## 3 Results and Discussion
### GC-MS Analysis of Extracts
#### 3.4.3 GC/MS Analysis of the Extract from the MAE  
The TIC for the extract obtained by MAE is shown in Figure 5. In total, 75 substances were detected in the extract, which are provided in Table S3 in Supplementary Materials; categorially, these 75 substances are also listed in Table 2. The most salient future is that the number of the extracted substances was increased drastically whereas the increased substances are from the class of oxygen-containing compounds (OCOCs); the total amount of OCOCs is also the highest among the three extraction methods. On the other hand, the level of aromatic compounds was remained almost unchanged compared to the UAE. In addition, a small portion of NAS (1.19%) was found, which is in contrast to the other extraction methods. Moreover, a few biomarker compounds were also identified including hexaoxane (Peak 44), 2-methylcholest-3-ene (Peak 63), 6,9,12-tripropyheptadecane (Peak 65), and 17-alfa-21-beta-21-beta-28,30-bisonrophane (Peak 69) in Figure 5. Figure 6 illustrates the compositions according to the categories. The extracted relative content and species of the three extraction methods are shown in Figure 6. Compared with other two kinds of extraction, the species extracted by MAE is more, but the content of COCs is less, which might be attributable to carbon tetrachloride as a non-polar solvent is not conducive to the transmission of microwave energy, and will slow down accelerated movement of polar molecular reactants, and can also reduce the collision frequency between reactants. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Number of Organic Species (Abbreviation)**} & \multicolumn{3}{c}{**Extraction Method**} \\ \cline{2-4}  & **SE** & **MAE** & **UAE** \\ \hline n-Alkanes (NAs) & 0 & 1 (1.19) & 0 \\ Branched paraffin (BAs) & 1 (3.20) & 5 (7.99) & 3 (7.22) \\ Chlorinated organic compounds (COCs) & 8 (94.21) & 3 (13.12) & 5 (33.97) \\ Oxygen-containing organic compounds (OCOCs) & 1 (2.59) & 53 (39.50) & 13 (18.72) \\ Aromatic compounds (ACs) & 0 & 13 (38.19) & 16 (39.97) \\ Types of organic compounds & 10 & 75 & 37 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Organic substances identified in the extracts from the coal samples with different extraction methods. Figure 4: TIC of the extract by use of UAE. (**a**) TIC of 5–26 min; (**b**) TIC of 26–37 min. NAs and Bas are extracted more than the other two extractions. Therefore, the MAE method is especially suitable for the extraction of compounds with high solubility in nonpolar solvents. Figure 5: TIC for the extract obtained by the MAE process. (**a**) TIC of 5–30 min; (**b**) TIC of 30–43 min; (**c**) TIC of 43–55 min; (**d**) TIC of 55–65 min. It is commonly accepted that coal is a heterogeneous and complex mixture, consisting mainly of macromolecules that are linked globally by weak interactions including hydrogen bonds and van der Waals interactions. Among the three extraction methods, the MAE gave the most substances, followed by UAE, while the SE gave the least. The acquisition of most substances from the MAE is probably originated from the extraction mechanism. In the extraction process, the microwave radiations can pass through the medium (namely CCl\({}_{4}\)) and reach the deep frame of the coal. On the other hand, the microwave heating can accelerate the penetration of CCl\({}_{4}\) into the matrix of the coal and can also accelerate the diffusion rate of the extracted molecules from the solid to the solid-liquid interface [23]. The electromagnetic fields of microwave have a few functions including polarizing instantly the molecules in the coal, destroying the hydrogen bonds, and breaking down the van der Wheels interactions. Consequently, some small \"fragments\" will dissolve in the solvent [24]. In the SE process, chlorinated compounds dominate the extracted substances (94.2%), indicating some chlorination reactions occurred. This is caused probably by the long extraction time (10 days) and in this period of time, chlorination reactions underwent.