Article pubs.acs.org/ac
Probing Combustion Chemistry in a Miniature Shock Tube with Synchrotron VUV Photo Ionization Mass Spectrometry Patrick T. Lynch,†,§ Tyler P. Troy,‡ Musahid Ahmed,‡ and Robert S. Tranter*,† †
Chemical Science and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
‡
S Supporting Information *
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 highpressure 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 (T > 600 K, P < 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% CH3OCH3/Ar) was observed behind the reflected shock (1400 < T5 < 1700 K, 3 < P5 < 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. flux, making them very difficult to implement with ST/TOFMS apparatuses. The latter two aspects make it especially difficult to probe the molecular beam with sufficient frequency to accurately resolve the rapidly changing composition and result in low signal intensities. Additionally, mass spectra from PI sources are often very weak and signal averaging over many experiments is necessary. Conventional ST/TOF-MS experiments are inherently single shot events that cannot be easily reproduced with sufficient accuracy or frequency to allow for signal averaging to improve signal/noise in weak spectra. Thus, while PI has the potential to augment ST/TOF-MS experiments, it cannot easily be implemented with current lab sources and conventional shock tubes. Synchrotron-based tunable PI mass spectrometry (PIMS), however, has been used in a number of critical experiments in gas-phase kinetics to measure time-dependent concentration profiles and identify isomers via PI energy scans. The range of reactors employed includes flames,9−12 flow reactors,13 tubular nozzles,14 and jet-stirred reactors.15,16 These are relatively low pressure techniques (typically 1000 K) studies targeting elementary gas kinetics, the reactor of choice is a shock tube (ST).3,5−8 All current shock tube mass spectrometry experiments use time-of-flight mass spectrometers (TOF-MS), which allow complete mass spectra (typically m/z < 300) to be obtained at least every 10 μs. This time resolution is normally sufficient to accurately capture the rapid changes in concentration that occur in the first 100 to 200 μs of reaction of a ST/TOF-MS experiment. Currently, all conventional ST/TOF-MS experiments use electron ionization (EI) to obtain high sampling rates, and the resultant spectra are complicated by fragmentation of molecules and radicals in the ion source. Potentially, photoionization (PI) could simplify the mass spectra by reducing fragmentation. However, current PI sources suitable for lab use have limitations with respect to energy resolution, pulse rate, and © 2015 American Chemical Society
Received: November 7, 2014 Accepted: January 16, 2015 Published: January 16, 2015 2345
DOI: 10.1021/ac5041633 Anal. Chem. 2015, 87, 2345−2352
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Analytical Chemistry composition) in a controllable manner, their size (several meters long) and duty cycles (typically
