Structural Characterization of Petroleum Molecules by CID FT-ICR MS with Narrow Isolation Window

Liu Ling; Zhang Yiwei; Zhang Zhihua; Hou Huandi; Zhang Qundan; Tian Songbai; Wang Wei

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Abstract: High resolution tandem mass spectrometry has been applied to obtain the structure information of petroleum samples. Here, we report a method for structural characterization of crude oil molecules by the collision-induced dissociation (CID) technology coupled with the high-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The ion isolation window was narrowed down to 1 Da to distinguish the complex homologues contained in petroleum.Aromatic model compounds and crude oil samples were measured by CID FT-ICR MS at different collision energy levels.The fragmentation of model compounds with alkyl side-chains was found to be related to the size of the aromatic rings. The fragmentation of model compounds with archipelago structures depended on the length of the bridge alkylene chain. The prevalent reaction pathway of model compounds with naphthenic rings was mainly determined by the position of naphthenic rings in the molecules. On the basis of the fragmentation pathways, the structure differences of two crude oils were recognized as different content of naphthenic rings by CID technology with 1 Da isolation window. The NMR analysis was also applied to confirm the CID results. This study exhibits the great potential of CID FT-ICR MS with narrow isolation window in the structural characterization of crude oil molecules.

Key words: FT-ICR MS; collision-induced dissociation (CID); naphthenic rings; crude oil; NMR

1 Introduction

The high-resolution mass spectrometry (HR MS) has been widely applied in the characterization of petroleum samples[1-3]. HR MS can provide the elemental formulas of petroleum molecules, which are valuable for the refining processes[4-9]. However, elemental formulas of compounds are generally insufficient for interpreting the behaviors of petroleum samples from different sources,mainly because of the existence of numerous isomers[10].The different forms of alkyl side-chains, naphthenic rings,and aromatic rings in the compounds would affect the bulk properties of petroleum samples greatly.

The collision-induced dissociation (CID) is an important tandem mass spectrometry technology, which is used to reveal the structural information of complex mixtures[11].The structures of vacuum residues and asphaltenes have been investigated by the CID Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)with a relatively wide ion isolation windows[12-16].

As one of the most complex natural organic mixtures,petroleum samples consist of a series of homologues,and the distance of the closest homologues in mass spectra is only 2 Da. As a result, the isolation window of CID spectra should not be wider than 2 Da in order to distinguish different homologues contained in petroleum. Recently, the CID technology with narrow isolation window (≤2 Da) has been applied on the linear quadrupole ion trap (LQIT)[17-18], Orbitrap[19-20], and FTICR MS[21-22]for characterizing the fine structures of petroleum molecules. However, most of the reports focused on the aromatic rings, while the naphthenic rings in the aromatic fractions of petroleum samples have been rarely addressed.

In this work, a narrow isolation window of 1 Da was achieved by 15T FT-ICR MS. The CID mass spectra of model compounds and petroleum samples were measured by the atmospheric pressure photo ionization (APPI) FT-ICR MS. The fragmentations of CID mass spectra under current condition were proposed according to the results for study of model compounds. The differences of the naphthenic rings in the aromatic fractions of two similar crude oils were compared by the CID mass spectra with narrow isolation window. The result was consistent with the average structural information obtained by the NMR spectrometry.

2 Experimental

2.1 Model compounds and petroleum samples

The sample information of selected model compounds is listed in Table 1. Two crude oils from Chad, namely crude oil A and crude oil B, were separated according to ASTM D2549 and the aromatic fractions were used for MS and NMR analyses. The bulk properties of crude oils A and B are shown in Table 2. The model compounds were dissolved in toluene (HPLC grade, Fisher Chemical)and diluted to 0.001—0.005 mg/mL for FT-ICR MS measurements. The aromatic fractions of crude oils were also dissolved in the HPLC-grade toluene with the concentrations adjusted to 0.5 mg/mL for FT-ICR MS analyses.

