The chemical evolution of soot precursor particles on the centerline o
f the laminar ethene diffusion flame has been analyzed using laser mic
roprobe mass spectrometry (LMMS) as they undergo the transition to car
bonaceous aggregates. LMMS is a reliable microanalytical technique for
the detection of intermediate and heavy polycyclic aromatic hydrocarb
ons (PAHs) in particulate material. The analyses show that many of the
masses present within the precursor particles coincide with those pre
dicted by Stein and Fahr (1985) to be most thermodynamically stable (s
tabilomers). The stabilomer PAHs that consist solely of six-membered r
ings, the benzenoid PAHs, prove to be the most important members of th
e stabilomer grid. Pericondensed PAHs as large 472 amu, which is attri
buted to the molecule C38H16 with 12 hexagonal rings, are found to be
constituents of the precursor particles. The PAH mass distribution div
erges to the larger sizes with increasing height in the flame, and inc
ludes many of the species identified by others as gas-phase PAH consti
tuents in hydrocarbon flames. Carbonization on the centerline of the f
lame occurs abruptly between 35 and 40 mm above the burner where the p
article metamorphosis (from single precursor liquid-like particles to
fused aggregates) and the decrease in hydrogen mole fraction (from 0.3
5 to 0.15) simultaneously occur. The presence of stabilomer PAHs repor
ted by others in the particulate combustion product of a variety of fu
els-aliphatic and aromatic gases, diesel fuel, elude oil, kerogen, car
bon black feed stuck, cigarette tobacco, and biomass-suggests that the
stabilomer grid represents the common path for the growth of PAHs whi
ch contribute to the formation of carbonaceous soot in these diverse i
nstances. This observation can account for the previously noted invari
ance of the soot product of combustion from diverse fuels and devices.
(C) 1998 by The Combustion Institute.