Application of Atmospheric Pressure Photo Ionization Hydrogen

Sep 13, 2013 - Application of Atmospheric Pressure Photo Ionization Hydrogen/Deuterium Exchange High-Resolution Mass Spectrometry for the Molecular Le...
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Application of Atmospheric Pressure Photo Ionization Hydrogen/ Deuterium Exchange High-Resolution Mass Spectrometry for the Molecular Level Speciation of Nitrogen Compounds in Heavy Crude Oils Yunju Cho,† Arif Ahmed,† and Sunghwan Kim*,†,‡ †

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

‡

S Supporting Information *

ABSTRACT: We report here for the first time the application of atmospheric pressure photo ionization hydrogen/deuterium exchange (APPI HDX) coupled to high-resolution mass spectrometry for molecular level speciation of nitrogen containing compounds in crude oils. The speciation was done based on different combinations of ions produced from nitrogen containing compounds with various functional groups. To prove the concept, 20 nitrogen containing standard compounds were analyzed. As a result, it was shown that the nitrogen containing compound (M) with a primary amine functional group mainly produced a combination of [M − 2H + 2D]•+ and ([M − 2H + 2D] + D)+ ions, one with a secondary amine including alkylated or phenylated pyrrole a combination of [M − H + D]•+ and ([M − H + D] + D)+, one with a tertiary amine including N-alkylated or phenylated pyrrole a combination of [M]•+ and [M + D]+, and one with a pyridine functional group mostly [M + D]+ ions. The concept was successfully applied to do nitrogen speciation of resins fractions of two oil samples. Combined with the subsequent investigation of double bond equivalence distribution, it was shown that resins of Qinhuangdao crude oil sample contained mostly alkylated pyrrole and N-alkylated pyrrole type compounds but resins of shale oil extract contained mostly pyridine type nitrogen compounds. It was also shown that the speciation of individual elemental composition was also possible by use of this method. Overall, this study clearly shows that atmospheric pressure photo ionization hydrogen/deuterium exchange (APPI HDX) coupled to high-resolution mass spectrometry is a powerful analytical method to do nitrogen speciation of crude oil compounds at the molecular level.

M

structures of the nitrogen compounds in heavy crude oils, especially at a molecular level, remains a challenge.26 The double bond equivalents (DBE) values calculated from the formulas or information derived from tandem mass spectrometry can provide some information on the chemical structures of the compounds found in crude oil.27−30 However, specific information on the chemical structures at the molecular level cannot be provided by these methods. Hydrogen/deuterium exchange coupled with the traditional chemical ionization has been used to study structures of nitrogen compounds.15,31,32 However, the method has limitation in studying heavy compounds because the chemical ionization cannot be applied to high molecular weight and/or polar compounds. In contrast, atmospheric pressure photo ionization (APPI) and modern atmospheric pressure chemical ionization (APCI) can be used

odern society is heavily dependent upon crude oil as an energy source and will be for the foreseeable future. Therefore, crude oil is one of the most important raw materials in the world. High-resolution mass spectrometry (Fourier transform ion cyclotron resonance mass spectrometry, FTICR MS) has greatly improved our understanding of crude oils by providing the chemical formulas of thousands of compounds that are present.1−9 Knowledge of these elemental compositions can be used to predict the chemical and physical properties of the oils.10−14 Nonetheless, finding elemental compositions is not enough to fully understand the chemical reactivity of the crude oil compounds.15−19 Gaining structural information on the compounds is also very important. For example, nitrogencontaining compounds in crude oils have important implications on catalyst deactivation during oil processing.20,21 Basic nitrogen compounds are particularly influential on catalyst deactivation.22−24 Hence, many studies have been devoted to discerning the chemical structures of nitrogen compounds in crude oil.15−17,20,25,26 However, understanding chemical © 2013 American Chemical Society

Received: July 15, 2013 Accepted: September 13, 2013 Published: September 13, 2013 9758

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to study polar and higher molecular weight compounds typically up to 2 000 Da. Herein, we report a new strategy for discovering functional group distribution of nitrogen containing compounds in complex mixture such as crude oil. The method utilizes atmospheric pressure photo ionization hydrogen/deuterium exchange mass spectrometry (APPI HDX MS)33 and can be applied to study even heavy components of crude oils at the molecular level.

assigned based on m/z values within a 1-ppm error range. Typical conditions for crude oil analysis13 plus up to four deuterium atoms were considered for these calculations. The spectra were calibrated and chemical formulas were calculated from the calibrated mass numbers. The DBE was calculated from the formulas using the following equation:35

