Characterization of Crude Oils at the Molecular Level by Use of Laser

Article pubs.acs.org/ac

Characterization of Crude Oils at the Molecular Level by Use of Laser Desorption Ionization Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry Yunju Cho,† Matthias Witt,‡ Young Hwan Kim,∧ and Sunghwan Kim*,†,§ †

Kyungpook National University, Department of Chemistry, Daegu, 702-701 Republic of Korea Bruker Daltonik GmbH, Bremen, Germany ∧ Division of Mass Spectrometry Research, Korea Basic Science Institute, Ochang, 863-883, Republic of Korea § Green-Nano Materials Research Center, Daegu, 702-701 Republic of Korea ‡

S Supporting Information *

ABSTRACT: In this study, laser desorption ionization (LDI) coupled to Fourier-transform ion cyclotron resonance mass spectrometry (FTICR MS) was applied to study crude oils at the molecular level. Molecular ions were the major type of ion detected by (+) mode, and deprotonated and radical anions were the major ions observed by (−) mode LDI FTICR MS. N1 and hydrocarbon classes were dominant in the class distribution plots obtained by (+) LDI FTICR MS, but other heteroatom classes, including Ox and S1, were abundant in plots obtained by (−) LDI FTICR MS. Detailed analysis of double-bond equivalence (DBE) vs carbon number plots revealed that LDI FTICR MS is more sensitive toward polyaromatic compounds than mono- or dicyclic-aromatic compounds. However, nonaromatic and aromatic O2 compounds could be detected simultaneously. An abundance of nonaromatic O2 compounds (presumably naphthenic acids) are correlated with total acid numbers, but O2 compounds with condensed structures are not. Overall, this study shows that LDI FTICR MS can be successfully used to study crude oils at the molecular level.

A

candidates and hence it would be difficult to identify a unique elemental composition from the numbers.5 However, even using FTICR MS, a complete understanding of the chemical composition of crude oil is not currently available. One of the problems limiting our knowledge is discrimination occurring during the ionization process. A single ionization technique can ionize only certain compounds, and the other compounds existing in crude oil are not detected. For example, electrospray ionization (ESI) is very efficient at ionizing polar compounds with nitrogen or oxygen atoms,1,7,8 but sulfur class compounds cannot be ionized effectively using this method unless they are chemically derivatized.9 No single ionization technique has been developed that can ionize all the compounds in

s the remaining global crude oil deposits become heavier, technological advancements are required to better utilize heavy crude oil. Many studies have analyzed the chemical composition of crude oil in order to better understand and predict the properties and behaviors of heavy petroleum.1−3 Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) is a powerful technique that can resolve crude oils at the molecular level. Using FTICR MS, resolving power up to 20 000 000 has been reported, which can be used to detect mass differences even smaller than an electron.4 Resolution over 600 000 at m/z 400 can be routinely achieved in analyses of crude oils to detect small mass differences between 12C3 versus 32S1H4 differing only by 3.4 mDa.5,6 In addition, the highly resolved peaks are internally calibrated, and sub ppm mass accuracy is possible. Resolution and mass accuracy is very important because mass numbers obtained from low resolution and mass accuracy would result in many possible © 2012 American Chemical Society

Received: June 12, 2012 Accepted: September 7, 2012 Published: September 7, 2012 8587

