Microwave Radiation Heating in Pressurized Vessels for the Rapid

22 Sep 2014 - This study presents the development of a fast approach using a pressurized vessel system with either a hot air oven or microwave radiati...
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Microwave Radiation Heating in Pressurized Vessels for the Rapid Extraction of Coal Samples for Broad Spectrum GC−MS Analysis Rajendra K. Mahat, Wesley Rodgers, and Franco Basile* Department of Chemistry, University of Wyoming, 1000 E. University Ave (Dept. 3838), Laramie, Wyoming 82071, United States S Supporting Information *

ABSTRACT: Soxhlet extraction has been successful at processing difficult to extract compounds from a variety of solid samples; however, the extraction is often time-consuming, uses large volumes of solvent, and can only process one sample at a time. This has been more evident in the sample preparation of coal and other complex geochemical samples for analysis by gas chromatography−mass spectrometry (GC−MS), where 72-h Soxhlet extractions are the norm. This study presents the development of a fast approach using a pressurized vessel system with either a hot air oven or microwave radiation heating. The techniques were tested with sub-bituminous (Powder River Range, Wyoming, U.S.A.) and bituminous (Fruitland Formation, Colorado, U.S.A.) coal samples. Performance of the pressure-vessel techniques in terms of extraction efficiency and extracted compound profiles (via GC−MS) were compared to that of a Soxhlet extraction. Overall 30−40% higher extraction efficiencies (by weight) were obtained with a 4 h hot air oven and a 20 min microwave-heating extraction in a pressurized container (using 5 mL of solvent and 1 g of coal sample) when compared to a 72 h Soxhlet extraction (using 125 mL of solvent and 25 g of coal sample). Analyses by GC−MS detected a wide range of nonpolar compounds including n-alkanes and diterpanes (bi-, tri-, and tetracyclic) in the sub-bituminous sample and n-alkanes and alkyl aromatic compounds (benzyl, naphthyl, fluorenyl, and phenanthryl) in the bituminous coal sample. The pressurized microwave heating extraction method for coal samples was found to yield extraction efficiencies that were mostly solvent independent and believed to be a result of the larger tan δ value of the coal relative to the tan δ values for the solvents tested. Advantages of the developed pressurized microwave-radiation heating method include a factor of 25 reduction in the use of solvent volume and coal sample, a 216-fold reduction of the extraction time, feasibility of parallel extractions (i.e., replication), and the ability for fully automated and safe operation of the sample preparation step.

1. INTRODUCTION Sample preparation is the backbone of most analytical procedures as it must accomplish the isolation of target constituents in a measurable state. A well-formulated sample preparation procedure is even more important when performing a broad spectrum analysis, given the expected wide range of molecular mass, polarity, reactivity, and so forth.1 These broad spectrum analyses usually form part of “omics” technologies like proteomics, metabolomics, lipidomics, and more recently, petroleomics.2 In particular, the field of petroleomics involves the broad spectrum analysis of fossil fuel derived components, generally performed with mass spectrometry (MS).2,3 Because of this extensive and complex range of molecular properties of the target analytes, the possibility of bias during the sample preparation step is high and most often goes unnoticed due to the lack of suitable controls or knowledge about the composition of the sample. As a result, sample preparation steps geared toward a broad spectrum analysis often rely on several parallel or serial extraction steps that bracket the wide range spectrum of molecular properties. This strategy can be effective when the sample extraction protocol is rapid; however, it becomes impractical when a single preparation protocol involves an extraction step of 24 h or more and can only process one sample at a time. The analysis of coal samples by gas chromatography (GC)− MS is one of such cases. Extracting nonvolatile compounds from the complex coal polymeric matrix is difficult and usually © 2014 American Chemical Society

