Evaluation of Laser Desorption Ionization Coupled to Fourier

Article pubs.acs.org/EF

Evaluation of Laser Desorption Ionization Coupled to Fourier Transform Ion Cyclotron Resonance Mass Spectrometry To Study Metalloporphyrin Complexes Yunju Cho,† Matthias Witt,‡ Jang Mi Jin,§ Young Hwan Kim,§ Nam-Sun Nho,*,∥ and Sunghwan Kim*,†,⊥ †

Department of Chemistry, and ⊥Green-Nano Materials Research Center, Kyungpook National University, Daegu 702-701, Republic of Korea ‡ Bruker Daltonik GmbH, 28359 Bremen, Germany § Division of Mass Spectrometry Research, Korea Basic Science Institute, Ochang 363-883, Republic of Korea ∥ Climate Change Technology Research Division, Korea Institute of Energy Research, Daejeon 305-343, Republic of Korea S Supporting Information *

ABSTRACT: In this study, the applicability of positive-ion (+) laser desorption ionization (LDI) as an ionization method for metalloporphyrin complexes was evaluated. The evaluation was performed by analyzing standard compounds and a series of crude oils with various V4+ and Ni2+ contents by (+) LDI and (+) atmospheric pressure photoionization (APPI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The analysis of standard compounds showed that molecular ions were main ionic species in (+) LDI. Fragmented ions because of the loss of CH3 were also observed. The analysis of crude oils showed that the sensitivity of (+) LDI toward metalloporphyrin complexes is greater than that of (+) APPI. Furthermore, five types of ion VO2+ and Ni2+ porphyrin complexes (etio, DPEP, rhodo-etio, rhodo-DPEP, and di-DPEP) were observed with (+) LDI, but only three types were observed with (+) APPI. Nickel porphyrins were observed in unfractionated oils by (+) LDI but not by (+) APPI. The summed relative abundance of peaks corresponding to VO2+ and Ni2+ porphyrins observed by (+) LDI was shown to be correlated with the V4+ and Ni2+ metal contents of the oils in general. However, the abundance of DPEP porphyrins did not correlate well with the metal content because it depends upon the maturity or state of biodegradation of oils. Also, 1 μL of oil sample was sufficient to perform (+) LDI FT-ICR MS analysis. Therefore, (+) LDI FTICR MS is a sensitive method to detect metalloporphyrins in petroleum using caution with respect to fragmented ions because of the loss of CH3.

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INTRODUCTION Global crude oil resources are becoming heavier as the world’s demand for crude oil continually increases. Therefore, research is required to develop new technologies to use heavy oil more efficiently.1−6 Much effort has been dedicated to advancing our knowledge of the molecular composition of crude oils.7−14 Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) is one method that has been applied toward this aim. Double-bond equivalence values calculated from the elemental compositions, tandem MS, and hydrogen−deuterium exchange, combined with ion mobility MS, can be used to structurally elucidate crude oil compounds.15−24 Such knowledge can be used to predict and understand the properties of heavy oils.25,26 Heavy crude oils contain significant amounts of metalloporphyrins, which have been extensively studied using various analytical techniques, such as chemical ionization MS,27 highpressure liquid chromatography,28−30 tandem MS,31 quadrupole ion trap MS,32 ultraviolet−visible spectroscopy,33−36 and time-of-flight (TOF) MS.37 Electrospray ionization (ESI) and atmospheric pressure photoionization (APPI) with FT-ICR MS have also been used to study metalloporphyrin complexes.29,38−42 Recently, VO2+ and Ni2+ porphyrin compounds were detected directly from crude oils with ultrahigh-resolution MS coupled to APPI.42 © 2014 American Chemical Society

In MS, having available a wide variety of ionization techniques is advantageous because no single technique can ionize all types of molecules simultaneously.6,29,37 Recently, laser desorption ionization (LDI) coupled with FT-ICR MS has been successfully used to study heavy crude oils.43−45 LDI is a useful technique for the ionization of porphyrin complexes because porphyrin contains aromatic ring structures with nitrogen atoms, and LDI has been shown to be effective toward aromatic compounds containing nitrogen. In fact, LDI coupled to lower resolution mass spectrometers was used to study metalloporphyrin complexes.45−49 However, LDI coupled to FT-ICR MS has never been evaluated as a method for analyzing metalloporphyrin complexes. In this study, LDI was evaluated as an ionization source of FT-ICR MS to study the molecular composition of metalloporphyrin complexes in petroleum.

