PAH isomers identification in complex mixtures by high resolution spectrofluorimetry at 15 K (Shpolskii effect). Application to methylated compounds in pyrene, phenanthrene, chrysene and azaphenanthrene series extracted

The identification and quantification of trace concentrations of polycyclic aromatic compounds (PAC) in environmental samples is of well-known importance as some of those compounds are toxic, mutagenic or carcinogenic. Structural isomers exhibit different levels of carcinogenic potential as function of the position of alkyls substituents in PAC. For example, the substitution of a methyl group at position 5 of chyrsene converts the weakly carcinogenic parent compound to a strong carcinogen 5-methyl chrysene while the others are only moderately carcinogenic. The presence of a heteroatom in the ring can also alter the carcinogenic properties. For instance, benzo vertical bar h vertical bar quinoline and benzo vertical bar f vertical bar quinoline are non carcinogenic, however phenanthridine is a carcinogen. Thus, during the last years, significant advances in analytical methodologies characterized by high sensitivity have been developed to distinguish between alkylated species of the same PAC. Methods based on gas chromatography or capillary gas chromatrography coupled with mass spectrometry, or high performance liquid chromatrography with UV or fluorescence detection have been widely employed for these purposes. Luminescence spectrometry and particularly fluorescence spectroscopy because of its high sensitivity has been also used but at room temperature, emission or excitation broadband spectra of similar compounds (i.e. structural or alkylated isomers) present in the sample suffer interference making individual identification difficult. One way to overcome these limitations is by utilizing the Shpolskii effect, where a sharpening of fluorescence emission spectra is observed when aromatic compounds are incorporated in a suitable polycrystalline n-alkane solvent, frozen at very low temperature (under 20 k since at 77 K an important phonon wing is generally sur-imposed on quasi-linear spectra)