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 na me and mass analyzed by laser microprobe mass spectrometry, Mass spect ra of the precursor particles formed in an ethene diffusion name have indicated the presence of polycyclic aromatic hydrocarbons (PAHs) in t he 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 (CxHy) f or each PAH peak and the possibilities of spectral interferences. To d etermine the chemical formula that can be assigned to each molecular i on peak, an isotopically pure deuterated ethene (C2D4) fuel was burned in place of normal ethene (C2H4) in the diffusion flame. For the norm al ethene fuel, mass peaks tentatively identified as C16H10 to C38H16 were obtained, Accordingly, deuterated PAH peaks ranging from C16D10 t o C38D16 were found when C2D4 was burned. These m/z values correspond to molecular ion, Mi, peaks for an array of PAH compounds. The deutera ted PAH mass peaks (CxDy) were entirely consistent with a mass shift o f y mass units with respect to the normal PAH mass peaks. The carbonac eous particle aggregates collected from the upper name region have mas s peaks characteristic of C-x(+) and CxH+, while the deuterated soot h as C-x(+) and CxD+. The deuterated ethene experiment has verified the identities of x and y in the PAH (CxHy) compounds present in the precu rsor particle samples. No prior experiment using pure deuterium-based fuel as a combustion diagnostic to form aerosol-containing deuterated PAH compounds has been reported.