Interpreting Chemical Structures of Compounds in Crude Oil Based on

Jun 12, 2017 - In this study, the tandem mass spectra of 25 standard compounds, obtained at a normalized collision energy of 50, were used to investig...
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Interpreting Chemical Structures of Compounds in Crude Oil Based on the Tandem Mass Spectra of Standard Compounds Obtained at the Same Normalized Collision Energy Jihyun Ha,† Eunji Cho,† and Sunghwan Kim*,†,‡ †

Department of Chemistry, Kyungpook National University, Daegu 702-701, Republic of Korea Green-Nano Materials Research Center, Daegu 702-701, Republic of Korea

‡

S Supporting Information *

ABSTRACT: In this study, the tandem mass spectra of 25 standard compounds, obtained at a normalized collision energy of 50, were used to investigate the structures of crude oil. The product ion distributions of these standard compounds were dependent on the number and length of their alkyl side chains. Long alkyl side chains (CnH2n, n ≥ 5) were easily lost during molecular collision, leaving behind methyl groups on the aromatic cores. In contrast, compounds with short interlinking alkyl chains (CnH2n, n ≤ 1) were not significantly fragmented, and a decrease in the double bond equivalent (DBE) value was observed when compounds had saturated rings. Based on the derived observations, the possible structures of S1-class compounds with DBE values of 6, 7, 8, and 9 were suggested. It was also determined that, in the tandem mass spectra of crude oil fractions, (1) a series of peaks separated by CH2 groups were generated from isomers with different numbers of alky side chains, (2) the number of methyl groups left after molecular collision was positively correlated to the quantity of branched molecules, and (3) significant fragmentation did not occur for aromatic cores linked by short alkyl chains (CnH2n, n ≥ 2), and hence, archipelago structures with short alkyl linkage could not be excluded based solely on tandem mass spectra. This study clearly shows that a systematic analytical approach using a well-defined set of standard compounds combined with tandem mass spectrometry can significantly improve our understanding on the chemical structures of crude oil compounds.

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INTRODUCTION Mass spectrometry (MS) is one of the most important analytical techniques used to study chemical constituents in food, drug, and environmental samples.1−3 In particular, highresolution mass spectrometry (HR-MS) is an important technique that provides elemental formulas of unknown compounds in crude oils.4,5 The elemental formula is an important piece of information in understanding the chemistry of crude oils, and HR-MS also plays an important role in petroleomics.6−10 However, establishing the elemental formulas of compounds on their own is not enough to fully elucidate the chemistry of crude oils. To further understand the chemistry and reactions of crude oils, the corresponding structures of the assigned elemental formulas need to be identified. Therefore, double bond equivalent (DBE) distribution, planar limit, hydrogen− deuterium exchange, chromatographic separation, and ion mobility mass spectrometry have been used to study the structures of chemical constituents in crude oils.11−22 Owing to the complexity of crude oil, an accurate identification of chemical structures can be difficult, and hence, it is necessary to develop more reliable analytical methods. Tandem mass spectrometry has been used to study the structures of aromatic compounds in heavy crude oils.12,14,23,24 A series of peaks, separated exactly by a differing number of -CH2 groups, were previously observed from the tandem mass spectra of analyzed oil samples, and it has been proposed that such peak distribution is obtained via the breakage of -CH2− CH2- bonds on the alkyl side chains.14,24 The removal of the © XXXX American Chemical Society

alkyl side chains leaves a core structure, which can then be identified using its resulting mass spectrum. For example, in previous studies of crude oil compounds, it has been suggested that archipelago structures are more dominant than island structures.24 However, we believe that the potential of tandem mass spectrometry in identifying the structures of chemical constituents in crude oils has not been fully explored. A systematic approach has not been developed for comparing the tandem mass spectra of crude oils with those obtained from a well-defined set of standard compounds. In this study, the tandem mass spectra of 25 standard compounds with various functional groups were obtained, and the results were used to interpret the structures of crude oil compounds. The standard compounds were chosen so that they could represent chemical structures that had been reported in the previous study.25

