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

Abstract

a b s t r a c tFlame-sampling molecular-beam mass spectrometry of premixed, laminar, low-pressure flat flames has been demonstrated to be an efficient tool to study combustion chemistry. In this technique, flame gases are sampled through a small opening in a quartz probe, and after formation of a molecular beam, all flame species are separated using mass spectrometry. The present review focuses on critical aspects of the experimental approach including probe sampling effects, different ionization processes, and mass separation procedures. The capability for isomer-resolved flame species measurements, achievable by employing tunable vacuum-ultraviolet radiation for single-photon ionization, has greatly benefited flame-sampling molecular-beam mass spectrometry. This review also offers an overview of recent combustion chemistry studies of flames fueled by hydrocarbons and oxygenates. The identity of a variety of intermediates in hydrocarbon flames, including resonantly stabilized radicals and closed-shell intermediates, is described, thus establishing a more detailed understanding of the fundamentals of molecular-weight growth processes. Finally, molecular-beam mass-spectrometric studies of reaction paths in flames of alcohols, ethers, and esters, which have been performed to support the development and validation of kinetic models for bio-derived alternative fuels, are reviewed.

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

“…47,48 While the overall design of the instrument 49 follows well established techniques, a key innovation of this instrument is the use of tunable VUV photoionization mass spectrometry for flame analysis. This approach allows researchers to examine the isomeric composition of stable molecules and radical species generated in a flame essentially in situ.…”

Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes

“…A breakthrough in combustion-chemistry research was achieved with the use of flame-sampling mass spectrometry at synchrotron light sources where tunable vacuum-ultraviolet (VUV) radiation is used for near-threshold single-photon ionization 5,6 . In the flame experiments at the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory, which are shown in the accompanying video, gas samples are withdrawn from within the premixed flames by a quartz cone, expanded into higher vacuum, and ionized by VUV photons 1,5 . The experimental set-up is shown schematically in Figure 1.…”

“…The experimental set-up is shown schematically in Figure 1. The key to the success of this experiment has been the ability to tune the energy of the ionizing photons in an appropriate range to minimize or even avoid photofragmentation and to allow isomer specificity 1,3,5,18 . As shown in the video, photoionization efficiency (PIE) curves can be recorded by tuning the photon energy 19 , which allow us to identify specific isomeric species in the complicated flame mixture.…”

“…These ALS-based combustion experiments have been focused on soot-formation processes in hydrocarbon flames and on the oxidation of oxygenated, next-generation, bio-derived fuels 1,20 . With regards to the soot-formation problem, the experiments revealed many new insights.…”

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AbstractThe following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics Beamline of the Advanced Light Source (ALS) of the Lawrence Berkeley National Laboratory [1][2][3][4] . This video demonstrates how the complex chemical structures of laboratory-based model flames are analyzed using flame-sampling mass spectrometry with tunable synchrotrongenerated vacuum-ultraviolet (VUV) radiation.

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