2.2 Mass spectrometry analysis

All the model compounds and crude oil fractions were measured by a 15T Bruker SolariX XR FT-ICR MS. The positive mode APPI was used as the ionization source coupled with a Kr lamp. Prepared solutions were loaded into a metal-coated silica capillary and sprayed at a flow rate of 360 μL/h. The temperature of the drying gas was 200 °C and the flow rate was 2.0 L/min. The nebulizing gas flow rate was 1.0 L/min, while the temperature of vaporizer was 400 °C. The RF amplitude of the ionfunnels was 150 Vpp. The RF amplitudes of the octopole,collision cell hexapole, and transfer hexapole were 350 Vpp, 1 200 Vpp, and 350 Vpp, respectively. The time-of-flight was set at 0.6―1.0 ms according to the m/z value and the intensity of selected parent ions. The range of m/z covered from 50 Da to 1 000 Da. The MS/MS analyses were carried out with the collision energy of 0,10, 20, 30, and 40 eV, respectively. The collision gas was helium. Ions were accumulated for 2 s in the hexapole collision cell before being introduced to the ICR cell. The width of the isolation window was 1 Da (M ±0.5 Da).Each spectrum was measured with 64 co-added scans and 8 M data points per transient.

Table 1 Sample information of the model compounds

Table 2 Bulk properties of the crude oils

2.3 Mass calibration and data analysis

The APPI FT-ICR MS spectra under CID conditions were externally calibrated by tetradecylbenzene and its fragment ions. The peaks with a relative abundance greater than six standard deviations of baseline RMS noise (6σ) were exported for the data analysis. In the CID mass spectra, deprotonation was dominant for the fragment ions. As a result, m/z of the fragment ions was converted by adding the mass of a proton before data analysis. Chemical formulas (CcHhNnOoSs)were calculated according to the m/z values within the precision of ±1×10-6. Double bond equivalent (DBE)was calculated according to the following formula: DBE= c-h/2+n/2+1.

2.4 NMR Analysis

The aromatic fractions of the crude oils were dissolved in CDCl3. The1H-NMR and13C-NMR measurements were performed on an Agilent 700 MHz spectrometer at room temperature.1H-NMR spectra were recorded with a spectral width of 12 kHz and 100 scans by using a repetition delay of 5 s.13C-NMR spectra were measured with a spectral width of 45 kHz and 15000 scans, with the repetition delay equating to 5 s.

3 Results and Discussion

3.1 Aromatic model compounds with side-chains

The MS/MS spectra of aromatic model compounds have been widely studied previously[12,19-20,22-23].Additional model compounds are studied here to supply more information about the CID mechanism of model compounds. The CID mass spectra and suggested fragmentation pathways of model aromatic compounds with different side-chains are shown in Figures 1. When the collision energy is 0 eV, fragment ions are not observed for tetradecylbenzene (C20H34)and 1-dodecylpyrene (C28H34). As for C20H34, significant fragmentation occurs at a low collision energy of 10 eV.The length of the side-chains of the ions varies from C1to C7at the collision energy of 10―20 eV, indicating to a random cracking of the side-chains at low collision energy. When the collision energy increases to 30―40 eV, the side-chain length of the fragment ions turns to be shorter in the range of C1to C4, and the C1-substituted benzene is the most abundant species. The absence of the fragment ion C6H5+(m/z=77.0386) indicates that the strong aryl-alkyl C-C bond is not ruptured for C20H34[24].Meanwhile, the cracking of C28H34is more selective, and the α-cleavage (bond cleavage between Cαand Cβ) is the only approach, when the collision energy is above 10 eV.The differences between C20H34and C28H34are attributed to the size of the aromatic ring. The radical ion of the C1-substituted pyrene can be better stabilized by the relatively large pyrene ring, which has more π electrons, and then the α-cleavage is preferred to other routes. It is also noted that the loss of two hydrogen atoms in the pyrene ring can occur, when the collision energy rises to 40 eV[25].