involves either the use of harsh solvents or long extraction times. Case in point is the standard protocol for coal extraction, which uses Soxhlet extraction with an appropriate solvent for 48 to 72 h.4−6 This process, although effective, requires an extensive time investment for the preparation of a single sample that limits the rapid analysis of replicates or the optimization of extraction variables because of its low sample throughput. The standard glassware for Soxhlet extraction requires at least 150 mL of solvent, which necessitates an evaporation/concentration step and produces large amounts of solvent waste.7,8 This large solvent volume requirement also makes the processing of small samples difficult. Moreover, most broad spectrum analyses unquestionably involve parallel processing (replicates), which in turn requires separate glassware setups and hood space, which logistically can become more problematic if the extractions are carried out with solvents covering a wide range of polarities. As a result, there is a clear need for the development of alternative rapid sample preparation/extraction procedures suitable for broad spectrum analysis of coal samples by GC−MS. Pressurized heating is an efficient and rapid extraction technique that involves heating a small amount of solid sample in a definite proportion of solvent in a closed vessel. The Received: July 22, 2014 Revised: September 15, 2014 Published: September 22, 2014 6326

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sample degradation is prevented even though it is heated beyond the boiling temperature of the solvent.19 In this work, we present the development and characterization of a simple method for the rapid and efficient extraction of coal samples for their subsequent analysis by GC−MS. The technique involves pressurized sample/solvent heating with either a hot air oven or microwave radiation. Gravimetric and GC−MS results are presented that demonstrate the ability of this technique to extract compounds from coal samples rapidly, efficiently, and reproducibly yielding results comparable to the lengthy and traditional Soxhlet extraction method.

advantages of this method are reduced consumption of solvent and sample9 and the possibility of high throughput sample preparation.10 When heating at a constant volume, the pressure inside the vessel increases due to the increased vapor pressure of the solvent, thus enabling the temperature of the solution to rise above the solvent’s boiling point. Richter et al.11 have described that a number of different physicochemical parameters of solvents undergo alteration because of the mutual relationship existing between pressure and temperature inside a pressurized vessel system. These parametric modifications are ultimately responsible for the enhanced extraction rate. They point that a rise in temperature is instrumental in bringing changes such as disruption of noncovalent solute−matrix and solute−solute interactions such as hydrogen bonds, London dispersion forces and dipole−dipole interactions. Higher temperatures, aided by pressure buildup, also bring about lower solvent viscosity and surface tension that lead to enhanced solvent penetration into the sample matrix and thus enhance mass transfer of the solutes. Since the first analytical use of microwave in 1975 by AbuSamra et al.,12 microwave radiation heated extraction13 has been extensively used in the analytical laboratories in the extraction of a broad variety of sample types, including coal samples. The main advantage of microwave radiation heating is the rapid and efficient rise in temperature of the actual sample solution without affecting the surrounding container. That is, in conventional radiative heating, the thermal energy applied to the system has to heat the sample container before the sample within is heated. Conversely, microwave radiation is absorbed by the sample directly via dipolar polarization and ionic conduction, and heat is lost (i.e., sample heating) due to molecular friction and dielectric loss.14,15 The nature of the solvent (e.g. its polarity, dipole moment, protic/nonprotic, etc.) has a large effect on the efficiency of microwave radiation heating of the sample. The ratio of solvent dielectric loss to the dielectric constant, known as the tangent delta (tan δ), loss tangent factor or loss factor, can quantitate the efficiency of the solvent in converting microwave radiation into heat.16 Here, the dielectric constant is a measure of the polarizibility of a molecule in an electric field responsible to cause change in dipole moment, and the dielectric loss is the fraction of absorbed microwave energy irradiated by the sample as heat.14,16,17 Hence, solvents with large values of tan δ are expected to convert microwave radiation more effectively to heat, which in turn translates into shorter heating times to achieve a given desired temperature. For example, water, with a tan δ of 0.123, is expected to reach a set temperature of 70 °C much faster than ethyl acetate, with a tan δ of 0.059 (for a list of tan δ values for different common polar and nonpolar solvents see ref 17). As a result of this direct sample heating by microwave radiation, the temperature gradient between sample/solvent and the container is maintained to a minimum.16 In the case where a closed vessel (i.e., pressurized) is used in conjunction with microwave radiation heating, improved diffusion of the solvent into the sample matrix core is believed to be aided by a combination of temperatures surpassing the boiling point of the solvent and by “localized molecular superheating”.14,18 This in turn leads to enhanced desorption of compounds from the sample matrix. Because this enhancement in extraction efficiency often comes with a concomitant decrease in the extraction time, it is reported that

