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High-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) combined with other analytical techniques and aromatic structure analysis were employed to characterize and identify large polycyclic aromatic hydrocarbons (PAHs) in coal tar pitch and its hexane-soluble fraction. Nonsubstituted aromatics in high abundance up to 850 Da can be extracted successfully with defined molecular formula from MALDI spectra, and mapped systematically into a 50/24 matrix, indicating a structural growth pattern by cooperative ortho-condensation and bay-condensation. The matrix provided a quantitative summary, with their quantity and chemical structures defined essentially by MS intensity and molecular formula of PAHs isomers, respectively. It also revealed a trend that the PAHs grown alternatingly at ortho and bay positions exhibit much higher concentrations than those by ortho- or bay-condensations alone, even though their relative abundance by MS intensity may drop as aromatic rings grow with condensation polymerizations. Such compositional insights may be valuable for better utilization of coal tar pitch as well as understanding the overall reactions that lead to the formation of such a complex mixture including very small and very large polycyclic aromatic compounds.

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Four fractions separated sequentially from a coal tar pitch by two different methods are examined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF-MS) and 50/24 matrix analysis, to evaluate both separation effectiveness and characteristics of large polycyclic aromatic hydrocarbons (PAHs) in variable fractions. Both Soxhlet extraction and anti-solvent precipitation can sequentially produce four fractions that have narrower molecular mass distribution with specific structural features of PAHs. The results show that the overall molecular mass (MM) and ring size of large PAHs abundant in the fractions gradually shift to higher values as solvent strength increases, with some overlap between two adjacent fractions. A broad range of peaks in MS spectra has been resolved as 4-to about 25-ring PAHs by 50/24 matrix analysis, with some detailed molecular structure elucidated. For example, the PAHs in the light HS fraction include 4 to 11-ring aromatics with more ortho-condensed structure. On the other hand, heavier fractions contain more aromatic rings fused by bay-condensations that result in quaternary aromatic carbons. Besides structural insights, SEM morphologically reveals differences in the extraction effectiveness between Soxhlet and antisolvent, due to mass transfer and diffusion limitations in bituminous material.

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