dx.doi.org/10.1021/ac301615m | Anal. Chem. 2012, 84, 8587−8594

Analytical Chemistry

Article

atmospheric pressure photo ionization (APPI),5,8,14,18,21,22 atmospheric pressure chemical ionization (APCI),14,19,23,24 atmospheric pressure laser ionization (APLI),14,25,26 easy ambient sonic-spray ionization mass spectrometry (EASI),27 laser-induced acoustic desorption (LIAD),28,29 and atmospheric pressure laser-induced acoustic desorption chemical ionization (AP/LIAD-CI)30 have been used to study chemical compositions of oils. Therefore, it is very important to apply different ionization techniques and understand the benefits and disadvantages of each technique for oil analysis. Laser desorption ionization (LDI) is another important ionization methods to study crude oils. LDI can be a useful technique to study crude oil because heavy components of oils contain aromatic ring structures that can efficiently absorb laser photons.19,31−36 However, in previous studies,19,31−36 LDI has been coupled to lower resolution mass spectrometers that cannot provide detailed chemical information, and hence the applicability of LDI for oil analysis has not been fully investigated. In this study, LDI coupled to FTICR MS has been applied to study crude oils at the molecular level. The major aim of this study is to evaluate the potential of LDI FTICR MS for crude oil analysis. To the best of our knowledge, this is the first paper that applies LDI FTICR MS to study petroleum at the molecular level.

crude oil simultaneously. The selectivity of ionization methods can be useful when specific structures or chemical functionalities are targeted. However, the selectivity can cause a serious problem when quantitative understanding for broad range compounds is intended. To circumvent this problem, it is beneficial to use as many ionization techniques as possible. One may employ different ionization techniques and later combine the results. Considering the importance, it is not surprising that there have been many studies devoted to utilizing various ionization techniques for oil research. Electron ionization,10,11 field desorption ionization,12,13 electrospray ionization (ESI),8,14−19 nanospray desorption electrospray ionization (nano-DESI),20 Table 1. Properties of Crude Oils Used in This Study source crude no. 1 crude no. 2 crude no. 3 a

origin

N S (ppm)a (ppm)b

TANc

APId

Napo

Ecuador

20 100

4 011

0.18

19.5

Qinhuangdao

China

2 500

4 405

3.15

16.1

Doba

Congo

1 100

1 884

4.26

22

Reference 46. bReference 47. cReference 48. dReference 49.

Figure 1. Broadband (left) and expanded (right) spectra of crude oil no. 1, no. 2, and no. 3 obtained by (a) (+) and (b) (−) mode LDI FTICR MS. 8588



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Figure 2. Distribution of heteroatom classes observed by (a) (+) and (b) (−) LDI FTICR. Relative abundances of each class in the forms of radical and protonated or deprotonated ions are separately noted in the bar graph. The summed abundance of molecular and protonated ions is shown as an inset.

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EXPERIMENTAL SECTION Mass Spectrometry Analysis. Three crude oils (crude oil no. 1, no. 2, and no. 3) were analyzed using LDI FTICR MS. Detailed information on the crudes oils used in this study is provided in the Table 1. In total, 10−20 mg of the sample was dissolved in 1 mL of dichloromethane (DCM) to get a final concentration of 10−20 μg/μL. A volume of 10 μL of this solution was spotted on a stainless steel target to get a thin layer of the oil for LDI measurements. Roughly 100 μg of the oil was spotted on the MALDI target with a spot size of about 5 mm diameter. Analysis was performed in LDI positive (+) and negative (−) ion mode on a solariX FTICR mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with a 12 T refrigerated actively shielded superconducting magnet (Bruker Biospin, Wissembourg, France) and a frequency-tripled Nd:YAG laser (355 nm) with repetition rates up to 1 kHz. Spectra were acquired with a 4 MW transient size. The signalto-noise ratio (S/N) was enhanced by summing 200 time domain transients for each spectrum. The transient length was roughly 2.5 s with a resolving power of about 600 000 at m/z 400. This resolving power was achieved in positive and negative ion mode. The data was zero-filled once to smooth the mass peaks. The detected mass range is affected by the flight time to transfer the ions from the accumulation (collision) cell to the analyzer (ICR) cell and by the frequency and amplitudes used in the ion guides and accumulation cell. However, the instrument was tuned for the mass range m/z 200−800. To compare the data of different samples, all samples were measured with the same instrument parameters (same instrument tuning). Mass lists have been obtained based on S/N > 5 using the Bruker Daltonik processing software DataAnalysis 4.0. The laser power and number of laser shots were adjusted to obtain a high S/N and low fragmentation (typically 35% laser power with a medium spot size of roughly 80 μm and 10−30 laser shots with a laser repetition rate of 50 Hz).37 The mass spectra were externally calibrated using an arginine cluster in the ESI mode. Internal