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EXPERIMENTAL SECTION

Sample Preparation. A total of 25 standard compounds were purchased from Sigma-Aldrich (St. Louis, MO, USA) and TCI (Tokyo, Japan). Details of the standard compounds used in this study were tabulated and provided in the Supporting Information (Table 1S). All samples were dissolved in toluene (high-performance liquid chromatography (HPLC) grade, Honeywell Burdick & Jackson, Ulsan, Korea) at a concentration of 100 ppm (0.1 mg/mL) before undergoing mass spectrometry analysis. The Kuwait oil was Received: March 27, 2017 Revised: June 7, 2017 Published: June 12, 2017 A

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Figure 1. Tandem mass spectra for the isomers of C10H14 [(a) butylbenzene, (b) isobutylbenzene, (c) 1,2-diethylbenzene, and (d) 1,2,4,5tetramethylbenzene], of C18H30 [(e) 1-phenyldodecane, (f) 1,2,4,5-tetraisopropylbenzene, and (g) hexaethylbenzene], and of C19H20 [(h) 9pentylphenanthrene and (i) 1,4,5,8,9-pentamethylbenzene], obtained at a normalized collision energy value of 50. Molecular ions were selected for collision induced dissociation, and the parent molecules have been listed at the right side of each spectrum. according to eq 1.28 NCE is used to optimize the collisional fragmentation of ions with different m/z values.

fractionated prior to MS analysis according to a previously reported HPLC method.26,27 Briefly, the oil was first dissolved in heptane at a concentration of 4500 ppm (4.5 mg/mL), and it was then injected into a separation system that consisted of two automated six-port valves (Rheodyne, CA, USA) and two different columns, i.e., a propylaminocyano column (250 mm × 4.6 mm, Phenomenex, CA, USA) and a dinitroanilinopropyl column (250 mm × 4.6 mm, ES Industries, Berlin, NJ, USA). Compounds in this sample were separated into five different fractions. Thereafter, the collected fractions were dried out and re-dissolved in toluene for analysis. Mass Spectrometry. Oil fractions and standard compounds were analyzed using a Q-Exactive mass spectrometer (Thermo Fisher Scientific Inc., Rockford, IL, USA) that utilizes atmospheric pressure photoionization (APPI) in the (+)-mode. Sample solutions were injected directly into the APPI source, and selected ions with a signal width of m/z 1 (M ± 0.5 m/z) were collected in the collision cell. The normalized collision energy (NCE) was set to various values, ranging from 10 to 200, and the charge factor was fixed at 1. NCE is a dimensionless number, and it is converted into collision energy (eV)

collision energy/eV =

(setting NCE) × (isolation center) (500 m/z) × (charge factor)

(1)

The aux gas flow rate was 5, while the sheath gas flow rate was 10 in arbitrary units. The vaporizer and capillary temperatures were set at 350 and 320 °C, respectively. The S-lens radio frequency (RF) level was set at 20 Hz. For the interpretation of data obtained from the crude oil fractions analyzed, an in-house software“statistical tool for organic mixture spectra” with an automated peak-picking algorithm was used.29,30 Double bond equivalent values were calculated using eq 2 for the elemental formula CcHhNnOoSs.31

DBE = c − h/2 + n/2 + 1 B

(2) DOI: 10.1021/acs.energyfuels.7b00882 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 2. Tandem mass spectra of aromatic compounds with long alkyl chains: (a) 1-phenyldecane (C16H26), (b) 1-phenylheptadecane (C23H40), (c) 1,4-didodecylbenzene (C30H54), and (d) 1-dodecylnaphthalene (C22H32), obtained at a normalized collision energy value of 50. Molecular ions were selected for collision induced dissociation, and the parent molecules have been listed at the right side of each spectrum.

