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Bitumen (Pitch, Asphalt) is composed of Asphaltene and Maltene. The heavy insoluble Asphaltene molecules give Bitumen its body and softening point while the Maltenes are oily and make Asphalt flexible. Asphaltene molecules are soluble in Aromatic Solvents like Toluene but insoluble in Heptane. The Maltenes are Heptane-soluble molecules and less aromatic.

Asphaltenes are highly aromatic; some are Hydrocarbons but many molecules contain odd heteroatoms, ie N, O, and S. There are also some Metals present - Vanadium, Nickel and Iron. 

The Asphaltene mixtures are complex combinations of Molecules, but some structures are known. Reported formula masses have been controversial because of a tendency of these molecules to aggregate, but mass spectroscopic techniques shows that the most probable M_r is ca 600, with a spread in masses to above 1000.