The model compounds 1,3,5-triisopropylbenzene (C15H24)and 1,6-didodecylpyrene (C40H58) can represent the multichain substituted aromatic compounds. Dealkylation of[M-CH3]+and [M-C3H7]+occur at a collision energy of 0 eV for C15H24, indicating that the methyl- and isopropylgroups are not very stable under the current condition[26-27].When the collision energy increases to 10 eV, the intensity of molecular ions (m/z = 240.1873) reduces sharply and the most abundant peaks are assigned to the loss of methyland isopropyl-groups. A series of fragment ions with sidechains of C1―C6and C8are observed at the collision energy of 20 eV. The length of the side-chains reduces to C1—C4at the collision energy of 30 eV. Only C1and C3-substituted benzene is observed at a highest collision energy of 40 eV. The peak with a m/z value of 117.0699 identified at a high collision energy of 30―40 eV is also attributed to the loss of hydrogen atoms.

The CID behavior of C40H58is much simpler than that of C15H24. The molecular ions of C40H58(m/z = 538.4535) are quite stable and are the dominant species at the collision energy of 0―20 eV. The dealkylation of C40H58also follows the route of α-cleavage. The cracking of the first alkyl side-chain begins at 20 eV, while the cracking of the second side-chain occurs at 30 eV. At a collision energy of 40 eV, the fragment ion of C1-substitued pyrene turns to be dominant. The loss of two hydrogen atoms in the pyrene ring is also observed at 40 eV.

Although the CID spectra of aromatic compounds with alkyl side-chains are very complex and sensitive to the experimental conditions, some general principles can still be suggested by combining the results reported here and previously. The fragmentation of alkyl substituted aromatic hydrocarbons is strongly associated with the size of the aromatic ring. When the aromatic ring originates from benzene and naphthalene[12,19], the CID mechanism is the random cracking at low collision voltage and is mainly related to the α-cleavage at high collision voltage.When the aromatic ring originates from anthracene/phenanthrene or larger molecules[15,19], α-cleavage is the dominant process. Besides, the scission between arylalkyl linkage is also observed at high collision energy for the isopropyl substituted and the multi-chain substituted aromatic compounds[12,15,19].

Figure 1 The CID mass spectra and bond cleavage mechanisms of aromatic model compounds with different sidechains at different collision energy: (a) tetradecylbenzene, (b) 1-dodecylpyrene, (c) 1,3,5-triisopropylbenzene, and (d)1,6-didodecylpyrene

3.2 Aromatic model compounds with bridge alkylene chain

The CID mass spectra and suggested fragmentation pathways of two aromatic model compounds di-9-anthrylmethane (C29H20) and 1,2-di-9-anthrylethane(C30H22) with bridge alkylene chain are shown in Figures 2a and 2b. The most abundant fragment ion of C29H20is C28H+17(m/z=353.1325) at 10―20 eV and(m/z=352.1247) at 30―40 eV as shown in Figure 2a, indicating to the loss of the CH2bridge chain. This may occur due to the migration of hydrogen and complex rearrangement of aromatic ring according to the previous report[28]. The suspected structure of C28H17+is shown in Figure 2a. This structure can be further confirmed by the formation of(m/z=339.1169) at high collision energy, which is formed by a further loss of CH2group. A small amount of the fragment ions C23H15+(m/z=291.1170)and C23H13+(m/z=289.1013) is found in Figure 2a,which is attributed to the cracking of C28H17+. The CID behavior of C30H22is less complicated than C29H20, and α-cleavage is the major process as shown in Figure 2b.As a result, the DBE of main fragment ions of di-9-anthrylmethane is almost unchanged, while the DBE of the fragment ions of 1,2-di-9-anthrylethane (C30H22)reduces sharply from 20 to 10. These results are consistent with the previous reports[15,19].

