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

Abstract

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, C60, C70, and C84 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 wavelength of 265 nm with laser power densities of 0.5 to 1 MW/cm2 are suitable. For larger PAHs, C60. and C70 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.

Combustion-formed nanoparticles

“…They had evidence of ''reactive polycyclic hydrocarbons probably with sidechains, containing more hydrogen than aromatic (mass range %150 to >550)". Their presence has been confirmed by MB-TOF-MS studies of ions performed by Homann and co-workers [62][63][64]66] and lately by the photo-ionization TOF-MS analysis of Grotheer [67] on neutral species. Homann hypothesized that the high-mass species with molecular masses ranging from 600 to 2500u were PAHs containing 5-membered rings held together mainly by van der Waals-forces or occasionally by a C-C bond or chain.…”

“…MB-TOF coupled with a mass spectrometer (MB-TOF-MS) is a useful tool for studying nanoparticle formation in combustion. Naturally ionized particles/molecules are immediately detected by a TOF-MS [62][63][64], whereas neutral particles present in the molecular beam need to be ionized first by a multiphoton ionization process and then they can be analyzed by a TOF-MS [65][66][67][68]. Different photo-ionization thresholds can be used to distinguish between particles having the same molecular mass but different chemical nature.…”

Recent contributions of flame-sampling molecular-beam mass spectrometry to a fundamental understanding of combustion chemistry

“…For the detection of large molecules, including PAHs, fullerenes, and small soot particles, Ahrens et al described a flow sublimator that can be used for generating exact gas-phase concentrations of compounds with low volatility [82]. Their device monitors the weight loss per time during sublimation of the sample at a definite temperature, allows for further mass-spectrometric investigation of the target species, and thus enables the quantitative analysis of large molecules when sampling fuel-rich flames.…”

“…A 4 ¼ 2.0 benzene-oxygen flame at low pressure was also investigated by Homann and co-workers [82,83,86]. They combined molecular-beam sampling with REMPI and time-offlight mass spectrometry to study the formation of PAHs with 18-70 carbon atoms per molecule.…”

“…This so-called ''soft ionization'' produces virtually fragment-free mass spectra. In flame studies, REMPI processes have been extensively used for the detection of aromatic species like benzene, substituted benzenes, and PAHs [29,[81][82][83][84][85][86][87][88]. As an example, the upper trace of Fig.…”