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MATERIALS AND METHODS

MS. Six crude oils were dissolved in dichloromethane (5 mg/mL), and approximately 2 μL of each solution was spotted on the stainlesssteel target without a matrix. Table 1 lists detailed information on the

Received: September 30, 2013 Revised: October 1, 2014 Published: October 2, 2014 6699

dx.doi.org/10.1021/ef500997m | Energy Fuels 2014, 28, 6699−6706

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Table 1. Properties of Crude Oils Used in This Study

a

abbreviation

source

origin

S (%)a

N (ppm)b

TANc

APId

V (ppm)e

Ni (ppm)f

NAP LAT RGB EOC RAT Duri

Napo Light Latam Blend Ras Gharib Eocene Ratawi Sumatra

Ecuador Latin America Egypt Kuwait Kuwait Indonesia

2.01 1.85 3.57 4.79 3.77 0.2

4011 3536 4019 2136 1620 3231

0.18 0.25 0.45 0.27 0.3 1.46

19.5 19.6 19.3 18.3 24.1 20.7

329.8 308.7 93.5 54.6 43.8 1.3

129.0 128.6 71.0 21.4 21.7 49.1

From ref 53. bFrom ref 54. cFrom ref 55. dFrom ref 56. eFrom ref 57. fFrom ref 58.

Table 2. List of Vanadyl and Nickel Porphyrin Standards Used in the Study

Figure 1. (+) LDI-TOF mass spectra of vanadyl and nickel porphyrin standards. The labels a−d correspond to the entries in Table 2. crude oils used. The samples were analyzed using LDI in positive-ion (+) mode on a solariX FT-ICR mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with a frequency-tripled Nd:YAG laser (355 nm) with repetition rates up to 1 kHz. Spectra were acquired with a transient size and length of 4 MW and 2.3 s, respectively, resulting in a resolving power of roughly 600 000 resolving power at m/z 400. A total of 500 signal transients were added for the final mass spectrum to enhance the signal-to-noise (S/ N) ratio. Each sample was analyzed 5 times to obtained replicate data. The laser power was up to 17%, and 30 laser shots with a laser repetition rate of 800 Hz were used to obtain a high S/N ratio and low fragmentation. Mass lists were obtained on the basis of a S/N ratio of >4 using the Bruker Daltonik processing software Data Analysis 4.0.

The mass spectra were externally calibrated using arginine cluster in ESI mode. Internal recalibration of the mass spectra was performed using Data Analysis 4.0 with radical cations of the N1 series in (+) mode. The list of N1 series compounds is provided in Table 1S of the Supporting Information. Two types of vanadyl (VO2+) and nickel (Ni2+) porphyrin standards were analyzed with a Bruker Daltonics Autoflex III matrix-assisted LDI (MALDI) TOF MS equipped with the 355 nm, 1 kHz Nd:YAG laser to determine the ionization behavior of these standards. The standards were purchased from Sigma-Aldrich (St. Louis, MO), and Table 2 lists specific information on these standards. The standards were dissolved in dichloromethane to a final concentration of 50 μM, and 6 μL aliquots were loaded onto a polished steel target. For calibration of the 6700


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Figure 2. (a) FT-ICR mass spectra of six crude oils (for abbreviations, refer to Table 1) obtained using (left) LDI and (right) APPI in positive-ion mode (+). Insets show the time-domain signal. (b) Distribution of abundant compound classes in the six crude oils, as determined by (+) FT-ICR MS using (left) LDI and (right) APPI. mass spectra, a mixture of 0.5 μL of peptide calibration standard II (Bruker Daltonics, Bremen, Germany) and 0.5 μL of the matrix solution [20 mM α-cyano-4-hydroxycinnamic acid in 70% acetonitrile/ 0.1% trifloroacetic acid (v/v)] was also spotted without premixing. LDI-TOF spectra were acquired in the m/z range of 420−2000, and 400 such spectra were summed. This process was replicated 3 times. FlexAnalysis version 4.0 software was used for the data processing. Spectral Interpretation. The spectra were interpreted using the homemade software “Statistical Tool for Organic Mixtures’ Spectra” (STORM).50 The software was equipped with an automated peakpicking algorithm.51 Elemental formulas were calculated from the calibrated peak list and assigned on the basis of m/z values within an error range of 1 ppm. Typical conditions for petroleum (CcHhNnOoSs, where c and h are unconstrained, 0 ≤ n ≤ 5, 0 ≤ o ≤ 5, and 0 ≤ s ≤ 4)