EXPERIMENTAL SECTION Sample Preparation. All the chemicals used in this study were purchased from Sigma-Aldrich (St. Louis, MO) and used without further purification. All of the standard samples were dissolved in perdeuterated toluene to a final concentration of about 10 μM. The Qinhuangdao (QHD) crude oil and shale oil from the Anvil Points Mine (APM) were used in this study; detailed information on them is provided in Table S-1 in the Supporting Information. The crude oils were divided into their saturated, aromatic, resin, and asphaltene (SARA) fractions according to procedures previously reported.13 The resin fractions were dissolved in perdeuterated toluene at 1.0 mg/ mL, which was used to minimize the generation of protonated ions. Each of the analyte solutions dissolved in perdeuterated toluene and deuterated methanol was pumped separately with two different syringe pumps. The solvent and solutions were mixed 90−700 ms before they were injected into the ionization source. Mass Spectrometry Analysis. A positive-mode APPI QExactive mass spectrometer (Thermo Fisher Scientific Inc., Rockford, IL) was used to analyze all of the standards. A model 100 Fusion syringe pump (Chemyx, Stafford, TX) was used to inject the solutions directly into the APPI source at flow rates of 250 and 2000 μL/h. The vaporizer temperature was 400 °C. The sheath, auxiliary, and sweep gas flow rates were 10, 5, and 0 arbitrary units, respectively. The capillary temperature was 300 °C, and the S-lens voltage was 50 V. Boil-off from liquid nitrogen was used as both the sheath gas and the auxiliary gas for the ionization source. A resolving power of about 140 000 at 200 m/z was achieved in the positive mode. Pierce Velos calibration solution (Thermo Fisher Scientific Inc.) was used by the (+)-electrospray ionization (ESI) source for the external calibration. A 15-T Fourier transform ion cyclotron resonance mass spectrometer (FTICR MS, Bruker Daltonics, Billerica, MA) was used for the analysis of the crude oil samples. A nebulizing gas temperature of 450 °C and a flow rate of 2.0 L/min were used. The drying gas temperature was 210 °C at a flow rate of 2.3 L/min. The capillary voltage was 3 600 V. The continuous accumulation of selected ions mode with 320−530 and 520− 720 m/z windows were used for the data acquisition. The collision cell radio frequency (rf) voltage and energy were 1 500 V and −3.0 eV, respectively. Spectra were acquired with a 4MW transient size and summed over 200 time-domain transients to improve the signal-to-noise ratio. Internal recalibration was performed using the radical cations of the N1 and N1D1 series in the (+) mode. Spectral Interpretation. Spectral interpretation was performed using in-house-developed software of Statistical Tool for Organic Mixtures’ Spectra for Hydrogen/Deuterium eXchange (STORMS-HDX) with an automated peak-picking algorithm for more reliable and faster results.34 Elemental formulas were calculated from the calibrated peak list and

RESULTS AND DISCUSSION Comparison of Various Flow Rates. In APPI HDX MS, crude oil samples are dissolved into perdeuterated toluene, mixed with deuterated methanol, and ionized by APPI. Throughout this paper, M•+ designates a radical ion, [M − H + D]•+ a radical ion with one H/D exchange, [M − 2H + 2D]•+ a radical ion with two H/D exchanges, [M] + D+ an ion with D+, [M − H + D] + D+ an ion with one H/D exchange and D+, and [M − 2H + 2D] + D+ an ion with two H/D exchanges and D +. To examine whether the HDX reaction is flow rate dependent, crude oils were analyzed with various flow rates and the results were presented in Figure S-1 in the Supporting Information. The flow rate of the crude oil solution dissolved in perdeuterated toluene and deuterated methanol were controlled independently by the use of two separate syringe pumps. Flow rates of analyte solution and methanol are noted in the figures. Significant change in the spectra was observed when the flow rate of deuterated methanol was varied with the flow rate of analyte solution fixed. For an example, the increased abundance of [M] + D+ ions compared to M•+ ions was observed at a higher flow rate. However, a less significant change was observed in the case when the flow rate of deuterated methanol was fixed and that of the analyte solution was varied. As methanol would be expected to be a better proton donor, it would similarly be expected that deuterated methanol is more suitable for deuterium exchange than deuterated toluene. In fact, Purcell et al. analyzed nitrogen compounds dissolved in deuterated toluene with APPI FTICR MS and the deuterated ion was a relatively small contribution.36 Therefore, it was concluded that the flow rate of deuterated methanol is an important factor determining the pattern of APPI HDX. All the spectra shown in this study were obtained with at least two flow rates of deuterated methanol. Application of Standard Compounds. During the ionization process, some hydrogen atoms in nitrogen containing compounds are exchanged with deuterium atoms in deuterated methanol. The number of exchanged hydrogen atoms is dependent on the structures of the nitrogen containing compounds. As a result, exchange behaviors of nitrogen containing compounds are structure dependent and that can lead us to identify the functional groups of them. To prove this point, 20 nitrogen containing compounds (Table S-2 in the Supporting Information) with various chemical structures were analyzed with APPI HDX and the observed spectra are listed in the Supporting Information (Figure S-2). For compounds having primary amine functional groups, both [M − 2H + 2D]•+ and [M − 2H + 2D] + D+ ions were abundant at the lower flow rate, but the [M − 2H + 2D] + D+ ions became more important as the flow rate was increased (refer to Figure S-2 graphs 1−5 in the Supporting Information). For those compounds containing a secondary amine group, both [M − H + D]•+ and [M − H + D] + D+