2. EXPERIMENTAL SECTION 2.1. Coal Samples. Sub-bituminous coal (Bridle Bit Ranch Mine, Powder River Basin, 60 miles south of Gillette, WY, U.S.A.) sample was used to compare the efficiency of different extraction techniques mentioned in this study. A bituminous coal sample was also used to determine the extent of chemicals and biomarkers extracted by the pressurized techniques (Durango, Fruitland Formation, CO, U.S.A.). Coal samples were dried and pulverized prior to sample extraction and analysis. 2.2. Chemicals. Solvents used in the extraction processes include chloroform, dichloromethane, ethyl acetate, and hexanes and were all of HPLC grade and used as obtained and without further purification. Chloroform, dichloromethane, and hexanes were purchased from Fischer Chemicals (Pittsburgh, PA, U.S.A.), and ethyl acetate was purchased from J. T. Baker (Phillipsburg, NJ, U.S.A.). 2.3. Soxhlet Extraction of Coal. A preweighed thimble loaded with 25 g of dried and pulverized coal sample was placed inside the Soxhlet extractor (Condenser, extractor, and flask, P/ N’s C243445, E103424, F302425 respectively, from Synthware, Beijing, China; thimble, P/N CG-1372-01, from Chemglass, NJ, U.S.A.) and extracted with 125 mL of dichloromethane at 75 °C. Three different heating times of 24, 48, and 72 h respectively were used. A single extraction was carried out for each extraction time. The Soxhlet extract was collected in 120 mL amber glass bottle (Supelco, PN 23230-U) and stored at 4 °C before GC−MS analysis. The coal in the thimble was allowed to dry and the weight of the coal after the extraction was recorded to determine extraction efficiency. 2.4. Pressurized Heating Extraction of Coal. Pressurized heating was carried out by employing either a regular hot air oven or a software-controlled analytical microwave oven (Discover oven and Synergy software, CEM, Matthews, NC, U.S.A.). The hot air heating was accomplished with a decommissioned GC oven (5890, HP, Santa Clara, CA, U.S.A.). In both cases, approximately 1 g of dried and pulverized coal sample was transferred to a 10 mL glass vial (CEM, P/N 908035; crimped with a PTFE septum lined aluminum cap; CEM, P/N 908040) and 5 mL of the appropriate solvent was added. The hot air oven temperature was set at 110 °C for 1, 2, and 4 h, respectively. For the microwave radiation heating method, a maximum microwave power of 150 W was used for extraction times of 10 and 20 min. The maximum temperature and pressure limits were set at 110 °C and 150 psi, respectively. The microwave oven heating profile was as follows: an initial heating ramp was set for 2 min to achieve the desired maximum temperature of 110 °C, which was then held for either 10 and 20 min, respectively, followed by cooling with nitrogen gas (to a temperature below the boiling point of the corresponding extraction solvent). For each 6327

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Table 1. Parameters of Solvents That Affect the Absorption of Microwave Energy solvent chloroform (CHLOR) dichloromethane (DCM) ethyl acetate (EA) hexane

dielectric constant (ε′)17

dielectric loss (ε″)17

loss tangent factor17 tan δ=ε″/ε′

dipole moment17 (μ, Debye)

boiling point (°C)

eluent strength (εo)21

4.8 9.1

0.437 0.382

0.091 0.042

1.15 1.60

61 40

0.26 0.30

6.0 1.9

0.354 0.038

0.059 0.020

1.78 0.08

78 69

0.48 0.01

single chromatogram for the following n-alkane hydrocarbon ranges: C11−C17, C13−C19, C15−C21, C17−C23, and C19−C25. Signals were obtained by manually selecting and integrating the peak area under each chromatogram (XIC m/z 57 for n-alkane, and XIC m/z 178, 192 for methylphenanthrenes) using the Genesis peak detection algorithm in the Xcalibur software (Thermo Scientific).