were used for these calculations. Double-bond equivalent (DBE) represents the number of saturated and aromatic rings plus the number of double bonds in a given molecular formula. DBE values were calculated by the following equation for the elemental formula CcHhNnOoSs:52

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DBE = c − h/2 + n/2 + 1

(1)

RESULTS AND DISCUSSION Study of Metal Porphyrin Standards by LDI-TOF. To study the applicability of (+) LDI as an ionization method for metal porphyrin complexes, standard compounds were analyzed with (+) LDI-TOF MS. Autoflex III MALDI TOF 6701


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Table 3. S/N Ratios of (a) Vanadyl Etio- and DPEP-Porphyrin Peaks and (b) Nickel Etio- and DPEP-Porphyrin Peaks Observed by (+) FT-ICR MS Using LDI or APPIa S/N ± SD of etio

(a) [C29H30N4OV]+

content V (ppm) Duri RAT EOC RGB LAT NAP

1.3 43.8 54.6 93.5 308.7 329.8 (b)

LDI 24.9 22.1 33.4 95.6 126.8

± ± ± ± ±

APPI

Ni (ppm) EOC RAT Duri RGB LAT NAP a

21.4 21.7 49.1 71.0 128.6 129.0

LDI

[C28H28N4Ni]+ LDI

8.4 14.7 16.4 21.0

± ± ± ±

14.9 ± 0.8 16.0 ± 1.7

[C29H30N4Ni]+ APPI

2.2 2.5 1.1 1.6

[C31H32N4OV]+

APPI

26.4 ± 4.8 23.3 ± 0.6 34.9 ± 3.0 16.0 ± 2.2 73.9 ± 7.4 16.0 ± 1.6 106.1 ± 9.4 S/N ± SD of etio

4.2 2.2 3.5 3.4 12.7

content

S/N ± SD of DPEP [C30H32N4OV]+

LDI

31.48 33.8 107.2 58.3 82.0

± ± ± ± ±

APPI 3.5 2.7 8.5 4.0 9.8

10.0 ± 1.5 7.2 ± 0.5 8.0 ± 1.3 S/N ±

[C30H30N4Ni]+ APPI

14.6 ± 1.8 12.2 ± 1.0 15.2 ± 1.6

LDI

[C32H34N4OV]+

LDI

26.0 ± 3.3 11.0 ± 1.2 14.7 ± 1.6

LDI 25.9 ± 27.3 ± 101.7 ± 37.4 ± 49.7 ± SD of DPEP

APPI 4.4 1.6 6.3 3.6 5.5

9.5 ± 1.5 5.4 ± 1.1 6.0 ± 0.7

[C31H32N4Ni]+ APPI

LDI

APPI

29.7 ± 4.2 12.5 ± 2.7

The raw data for the detailed spectra are presented in Figure 2S of the Supporting Information. For crude oil abbreviations, refer to Table 1.

Figure 3. (a) S/N ratio of peaks corresponding to vanadyl (1) etio- and (2) DPEP-porphyrin complexes in six crude oils as a function of the vanadium content of the oils. (b) S/N ratio of peaks corresponding to nickel (1) etio- and (2) DPEP-porphyrin complexes in six crude oils as a function of the nickel content of the oils.