recalibration of the mass spectra was performed using Data Analysis 4.0 with radical cations of the N1 series in (+) mode and the fatty acids (O2 series) in (−) mode. The list of peaks used to calibrate the spectra is provided in the Supporting Information (Tables 1S and 2S). Bruker Daltonics Autoflex III MALDI time-of-flight (TOF) MS equipped with the 355 nm-1 kHz Nd:YAG laser was used to evaluate ionization behavior of standard compounds. Pyrrole, indol, carbozole, acridine, and dibenzothiophene were purchased from Sigma Aldrich (St. Louis, MO) and used as the standards. For sample preparation, standard samples were diluted to 100 μM with a solution of DCM. The final concentration of standard solutions were between 0.25 and 0.33 μM. LDI-TOF spectra were acquired in the m/z range between 0 and 1500 by summing 200 spectra. The acquired spectra were repeated three times. FlexAnalysis, version 3.0 software was used for data processing. Spectral Interpretation. Spectral interpretation was performed using the Statistical Tool for Organic Mixtures’ Spectra (STORMS 1.0) software with an automated peak-picking algorithm for more reliable and faster results.38 Elemental formulas were calculated from the calibrated peak list and assigned based on m/z values within a 1-ppm error range. The rms mass errors for the assignment of major classes are listed in the Supporting Information (Table 3S). rms mass errors for assignments were smaller than 0.27 ppm. Normal conditions for petroleum data (CcHhNnOoSs, c unlimited, h unlimited, 0 ≤ n ≤ 5, 0 ≤ o ≤ 5, 0 ≤ s ≤ 4) were used for these calculations. Double-bond equivalence (DBE) values represent the number of rings plus the number of double bonds to carbon in a given molecular formula. DBE values can be calculated using the following equation:39 DBE = c − h/2 + n/2 + 1

(1)

for elemental formulas of CcHhNnOoSs. 8589



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Figure 3. Two-dimensional DBE vs carbon number (left) and DBE (right) distribution plots of (a) N1 class, (b) HC class, and (c) O1 class compounds observed by (+) mode (top) and (−) mode (bottom) LDI FTICR MS. DBE values are marked in DBE distribution plots.

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RESULTS AND DISCUSSION Characterization of (+) and (−) Mode LDI FTICR MS Spectra. Figure 1 shows time domain signals, and broadband and expanded spectra of three crude oils obtained by (a) (+) and (b) (−) mode LDI FTICR MS. More than 11 000 peaks with S/N ratios over 5 were assigned for each spectrum. The expanded spectra show that the peak distributions are quite different among the observed samples. For example, the O2 peak (C32H59O2) noted in the expanded spectra of Figure 1 is very abundant in crude oil no. 3, fairly abundant in crude oil no. 2, but almost absent in crude oil no. 1. It appears that the O2 peak is closely related with the total acid number (TAN) values listed in Table 1. The TAN number is highest for crude oil no. 3 (TAN = 4.26) and lowest for crude oil no. 1 (TAN = 0.18). The class distribution observed in (+) mode LDI FTICR MS spectra is shown in Figure 2a. In the graph, N1+• and [N1 + H]+ represent molecular ions and protonated ions of the N1 class, respectively. The same notation was used for the other classes of compounds. In (+) mode LDI spectra, molecular ions of the N1+• and HC+• class compounds are abundant. Summed abundances of N1S1+• and S1+• class compounds were in the order of crude oil no. 1 > no. 2 > no. 3, which agrees with the