DBE = c − (h + d)/2 + n/2 + 1

(1)

for CcHhDdNnOo elemental formulas.

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Table 1. Major Types of Ions Observed From APPI HDX MS Analysis of Nitrogen Containing Compounds M•+ [M] + D

+

[M − H + D] + D+

[M − H + D]•+

[M − 2H + 2D]•+

3° amines (including N-alkyl or phenyl pyrroles)

none pyridine ring-type compounds

2° amines (including C-alkyl or phenyl pyrroles)

[M − 2H + 2D] + D+

1° amine

Figure 1. Bar graphs showing the summed relative abundances of N-containing classes observed from (a) QHD and (b) APM resins at two flow rates. Graphs in the left and right columns were generated from spectra acquired with m/z windows of 320−530 and 520−720.

ions were about equally abundant at the lower flow rate but the [M − H + D] + D+ ions became more abundant at the higher flow rate (see Figure S-2 graphs 6 and 8 in the Supporting Information). Pyrrole-type compounds with alkyl (C-alkyl pyrroles) or phenyl groups attached to the aromatic ring (Cphenyl pyrroles) behaved like secondary amines, yielding mostly [M − H + D] + D+ ions at the higher flow rate (see Figure S-2 graphs 7 and 9 in the Supporting Information). For tertiary amines, both M•+ and [M] + D+ were abundant at the lower flow rate but [M] + D+ became more abundant at the higher flow rate (see Figure S-2 graphs 10−15 in the Supporting Information). For N-methylated pyrrole-type compounds, the observed ions were similar to those of tertiary amines (see Figure S-2 graph 11 in the Supporting Information). For N-phenyl pyrrole-type compounds, both M•+ and [M] + D+ were abundant at the lower flow rate, but at the higher flow rate, H/D exchange occurred between the hydrogen atoms attached to the benzene ring. Even multiply exchanged ([M − nH + nD] + D+) ions appeared (see Figure S-2 graph 13 in the Supporting Information). The [M] + D+ type ions dominated regardless of the flow rate for compounds having the pyridine structure (see Figure S2 graphs 16−20 in the Supporting Information). The major types of ions observed in the spectra are summarized in Table 1. More detailed information depending on two different flow rates is provided (Table S-3 in the Supporting Information). From inspection of the table, compounds with different chemical structures clearly produced different combination of

ions by APPI-HDX. Hence, the chemical structures of ions could be predicted by examining the various combinations To establish whether the information in Table 1 could be used to identify chemical structures of nitrogen compounds present in crude oil, resin fractions of crude oil samples were analyzed by APPI-HDX-MS. The resin fraction was chosen for this study because nitrogen-containing compounds are reportedly abundant in this fraction.13,37,38 H/D exchange was performed at two different sample flow rates. To improve the dynamic range of the analysis, accumulation of selected ions with 320−530 and 520−720 m/z windows were used for the data acquisition. Two m/z windows from 320 to 530 and 520 to 720 each presenting lower and higher mass regions were used in this study. Two mass windows were selected to check whether the outcome of APPI-HDX was mass dependent. The obtained spectra are provided in the Supporting Information (Figure S-3). Comparison of N-Containing Class Distributions. The summed relative abundances of the nitrogen-containing classes observed for the crude oils are listed in Figure 1. The data obtained in both of the m/z windows had similar distributions. Although perdeuterated toluene and deuterated methanol were used, the protonated ion ([M] + H+) was observed; these protons may have originated from the petroleum compounds themselves.17 The protonated ions were not further considered in this study. Other than the protonated ion, M•+, [M − H + D]•+, [M] + D+, and [M − H + D] + D+ were the major types of ions observed for the crude oils. Ions with multiple 9760

