Article pubs.acs.org/EF
Characterizations of the Extracts from Geting Bituminous Coal by Spectrometries Da-Ling Shi, Xian-Yong Wei,* Xing Fan, Zhi-Min Zong, Bo Chen, Yun-Peng Zhao, Yu-Gao Wang, and Jing-Pei Cao Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), China University of Mining & Technology, Xuzhou 221116, Jiangsu, People’s Republic of China S Supporting Information *
ABSTRACT: Geting bituminous coal (GBC) was sequentially extracted with petroleum ether, carbon disulfide (CDS), methanol, acetone, and isometric CDS/acetone mixed solvent at room temperature to afford extracts 1−5 (E1−E5) and residue. Detailed characterizations of the extracts were performed with a gas chromatography/mass spectrometer (GC/MS), Fourier transform infrared (FTIR) spectrometer, and direct analysis in real-time ionization source (DARTIS) coupled to an ion-trap mass spectrometer (ITMS). GBC and its residue were also analyzed with the FTIR spectrometer. Particle sizes of the residue were significantly reduced compared to those of GBC according to the observation with a scanning electron microscope. Arenes with 1−4 rings and more condensed arenes were enriched into E1 and E2, respectively, while more heteroatom-containing organic species were detected in other extracts, especially in E3 and E4 according to GC/MS analysis. The extracts, especially E1−E4, contain more aliphatic moieties and less aromatic moieties compared to GBC and its residue based on FTIR analysis. DARTIS/ITMS proved to be a powerful tool for analyzing thermally labile and/or involatile species, which are difficult to be identified with GC/MS, in the extracts.
1. INTRODUCTION As a nondestructive and separable method, fractional extraction was widely used to isolate trace species1,2 and group components3,4 in organic matter (OM) from coals and their derivatives. Many extractable species from coals and their derivatives can be identified by subsequent analyses; among them, gas chromatography/mass spectrometer (GC/MS) is most commonly used. However, GC/MS is only effective for analyzing thermally stable and relatively volatile species. Fourier transform infrared (FTIR) spectrometer is also frequently used as a supplementary tool for providing information on functional groups of detected species. As an ambient ionization technique undergoing rapid development, direct analysis in real-time ionization source coupled to iontrap mass spectrometer (DARTIS/ITMS) was first reported by Cody et al. in 2005.5 It creates ions outside the instrument, allowing for the rapid and non-contact analysis of solid, liquid, and gaseous samples without preparations, such as derivatizations, chromatographic separations, and other time- and chemicalconsuming treatments.6−9 Applications of this technology have been extensively reported in the analysis, characterization, and identification of pharmaceuticals,8,10,11 drugs of abuse,7,12−14 forensics,7,15 biocides,16 self-assembled monolayers,17 explosives,18,19 food,16,20 fragrances,21 cosmetic products,22 and biomolecules.7,23 In the field of coal chemistry, this technology was first used to analyze coal degradation products.24 Analyses with multiple tools are needed for understanding complex compositions of OM in coals. In this work, we analyzed the extracts and residue from Geting bituminous coal (GBC) with GC/MS, FTIR, and DARTIS/ITMS, along with a scanning electron microscope (SEM), to understand molecular composition of the extractable species, including macromolecular species. © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Coal Sample and Solvents. GBC was collected from Geting Coal Mine, Shandong Province, China. It was pulverized to pass through a 200-mesh sieve (particle size of 10.51
0.42
0.7936
a
diff, by difference; daf, dry and ash-free basis; Mad, moisture (air-dried basis); Ad, ash (dry basis, i.e., moisture-free basis); Vdaf, volatile matter (dry and ash-free basis); St,d, total sulfur (dry basis). analyzer with a m/z range from 30 to 500 and operated in electron-impact (70 eV) mode. Its temperature was raised from 60 to 130 °C at a rate of 5 °C/min, then raised to 300 °C at a rate of 3 °C/min, and held at that temperature for 20 min. Data were acquired and processed using software of Agilent MSD Productivity Chemstation. The compounds were identified by comparing mass spectra to NIST05 library data. 2.4. FTIR and DARTIS/ITMS Analyses. All of the extracts and residue were analyzed with a Nicolet Magna IR-560 FTIR spectrometer by collecting 50 scans at a resolution of 8 cm−1 in reflectance mode with measuring regions of 4000−400 cm−1 and a SVP-100 ion source (IonSense, Danvers, MA) coupled with an Agilent XCT ITMS. The mechanism for the formation of positive ions is known as Penning ionization:7,25,26
He(23S) + nH 2O → [(H 2O)n − 1 + H]+ + OH• + He(11S)
Figure 2. Yields of the extracts from GBC.
