Mass Spectrometry of Particles Formed in a Deuterated Ethene Diffusion Flame

Nanometer-sized spherule soot precursor particles have been collected by thermophoretic sampling from the interior of a laminar diffusion flame and mass analyzed by laser microprobe mass spectrometry. Mass spectra of the precursor particles formed in an ethene diffusion flame have indicated the presence of polycyclic aromatic hydrocarbons (PAHs) in the m/z range of 202-300 and higher mass peaks extending out to m/z 472. The mass resolution of the time-of-flight mass spectrometer used did not provide conclusive identification of PAHs because of ambiguities in assignment for the relative amounts of carbon and hydrogen (C(x)H(y)) for each PAH peak and the possibilities of spectral interferences. To determine the chemical formula that can be assigned to each molecular ion peak, an isotopically pure deuterated ethene (C(2)D(4)) fuel was burned in place of normal ethene (C(2)H(4)) in the diffusion flame. For the normal ethene fuel, mass peaks tentatively identified as C(16)H(10) to C(38)H(16) were obtained. Accordingly, deuterated PAH peaks ranging from C(16)D(10) to C(38)D(16) were found when C(2)D(4) was burned. These m/z values correspond to molecular ion, M•(+), peaks for an array of PAH compounds. The deuterated PAH mass peaks (C(x)D(y)) were entirely consistent with a mass shift of y mass units with respect to the normal PAH mass peaks. The carbonaceous particle aggregates collected from the upper flame region have mass peaks characteristic of C(x)(+) and C(x)H(+), while the deuterated soot has C(x)(+) and C(x)D(+). The deuterated ethene experiment has verified the identities of x and y in the PAH (C(x)H(y)) compounds present in the precursor particle samples. No prior experiment using pure deuterium-based fuel as a combustion diagnostic to form aerosol-containing deuterated PAH compounds has been reported.