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RESULTS AND DISCUSSION Aromatic Compounds with Alkyl Side Chains. Tandem mass spectrometry data were obtained from nine compounds with alkyl side chains at various collision energy values. The tandem mass spectra are all presented in the Supporting Information (Figure 1S). Spectra with an NCE value of 50 (20−30 eV collision energy) were selected and presented in Figure 1. Assigned structures corresponding to the most abundant peaks are also presented in the same figure. NCE of 50 was chosen in this study because significant fragmentation could be observed from the standard compounds and at the same time different fragmentation could be obtained from isomers. The spectra obtained from isomers were very similar when very high collision energy was used. For an example, the spectra of butylbenzene and isobutylbenzene was very similar when NCE of 200 was used (compare Figure 1Sa,b). The tandem mass spectra of four compounds with a chemical formula of C10H14 (butylbenzene, isobutylbenzene, 1,2diethylbenzene, and 1,2,4,5-tetramethylbenzene) are presented in Figure 1a−d. Even though these compounds had the same core structure and m/z values, their respective product ion distributions were quite different. The methylbenzene ion (m/z 91.055) was the most abundant species in the tandem mass spectrum of butylbenzene (Figure 1a) whereas the dimethylbenzene ion (m/z 105.070) appeared as the main species in the spectrum of 1,2-diethylbenzene (Figure 1c). In the case of 1,2,4,5-tetramethylbenzene, the trimethylbenzene ion (m/z 119.086) was the most abundant species (Figure 1d). Therefore, the data presented in Figure 1 suggest that the product ion distribution is dependent on the number and length of the alkyl side chain. With an increasing number of branched molecules, more methyl groups are left behind after undergoing molecular collisions at an NCE value of 50. The

difference in fragmentation pattern can be explained by the difference in bond dissociation energies of CH3−CH3 (∼88 kcal/mol) and C6H5−CH3 (∼100 kcal/mol) bonds.32 Meanwhile, the tandem mass spectra of compounds with a chemical formula of C18H30 (1-phenyldodecane, 1,2,4,5tetraisopropylbenzene, and hexaethylbenzene) are shown in Figure 1e,f,g. Different fragmentation patterns were also observed for compounds with the same core structures. The methylbenzene ion (m/z 91.055) was the most abundant species for 1-phenyldodecane (Figure 1e) but not in the spectra presented in Figure 1f,g. The tandem mass spectra of compounds with a chemical formula of C19H20 (9-pentylphenanthrene and 1,4,5,8,9-pentamethylanthracene) are shown in Figure 1h,i. The 9-methylphenanthrene ion (m/z 191.085) was the most abundant product ion generated by 9-pentylphenanthrene, while pentamethylanthracene afforded the tetramethylanthracene ion (m/z 233.132) as the main species. It is interesting to note that long alkyl side chains are easily lost, thus mostly leaving methyl groups behind after undergoing collisions at an NCE value of 50 (refer to Figure 1a, e, h. To further investigate the behavior of aromatic compounds with long alkyl chains (CnH2n, n ≥ 5), four compounds (i.e., 1,4didodecylbenzene, 1-phenyldecane, 1-phenylheptadecane, and 1-dodecylnaphthalene) were analyzed, and the obtained spectra are presented in Figure 2. The methylbenzene ion (m/z 91.055) was the most abundant species for both 1-phenyldecane and 1-phenylheptadecane (Figure 2a,b), whereas the dimethylbenzene ion (m/z 105.070) was the primary species detected for 1,4-didodecylbenzene (Figure 2c). In the meantime, the methylated naphthalene (m/z 141.070) ion was the most abundant species for 1-dodecylnaphthalene (Figure 2d). The data presented in Figures 1h and 2 clearly show that long alkyl chains (CnH2n, n ≥ 5) are easily lost after undergoing C

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Figure 3. Tandem mass spectra of aromatic compounds with alkyl interlinkage between opposing cores: (a) biphenyl (C12H10), (b) diphenylmethane (C13H12), (c) dibenzyl (C14H14), (d) 1,3-diphenylmethane (C15H16), (e) 1-phenylnaphthalene (C16H12), (f) 1-benzylnaphthalene (C17H14), (g) 1-(2-phenylethyl)naphthalene (C18H16), and (h) 1,2-di(1-naphthyl)ethane (C22H18), obtained at a normalized collision energy value of 50. Molecular ions were selected for collision induced dissociation, and the parent molecules have been listed at the right side of each spectrum.

relative abundance of m/z 91 and m/z 92 ions in Figure 1 show that relatively high energy collision was used in this study. Aromatic Compounds with Alkyl Interlinkage between Cores. Eight compounds with an alkyl interlinkage between their respective aromatic cores, namely, biphenyl, diphenylmethane, dibenzyl, 1,3-diphenylpropane, 1-phenylnaphthalene, 1-benzylnaphthalene, 1-(2-phenylethyl)naphthalene, and 1,2-di(1-naphthyl)ethane, were analyzed by tandem mass spectrometry, and their spectra, which were obtained at various collisional energies, are shown in Figures 3 and 2S. Structures of the observed product ions are indicated in the figures. Panels a, b, e and f of Figure 3 show the data of compounds whose aromatic rings are linked by no or only one carbon atom between the rings. It was observed that the linkage between the aromatic rings was not broken at an NCE value of 50. The data presented in Figure 2S show that the breakage of an alkyl linkage did not occur even at higher collision energy. Meanwhile, the spectra shown in Figure 3c,d,g,h were obtained