MS was used because it has the same laser system as the FTICR MS instrument used in this study. Two VO2+ and two Ni2+ porphyrins (Table 2) were analyzed, and the obtained spectra are shown in Figure 1. In the case of etio-porphyrin (panels a and c of Figure 1), both fragmented and intact molecular ions were observed. The fragmented ions exhibited a difference of m/z 15 from the molecular ions, suggesting the loss of a methyl

group (−CH3). A possible pathway for the loss of a methyl group is proposed in Figure 1S of the Supporting Information. No fragmented ions were observed for the tetraphenyl porphyrins (panels b and d of Figure 1). The data presented in Figure 1 show that metalloporphyrin complexes can be analyzed with (+) LDI and that CH3 loss ions can be observed along with intact molecular ions. However, the difference of 6702


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Table 4. Examples of (a) Vanadyl and (b) Nickel DPEP-Porphyrin Peaks Observed by (+) LDI FT-ICR MS in RGB and LAT Crude Oilsa (a) index 1 2 3 4 5 6 7 8 9 10 11 12 (b)

RGB calculated mass

assigned formula

measured mass (average)

485.15408 499.16973 513.18538 527.20103 541.21668 555.23233 569.24798 583.26363 597.27928 611.29493 625.31058 639.32623

C28H26N4OV C29H28N4OV C30H30N4OV C31H32N4OV C32H34N4OV C33H36N4OV C34H38N4OV C35H40N4OV C36H42N4OV C37H44N4OV C38H46N4OV C39H48N4OV

485.1541 499.1698 513.1854 527.2011 541.2167 555.2323 569.2480 583.2637 597.2793 611.2948 625.3105 639.3263

index

calculated mass

assigned formula


1 2 3 4 5 6 7

462.13490 476.15055 490.16620 504.18185 518.19750 532.21315 546.22880

C27H24N4Ni C28H26N4Ni C29H28N4Ni C30H30N4Ni C31H32N4Ni C32H34N4Ni C33H36N4Ni

462.1350 476.1507 490.1662 504.1818 518.1975 532.2132 546.2289

a

LAT

S/N ± SD 38.3 ± 63.1 ± 90.1 ± 107.2 ± 101.7 ± 49.7 ± 33.7 ± 17.8 ± 12.3 ± 8.8 ± 7.8 ± 6.9 ± RGB

mass error (average ppm)


0.01 0.05 −0.01 0.05 0.07 0.02 0.12 0.04 0.08 −0.24 −0.07 0.09

485.1541 499.1697 513.1853 527.2010 541.2167 555.2323 569.2480 583.2636 597.2793 611.2948 625.3105 639.3263

2.7 3.9 5.7 8.5 6.3 5.1 4.0 1.6 1.8 1.3 1.0 0.5

S/N ± SD



± ± ± ± ± ± ±

0.29 0.31 −0.04 −0.06 −0.07 0.08 0.14

462.1349 476.1506 490.1661 504.1818 518.1974

8.0 15.5 19.3 26.0 29.8 26.1 12.6

0.7 1.8 1.4 3.3 4.2 2.4 0.9

S/N ± SD 79.0 ± 3.1 78.7 ± 5.3 67.6 ± 5.5 58.3 ± 4.0 37.4 ± 3.6 18.0 ± 1.1 12.0 ± 1.1 7.7 ± 1.0 5.7 ± 1.0 4.8 ± 0.3 4.9 ± 0.4 5.0 ± 0.5 LAT

mass error (average ppm) −0.06 −0.01 −0.07 −0.02 0.02 −0.03 0.07 0.00 0.06 −0.26 −0.15 0.11

S/N ± SD


± ± ± ± ±

0.01 0.07 −0.22 −0.17 −0.27

11.4 16.9 13.0 11.0 9.5

1.2 1.1 1.8 1.2 0.0

For abbreviations, refer to Table 1.

than those observed using (+) APPI. Note that Ni2+ porphyrin complexes have been observed from unfractionated crude oils using (+) LDI but not using (+) APPI in this study. Therefore, we conclude that the sensitivity of detecting metalloporphyrin complexes is greater with the use of (+) LDI than with (+) APPI. Correlation between the Metal Content of Oils and the S/N Ratios of Peaks Corresponding to MetalContaining Porphyrins. The V4+ and Ni2+ contents of oils and the S/N ratios observed using (+) LDI are plotted and presented in the Supporting Information (refer to Figure 3). In the case of VO2+ and Ni2+ etio-porphyrin complexes, the S/N ratios were closely correlated with the corresponding V4+ and Ni2+ contents of the oils (refer to Figure 3a and Table 3a). The S/N ratios observed in oils containing more than 300 ppm V4+ were approximately 4−5 times greater than those of samples with a V4+ content of approximately 50 ppm. No VO2+ etioporphyrin complex above the S/N ratio of 4 was observed from Duri crude oil, which had the smallest V4+ concentration (∼1 ppm). Even though Ni2+ porphyrin complexes were observed in unfractionated oils using (+) LDI, their S/N ratios were significantly lower than those of the VO2+ porphyrin complexes. For an example, the EOC sample with a V4+ content of 54.6 ppm produced an etio VO2+ porphyrin peak with a S/N ratio of 22.1. However, a Ni2+ content as high as 129 was needed to observe an etio Ni2+ porphyrin peak with a similar S/N ratio (refer to parts a and b of Table 3). Therefore, a preconcentration step, such as a silica-gel cyclograph, or resolving power over a million may be needed to study Ni2+ porphyrins more effectively.40 In the case of VO2+ DPEP-porphyrin complexes, a correlation between the V4+ content of the oils and the S/N ratios existed, except for the RGB crude oil (refer to Figure 3b

