Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration under contract DE-AC04-94-AL85000. The work was also supported by the U.S. Department of Energy, Office of Basic Energy Sciences under the Single Investigator Small Group Research project (Grant No. DE-SC0002619) of Prof. Violi (University of Michigan, Ann Arbor). KRW is supported by the Department of Energy, Office of Science, Early Career Research Program under U.S. Department of Energy Contract No. DE-AC02-05CH11231. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. KKH acknowledges continuing support of part of this research by the DFG under contract KO 1363/18-3.

1. Gas-phase Experiments
<ol>
	<li>Igniting the Low-pressure Premixed Flame
	<ol>
		<li>Make sure that the cooling water is flowing through the burner and flame chamber wall and that the flame chamber is pumped down to ~0.1 mbar.</li>
		<li>Establish a 1 L/min flow of argon and a 1.5 L/min flow of oxygen through the burner surface and maintain a pressure of 80 mbar (60 Torr) in the flame chamber.</li>
		<li>Position the hot-wire igniter over the burner surface; set the hydrogen flow to 0.4 L/min, and quickly activate the igniter.</li>
		<li>After ignition, turn off the hot-wire igniter, and reposition it away from the burner.</li>
		<li>Establish the desired flows of argon, oxygen, hydrogen, and fuel. Adjust the pressure to match the conditions of the target flame [typically&#160;20-40 mbar (15-30 Torr)]. Note: Flame conditions for individual flames are provided in the original literature. For example, flows for the stoichiometric allene and propyne flames are listed in Hansen&#160;et al22.</li>
	</ol>
	</li>
	<li>Acquisition of Photoionization Efficiency (PIE) Curves &#8211; Energy Scan
	<ol>
		<li>When the pressure in the ionization chamber is &#8804;10-6 mbar, apply the voltages to the ion optics of the time-of-flight mass spectrometer and microchannel plate detector and open the beamline valves. Note: Earlier calibration experiments (not shown in the video) were used to find the voltage settings for optimal performance of the mass spectrometer.</li>
		<li>Start the Labview data acquisition program &#8220;General Interface.vi&#8221; (Figure 2) and move the burner to the desired position using the &#8220;Motor&#8221; tab in the software. Note: This Labview code was developed at the beamline and is available upon request.</li>
		<li>Use the &#8220;General&#8221; tab to define the scan parameters, i.e., the number of steps per eV of photon energy. Typically, a step size of 0.05 eV is used.</li>
		<li>Use the &#8220;ALS&#8221; tab to set the photon energy to the desired start value, and define the &#8220;ALS Energy&#8221; to be &#8220;active&#8221;.</li>
		<li>On the &#8220;Control&#8221; panel, activate &#8220;K6485&#8221; to read out the photocurrent measured by the photodiode.</li>
		<li>On the &#8220;P7886&#8221; tab, use the &#8220;Set Parameters&#8221; button to set the number of sweeps (normally between 219 and 221), the number of bins (normally 48k), and the bin width (500 psec).</li>
		<li>Enter a valid file path and name, and click &#8220;Start&#8221; to start the computer-controlled data acquisition process.</li>
	</ol>
	</li>
	<li>Acquisition of Mass Spectra &#8211; Burner Scan
	<ol>
		<li>Apply the voltages as for the energy scans to the ion optics of the time-of-flight mass spectrometer and microchannel plate detector.</li>
		<li>Open the beamline valves to allow the photon beam in the chamber.</li>
		<li>Open the Labview data acquisition program &#8220;General Interface.vi&#8221;.</li>
		<li>Using the &#8220;Motor&#8221; tab in the software, use the &#8220;Jogger&#8221; to move the burner surface as close as possible to the sampling cone, and define that position as the &#8220;Origin&#8221;. Define the motor to be &#8220;active&#8221;.</li>
		<li>Use to &#8220;General&#8221; tab to define the scan parameters, i.e., the number of steps per mm of burner movement. Typical values are 0-5 mm in 20 steps, 5-20 mm in 15 steps, and 20-30 mm in 5 steps.</li>
		<li>Use the &#8220;ALS&#8221; tab to set the photon energy to the desired value. Typical values are&#160;8-16.65 eV.</li>
		<li>On the &#8220;Control&#8221; panel, activate &#8220;K6485&#8221; to read out the photocurrent measured by the photodiode.</li>
		<li>On the &#8220;P7886&#8221; tab, use the &#8220;Set Parameters&#8221; button to start a sub-VI (Figure 3) to set the number of sweeps, i.e., the number of mass spectra added on top of each other at every burner position (normally between 219 and 221), the number of bins (normally 48k), and the bin width (500 psec).</li>
		<li>Provide a valid file path and name, and click &#8220;Start&#8221; to begin the automated data acquisition process.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 2" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig2highres.jpg" width="500" /> 
Figure 2.&#160;Graphical user interface of the data acquisition program. <a href="https://www.jove.com/files/ftp_upload/51369/51369fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig3highres.jpg" width="500" /> 
