Instrumental improvements for the trace analysis of structural isomers of polycyclic aromatic hydrocarbons with molecular mass 302 Da.

Polycyclic aromatic hydrocarbons (PAHs) are some of the most common environmental pollutants encountered worldwide. Eco-toxicological studies attribute a significant portion of the biological activity of PAH contaminated samples to the presence of high molecular weight PAHs (HMW-PAHs), i.e. PAHs with molecular mass (MM) greater than 300 Da. The research presented here focuses on the analysis of PAH isomers of MM 302 Da. This is not a trivial task. There are 23 isomers with MM 302 Da available to commercial and academic researchers. Many of them are difficult to separate in the chromatographic column and have virtually identical fragmentation patterns. The selectivity of HPLC absorption and fluorescence detectors is modest for resolving co-eluting isomers. Previous work in our lab demonstrated the potential of laser excited time-resolved Shpol'skii spectroscopy (LETRSS) for the analysis of 302 Da isomers in HPLC fractions. The main limitation of the technique was instrumental and due to the narrow range of excitation wavelengths of the tunable dye laser used for sample excitation. Herein, we remove this limitation with an optical parametric oscillator (OPO)-based wavelength tuning laser that covers the whole excitation range of 302 Da isomers. It is possible now to excite each isomer at its excitation wavelength for maximum fluorescence emission and reach limits of detection at the parts-per-trillion level (pg.mL-1). The excitation bandwidth of the OPO laser (0.2 nm) is a good match for the narrow excitation spectra of 302 Da isomers in n-octane. This feature, associated to unique vibrational fluorescence profiles and lifetime decays, allows for the unambiguous identification of co-eluting isomers in RPLC fractions. The same is true for their quantitative analysis in coal tar samples.