solvent and each method (and time), four replicate experiments were carried out. In both pressurized microwave and hot air heating techniques, after heating, the sample vials were allowed to cool to room temperature and the slurry was filtered using preweighed Whatmann filter paper no. 1. The filter paper was rinsed with the corresponding fresh solvent (3−4 mL) until the filtrate was colorless. The combined filtrates were saved in 7 mL glass vials (Supelco, PN 27151) equipped with PTFE lined screw caps for subsequent GC−MS analysis (see below). The rinsed coal sample/filter paper was dried at room temperature inside a desiccator overnight and weighed. The amount of material extracted from coal, and thus the percent extraction efficiency for each solvent, was then calculated. 2.5. Sample Preconcentration and GC−MS Analysis. The extractant phases (∼8−9 mL) from both pressurized heating methods were concentrated down to 200 μL by purging with a gentle stream of nitrogen gas (∼20 psi) using a Minivap evaporator (Supelco, PN 22971) and collected in 2 mL glass vials equipped with 200 μL glass inserts. Extractant phases from the Soxhlet method (∼120 mL) were reduced down to 5 mL (under a gentle stream of nitrogen gas), and aliquots were collected in 2 mL glass vials for subsequent analysis by GC− MS. The GC−MS analysis was carried out using a gas chromatograph (Trace GC-Ultra, Thermo Scientific, Austin, TX) equipped with an autosampler (Triplus, Thermo Electron, Milan, Italy) and coupled to a single quadrupole mass spectrometer (DSQ II, Thermo Scientific, Austin, TX). The chromatographic separation was carried out using a 30 m × 0.25 mm I.D. fused-silica capillary column with 0.25 μm film thickness (5% diphenyl−95% dimethyl-polysiloxane; ZB-5 Phenomenex, Torrance, CA, U.S.A.). Helium carrier gas was set at a constant flow rate of 0.8 mL/min. A 1 μL aliquot of sample was injected with a split ratio of 1:70. The GC oven was programmed at an initial temperature of 35 °C held for 2 min and increased at 8 °C/min to a final temperature of 300 °C and held for 5 min. Injector and transfer line were set at 250 and 300 °C, respectively. Each sample was injected twice and a solvent blank was measured between two different sample injections to check for possible sample carryover. Mass spectrometric analysis was carried out in positive electron ionization (EI+) mode with an electron energy of 70 eV and the ion source temperature kept at 250 °C. The quadrupole mass analyzer was scanned at the rate of 2.5 scans/s in the mass range of 50−550 u and operated with a solvent delay of 3 min. Chromatograms and mass spectra were collected with the Xcalibur software (ver. 2.0.7; Thermo Scientific) and tentative chemical assignments were performed with the NIST mass spectral database (ver. 2008) and by comparison with published mass spectra. 2.6. Carbon Preference Index (CPI) and Methylphenanthrene Indices Calculations. The CPI was calculated according to the revised equation by Olson et al.20 Average CPI values from a single chromatogram were calculated from a

3. RESULTS AND DISCUSSION Results are first presented on the development of the pressurized vessel extraction protocols with both hot air oven and microwave radiation heating, followed by their comparison with the standard Soxhlet extraction. This comparison specifically focuses on (1) the amount of mass extracted from coal by each technique and (2) the resulting GC−MS profiles of the extracts obtained using each method. However, because this work uses low-resolution MS detection (i.e., single quadrupole), only tentative compound identifications and class classifications were attempted. 3.1. Pressurized-Vessel Method Development. Using a set hot air oven temperature of 110 °C (above all solvents’ boiling points; Table 1) and the pressurized vessel, the percent weight extracted from the Bridle Bit coal sample (subbituminous) was measured using several solvents and extraction times. The data illustrated in Figure 1 shows the percent

Figure 1. Percent weight extracted (i.e., extraction efficiencies by weight) for different solvents at different extraction times using pressurized heating employing a hot air oven, where extraction efficiency increases with time. Highest extraction efficiency obtained for dichloromethane (DCM) at 4 h of extraction (error bars = ±1σ, n = 4). Sub-bituminous coal sample (Bridle Bit, Powder River Basin, WY, U.S.A.). CHLOR, chloroform; DCM, dichloromethane; EA, ethyl acetate.