sulfur contents of crude oil listed in Table 1. However, despite the fact that sulfur is more abundant than nitrogen in crude oil no. 1, the summed abundance of N1+• and [N1 + H]+ is about 7 times higher than the summed abundance of S1+• and [S1 + H]+. This comparison suggests that (+) mode LDI is more sensitive toward nitrogen-containing compounds than sulfur compounds. To confirm this finding, a standard compound mixture of carbazole, acridine, and dibenzothiophene was prepared and analyzed by (+) mode LDI-TOF MS with the same laser system as LDI FTICR MS used in this study. The obtained spectra were processed and intensities of standard compounds were presented as a bar graph in the Supporting Information (Figure 1S). The comparison clear showed that (+) mode LDI is more sensitive for nitrogen-containing compounds than sulfur compounds. This also implies that there is a significant difference in ionization efficiencies of LDI and APPI toward the molecular species. It is commonly observed that APPI is more sensitive toward sulfur-containing compounds than nitrogencontaining compounds.9,21 In Figure 2a, the summed abundance of N1+• is significantly higher than that of [N1 + H]+. This also shows the difference between LDI and APPI methods. It is commonly observed that 8590



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protonated nitrogen compounds are more abundant than molecular ions for (+) mode APPI.40 Although both LDI and APPI use photons to ionize samples, the ionization preference for types of compounds and resulting ions differ significantly. In Figure 2b, N1−•and [N1 − H]− represent N1 class radical anions and deprotonated anions, respectively. The same notation was used for the other classes of compounds. For (−) mode spectra, deprotonated anions of N1 ([N1 − H]−), hydrocarbon ([HC − H]−), and Ox ([Ox − H]−) classes are abundant. This corresponds with a previous study in which deprotonated anions were dominant for PAH ions generated by LDI.31 In the previous study,31 it was reported that observation of a molecular anion could be dependent on the source configuration of mass spectrometers. N1 class compounds were also abundant in (−) mode LDI spectra; however, they were not as dominant as in (+) mode. Instead, other heteroatom classes, including Ox, NS, and S1, were abundant in (−) LDI FTICR MS (refer to Figure 2S in the Supporting Information). This suggests broader types of heteroatom classes could be observed using this technique (compare parts a and b of Figure 2). DBE and Carbon Number Distribution of Observed Classes. DBE and two-dimensional DBE vs carbon number distributions of N1, HC, and O1 classes observed in this study are shown in Figure 3. The relative abundance weighted average DBE values were calculated from the distribution by the following equation: DBEaverage =

Table 2. Ionization Energies of Compounds

∑i Ii × (DBE)i ∑i Ii

a

N class

formula

DBE

IEa (eV)

pyrrole pyridine indole quinoline carbazole acridine benzo[b]carbazole dibenzocarbazole S class

C4H5N C5H5N C8H7N C9H7N C12H9N C13H9N C16H11N C20H13N formula

3 4 6 7 9 10 12 15 DBE

8.21 9.34 7.76 8.63 7.50 7.80 7.10 7.10 IEa (eV)

benzo[b]thiophene dibenzothiophene HC class

C8H6S C12H8S formula

6 9 DBE

8.17 7.93 IEa (eV)

benzene naphthalene anthracene naphthacene benzo[b]chrysene pentacene benzo[a]pentacene O class

C6H6 C10H8 C14H10 C18H12 C22H14 C22H14 C26H16 formula

4 7 10 13 16 16 19 DBE

9.24 8.14 7.44 6.97 7.20 6.63 6.61 IEa (eV)

phenol benzofuran 1-naphthalenol 1-hydroxyanthracene

C6H6O C8H6O C10H8O C14H10O

4 6 7 10

8.49 8.36 7.76 7.70

Reference 50.