molecular collision and, subsequently, methylated or dimethylated ions become the most abundant species in tandem mass spectra obtained at an NCE value of 50. A series of peaks separated by 14 CH2 groups was observed in the spectra, and they are indicated in Figures 1 and 2. A difference in the number of CH2 groups, as observed from the alkylated aromatic compounds studied, has been reported previously.14,24 However, the series of peaks separated by 14 CH2 groups was not prominent in the spectra of compounds with long alkyl chains (refer to Figure 2). In contrast, compounds with three or more side alkyl chains prominently showed a series of peaks evenly spaced by 14 CH2 groups (Figure 1f,g). It is important to note that m/z 91 ion was more abundant than m/z 92 ion in the tandem mass spectra of butylbenzene presented in Figure 1a. It has been well-documented that m/z 92 ion is more abundant at low energy but m/z 91 ion is at high energy and hence m/z 91/92 ratio has been suggested as an indicator of variation of parent ion internal energy.33,34 The D

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Figure 4. Tandem mass spectra of aromatic compounds with saturated cyclic ring(s): (a) 1,2,3,4-tetrahydronaphthalene (C10H12), (b) 1,2,3,6,7,8hexahydropyrene (C16H16), (c) 9,10-dihydroanthracene (C14H12), and (d) 4,5,9,10-tetrahydropyrene (C16H14), obtained at a normalized collision energy value of 50. Molecular ions were selected for collision induced dissociation, and the parent molecules have been listed at the right side of each spectrum.

tandem mass spectra of crude oils, and it is attributed to the loss of alkyl chains. However, the data presented in Figure 4 show that compounds with saturated rings can also generate these peaks. For example, the peaks at m/z 179.085 and 165.070 were observed in the tandem mass spectra of 9,10dihydroanthracene (Figure 4c). The loss of a saturated ring was observed in 1,2,3,4tetrahydronaphthalene, 1,2,3,6,7,8-hexahydropyrene, and 4,5,9,10-tetrahydropyrene. For example, the fragmentation of 4,5,9,10-tetrahydropyrene with a DBE value of 10 resulted in ions with a DBE value of 9 at m/z 165.070 (Figure 4d). In addition, the peak at m/z 181.101 in Figure 4b also shows a decrease in the DBE value possibly resulting from stepwise ring opening followed by the C2H4 loss. Interpreting the Tandem Mass Spectra of Crude Oil. One and two aromatic ring fractions obtained from the crude oil sample were analyzed, and molecular ions of five sulfurcontaining peaks with m/z 316.222 ([C21H32S]+), 314.206 ([C 2 1 H 3 0 S] + ), 312.190 ([C 2 1 H 2 8 S] + ), and 324.190 ([C22H28S]+) (each having a DBE value of 6, 7, 8, and 9) were subjected to tandem mass spectrometry analysis. Spectra acquired at an NCE value of 50 are shown in Figure 5, while the rest (obtained at various collision energies) are shown in Figure 4S. A selection window of m/z 1 was used, and the peaks were chosen so that only one major peak exists in the selected m/z range. The elemental formulas of the peaks and the energy used for fragmentation were marked in the figure. It is important to note that a series of peaks separated by m/z 14.014 (a difference of one CH2 group) was observed in all the spectra presented in Figures 5 and 4S. Based on published tandem mass spectra of oil compounds, peaks that are separated by an m/z value of 14.014 are well-documented, and they have been attributed to CH2 groups that are lost from the alkyl chain attached to the aromatic core.14,24 In Figure 2, it is shown that compounds with long alkyl chains could easily lose their chains but they did not produce a significant series of