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exchanges such as [M − 2H + 2D]•+ and [M − 2H + 2D] + D+ had low abundances, which suggested that compounds with primary amine groups were not as abundant as other types of nitrogen compounds in the samples. For the data obtained for the resin fraction of the QHD crude oil sample, the M•+ and [M − H + D]•+ ions were quite abundant at the lower flow rate. However, at the higher flow rate, the abundances of the M•+ ions were much lower, while those of the M + D+ and [M − H + D] + D+ ions were greater. The combination of a high abundance of M•+ at the low flow rate, with a decrease in the abundance of the M•+ and an increase in the abundance of the [M] + D+ ions, was very similar to the behavior of tertiary amines, N-alkylated or Nphenyl pyrroles presented in Table 1. Additionally, the combination of abundant [M − H + D]•+ ions at the low flow rate and an increase in the abundances of the [M − H + D] + D+ at the high flow rate was very similar to the behavior of secondary amines, C-alkyl or C-phenyl pyrroles. Therefore, the distribution of ions presented in Figure 1a suggests that tertiary amines including N-alkylated or N-phenyl pyrroles and secondary amines including C-alkyl or C-phenyl pyrroles are abundant in the QHD resin fraction. The [M] + D+ ions dominated regardless of the flow rate for the resin fraction of the crude shale oil from the APM (Figure 1b). The [M] + D+ ions were observed for compounds having tertiary amine or pyridine ring-types (Table 1). The tertiary amines also produced significant numbers of M•+ ions, especially at the low flow rate, but the M•+ ions were not abundant in this resin fraction. Therefore, the combined distributions of the [M] + D+ and M•+ ions indicated that compounds containing pyridine structures were abundant in the APM resin fraction. DBE Distribution of N-Containing Classes. To further support this conclusion, the DBE distributions of the observed ions were examined (Figure 2). The data obtained in either m/ z window (320−530 and 520−720) had similar DBE distributions at the lower flow rate. The DBE distribution of the [M − H + D]•+ ions observed in the QHD resin is shown in Figure 2a. The first DBE value with a significant summed abundance was 9. Carbazole, a compound with the pyrrole structure, also had a DBE of 9. The summed abundance decreased with the DBEs of 10 and 11 but increased at DBE 12. The summed abundance decreased again at DBE values of 13 and 14 but increased again at a DBE of 15. The difference in DBE values between 9, 12, and 15 was 3; this 3-DBE step was attributable to benzene rings: linear addition of a benzene ring to an existing structure increased the DBE by 3 for each addition.2,7,27,39 The DBE values 12 and 15 are assigned to benzocarbazole and dibenzocarbazole, respectively. Therefore, the DBE distributions indicated that compounds having pyrrole structures were abundant in [M − H + D]•+ ions, in good agreement with the interpretation shown in Figure 1a. The DBE distribution of [M − H + D] + D+ ions is displayed in Figure 2b. The DBE distribution showed that protonated carbazole (DBE of 8.5) and benzocarbazole (DBE of 11.5) was most abundant. Therefore, the DBE distribution of [M − H + D] + D+ ions agreed well with that of [M − H + D]+ ions. The fact that the DBE distribution of [M − H + D] + D+ ions is very similar to that of [M − H + D]•+ ions agrees with the summarized information in Table 1 because both of [M − H + D] + D+ and [M − H + D]•+ ions are uniquely found with secondary amine compounds. Although compounds with pyrrole structures are presumably predominant judging by the

Figure 2. Bar graphs showing the DBE distributions of (a) [M − H + D]•+, (b) [M − H + D] + D+, (c) [M]•+, (d) [M] + D+ in QHD resins and (e) [M] + D+ in APM resins at the m/z windows of 320−530 and 520−720.