[(H 2O)n + H]+ + M → [M + H]+ + nH 2O
with PE, CDS, methanol, acetone, and isometric CDS/acetone mixed solvent to afford extracts 1−5 (E1−E5) and residue (i.e., isometric CDS/acetone mixed solvent-inextractable portion). Each extraction with the same solvent (900 mL) was conducted for 2 h and repeated at least 30 times to extract the soluble species as exhaustively as possible. Both GBC and its residue were observed with a Quanta250 SEM using an electron transfer detector with a high voltage of 30 kV. The samples were mounted on brass stubs using silver paste as glue and coated with gold (thickness of about 400 Å). The magnification ranged from 250 to 2500. 2.3. GC/MS Analysis. All of the extracts were analyzed with a Hewlett-Packard 6890/5973 GC/MS, which is equipped with a HP-5MS capillary column (cross-link 5% PH ME siloxane, 30 m × 0.25 mm inner diameter, 0.25 μm film thickness) and a quadrupole
The moisture in air is ionized by helium in metastable 23S. Then, the resulting protons are transferred to analyte to form protonized ions (i.e., [M + H]+), which are called quasi-molecular ions (QMIs). The QMIs are detected by a mass spectrometer. No multiply charged ions are formed during the process.24 An atmospheric pressure glow discharge is initiated by applying a kilovolt potential between a needle electrode and a grounded counter electrode. During the process, the mass spectra are affected by a series of factors, such as solvents, containments, temperature, gas flow, pressure, and electrical potential. We set the needle voltage of DARTIS to 4000 V and both the discharge and grid electrode to 280 V. Fast-scan mode was settled from m/z 50 to 1000 with a step size of m/z 0.1 units. Helium (purity of >99.999%) was used as the discharge gas,
Figure 3. SEM micrographs of GBC and its residue. 3710
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Figure 4. Total ion chromatograms (TICs) of E1−E5 from GC/MS analysis. and nitrogen was used as alternative gas, operating at 400 °C with a flow rate of 2 L min−1. The sample was adhered to a sealed end of a capillary column and then placed between the outlet of DARTIS and the inlet of ITMS (1.1 cm of gap between the outlet and inlet). The ionized species from the sample were led into the ITMS with helium stream. Some specific ionized molecules (i.e., parent ions) observed in mass spectra were selected for tandem mass spectral analysis. For data acquisition and processing, LC/MSD Trap software (version 5.2, Agilent, Santa Clara, CA) was used. All of the mass spectra were recorded after background subtraction to avoid false information.