extraction efficiency (or percent weight extracted) for the four solvents tested, chloroform (CHLOR), dichloromethane (DCM), ethyl acetate (EA), and hexane, at three extraction times (1, 2, and 4 h). For all solvents tested, the percent extraction efficiency is observed to increase with increasing extraction time, with the highest extraction efficiency obtained 6328

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with the dichloromethane solvent (7.6 ± 3.1% by weight) at 4 h. On the other hand, chloroform solvent yielded the lowest and least consistent extraction efficiency (4.1 ± 8.6%) for a 4 h extraction. At 1 h extraction time, the measured percent extraction was highest for ethyl acetate, whereas the efficiencies for the remainder of the solvents were statistically indistinguishable (i.e., within experimental error). Although no clear correlation is observed between the extraction efficiencies obtained using the hot air method and solvent properties listed in Table 1, it is evident that the largest fraction of extractable compounds from coal is most soluble in the dichloromethane solvent. For the pressurized-vessel extraction using microwave radiation heating, two different heating times, 10 and 20 min, were tested with a maximum temperature and pressure limits set at 110 °C and 150 psi, respectively, and with a constant microwave power setting of 150 W. (The effect of microwave oven power on extraction efficiency was also investigated. Results showed that the extraction efficiency increased with increasing microwave power, with a maximum extraction efficiency at 150 W. See Supporting Information Figure S1). The resulting coal percent weight extracted using these microwave heating parameters and different extraction solvents are shown in Figure 2.

or a 30% increase when compared to the 10 min extraction efficiency. The highest extraction efficiency at 20 min was achieved with the dichloromethane (DCM) solvent (9.7 ± 0.6%), although this value was found to be statistically indistinguishable (95% confidence limit) from that of chloroform (CHLOR). Interestingly, the extraction efficiencies using the microwave radiation heating method for the solvents chloroform, ethyl acetate, and hexane were statistically indistinguishable from each other, unlike the conventional heating process (Figure 1). That is, the extraction efficiencies obtained with the microwave radiation heating method are mostly independent of the nature of the solvent and do not follow the expected trend of increasing extraction efficiencies with increasing loss tangent (tan δ) values (see Table 1). A rationalization for the apparent lack of solvent effect and the observed extraction efficiencies of coal samples heated by microwave radiation can be derived from temperature−time and microwave power−time curves during the microwave heating process. It was observed that when a solvent blank (i.e., no coal sample present) was heated via microwave radiation, different times were required to achieve the set temperature. For example, chloroform reached the preset temperature limit of 110 °C the fastest (in 2 min 20 s) followed by ethyl acetate (3 min 7 s). The solvents dichloromethane and hexane took more than 9 min each to achieve the set temperature limit. That is, the microwave heating time to reach the set temperature of 110 °C for the solvent blanks correlated with their corresponding tan δ values (Table 1). In addition, the highest vapor pressure during microwave heating was observed for dichloromethane in accordance with its low boiling point (and despite its comparatively lower tan δ value). On the other hand, when coal sample was added to the solvent and heated via microwave radiation, this heating time profile no longer correlated with the solvent’s tan δ factors. In fact, the heating time pattern was observed to invert, with hexane heating the quickest (2 min 5 s to reach 110 °C), and dichloromethane the slowest (6 min; See Supporting Information, Table S1, and temperature−time profiles for coal−solvent heating with microwave radiation, Supporting Information Figure S2). In addition, the microwave radiation (i.e., the microwave power) was constantly adjusted by the microwave oven software in order to achieve the set temperature of 110 °C. Microwave power versus heating time curves showed (Figure S3 in Supporting Information) that the microwave power used is constant and close to the set maximum value of 150 W for the two solvents with the highest extraction efficiencies (dichloromethane and chloroform), whereas the adjusted power decreased considerably ( 1, odd carbon number preference) or below unity (CPI 1.0%, whereas the MPI-1 index correlates with maturity at Ro < 1.35.31 Table 3 list these values as calculated from GC−