3.3 CID mechanism of aromatic model compounds with naphthenic rings

The model compounds with naphthenic rings can be divided into three types according to the position of naphthenic rings. In type A, the naphthenic ring shares two bonds with the adjacent aromatic rings. In type B, each naphthenic ring is linked to the aromatic ring by one side. There are two naphthenic rings linked together in type C. Three compounds, viz.9,10-dihydrophenanthrene (C14H12), 1,2,3,4,5,6,7,8-oct ahydrophenanthrene (C14H18), and dehydroabietylamine(C20H31N1), are selected to represent the type A, B, and C, respectively. The CID spectra and possible routes to generate fragment ions from these model compounds are outlined in Figures 3.

Figure 3a demonstrates the main fragment ions of C14H12are C14H10+(m/z=178.0781, DBE+1) and(m/z=165.0702, DBE+0) at collision energy in the range of 20―30 eV, indicating to the formation of phenanthrene ring and fluorene ring, respectively. The opening of the naphthenic ring is not observed even at high collision energy of 40 eV. It is suspected that compounds with the type A structure are more likely to form fully conjugated structures through losing hydrogen atoms (DBE+1) or CH2group (DBE+0). The resulting phenanthrene and fluorene rings continue to lose two hydrogen atoms, when the collision energy rises to 40 eV[29].

Figure 2 The CID mass spectra and bond cleavage mechanisms of aromatic model compounds with bridge alkylene chain at different collision energy: (a) di-9-anthrylmethane, and (b) 1,2-di-9-anthrylethane

As shown in Figure 3b, the main changes of C14H18involve the loss of hydrogen atoms (DBE+1), the loss of CH2group (DBE+0), and the opening of naphthenic ring (DBE-1). It is also noted that the opening of the naphthenic ring is always accompanied with the loss of hydrogen atoms in the type B structure. As a result, the abundance of fragment ions C11H13+(m/z=145.1015, DBE-1) and C9H9+(m/z=117.0701, DBE-1) is much less than that of C11H9+(m/z=141.0702,DBE+1) and C9H7+(m/z=115.0504, DBE+0) at high collision energy.

As aromatic hydrocarbons with the type C structure are not easy to be obtained, dehydroabietylamine is applied here as an alternative. The amino group in dehydroabietylamine can be easily removed at low collision energy and the fragment ions are mainly hydrocarbons[30]. The reactions of the fragment ions are quite complicated, as shown in Figure 3c. The high abundance of C13H17+(m/z=173.1324, DBE-1) and(m/z=131.0855, DBE-1) indicates that the opening of naphthenic ring is more readily than the loss of hydrogen atoms in the type C structure. The existence of C7H7+(m/z=91.0542, DBE-2) at high collision energy proves that the two naphthenic rings can even be fully cracked in the type C structure.

Figure 3 The CID mass spectra and bond cleavage mechanisms of aromatic model compounds with naphthenic rings at different collision energy: (a) 9,10-dihydrophenanthrene, (b) 1,2,3,4,5,6,7,8-octahydrophenanthrene, and (c) dehydroabietylamine

As described above, the fragmentations of the naphthenic rings in aromatic model compounds are found to be related with the position of the naphthenic rings. Then, the CID behavior of model compounds with naphthenic rings can be briefly summarized in Figure 4, which illustrates the DBE variation of compounds with the type A, B, and C structures at high collision energy. This information will be helpful for interpreting the structures of petroleum molecules.

Figure 4 The DBE variation of typical compounds with naphthenic rings at high collision energy

3.4 Structures of crude oils measured by CID FTICR MS with narrow isolation window

Two crude oils from Chad (crude oil A and crude oil B)were measured by APPI FT-ICR MS. The broadband mass spectra and isolated mass spectra (m/z = 402.3 ±0.5 Da) are shown in Figure 5. It can be seen clearly that the chemical compositions of crude oils A and B are very similar. Moreover, only a few precursor ions are introduced to the detector of FT-ICR MS after isolation,which would greatly facilitate the assignment of the fragment ions. The O1species generated by APPI+source are ascribed to polyaromatic oxygen compounds, such as dibenzofurans and benzonaphthofurans[31-32]. These compounds will not affect the fragment ions of aromatic hydrocarbons due to their low intensity, different elemental composition, and stable chemical structures.