DBE pattern, it should be noted that isomers may also be present. M•+ ions were also abundant in the QHD resin fraction; the DBE distribution of the M•+ ions is presented in Figure 2c. Significant peaks were observed for DBE values of 6 and 9, 9761

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Table 2. Examples of the Information on the Individual Compoundsa structural assignment 2° amines (including C-alkyl or phenyl pyrroles)

formulas

CD3OD flow rate (μL/h)

observed m/z

relative abundance (%)

mass accuracy (ppm)

250 2000 250 2000 250 2000 250 2000 250 2000 250 2000

442.3451 442.3450 444.3591 444.3592 461.4014 461.4013 463.4155 463.4155 399.3857

10.42 8.09 1.09 5.28 3.41 1.86 2.74 8.12 2.42

−0.2 −0.4 −0.2 0.1 −0.4 −0.7 −0.2 −0.2 −0.6

401.3997 401.3999

46.18 58.13

−0.6 −0.1

[C32H43N1 − H + D]•+ [C32H43N1 − H + D] + D+

3° amines (including N-alkyl or phenyl pyrroles)

[C33H51N1]•+ [C33H51N1] + D+

pyridine ring-type compounds

[C28H49N1]•+ [C28H49N1] + D+

a

The raw data for the detailed spectra are presented in Figure S-4 in the Supporting Information.

The compounds were concluded to contain tertiary amine structures such as N-alkyl or phenyl pyrroles (Table 1). The examples presented in Table 2 and Figure S-4 in the Supporting Information clearly demonstrates that the APPI-HDX technique can be used to predict the chemical structures of individual compounds.

which match the DBE values of benzopyrrole and carbazole, respectively. Therefore, the DBE distribution shown in Figure 2c suggests that the M•+ ions have pyrrole-type structures. The spectra of N-methylated pyrrole compounds showed that the N-alkylated pyrrole structure produced M•+ ions. Therefore, compounds containing N-alkylated pyrrole structures were also abundant in the QHD resin fraction. DBE distribution of [M] + D+ ions observed in QHD and APM resins are displayed in Figure 2d,e. Unlike the distributions of the [M − H + D]•+ and M•+ ions, the DBE distribution of the [M] + D+ ions started at 3.5. The deuterated pyridine compound has a DBE value of 3.5. Therefore, the DBE distribution suggests that compounds containing pyridine structures are responsible for the [M] + D+ ions. Purcell et al. studied N1 class compounds in crude oil by (+) mode ESI MS and showed that DBE distribution of pyridinic compounds starts from 3.5, which agrees well with the finding of this study.17 However, in the case of [M] + D+ ions observed from QHD resins, the peak at DBE 8.5 was most abundant and deuterated carbazole had a DBE value of 8.5. Therefore, it is concluded that pyridinic compounds are abundant for [M] + D+ ions observed in APM resins but pyrrole compounds are more abundant for [M] + D+ ions observed in QHD resins which is in good agreement with the interpretation discussed in the previous section. In summary, the DBE distributions of crude oils presented in Figure 2 are in good agreement with the interpretation shown in Figure 1. Information on the Individual Compounds. The advantage of APPI-HDX combined with high-resolution mass spectrometry is that the combined technique can provide structural information on individual compounds. Examples of such structural details are provided in Table 2. The raw data for this table can be found in the Supporting Information (Figure S-4). In the first example, peaks corresponding to the formulas of [C32H43N1 − H + D]•+ and [C32H43N1 − H + D] + D+ were observed. The relative abundance of [C32H43N1 − H + D]•+ remained unchanged but that of [C32H43N1 − H + D] + D+ increased when the flow rate was increased. The compounds were concluded to contain secondary amine structures such as C-alkyl or phenyl pyrroles. In the second example, the peak corresponding to the elemental formulas of [C33H51N1]•+ and [C33H51N1] + D+ were observed. The relative abundance of [C33H51N1]•+ was higher than that of [C33H51N1] + D+ at the lower flow rate but the opposite was true at the higher flow rate.

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CONCLUSIONS In summary, an experimental strategy using hydrogen− deuterium exchange with APPI-HDX coupled to highresolution mass spectrometry was developed and used to elucidate the chemical functional groups of nitrogen-containing compounds in crude oils. The proposed method could differentiate two different oil samples by their functional group distributions. Finally, the application of the proposed method is not limited to crude oil: it could be used to study any mixture, including humic substances and foods.

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ASSOCIATED CONTENT

* Supporting Information S

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

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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 The authors thank Ms. MinHee Lee from Scinco Korea Corporation for providing help with the Q-Exactive mass spectrometer. 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). This work was also supported by NRF (National Research Foundation of Korea) Grant funded by the Korean Government (Grant NRF-2011-Fostering Core Leaders of the Future Basic Science Program).

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