(2)

where Ii and (DBE)i are the relative abundance and DBE value of peak i. The average DBE values calculated with (−) mode data were higher than those with (+) mode. For the N1 class compounds, the lowest DBE value observed with summed relative abundance more than 10% of most abundant DBE was 9 for (+) mode and 9.5 for (−) mode data. Considering that subtraction of hydrogen to generate deprotonated anions ([M − H]−) increases DBE values by 0.5 over radical anions ([M]−•), the same minimum DBE value of 9 for neutral molecules were observed in both (+) and (−) modes. In addition, (+) mode shows a continuous DBE distribution (top right column of Figure 3a); however, (−) mode data show specific patterns in which the relative abundance increases for an increased DBE value of 3 (bottom right column of Figure 3a). For example, the summed relative abundance increased as DBE values increased to 9.5, 12.5, 15.5, and 18.5. This corresponds to DBE values of 9, 12, and 15 for neutral molecules. This pattern may be related to the structures of the observed molecules.23 In this case, the pattern corresponds to carbazole (DBE = 9), benzocarbazole (DBE = 12), and dibenzocarbazole (DBE = 15). The relative abundance of N1 class compounds with DBE values larger than 9 were dominant in the distribution. The ionization energies (IEs) of pyrrole, indole, carbazole, benzocarbazole, and dibenzocarbazole are presented in Table 2. The IEs of compounds with low DBE values (e.g., mono or dicylic aromatic compounds such as pyrrole and indole) are higher than 7 eV, and the IE values approach 7 eV as DBE values increase. Standard mixture of pyrrole (DBE = 3), indole (DBE = 6), and carbazole (DBE = 9) was analyzed by (+) and (−) mode LDI-TOF with the same laser system as LDI FTICR MS used in this study. The obtained spectra were processed, and intensities of standard compounds were presented as a bar graph in the Supporting Information (Figure 3S). Carbozole

was dominant, and intensities of pyrrole and indole were negligible in the spectra. This clearly shows that ionization efficiency is closely related to IE values of compounds. The third harmonic of the 355-nm Nd:YAG laser was used in this study for ionization. This is equivalent to energy of ∼3.5 eV. Absorption of two laser photons results in an absorption energy of ∼7 eV. Therefore, it is reasonable to expect that it would be easier for the high-DBE nitrogen compounds with lower IEs to be ionized by absorbing two photons. In fact, ionization by absorbing two photons has been suggested as one of the important ionization pathways for the MALDI process.41 This could explain the dominance of high-DBE compounds in the N1 DBE distribution observed by (+) LDI. The plots showing DBE and two-dimensional DBE vs carbon number distribution of the HC class are presented in Figure 3b. The relative abundance of compounds with DBE values lower than 9−10 was lower but the relative abundance of compounds increased greatly at DBE values of 12 or 13, which may be associated with their IE values listed in Table 2. It is clear that the IE values decrease as DBE value increases, and IE values were less than 7 eV for molecules with DBE > 10. Therefore, the same relationship between DBE, IE, and relative abundance observed for the N1 class could also be observed for the HC class. The IE values of commonly observed sulfur compounds are listed in Table 2. It is clear that benzothiophene and dibenzothiophene have higher IE values than N1 compounds with the same DBE values, such as indole and carbozole. The lower IE values of nitrogen compounds than those of sulfur compounds may be associated with a higher abundance of nitrogen compounds than sulfur compounds observed in Figure 2. However, there have been few reported values of IE of S class compounds in the literature, and more studies including quantum mechanical calculations of IE values would be required to verify the current findings. 8591



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Figure 4. Three-dimensional DBE vs carbon number (left) and DBE (right) distribution plots of the O2 class observed for (a) crude oil no. 1, (b) no. 2, and (c) no. 3 by (−) mode LDI FTICR MS. DBE values are marked in DBE distribution plots.

pounds with DBE values between 1 and 3) were fairly abundant in the O2 class distribution. A single aromatic ring has a DBE value of 4, and the compounds with DBE values between 1 and 3 can be considered as nonaromatic. It is likely that the electronegative nature of the oxygen atoms and the acidity of the OH group can play an important role in (−) LDI. Also, deprotonated anions of O1 and O2 classes (with halfinteger DBE values) are dominant at DBE values less than 12, but radical anions ([M]−•) (with whole number DBE values) become increasingly abundant as DBE values increased. Eventually the abundance of radical anions with DBE values over 20 is higher than that of deprotonated anions, as shown in Figures 3c and 4. For the N1 class, the radical anions were not dominant, even for high-DBE compounds (refer to bottom of Figure 3a).