Figure 3.&#160;Graphical user interface to input multichannel scaler parameters. <a href="https://www.jove.com/files/ftp_upload/51369/51369fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
2. Aerosol Experiments
<ol>
	<li>Igniting the Near-atmospheric Pressure Opposed-flow Flame
	<ol>
		<li>Make sure that the cooling water is flowing through the burner, that the reactant outlets are approx. 10 mm apart, and that the flame chamber is pumped down to the minimum achievable pressure (~2 mbar).</li>
		<li>Fill the flame chamber with argon and bring the pressure up to ~860 mbar (650 Torr).</li>
		<li>Place the igniter coil approx. in the center of the two burner outlets.</li>
		<li>Set the gas flows as follows (Ar should already be flowing because it was used to fill chamber): Oxidizer stream: O2 0.3 L/min and Ar 1.6 L/min; Oxidizer stream coflow: Ar 2.5 L/min; Fuel stream: H2 0.3 L/min, Ar 2.5 L/min; Fuel stream coflow: Ar 2.5 L/min.</li>
		<li>Open the hydrogen and oxygen valves, and immediately switch on igniter coil.</li>
		<li>Once the flame is ignited, turn off the igniter coil, and retract it.</li>
		<li>Establish the desired flows of oxygen, argon, and fuel. Turn off the flow of hydrogen, and set pressure and reactant outlet separation to desired values for target flame. Note: The flow rates for the propane flame, which is shown in the video, are provided in Skeen&#160;et al4.</li>
	</ol>
	</li>
	<li>Acquisition of an Aerosol Mass Spectrum
	<ol>
		<li>Apply the appropriate voltages to the ion optics and detector of the AMS. Note: The voltage settings for an optimal performance were found in earlier calibration experiments, which are not shown in the video.</li>
		<li>Open the Labview data acquisition program &#8220;General Interface-Opposed-Flow.vi&#8221;. Note: This VI is a modification of the &#8220;General Interface.vi&#8221;, in which the motor control has been updated to match the new needs.</li>
		<li>Use the &#8220;Jogger&#8221; application on the &#8220;Motor&#8221; tab to translate the opposed-flow burner so that the quartz microprobe is at the position nearest to the fuel stream outlet (Figure 4). While at this position, reset the motor step position to zero. This procedure defines the &#8220;origin&#8221; position.</li>
		<li>Slowly open the quarter-turn ball valve allowing flow from the flame-sampling line into the aerodynamic lens (ADL) system. Confirm that the pressure at the outlet of the ADL is near 1 x 102 mbar.</li>
		<li>Use the &#8220;General&#8221; tab to define the scan parameters, i.e., the number of steps per mm of burner movement (burner scan) or photon energy (energy scan).</li>
		<li>Use the &#8220;ALS&#8221; tab to set the desired photon energy, and use the &#8220;Motor&#8221; tab to move the burner to the desired burner position.</li>
		<li>Use the &#8220;P7886&#8221; tab and the &#8220;Set Parameters&#8221; button therein to set the acquisition parameters.</li>
		<li>Define &#8220;Motor&#8221; (burner scan) or &#8220;ALS Energy&#8221; (energy scan) to be &#8220;active&#8221;.</li>
		<li>Insert a valid file path and name in the appropriate fields, and click &#8220;Start&#8221;. Aerosol mass spectra are now automatically taken.</li>
	</ol>
	</li>
</ol>
<img alt="Figure 4" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig4highres.jpg" width="500" /> 
Figure 4.&#160;Graphical user interface for burner movement for the opposed-flow flame assembly. <a href="https://www.jove.com/files/ftp_upload/51369/51369fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemical+dynamics,+molecular+energetics,+and+kinetics+at+the+synchrotron.">Chemical dynamics, molecular energetics, and kinetics at the synchrotron.</a> Physical Chemistry Chemical Physics. 12, 6564-6578 (2010).</li><li>Skeen, S. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Near-threshold+photoionization+mass+spectra+of+combustion-generated+high-molecular-weight+soot+precursors.">Near-threshold photoionization mass spectra of combustion-generated high-molecular-weight soot precursors.</a> Journal of Aerosol Science. 58, 86-102 (2013).</li><li>Cool, T. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photoionization+mass+spectrometer+for+studies+of+flame+chemistry+with+a+synchrotron+light+source.">Photoionization mass spectrometer for studies of flame chemistry with a synchrotron light source.</a> Review of Scientific Instruments. 76, (2005).</li><li>Cool, T. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Selective+detection+of+isomers+with+photoionization+mass+spectrometry+for+studies+of+hydrocarbon+flame+chemistry.">Selective detection of isomers with photoionization mass spectrometry for studies of hydrocarbon flame chemistry.</a> Journal of Chemical Physics. 119, 8356-8365 (2003).</li><li>Hansen, N., Miller, J. A., Klippenstein, S. J., Westmoreland, P. 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The authors have nothing to disclose.

