Microwave Radiation Heating in Pressurized Vessels for the Rapid Extraction of Coal Samples for Broad Spectrum GC–MS Analysis

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.