Figure 5 The broadband APPI FT-ICR MS spectra and isolated mass spectra (isolation window is 1 Da) of crude oils A and B

Figure 6 illustrates the DBE versus carbon number distribution of the aromatic hydrocarbon fragment ions of crude oils A and B at the collision energy of 30 eV.The cracking of side chains and opening of naphthenic rings can readily occur at this collision energy, while the loss of hydrogen atoms can be restrained. Two major precursor ions of aromatic hydrocarbons (C30H42and C31H30) observed at the collision energy of 0 eV are also shown as the reference, which are labeled with red color. Although there is more than one precursor ion of aromatic hydrocarbon, the large difference of the DBE value makes it possible to distinguish the fragment ions respectively. The insets in Figure 6 display the DBE versus carbon number distribution of the fragment ions from C30H42. Structures of the smallest ions for each DBE are also listed in the insets. Two basic rules are obeyed for the suggested structures: 1) Each substitute group is no longer than C2-alkyl group;2) The core structure is mainly the conjugated aromatic structure without naphthenic rings. The DBE of the fragment ions of C30H42in crude oil A is in the range of 7―12, while the DBE range of the fragment ions in crude oil B is only 8―11. The wider DBE range of the fragment ions in crude oil A indicates the greater diversity of the molecular structures. Moreover,the fragment ion with DBE=7 is only observed in crude oil A, and the structure of the precursor ion is suspected as three naphthenic rings linked with the naphthalene ring.

Figure 7 illustrates the possible structures of C30H42and the most suggested structures of C30H42in crude oils A and B according to the CID mass spectra. Although there are numerous possible structures of C30H42, Figure 7 reveals that the most suggested structures of different crude oils can be restricted to a few candidates, which would be valuable for comparing the properties of different crude oils. For example, it can be suspected that there are more naphthenic rings in crude oil A than in crude oil B as evidenced by Figure 7. Furthermore, NMR measurement is also applied to determine the number of naphthenic carbon (CN)[33]in the aromatic fractions of crude oils A and B. The obtained CNof crude oil A is 7.25% while CNof crude oil B is 3.65%, which can match quite well with the CID mass spectra results.

Figure 6 DBE versus carbon number distribution of the fragment ions of aromatic hydrocarbons in crude oils A and B at collision energy of 30 eV (isolation window=402.3 ± 0.5 Da). Molecular ions at 0 eV are also listed in red color as references

Figure 7 Possible chemical structures of C30H42 and the most suggested structures of C30H42 for crude oils A and B

4 Conclusions

This study reports a convenient method to realize the CID mass spectra with narrow isolation window of 1 Da at the high-field FT-ICR MS platform. The much narrower isolation window than the conventionally used conditions could simplify the CID mass spectra and facilitate the interpretation of the obtained fragment ions. A series of typical model compounds were measured to provide more information about the CID behavior of aromatic hydrocarbons. The DBE variation of aromatic model compounds with naphthenic rings was systematically studied. The opening of the naphthenic ring, the loss of hydrogen atoms, and the loss of CH2groups in the naphthenic ring covered the major processes leading to generation of the fragment ions. The position of the naphthenic ring was a key factor for the formation of the fragment ions. The aromatic fractions of two crude oil samples were also measured by CID FT-ICR MS with narrow isolation window. The structural information of the crude oil molecules could be expatiated by combining the structures of the fragment ions. The differences in the naphthenic rings of the two crude oil fractions were compared by CID FT-ICR MS, and the result was consistent with the NMR measurements. This study showed that CID FT-ICR MS with narrow isolation window of 1 Da has a great potential in structural characterization of complex natural mixtures,which may be useful to the study of petroleum, coal derivatives, and biological samples.

Acknowledgement:This work was supported by the Key Laboratory of SINOPEC (KL17010) and the research project of Research Institute of Petroleum Processing (R16075).