The described combination of flame-sampling and synchrotron-based VUV single-photon ionization with mass spectrometry provides the most detailed look into the chemical composition of laboratory-based model flames currently possible. The mass spectrometer provides universal detection of all sampled flame species simultaneously with high sensitivity (ppm range) over a wide dynamic range. Instrumental for the success of this technique is the use of synchrotron-generated VUV photons, whose energies can easily be tuned, to provide good selectivity between isomers and control of fragmentation. The latter factor is important when analyzing complex mixtures. The capabilities of the described experiment are unmatched by gas chromatography, which is commonly used for isomer-separation, and by conventional ionization techniques using energetic electrons. Limitations of the synchrotron-based technique arise from the fact that, especially for larger mass-to-charge ratios, many different isomers are conceivable, which then cannot be uniquely identified, and their contributions cannot be separated reliably1. The experimental results, in the form of isomer-resolved flame compositions, may yield improved kinetic models of combustion chemistry at an exceptionally detailed molecular level.
The described experiments are very complicated and a description of the troubleshooting procedures is beyond of what can be documented in the video and/or the protocol section of this manuscript. This fact is also true for the data analysis procedures. Modifications to the experimental set-up are normally done off-line in between the allocated &#8220;beamtime&#8221;. Because the emphasis of these experiments is on the quantitative determination of combustion intermediates, it is very critical to have stable and reproducible flames. Furthermore, it is necessary to wisely choose the photon energies and the other scan parameters to obtain an adequate set of experimental data that is sufficient for a reliable determination of the flame structure.
The flame experiments performed at the Advanced Light Source have successfully contributed to unravel the chemistry of benzene formation in hydrocarbon flames7.&#160;A prominent role of resonance-stabilized radicals as precursors has been established, for example, with the identification of the propargyl, allyl, and i-C4H5 radicals.
Because benzene formation is thought to be only the first step in the overall soot-formation process, additional efforts are under way at the Advanced Light Source to identify the chemical composition of flame-sampled soot particles. Compared to similar previous soot-sampling experiments28, this newly established aerosol-sampling experiment allows for recording near-threshold mass spectra, meaning that the photon energy can be tuned precisely to be only slightly above the components&#8217; ionization energies, thus avoiding fragmentation. Furthermore, fragmentations are also largely avoided by employing the process of flash-vaporization on the temperature-controlled copper block. However, the experiment is currently limited by not being able to provide quantitative data. Also, the recorded mass spectra are not particle specific, but averaged over many particles probably varying in composition and size. In addition, condensation can and does occur in the sampling probe, complicating the identification of species associated with particles in the flame. Furthermore, the species detected must be volatile enough to be vaporized at the temperature of the copper block (300-400 &#176;C) under vacuum. Nevertheless, the early qualitative data suggests that the compositions of soot precursor species are dependent on the chemical structure of the fuel and that soot-precursor-formation mechanisms are kinetically driven as opposed to thermodynamically. The aerosol mass spectrometry efforts are currently at the early stages, and the insights gained so far identify more research opportunities.
Future work on soot-formation processes is likely to focus on the chemistry beyond the first aromatic ring, i.e., the formation of indene, naphthalene, anthracene, etc, and their isomers. The ultimate goal is to understand the chemistry (and physics) of particle inception, and to develop a predictive model that can describe the entire soot-formation process (from fuel oxidation to particle coagulation).