from compounds with two or more carbons between their aromatic rings. For these types of compounds, a significant fragmentation of the interlinking bonds was observed. As a result, toluene and methylated naphthalene ions were the most abundant species in these spectra. Infrared multiphoton dissociation (IRMPD) has been used to investigate fragmentation patterns of oil compounds.12 It would be worthwhile to obtain IRMPD tandem mass spectra of the standard compounds used to generate Figure 3 and compare the results. In summary, the data presented in Figure 3 show that tandem mass spectrometry can be used to estimate the number of carbons in an alkyl chain [(CH2)n] that interlinks opposing aromatic groups. When n was less than 2, either an insignificant fragmentation or the loss of CH2 was observed at high collision energy. In contrast, when n was equal to or larger than 2, the interlinking alkyl chain underwent a significant fragmentation and the m/z values of the observed ions were reduced. In previous studies, it has been assumed that the lack of a significant breakage and a sudden shift in the m/z values to lower numbers would prove that crude oil compounds have island but not archipelago structures.24 However, the data presented in Figures 3 and 2S clearly show that this did not apply to compounds with short alkyl chains (e.g., n < 2). Therefore, the lack of a significant breakage, as observed by tandem mass spectrometry, can only exclude archipelago structures for compounds with long alkyl chains (CnH2n, n ≥ 2) connecting their aromatic cores. Tandem Mass Spectra of Aromatic Compounds with Saturated Rings. The tandem mass spectra of aromatic compounds with saturated rings (1,2,3,4-tetrahydronaphthalene, 1,2,3,6,7,8-hexahydropyrene, 9,10-dihydroanthracene, and 4,5,9,10-tetrahydropyrene) were obtained at various collision energies and shown in Figures 4 and 3S. Spectra acquired at an NCE value of 50 are shown in Figure 4, while the rest are shown in Figure 3S. Peaks separated by m/z values that are equivalent to multiple CH2 groups are commonly found in the E

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Figure 5. Tandem mass spectra of S1-class compounds with DBE values of (a) 6, (b) 7, (c) 8, and (d) 9, obtained at a normalized collision energy value of 50. Molecular ions with m/z 316.222 ([C21H32S]+), 314.206 ([C21H30S]+), 312.190 ([C21H28S]+), and 324.190 ([C22H28S]+) were subjected to collision induced dissociation.

desulfurization process. 36 Hexahydrodibenzothiophenes (C12H14S) also have a DBE value of 6. If hexahydrodibenzothiophenes (C12H14S) were abundant, we would expect to see the product ions of dihydrobenzothiophene (C8H8S) as the saturated cyclic ring could be lost via molecular collision (refer to Figure 4). However, no ions originating from dihydrobenzothiophene (C8H8S) or alkylated dihydrobenzothiophene (C9H10S) were observed in the spectra. Therefore, it was concluded that compounds with a hexahydrodibenzothiophene (C12H14S) core structure are not abundant in the observed spectra. The tandem mass spectrum of a compound with a DBE of 7 is presented in Figure 5b. Peaks assigned to the alkylated benzothiophene species (DBE = 6) were observed in the spectrum (m/z 161.042 and 175.058). As shown in Figure 4, a decrease in the DBE value was observed for compounds with saturated cyclic rings. Therefore, it was concluded that compounds with a DBE value of 7 have saturated cyclic rings. Based on the aforementioned information, tetrahydrodibenzothiophene (C12H12S) was suggested as the core structure for compounds with a DBE value of 7. Meanwhile, the peak at m/z 215.089 (one of the most abundant peaks in the spectrum) can be assigned to the dimethyltetrahydrodibenzothiophene ion ([C14H16S−H]+).

peaks that were separated by CH2 groups at the stipulated collision energy. Instead, compounds with shorter side chains generated obvious CH2-separated peaks (Figure 1g,h). A pattern of a series of peaks separated by CH2 are not likely to be produced by through losses of a series of CH2 groups because CH2 is a carbene that is unlikely to be produced. It is likely that the pattern is produced through various competitive fragmentations (for instance one loss of ethyl and one loss of propyl) from the different species in the mixtures. Therefore, it is concluded that the observed pattern of multiple CH2separated peaks shown in Figure 5 can be attributed to the loss of alkyl groups from a mixture of isomers that have various numbers of alkyl side chains attached to their aromatic cores. The tandem mass spectrum of a compound with a DBE value of 6 is presented in Figure 5a. It is well-known that there is an abundance of compounds with benzothiophene (C8H6S) structures in crude oil, and thiophene has a DBE value of 6.35 In Figure 5a, the product ion at m/z 175.058 was the most abundant species, which can be assigned to the trimethylbenzothiophene ion ([C11H12S−H]+) (refer to the structure shown in Figure 5a). This agrees with the previously described observation that mostly methyl groups are left behind after the parent compound undergoes collision at an NCE value of 50. Compounds with hexahydrodibenzothiophene (C12H14S) structures were reported in analyzed samples after a hydroF

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compounds with DBE values of 6, 7, 8, and 9. This study clearly shows the potential of using high-resolution tandem mass spectrometry to elucidate the structures of crude oil compounds. Moving forward, we aim to use a hyphenated analytical approach, by incorporating other techniques such as ion mobility mass spectrometry, to further verify our structural assignments.

