LASER EXCITED SHPOL'SKII SPECTROSCOPY FOR THE SELECTIVE EXCITATION AND DETERMINATION OF POLYNUCLEAR AROMATIC HYDROCARBONS

Laser excited Shpol'skii spectroscopy (LESS) was developed as an analytical technique for the qualitative and quantitative determination of polynuclear aromatic hydrocarbons (PAHs) in complex mixtures. An excimer laser pumped, dye-laser coupled with gated detection was employed in the LESS system. The sharp absorption bandwidth exhibited by PAHs in Shpol'skii hosts (n-alkanes) at 15 K facilitated the selective excitation and sensitive detection of the luminescence through narrow band laser excitation. It was shown that LESS is capable of distinguishing between structural isomers as well as alkyated substitutional isomers when these occurred in complex mixtures. Luminescence spectra, site population dependency on concentrations, analytical curves, and detection limits were studied. It was found that: (a) reproducible site population of PAHs at concentration levels below one ppm could be achieved through reproducible cooling regimes; (b) self-absorption of the 0-0 fluorescence lines could be alleviated on excitation with reduced laser power; (c) analytical calibration curves were linear over 3.5 decades in concentration; and (d) detection limits were on the order of parts-per-trillion. The applications of LESS to the direct qualitative and quantitative determination of PAHs in various liquid fuel samples were demonstrated. Quantitative data obtained via standard addition and internal reference approaches showed excellent agreement.