polynuclear aromatic hydrocarbons
total luminescence spectroscopy
Springer Online Journal Archives 1860-2000
Abstract Polynuclear aromatic hydrocarbons (PAHs) are a vast class of organic compounds. Many PAHs show carcinogenic effects, which in general are strongly dependent on their molecular structure. Hence, some, even structurally strongly related, PAHs may show a large difference in carcinogenicity. Low-temperature fluorescence spectroscopy of PAHs embedded in a polycrystalline n-alkane matrix (or Shpol'skii matrix) is a powerful technique to identify trace amounts of PAHs in complex samples. The fluorescence spectra show vibrational resolution, so that even very similar isomers can be discerned. Since the Shpol'skii effect is matrix-induced, both the fluorescence emission and excitation spectra display narrow lines, so that a maximum of information can be obtained via the application of fluorescence excitation–emission matrices. To demonstrate the possibilities of “high-resolution excitation–emission matrices (HREEMs),” a marine sediment sample has been investigated, containing, among others, benzo[a]pyrene and benzo[k]fluoranthene, PAHs with very similar spectral features, but the former being a notorious carcinogenic compound. The two PAHs could be clearly distinguished in the HREEM. Subsequently, a comprehensive set of data was obtained with a multitude of combinations of excitation and emission wavelengths, which allows for the screening of many PAHs, including all priority pollutants, in one analysis step. A simpler and also highly informative screening method, which can be applied over a wide wavelength region, is that of synchronous scanning: the fluorescence signal is obtained by scanning the excitation and emission monochromators simultaneously. When a fixed wavenumber difference is maintained during the scan, the screening can be done based on structural characteristics of the molecules of interest. High-resolution constant-energy fluorescence (CESF) spectroscopy also has been applied to acquire vibrationally resolved spectra for PAHs in the marine sediment sample. When a relatively small energy difference (〈800 cm−1) is applied between excitation and emission wavelength, simple CESF spectra are obtained, which allow for rapid and specific screening of PAHs. Larger energy differences lead to more complex spectra, with spectral features showing up due to various combinations of vibronic excitation and vibronic emission of the same molecule.
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