Craig Taatjes of Sandia National Labs, Livermore, US, and colleagues look inside the mysterious chemistry of combustion using photoionization mass spectrometry combined with synchrotron radiation.

A visual interpretation of multidimensional data can give a glimpse of chemical mechanisms - if you know what to look for

Combustion is an ancient technology, but it remains the dominant means of releasing stored energy in the world today. When Nikolaus Otto invented the four-stroke engine in 1876 and when Rudolf Diesel patented his engine in 1898, scientists did not even have the basic knowledge that combustion was driven by chain-reaction chemistry, an insight that would yield Nikolay Semenov the Nobel Prize decades later. The science of combustion systems, with interacting fluid mechanics and complex chemistry, is a daunting multiscale and multiphase problem that continues to tax state-of-the-art computational and experimental methods. There is an intensifying emphasis on reducing pollutant levels, and new engine technologies are emerging that rely on chemical kinetics for ignition timing. These have made understanding the fundamentals of combustion a key to designing new practical devices and have brought detailed chemistry to the forefront of twenty-first-century combustion research.

Understanding the chemistry behind some of the most stubborn problems in combustion technology relies on knowledge of isomer-specific concentrations of products and intermediates. For example, the efficiency of advanced engines that rely on compression ignition of a premixed fuel-air charge depends on the particulars of alkylperoxy radical isomerizations. The problem of soot formation in hydrocarbon flames hinges on whether reactions of small hydrocarbon species in a flame form aromatic rings or not. Photoionization mass spectrometry is a powerful technique for probing chemical reactions and combustion processes, and the photon-energy dependence of the ionization can distinguish isomers. Recently, photoionization by synchrotron radiation has been combined with simultaneous detection of multiple masses to investigate the chemistry of low-pressure flames. The brightness of the synchrotron source and the multiplexed mass spectrometry permits rapid acquisition of three-dimensional data: signal as a function of the mass, the ionization photon energy, and either reaction time or distance from the burner surface.

'Slices' taken out of such datasets by integrating over two of the dimensions would correspond to more traditional measurements, such as profiles of species as a function of distance from the burner or kinetic concentration against time. However, interpretation of the multidimensional data directly from an image yields great insight into global chemical mechanisms and can highlight previously unknown reactive pathways. For example, an image of the evolving mass spectrum of molecules sampled from a rich ethene flame as a function of the distance from the burner shows soot precursor chemistry at a glance. Moving away from the burner, the fuel is consumed and higher-molecular-weight hydrocarbons begin to be formed; species with three carbon atoms appear very close to the burner, four-carbon species slightly farther away, five-carbon species farther still, and so on. Comparison with an image of a similarly rich flame of the oxygenated fuel dimethyl ether shows immediately that the growth of these soot precursors is almost completely absent in the ether flame, consistent with the soot-reducing effects of oxygenates.

Additionally, possession of the entire three-dimensional data set allows the measurements to be correlated in different ways. As one example, the change in the photon-energy dependence of signal at a given mass can be related to a change in the isomeric populations. Correlating this with time then allows the separation of contributions from different stages of the reactions. Spectra derived from such correlations offer means to detect important ignition-chemistry intermediates.

The investigation of increasingly complex chemical systems not only requires the ability to simultaneously monitor many channels, but demands new strategies to manage and visualize the resulting data. Tunable photoionization and simultaneous multiple-mass detection can effectively depict flame and chemical kinetic processes; continuing development of spectroscopic and visualization technologies will deliver new insights into combustion.

Read more in Craig Taatjes  et al.'s critical review '"Imaging" combustion chemistry  via multiplexed synchrotron-photoionization mass spectrometry' in issue 1, 2008 of  Phys. Chem. Chem. Phys.