CDS/acetone mixed solvent could also be related to the π−π interaction27 between CDS and acetone. In comparison to GBC, the particle sizes of the residue were substantially reduced, as shown in Figure 3, indicating that the extraction with isometric CDS/acetone mixed solvent led to significant destruction of GBC particles. 3.2. GC/MS Analysis. The results from GC/MS analysis are summarized in Figure 4 along with Figures S1−S5 and Tables S1−S5 of the Supporting Information. The organic compounds detected in the extracts can be largely grouped into alkanes, arenes, and heteroatom-containing organic species (HACOSs). As Figure 5 demonstrates, arenes are the main group component in both E1 and E2, but different from E1, in which arenes contain 1−4 rings (see Figure S1 and Table S1 of the Supporting Information), more condensed arenes (i.e., arenes with 5 and 6 rings) are included in E2 (see Figure S2 and Table S2 of the Supporting Information), suggesting that CDS is more effective than PE for extracting highly condensed arenes from GBC. The π−π interactions between CDS and the condensed arenes and
3. RESULTS AND DISCUSSION 3.1. Extract Yields and SEM Observation. As Figure 2 exhibits, the yields of E1−E5 are 0.3, 2.0, 2.1, 2.1, and 4.1%, respectively, and in total, more than 10% of OM was extracted from GBC. It is noteworthy that the yield of E5 is much higher than that of ether E2 or E4, implying that isometric CDS/acetone mixed solvent has a strong synergic effect on GBC extraction. Similar to isometric CDS/N-methyl-2pyrolidinone mixed solvent, the synergic effect of isometric 3711
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and E4 (Figure 5 and see Figures S3 and S4 and Tables S3 and S4 of the Supporting Information). The strong interactions of hydrogen bonds between the polar solvent (methanol or acetone) and the HACOSs should be the main reason for enriching the HACOSs into E3 and E4. In total, 18 phthalates and alkoxycarbonyl benzoic acids (AOCBAs) were detected in E4, but only 2 phthalates (peaks 65 and 77) can be identified (see Figures S4 and S6 and Table S4 of the Supporting Information). Thermal lability of the species led to the disappearance of molecular ions (MIs) and some important fragmental ions (IFIs) and thereby caused difficulty in their identification by GC/MS analysis. Phthalates and their isomers (i.e., iso- and terephthalates) could originally exist in coals as biomarkers.29 Although the yield of E5 is much higher than that of each other extract, only 19 compounds were identified (including 11 arenes, 4 NNKs, 3 esters, and aminonananol) in E5. This result could imply that most of the species in E5 are too polar and/or involatile to be detected by GC/MS analysis.30 3.3. FTIR Analysis. Obvious differences in the FTIR spectrum among GBC and its extracts and residue can be observed from Figure 6. The absorbances of aliphatic moieties (AMs) from the extracts around 2861 and 2924 cm−1 are much stronger than those from both GBC and its residue, indicating that AM-rich species in GBC are relatively easily extracted. The AMs are present in alkanes, alkenes, alkylarenes, alkanols, alkanals, alkanones, alkanoic acids, esters, and many other HACOSs. Among the extracts, the absorbances of AMs from E1 are the strongest, which is consistent with the highest content of alkanes in E1, as shown in Figure 5. In addition, most arenes and all of the HACOSs detected in E1 contain a methylene chain,
Figure 5. Distribution of group components in E1−E5 from GC/MS analysis.
low viscosity of CDS should be responsible for the effectiveness of CDS for extracting the condensed arenes from GBC. The arenes were reported to be originated from small molecules in vitrinites during the coal-forming process.28 Various HACOSs, including alkanols, arenols, arenofurans, aldehydes, non-nitrogen ketones (NNKs), nitrogen-containing ketones (NCKs), alkanoic acids, esters, amines, other nitrogencontaining species, and some sulfur-containing species, were detected. As HACOSs, only 2 NNKs and 9 methyl alkanoates were detected in E1 (see Figure S1 and Table S1 of the Supporting Information) and fluoren-9-ol, 4 arenofurans, naphtho[2,1,8,7klmn]xanthene, biphenylcarbaldehyde, 3 NNKs, and trimethylindenopyridine were observed in E2 (see Figure S2 and Table S2 of the Supporting Information), while most HACOSs, especially oxygen- and/or nitrogen-containing species, were enriched in E3
Figure 6. FTIR spectra of GBC and its extracts and residue. 3712
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Figure 7. Mass spectra of GBC and its extracts from DARTIS/ITMS analysis.
