Separation schemes for coal tar analysis — a comparison of methods

A standard tar fractionation scheme used to separate coal-derived materials at CRE involves a solvent separation based on solubility in pentane and benzene. This method is operator-dependant. The solvent fractionation has been followed by open-column liquid chromatography on silica/alumina to generate saturate, aromatic and polar fractions. Our published work has shown that the aromatic fraction contains both saturate and polar material. Alternative methods have been investigated using three tars to compare performance. An initial separation into pentane solubles, asphaltenes and pre-asphaltenes by sequential elution of tar coated on beads, appears to be reproducible and not subject to operator variations. The subsequent fractionation of pentane soluble material into chemical class fractions has been investigated by three methods. They are open column chromatography on alumina, open column chromatography on OPN Porasil-C and HPLC on a semi-preparative PAC10 column. The alumina and OPN Porasil methods are more satisfactory than the silica column method, giving heterocyclic fractions which contained very little aromatic material. OPN Porasil-C is the more expensive packing, but it can be reused. The HPLC method is best for isolation of PAH ring size fractions free from polar compounds. A solvent gradient can be used to elute polar material free from the aromatics but large volumes of relatively expensive solvents are needed. It is cheaper to back-flush the column after elution of the aromatics and collect the polars with no separation. Mass spectrometry has been used to evaluate the various class fractions.