CH3 may not be critical in understanding types of metalloporphyrin compounds existing in crude oils because the difference of CH3 will not affect the identification of core structures of porphyrins. Comparing LDI and APPI Spectra. Six crude oil samples containing various amounts of V4+ and Ni2+ (Table 1) were analyzed by (+) LDI and APPI coupled to FT-ICR MS. The obtained time- and m/z-domain spectra are shown in Figure 2a. The time-domain signals lasted up to 3 s with a resolution of 600 000 at m/z 400. This resolution was sufficient to distinguish between the common mass differences of 12C3 versus 32S1H4, which differ by 3.4 mDa. The class distributions of the obtained spectra are presented in Figure 2b. In agreement with previous studies, N1 class compounds were abundant in the spectra obtained using LDI and sulfurcontaining classes (S1 and S2) were abundant in the spectra obtained using APPI.43,44 Spectra expanded in selected ranges showing the VO2+ and Ni2+ porphyrin peaks are presented in Figure 2S of the Supporting Information. The m/z ranges of the spectra, showing eight peaks associated with V (C28H28N4OV, C29H30N4OV, C30H32N4OV, C31H34N4OV, C29H28N4OV, C30H30N4OV, C31H32N4OV, and C32H34N4OV) and another four peaks associated with Ni (C28H28N4Ni, C29H30N4Ni, C30H30N4Ni, and C31H32N4Ni) are shown as boxes in the figure. The etio (C28H28N4, C29H30N4, C30H32N4, and C31H34N4) and DPEP (C29H28N4, C30H30N4, C31H32N4, and C32H34N4) compounds were chosen because they were the most abundant porphyrin complexes observed in previous studies.29,39 The S/N ratios of the selected peaks shown in Figure 2S of the Supporting Information were determined by use of Data Analysis 4.0 and are listed in Table 3. As shown in the table, the S/N ratios observed using (+) LDI were 7−10 times greater 6703


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Figure 4. (a) Summed relative abundance of five types of vanadyl porphyrin complexes (etio, DPEP, rhodo-etio, rhodo-DPEP, and di-DPEP) in six crude oils (for abbreviations, refer to Table 1), as determined by (+) FT-ICR MS using (top left) LDI and (top right) APPI. (Bottom) Same data as panels top left and top right combining the relative abundances of all of the vanadyl porphyrins into one sum. (b) Summed relative abundance of five types of nickel porphyrin in six crude oils, as determined with (left) (+) LDI FT-ICR MS. (Right) Same data as the left panel combining the relative abundances of all of the nickel porphyrins into one sum. The linear correlation factors (R2) are marked in the figure.

and Table 3b). Although the V4+ content of RGB crude oil (93.5 ppm) was less than that of LAT and NAP crude oils (>300 ppm), the S/N ratios of the VO2+ DPEP-porphyrin complexes were greatest for RGB crude oil. The data obtained using (+) APPI also showed greater S/N ratios for VO2+ DPEP-porphyrin in RGB than in LAT and NAP crude oils (Table 3). The same trend was also observed from Ni2+ DPEPporphyrin complexes (refer to Table 3b). The content of Ni2+ DPEP porphyrins was higher for RGB than LAT and NAP, although it had lower Ni2+ contents. To confirm that the higher S/N ratios of peaks corresponding to VO2+ and Ni2+ DPEP-porphyrin in RGB crude oil were not the result of misassignment of the elemental formulas, the lists of peaks observed in the (+) LDI FT-ICR spectra of both RGB and LAT crude oils are presented and compared in Table 4. The assigned formulas, theoretical masses, and deviations (ppm) between the observed and theoretical masses are also listed in the table. The deviations are