The IE values of O1 class compounds are listed in Table 2, and the same relationship between DBE, IE, and signal abundance of the compounds observed in N1 and HC classes can be found for the O1 class observed by (+) LDI. The O1 class compounds with DBE > 12 and presumably IEs close to 7 eV or lower are dominant in spectra obtained by (+) LDI (refer to top row of Figure 3c). However, this differs from the distribution observed by (−) mode LDI (refer to bottom row of Figure 3c). The compounds with DBE values between 4 and 8 are fairly abundant in (−) mode LDI data. This suggests that a different ionization mechanism may play an important role for oxygen-containing compounds. The DBE distribution of O2 class compounds shown in Figure 4 also agrees with this hypothesis. The compounds lacking aromatic structure (e.g., com8592



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Similar trends of increased abundance of radical anions compared to deprotonated anions for high-DBE compounds could be observed in the HC class (refer to bottom of Figure 3b). The presence of radical anions strongly suggests that electron capture is an important ionization pathway for (−) mode LDI. Electron capture has also been suggested as a pathway to generate negative ions in MALDI.41 The dominance of radical ions for Ox and HC classes with high DBE values can be explained by the high electronegative nature of the oxygen atom and the fact that the electron affinity of aromatic compounds generally increases as the degree of unsaturation increases.42 Comparison of the class distribution shown in Figure 2 and properties of crude oils in Table 1 shows that the summed relative abundance of the O2 class is largest for crude no. 3 (highest TAN value) and smallest for crude no. 1 (lowest TAN value). The correlation between the O2 class and TAN values has been previously reported from data obtained by (−) mode APPI FTICR MS.2 Figure 4 provides more detailed information on the correlation. The red circle in the plots notes area where compounds with low DBE values are located, and the black circle indicates compounds located near the planar limit. The planar limit is the imaginary line generated by connecting compounds with the highest possible DBE values for a given carbon number, and hence compounds with the most condensed structures are located near the planar limit.43,44 The relative abundance of peaks in the red-circled area compared to the black-circled area decreases in accordance with TAN values. This demonstrates that O2 compounds with lower DBE values (presumably naphthenic acids) contribute more to the TAN number than O2 compounds with condensed structures. It is known that naphthenic acids with saturated cyclic ring structures contribute to the TAN number of crude oils.45 The results shown in Figure 4 confirm the general concept of naphthenic acids.

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CONCLUSIONS

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Knowledge Economy (MKE, Korea) and by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST) (Grant 2012044853). This work was also supported by a NRF (National Research Foundation of Korea) Grant funded by the Korean Government (Grant NRF-2011Fostering Core Leaders of the Future Basic Science Program).

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REFERENCES

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This study clearly shows that LDI FTICR MS can be successfully used to study crude oils at the molecular level. As with MALDI, the obvious advantage of LDI over other ionization techniques is the small amount of sample necessary for analysis. Microliter quantities of sample are sufficient to obtain LDI FTICR MS spectra. The other characteristics of LDI FTICR MS found in this study can be summarized as follows. First, (−) mode LDI FTICR MS can be used to characterize Ox class compounds because nonaromatic and aromatic compounds can be detected simultaneously using this technique. Therefore, we expect that (−) LDI FTICR MS can be a suitable method to study naphthenic acid and humic substances. Also, LDI FTICR MS can be a valuable tool to study nitrogen-rich samples (such as shale oils) because the method is very sensitive toward nitrogen classes. Finally, LDI FTICR MS with a 355-nm laser source efficiently detects polyaromatic compounds but is not effective for studying mono- or dicyclic aromatic compounds. This is presumably because the LDI efficiency is dependent on the ionization energies of molecules.

* Supporting Information S

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org. 8593



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Characterization of Crude Oils at the Molecular Level by Use of Laser

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