amounts of carbonyl group-containing species (CGCSs) were detected in E2. This result may suggest that most CGCSs are strongly polar and/or involatile species. The characteristic absorbance of aromatic rings is around 1612 cm−1. Such absorbance from GBC, E5, and the residue is significantly stronger than that from E1−E4, implying that most aromatic ring-containing species in GBC are macromolecular species, and exhaustive extraction of such species is very difficult. A more significant difference in the absorbances around 1040, 910, 540, and 470 cm−1, which are ascribed to the presence of mineral matter (MM),31−33 between the residue and extracts can be clearly observed. This fact suggests that the
an alkyl group, or alkyl groups (see Table S1 of the Supporting Information), which also increase the absorbances of AMs. The absorbance around 3412 cm−1 is attributed to bound −OH. It cannot be observed in the FTIR spectrum of E1, implying that there is almost no interaction between PE and bound −OH-containing species in GBC. In contrast, strong absorbance of other extracts around 3412 cm−1 suggests strong interactions between other solvents and bound −OH-containing species in GBC. The characteristic absorbance of the carbonyl group from aldehydes, ketones, and carboxylic acids is around 1699 cm−1. Such absorbance from E2 is very strong, but only very small 3713
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Figure 8. (a) TMS of the ion at [M + H]+/z 279.3 from DARTIS/ITMS analysis of GBC and (b) corresponding fragmentation mechanism.
Figure 9. (a) TMS of the ion at [M + H]+/z 419.5 from DARTIS/ITMS analysis of E4 and (b) corresponding fragmentation mechanism.
fractional extraction resulted in sufficient separation of partial OM from MM in GBC. 3.4. DARTIS/ITMS Analysis. All of the samples were analyzed with DARTIS/ITMS, but the mass spectrum was not obtained from the residue perhaps because of the difficulty in ionizing the very involatile sample. In other words, the extraction with isometric CDS/acetone mixed solvent is so exhaustive that no species can be detected in the residue even with
DARTIS/ITMS. As Figure 7 exhibits, molecular mass distributions of different samples are quite different. For example, molecular masses of detected species tended to decrease rather than increase from E1 to E3. We selected some species from Figure 7 as examples to understand their possible molecular structures by investigating their tandem mass spectra (TMS). As mentioned above, both phthalates and AOCBAs are thermally labile, leading to the loss of MIs and some IFIs during 3714
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Figure 10. (a) TMS of the ion at [M + H]+/z 463.3 from DARTIS/ITMS analysis of E4 and (b) corresponding fragmentation mechanisms.
Figure 11. (a) TMS of the ion at [M + H]+/z 100.3 from DARTIS/ITMS analysis of E5 and (b) corresponding fragmentation mechanisms.
2-(octadecyloxycarbonyl)benzoic acid in Figure 9] can also be clearly observed. The clear observations of both QMIs and QFIs greatly facilitate elucidating the corresponding fragmentation mechanisms and subsequent structural identification (Figures 8b and 9b). Both TMS shown in Figures 10a and 11a are very simple, but the difference in molecular mass between the two species detected is significant. We considered many possible structures for the two species. As a result, only compound I in Figure 10 and allylidenecarbamic acid (ACA) in Figure 11 are reasonable
GC/MS analysis. Different from GC/MS analysis, molecular masses of organic species in an analyte can be clearly observed according to [M + H]+/z values of QMIs from DARTIS/ITMS analysis (Figure 7). The selected QMIs are fragmented into quasi-fragmental ions (QFIs, i.e., [m + H]+) in a tandem mass spectrometer.24 As Figures 8a and 9a demonstrate, the most characteristic base peak at [m + H]+/z 149.2 (which is ascribed to protonized isobenzofuran-1,3-dione)29,34 and other characteristic peaks representing QFIs of 2 AOCBAs [i.e., 2-((7-methyl-octyloxy)carbonyl)benzoic acid in Figure 8 and 3715
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Postdoctoral Science Foundation (Grants 2011M500975 and 2012T50501), the Jiangsu Provincial Natural Science Foundation (Grant BK2011213), the Priority Academic Program Development of Jiangsu Higher Education Institutions. We also acknowledge ASPEC Technologies Limited for loaning the DART-100S system and Dr. Charles C. Liu for his thoughtful discussion.
for the TMS. During the fragmentations, compound I lost H2O to form compound II with [m + H]+/z 445.3, which either lost CH4 to form compound III with [m + H]+/z 429.3 or underwent ring opening to form compound IV with [m + H]+/z 360.2 and (Z)-1-iminobut-2-en-2-ol (Figure 10b), while ACA was subjected to the loss of either −OH or −CHCH2 (Figure 11b). Both compound I and ACA were not detected with GC/MS because of the involatility and strong polarity of compound I and the strong polarity of ACA.