similar for both RGB and LAT crude oils. Therefore, we conclude that the high abundance of metal DPEP-porphyrin in RGB crude oil and the deviation from the correlation are not a result of misassignment of elemental formulas but the result of the inherent nature of the particular sample. The overall distribution of VO2+ and Ni2+ porphyrin peaks observed in the (+) LDI FT-ICR MS spectra are shown in Figure 4. These distributions confirm the conclusions made in the previous sections. Five types of VO2+ porphyrin complexes (etio, DPEP, rhodo-etio, rhodo-DPEP, and di-DPEP) were identified in the (+) LDI FT-ICR MS spectra, but only the etioand DPEP-porphyrins were observed in the (+) APPI FT-ICR MS spectra (Figure 4a). In addition, the overall summed abundance of VO2+ porphyrins was significantly greater in the (+) LDI data (Figure 4a). These observations are in agreement with the conclusion that the sensitivity of detection of metalloporphyrin complexes is greater with (+) LDI than with (+) APPI. In the case of Ni2+ porphyrin complexes, the 6704


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same trend was also observed. Up to five types of Ni2+ porphyrin complexes were observed by (+) LDI FT-ICR MS, but none was observed by (+) APPI FT-ICR MS (Figure 4b). The graphs provided in Figure 4 also confirm that the summed relative abundance of peaks corresponding to VO2+ and Ni2+ porphyrins correlated with the V4+ and Ni2+ contents of the crude oils. The VO2+ and Ni2+ DPEP-porphyrins in RGB were an exception to the correlation. Th data obtained with both LDI and APPI were in agreement in showing that the observed abundance of VO2+ DPEP-porphyrins was larger in RGB than what was expected from its vanadium content. It strongly suggests that the larger than expected abundance of metal DPEP-porphyrins is not originated from error in LDI detection but from the nature of the oil sample. It has to be noted that applicability of data presented in Figure 4 cannot be judged solely by R2 values presented in the plots but the abundance or S/N ratio of peaks also has to be considered. Although the R2 value calculated with least-square method is slightly higher for Ni (R2 = 0.90) than V (R2 = 0.82) in panels a and b of Figure 4, the summed relative abundance of peaks containing V was about 10 times higher than that containing Ni (refer to scales of y axes). Therefore, as discussed in the previous section, it would be recommended to include a pre-concentration step to study Ni2+ porphyrins more semiquantatiativly or quantitatively.

Foundation (KOSEF) grant funded by the Korean government [Ministry of Education, Science and Technology (MEST)] (20110003796) and the Korea Institute of Energy Research. This work was also supported by NRF(National Research Foundation of Korea) grant funded by the Korean Government (NRF-2011-Fostering Core Leaders of the Future Basic Science Program).

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CONCLUSION The results presented in this study show that (+) LDI FT-ICR MS is a sensitive method that can be used to study the chemical compositions of metalloporphyrins in crude oils. Also, (+) LDI FT-ICR MS was shown to be applicable for the semiquantitative estimation of the vanadium content of crude oils. It is important to note that the porphyrines can be directly detected in crude oil without any purification by this method. Another advantage provided by LDI is that a sub-microliter sample is sufficient to acquire (+) LDI FT-ICR MS spectra. Therefore, we expect that (+) LDI FT-ICR MS will become a valuable tool in the study of metalloporphyrins in petroleum.

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ASSOCIATED CONTENT

S Supporting Information *

Proposed mechanism for the loss of a methyl group by metalloporphyrins during LDI (Figure 1S), expanded spectra of selected ranges showing the examples of vanadyl and nickel porphyrin peaks observed in this study (Figure 2S), and list of mass peaks used to calibrate the spectra in positive-ion mode (Table 1S). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: 82-42-860-3631. Fax: 82-42-860-3134. E-mail: [email protected]. *Telephone: 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 Korea Basic Science Institute (KBSIC33720 to Young Hwan Kim) for access to the 15 T FTICR MS instrument and a Korea Science and Engineering 6705


Energy & Fuels

Article

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