Large molecules, radicals, ions, and small soot particles in fuel-rich hydrocarbon flames - Part III: REMPI mass spectrometry of large flame PAHs and fullerenes and their quantitative calibration through sublimation

Large polycyclic aromatic hydrocarbons (PAHs) and fullerenes have been studied using nozzle/molecular beam sampling from flames and from a sublimator with subsequent resonance enhanced multi-photon ionization (REMPI) mass spectrometry. The mass spectrometer was of the time-of-flight type with an ion reflector. The sublimator for large molecules generated a definite gas phase concentration of the respective PAHs or fullerene which was controlled by determining the mass loss of the substance in a stream of heated nitrogen. The purpose of these calibrations is the quantitative analysis of these large molecules when sampling fuel-rich flames. Twenty-three PAHs with masses up to 596 u (dicoronylene), some oxygen-containing PAHs, C-60, C-70 and C-84 were used. Sublimation energies of low-volatility PAHs and of fullerenes were determined. Multi-photon ionization of PAHs and fullerenes was investigated as function of the effective laser power density in the ion source and the wavelength. Furthermore, ion fragmentation of large PAHs and fullerenes was studied for different power densities and wavelengths. For PAHs with up to about 20 C atoms a wave length of 265 nm with laser power densities of 0.5 to 1 MW/cm(2) are suitable. For larger PAHs, C-60,C- and C-70 a wavelength of 208 nm is more favorable. At the shorter wavelength PAHs have considerably higher ionization efficiencies and the fullerenes do not show delayed ionization and fragmentation. Quantitative relationships between the ion signal and the absolute number density for a large number of PAHs and fullerenes in the source were determined for standardized inflow and ionization conditions.