Synchronous Fluorescence as a Sensor of Trace Amounts of Polycyclic Aromatic Hydrocarbons.

Synchronous fluorescence spectroscopy (SFS) is a technique that involves the simultaneous detection of fluorescence excitation and emission at a constant wavelength difference. The spectrum yields bands that are narrower and less complex than the original excitation and emission bands. The SFS bands correspond uniquely to the fluorescing molecule. Our investigation focuses on evaluating the sensitivity of the SFS technique for the detection and quantitation of PAHs relevant to astrochemistry. Results are presented for naphthalene, anthracene, and pyrene in three different solvents: n-hexane, water, and ethanol. SF bands are obtained with a constant wavelength difference between the peak excitation and emission wavelength (Δλ = λex - λem) at a concentration ranging from 10-4 to 10-10 M. Limit of detection (LOD) and limit of quantitation (LOQ) calculations are based on integrated SF band areas at different concentrations. Spectra of 23 pg/g of anthracene, 16 pg/g, and 2.6 pg/g of pyrene are recorded using ethanol as the solvent. The PAHs exhibit detection limits in the fractions of parts-per-billion (ng/g) range. Through comparison with similar prior studies employing fluorescence emission, our findings reveal a better detectability limit, demonstrating the effectiveness and applicability of the SFS technique.

Vibrational fundamental and overtone spectra of C2H4 in cryogenic liquid solutions.

Vibrational Fundamental and overtone transitions of C2H4 around the Δυ = 1-3 in liquid Ar, Kr, Xe and N2 solutions have been obtained using a low temperature cryostat and a Fourier transform infrared spectrophotometer for wavenumbers between 800 and 10,000 cm-1. The visible CH spectra for Δυ = 6 of ethylene in cryogenic solutions of the same solvents and liquid ethane have been measured with a low temperature cell and the thermal lens technique. Spectra in solutions show great simplification of the bands with respect to gas phase absorption bands. Assignments have been made based on gas phase transitions. Peak positions (ν), wavenumber shifts (Δν), and full widths at half maximum (Δω1/2) are reported. Changes of C2H4 frequencies in liquid Ar, Kr, and Xe seem to correlate with an increase in molar volume, dielectric constant, temperature, and polarizability of the solvents. Influence of the solvent on some fundamental vibrational frequencies are explored using the Onsager model and the polarizable continuum model (PCM). When used in conjunction with calculated anharmonic frequencies, the PCM model shows qualitative agreement with frequency shifts.