The mass difference between compounds with a DBE value of 6 versus 7 is 2 Da; hence, the collision energy applied to the ions is almost identical. However, the product ion distributions shown in Figure 5a,b were quite different. Fragmentation was more significant for compounds with a DBE value of 6 compared to those with a DBE value of 7. For example, the [M−CH3]+ peak was significantly less abundant in Figure 5a in comparison to Figure 5b. This difference can be attributed to the number of alkylation sites. Benzothiophene (DBE = 6) has six possible alkylation sites whereas tetrahydrodibenzothiophene has eight potential sites. The hexamethylbenzothiophene ion ([C14H18S−H]+) appeared at m/z 217.104 with a mass difference of about 99 Da from its parent molecular ion, which corresponds to one CH3 and seven CH2 groups. In the case of octamethyltetrahydrodibenzothiophene ([C20H28S−H]+) at m/ z 299.183, it only has a mass difference of one CH3 group from its parent molecular ion. This means that the compound with a DBE value of 6 (Figure 5a) has a longer alkyl chain compared to that with a DBE value of 7. The tandem mass spectrum of a compound with a DBE value of 8 is presented in Figure 5c. Unlike the spectra in Figure 5a,b, there was a relatively low abundance of the alkylated benzothiophene (m/z 161.042 and 175.058) species. Therefore, it was concluded that benzothiophene is not the major structural motif. In the spectrum, the peak at m/z 213.073 had the highest abundance, and it was assigned to the dimethyldihydronaphthothiophene ion ([C 14 H 14 S−H]+ ). Meanwhile, the [M−-CH3]+ peak was abundant in the spectrum of Figure 5c, and it was assigned to the octamethyldihydronaphthothiophene ion ([C20H26S−H]+). Figure 5d shows the tandem mass spectrum of a compound with a DBE value of 9. It is well-documented that dibenzothiophene (C12H8S) is one of the major sulfurcontaining compounds observed in crude oil, with a DBE value of 9.35 In this case, the most abundant product ion appeared at m/z 225.073, and it was assigned to the trimethyldibenzothiophene ion ([C15H14S−H]+). The octamethyldibenzothiophene ion ([C20H24S−H]+) was also assigned and indicated in the figure. It is important to note that the structures in Figure 5 are suggested based on the observation that long alky chains are easily lost after collision with energy of NCE 50. Therefore, it was assumed that only short alkyl chains would survive. However, the suggested structures presented in Figure 5 are ones out of various possible structures. Further study possibly including advanced technique such as ion mobility mass spectrometry is required to further verify the chemical structures.

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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.7b00882. Table of compounds studied and figures of tandem mass spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 82-53-950-5333. Fax: 82-53-950-6330. E-mail: [email protected]. ORCID

Sunghwan Kim: 0000-0003-4880-4174 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIP), Grant No. 2014R1A2A1A11049946 and 2017R1A2B3003455.

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REFERENCES

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CONCLUSIONS In summary, the information obtained from the tandem mass spectra of 25 standard compounds show that (1) a series of peaks separated by CH2 groups in the tandem mass spectra of crude oil compounds originated from isomers with different numbers of alky side chains, (2) the distribution of tandem mass spectrometry peaks is dependent on the number and length of alkyl side chains that are attached to the aromatic cores, (3) archipelago structures with short alkyl linkages are not broken even at high collision energy, and (4) a decrease in DBE value can occur via the opening of saturated rings resulting from molecular collision. The information was then used to interpret the tandem mass spectra of crude oil fractions, resulting in the elucidation of the chemical structures of S1-class G

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DOI: 10.1021/acs.energyfuels.7b00882 Energy Fuels XXXX, XXX, XXX−XXX