from bulk measurements alone (i.e., percent mass changes) and does not reflect the nature of the chemical species extracted by each method. A series of GC−MS measurements is presented next in order to characterize the nature and range of the compounds extracted by each method. 3.3. GC−MS Analyses of Coal Extracts and Comparison of Extraction Methods. 3.3.1. GC−MS Analysis of a Sub-Bituminous Coal Sample. Analyses by GC−MS were performed on the resulting extracts obtained from the three extraction techniques on several coal samples. Figure 3 shows total ion chromatograms (TIC) for the Bridle bit coal sample, a low-rank sub-bituminous coal, prepared with microwave-oven (20 min), hot air oven (4 h), and Soxhlet (72 h) extraction techniques. Visual inspection of these chromatograms shows distinct features that are common in all three chromatograms, making their profiles indistinguishable from each other. This is evident in the retention time region between 13 and 17 min (inset), where the chromatogram profiles are very similar. However, it is worth noting that the absolute intensities are different for each chromatogram, and these values correlate with extraction efficiencies (by weight) listed in Table 2; that is, the highest mass spectral signal intensity is observed for the sample extracted with the pressurized microwave-oven heating technique. From results presented in Figure 3 it can be concluded that the pressurized microwave oven extraction procedure yields equivalent extractions as those achieved with the Soxhlet method, albeit with higher efficiency. Although not within the scope of this work, the nature of several classes of compounds extracted from the coal samples (prominent in the chromatograms in Figure 3) was determined by inspection of the extracted ion chromatograms (XIC’s) and comparison of their mass spectra to those of known fossil fuel biomarkers.24 Figure 4 shows several XIC’s derived from the GC−MS analysis of the Bridle Bit coal sample (subbituminous; Powder River Basin, WY, U.S.A.) to tentatively identify prominent signals in its chromatogram, including alkanes (XIC m/z 57)24 and bicyclic and tricyclic diterpanes (XIC m/z 123).25 A tetracyclic diterpane at a retention time (tr) of 23.08 min in the XIC m/z 123 was tentatively identified as either 16β(H) or 16α(H)-phyllocladane when compared to published mass spectra of synthesized reference compounds26 (mass spectrum and structures in Supporting Information section, Figure S4). The prominent peak at tr = 27.15 min was tentatively classified as an alkylphenanthrene (XIC m/z 234). Similar XIC’s for the other two methods, pressurized hot air oven and Soxhlet extraction, are shown in the Supporting Information section (Figures S5 and S6, respectively). Again, visual inspection of the resulting XIC’s showed similar and indistinguishable patterns for each extraction technique. It is worth noting that signals in the XIC m/z 57 are not exclusively derived from alkanes, as bicyclic diterpanes contribute considerably to the production of this fragment ion (e.g., signals in the 13−17 min retention time window are common to both groups of compounds). Overall, it can be concluded that these data demonstrate the equivalence of the pressurizedextraction methods to the Soxhlet method in their ability to extract the same range of compounds, albeit noting that only nonpolar compounds were detected by this approach. Finally, it is worth noting that the range of biomarker compounds extracted from this sample is consistent with the organic content in sub-bituminous coals characteristic of the Powder River coal field (Wyoming, U.S.A.).27−29

Table 3. Maturity Indices Calculated for the Durango Coal Sample Extracted with Different Techniques extraction technique Soxhlet (72 h) microwave-pressure (4 h) microwave-pressure (20 min)

CPIa (95% C.I.)

MPI1b

0.99 (±0.015) 0.98 (±0.014)

1.35 1.36

1.21 1.21

3.10 2.99

1.4 1.4

0.96 (±0.073)

1.59

1.35

2.97

1.4

Ro,calcc MPRd Ro,graphe

a

CPI calculated using equation in ref 30. Numbers in parentheses are the 95% confidence interval, n = 5. bMPI-1 calculated using equation in ref 20. cRo,calc = 0.60(MPI-1) + 0.40 (in ref 33). dMPR = 2MP/1MP (in ref 31); 2MP = 2-methylphenanthrene; 1MP = 1-methylphenathrene eRo,graph estimated graphically from Figure 13 in ref 31 and using MPR values calculated in this study.

