A selective fluorescence quenching method using nitromethane as the quenching agent is systematically studied in the absence and presence of absorption effects. Nitromethane is found to quench the emission intensity of polynuclear aromatic hydrocarbons (PAHs) with six-membered rings, while the emission of PAHs with five-membered rings is essentially unaltered. However, fluorescence attenuation caused by primary and secondary absorption may result in errors in the determination of dynamic quenching constants. In this work, a modified Stern-Volmer relationship is developed to distinguish and to compensate mathematically for absorption effects. Utilizing this expression, the Stern-Volmer quenching constant (Kq) is determined to be 125 and 0.15 M-1 for pyrene and fluoranthene, respectively, using nitromethane as the quenching agent in methanol at ambient temperature. Because of the large difference in quenching constants, this analytical methodology is applied for the class-selective identification of PAHs in coal-derived fluids by microcolumn liquid chromatography with laser fluorescence detection.
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