Probing Combustion Chemistry in a Miniature Shock
Tube with Synchrotron VUV Photo Ionization Mass Spectrometry
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Abstract
Tunable synchrotron-sourced photoionization
time-of-flight mass
spectrometry (PI-TOF-MS) is an important technique in combustion chemistry,
complementing lab-scale electron impact and laser photoionization
studies for a wide variety of reactors, typically at low pressure.
For high-temperature and high-pressure chemical kinetics studies,
the shock tube is the reactor of choice. Extending the benefits of
shock tube/TOF-MS research to include synchrotron sourced PI-TOF-MS
required a radical reconception of the shock tube. An automated, miniature,
high-repetition-rate shock tube was developed and can be used to study
high-pressure reactive systems (<i>T</i> > 600 K, <i>P</i> < 100 bar) behind reflected shock waves. In this paper,
we present results of a PI-TOF-MS study at the Advanced Light Source
at Lawrence Berkeley National Laboratory. Dimethyl ether pyrolysis
(2% CH<sub>3</sub>OCH<sub>3</sub>/Ar) was observed behind the reflected
shock (1400 < <i>T</i><sub>5</sub> < 1700 K, 3 < <i>P</i><sub>5</sub> < 16 bar) with ionization energies between
10 and 13 eV. Individual experiments have extremely low signal levels.
However, product species and radical intermediates are well-resolved
when averaging over hundreds of shots, which is ordinarily impractical
in conventional shock tube studies. The signal levels attained and
data throughput rates with this technique are comparable to those
with other synchrotron-based PI-TOF-MS reactors, and it is anticipated
that this high pressure technique will greatly complement those lower
pressure techniques