MS data for samples extracted using the pressurized microwave heating (microwave heating for 20 min and 4 h) and the Soxhlet extraction methods (72 h). All the CPI values presented in Table 3 are within unity and correlate well with the fact that the Durango coal from the northern section of the Fruitland Formation is thermally mature.34−36 The ability to dependably extract hydrocarbons from coal samples (e.g., Figure 5a) is clearly demonstrated since 6333

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material is available free of charge via the Internet at http:// pubs.acs.org/.

all CPI values calculated are indistinguishable from each other at the 95% confidence level, and further demonstrates the equivalence between the pressurized-microwave heating method and the standard Soxhlet method. The MPI-1 and MPR indices were calculated from the signals corresponding to phenanthrene and methylphenanthrene compounds (Figure 5d) for this bituminous coal sample, and their relative magnitudes are also comparable between the extraction methods tested.

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Corresponding Author

*E-mail: [email protected]. Telephone: 307-766-4376. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors would like to acknowledge NSF for the acquisition of the GC−MS instrument (NSF CAREER CHE-0844694) and the Research Partnership to Secure Energy for America (RPSEA) for their generous support of this work (RPSEA with The US Department of Energy, prime contract DE-AC 2607NT42677 and subcontract between RPSEA and Colorado School of Mines #07122-14).

4. CONCLUSIONS This study presented the optimization and characterization of a rapid pressurized-microwave heating/extraction method for the analysis of nonpolar biomarkers in coal samples, both bituminous and sub-bituminous. Gravimetric as well as GC− MS data were presented that clearly demonstrated the ability of the pressurized/microwave extraction technique to extract equivalent coal biomarkers as those extracted by the standard Soxhlet protocol. However, the pressurized/microwave approach yielded a factor of 216 reduction in extraction time and a 30−40% enhanced extraction efficiency, the latter demonstrated by both gravimetric and GC−MS analyses. These results were attributed to (1) the rapid and efficient microwave heating of the sample itself (and not the container) and (2) the elevated temperatures and pressures achieved in the pressurized vessels that offer a kinetic advantage over the Soxhlet extraction (more efficient solvent−solute mass-transfer between the coal matrix and bulk solvent).17,18 An unexpected finding of this study was that the extraction efficiencies were nearly independent of the nature of the solvent used when using microwave radiation heating. This effect was attributed to the higher tan δ value of coal when compared to that of the solvents tested, and thus, the effective tan δ value for the coal− solvent mixture remains practically unchanged with different solvent systems. This finding can be advantageous as the solvent used for extraction can be chosen to best fit the downstream analytical method. Analyses by GC−MS of a subbituminous coal sample from the Powder River Basin (Wyoming, U.S.A.) extracted by all the techniques tested showed the presence of linear alkane hydrocarbons, diterpanes (bi-, tri-, and tetracyclic) and a dimethylated phenanthrene, whereas the GC−MS analysis of a bituminous coal sample from the Durango Fruitland Formation (Colorado, U.S.A.) showed a higher range of alkane hydrocarbons and a high abundance of alkylated aromatic compounds (benzyl, naphthyl, fluorenyl, and phenanthryl). In addition to the time advantage, the pressurized/microwave extraction method also allowed a reduction on the amount of solvent and sample used by a factor of 25 when compared to the Soxhlet method. Combined, these advantages make feasible the extraction of several samples (replication) and the optimization of broad spectrum analyses requiring the use of a wide range of solvent polarities.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Information related to the optimization of the microwave extraction method, graphs of the heating characteristics (temperature, power−time curves) of solvent blank and coal sample, effect of moisture in sample matrix in the microwave extraction, and detailed mass spectra and XIC’s for samples prepared by Soxhlet extraction, pressurized hot air heating, and pressurized microwave radiation heating. This 6334

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dx.doi.org/10.1021/ef501659h | Energy Fuels 2014, 28, 6326−6335