction chromatogram was normalized by mass injection (1 μL of a 15 mg/mL solution in hexane) of each crude oil sample.

the most iconic oil spills in the GoM; Ixtoc I, and DWH. Coupling a high-resolution mass detector is a substantial analytical improvement to GC × GC systems, which enables very granular identification of numerous alkylated homologue series, including within heteroatom containing compound classes, demonstrating a promising potential of GC × GC− HRT to become a complementary tool to typical petroleomics instrumentation such as FTICR-MS, for the studies of lower

differences between these crude oils stem from specific genetic and geochemical attributes of each petroleum, as observed within sterane, hopane, and dibenzothiophene fingerprints.

4. CONCLUSIONS This work showcases the resolving capacity of comprehensive two-dimensional GC for exhaustive characterization of volatile petroleum species, using the example of crude oils from two of G

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molecular weight, GC-amenable petroleum species. In addition, HRT data facilitated the identification of unusual de-A-sterane biomarkers, present in Ixtoc I crude. These novel analytical capabilities can be leveraged in multiple innovative ways to advance oil spill science: more robust spill source apportionment in complex systems with concurrent spill sources, such as the GoM; identification of molecular transformations occurring during crude oil weathering process, for improved predictive models of the long-term fate of persistent petroleum hydrocarbons in the environment; better toxicity assessments, in particular, because of more comprehensive characterization of highly alkylated and heteroatombearing species, which often have higher toxic potential than the parent compounds.33 More broadly, expansion of the analytical window and identification of novel molecular marker compounds will advance all the areas of organic (geo)chemistry, for example, petroleum geochemistry as a field of research which will likely benefit from these types of highresolution analyses.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b04384. GC × GC−FID and −TOF methods; GC−FID traces of the three investigated oils; structures, molecular formulas, mountain plots and monoisotopic masses of base molecules and RDBEs of PASHs identified in Ixtoc I crude oil; comparison of Hopane Biomarker Ratios from Ixtoc I, DWHMacondo well (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Robert K. Nelson: 0000-0003-0534-5801 Author Contributions

R.K.N., D.J.H., S.A.M., and C.M.R. designed the study; C.M.R. conducted the DWH sampling; D.J.H., and S.A.M., supplied the archival Ixtoc I crude oil; R.K.N. and K.M.G. conducted laboratory measurements; A.G., R.K.N., D.J.H., S.A.M., J.R.R, and C.M.R participated in data analysis; R.K.N., J.R.R, and C.M.R. wrote the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative. org (DOI: 10.7266/n7-xx61-4h59, UDI: R4.x267.179:0021). The authors wish to thank Dr. John Farrington for useful information, insights, and discussion regarding the Ixtoc I oil spill. The authors also wish to thank Ray Clifford, Lorne Fell, Joe Binkley, Michael Mason, and Christina Kelly from LECO for technical support on all of our GC × GC instruments. H

DOI: 10.1021/acs.energyfuels.8b04384 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.8b04384 Energy Fuels XXXX, XXX, XXX−XXX