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4. CONCLUSION Multiple characterizations of the extracts from GBC were conducted. The extracts mainly consist of alkanes, alkylarenes, and various HACOSs according to GC/MS analysis. CDS is especially effective for extracting condensed arenes, including condensed alkylarenes, by π−π interactions with the arenes, while more HACOSs were extracted with either methanol or acetone by hydrogen-bond interactions between the solvent and HACOSs. Isometric CDS/acetone mixed solvent has a strong synergic effect for extracting more organic species from GBC, and the extract is rich in aromatics according to FTIR analysis. GBC and its residue are more rich in aromatic species, while the extracts, especially E1−E4, contain more AMs and less aromatic species than GBC and its residue according to FTIR analysis. DARTIS/ITMS analysis proved to be an effective approach for characterization of molecular composition of the extracts, especially thermally labile, involatile, and/or strongly polar species in the extracts. Such multiple characterizations could be applied to other complex organic mixtures, such as extracts from other coals, coal tar, coal liquefaction oil and residue, heavy petroleum, and bio-oil. Further development can be expected using more advanced techniques, such as DARTIS coupled to a tandem time of fight mass spectrometer.
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ASSOCIATED CONTENT
S Supporting Information *
TICs of E1−E5 from GC/MS analysis, GC/MS-detectable compounds, and mass spectra of phthalates or AOCBAs detected in E4 with GC/MS. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
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*Telephone: +86-516-83884399. E-mail: wei_xianyong@ 163.com.
NOMENCLATURE GBC = Geting bituminous coal CDS = carbon disulfide E1 = extract 1, i.e., petroleum ether-extractable portion from GBC E2 = extract 2, i.e., petroleum ether-inextractable but CDSextractable portion from GBC E3 = extract 3, i.e., CDS-inextractable but methanolextractable portion from GBC E4 = extract 4, i.e., methanol-inextractable but acetoneextractable portion from GBC E5 = extract 5, i.e., acetone-inextractable but isometric CDS/ acetone-extractable portion from GBC GC/MS = gas chromatography/mass spectrometer FTIR = Fourier transform infrared DARTIS = direct analysis in real-time ionization source ITMS = ion-trap mass spectrometer OM = organic matter DARTIS/ITMS = DARTIS coupled to ITMS SEM = scanning electron microscope PE = petroleum ether QMI = quasi-molecular ion HACOSs = heteroatom-containing organic species NNK = non-nitrogen ketone NCK = nitrogen-containing ketone AOCBA = alkoxycarbonyl benzoic acid MI = molecular ion IFI = important fragmental ion AM = aliphatic moiety CGCSs = carbonyl-group-containing species MM = mineral matter TMS = tandem mass spectrum or tandem mass spectra QFI = quasi-fragmental ion ACA = allylidenecarbamic acid TIC = total ion chromatogram REFERENCES
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research work was supported by National Basic Research Program of China (Grant 2011CB201302), National Natural Science Foundation of China (Grants 20936007, 51074153, and 21206187), the Fund from the Natural Science Foundation of China for Innovative Research Group (Grant 51221462), the Key Project of Coal Joint Fund from National Natural Science Foundation of China and Shenhua Group Corporation Limited (Grant 51134021), the Strategic Chinese−Japanese Joint Research Program (Grant 2013DFG60060), the Fundamental Research Funds for the Doctoral Program of Higher Education (Grant 20120095110006) and for the Central Universities (China University of Mining & Technology; Grants 2010LKHX09, 306 2012LWB27, and 2011QNA22), the Fund from China 3716
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