Establishing a consistent and predictive mechanism for molecular growth and soot formation processes is one of the greatest challenges in combustion chemistry research8,9. Combustion processes account for over half of the fine particle air pollution (PM2.5 &#8211; fine particles defined by an aerodynamic diameter of &#8804;2.5 &#956;m), and, to reduce the emission of these unwanted combustion byproducts, it is important to know their identities, concentrations, and formation pathways10. The nature of the combustion byproducts is influenced by the fuel and the conditions under which it is burned. Many studies have linked combustion emissions to acute environmental and health effects11-13. For example, combustion-generated particles have a strong influence on the air quality, atmospheric visibility, and the radiative balance of the Earth&#8217;s atmosphere. It is assumed that the chemical composition of the airborne combustion-generated particles determines their toxicity, which is commonly associated with polycyclic aromatic hydrocarbons (PAHs). The latter species are considered to be the molecular precursors of soot, and they are formed in incomplete combustion processes. Again, to identify these processes is still a challenging problem.
Generally speaking, the combustion reactions, which are at the origin of these emissions, follow complicated fuel decomposition and oxidation pathways, involving many different reactive species. They are connected within a network of hundreds or even thousands of reactions whose rates depend on temperature and pressure14,15.
Laminar, premixed, burner-stabilized flat flames, which can be established at pressures as low as 20-80 mbar (15-60 Torr), represent one of the standard combustion environments commonly used to unravel this complex chemical network and to investigate the pollutant potential of any given prototypical fuel16. In this configuration, the fuel and the oxidizer are already mixed when they reach the flame front; thus, the rate of combustion is dominated by chemical processes and not by mixing. By operating these flames at a sub-atmospheric pressure, the physical thickness of the reaction region is increased, allowing for improved spatial resolution of temperature and concentration gradients with laser-based or probe-sampling techniques1,17.
In order to precisely analyze the chemical composition of such flames, an analytical tool is required that provides universal detection of all species simultaneously, high sensitivity and dynamic range, good selectivity between isomers, and control of molecular fragmentation. 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 ionization5,6. 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 photons1,5. 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 specificity1,3,5,18. As shown in the video, photoionization efficiency (PIE) curves can be recorded by tuning the photon energy19, which allow us to identify specific isomeric species in the complicated flame mixture. The PIE curves for individual species generally have distinct features, i.e., ionization thresholds, shapes, and intensities. The video also shows the experimental approach used to determine mole-fraction profiles of the individual components as a function of the distance to the burner surface.
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 fuels1,20. With regards to the soot-formation problem, the experiments revealed many new insights. In summary, it is now understood that the chemical structure of the fuel influences the identity (and the quantity) of the precursor molecules and that consequently many different pathways can contribute to the first step of the overall soot-formation process7,21.
Even deeper insights into the soot-formation chemistry were gained when identifying the chemical components of flame-generated soot nanoparticles with an ALS-based aerosol mass spectrometer. In this new experiment, which is explained in the second half of the video, non-premixed (diffusion) flames are used. The experimental setup is also shown in Figure 1. In this configuration, a flame is established at near-atmospheric pressure [933 mbar (700 Torr)] between two opposed laminar jets of fuel and oxidizer. Because the fuel and oxidizer streams remain separated outside the reaction zone, this configuration provides a good opportunity to examine molecular growth processes. Flame-generated particles are withdrawn from the flame using a quartz microprobe and subsequently focused with an aerodynamic lens system onto a heated copper target, where the particles flash vaporize and break apart into their individual constituents. These molecular building blocks are then ionized by the VUV photons from the ALS, and the corresponding ions are mass selected4. Not all the necessary work can be shown in the video, but the aerosol data suggest that the soot-formation mechanisms might be kinetically and not thermodynamically controlled. Furthermore, the data also indicate that the widely accepted H-abstraction-C2H2-addition (HACA) mechanism, in which small aromatic species grow to larger polycyclic aromatic hydrocarbons (PAHs) by a repetitive sequence of H-abstraction and C2H2-addition reactions, cannot explain all the observed particle constituents.
Combined with the video, the following protocol details the data acquisition procedures.
<img alt="Figure 1" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig1highres.jpg" width="500" /> 
Figure 1.&#160;Schematic diagram of the flame-sampling molecular beam and aerosol mass spectrometry experiments at the Advanced Light Source of the Lawrence Berkeley National Laboratory. With permissions from Refs. 2 and 4. <a href="https://www.jove.com/files/ftp_upload/51369/51369fig1highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table><tbody><tr><td>Flame-sampling mass spectrometer</td><td></td><td></td><td>custom-built</td></tr><tr><td>Aerosol mass spectrometer</td><td></td><td></td><td>custom-built</td></tr></tbody></table>

flame experiments at the advanced light source: new insights into soot formation processes

The 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 Laboratory1-4. This video demonstrates how the complex chemical structures of laboratory-based model flames are analyzed using flame-sampling mass spectrometry with tunable synchrotron-generated vacuum-ultraviolet (VUV) radiation. This experimental approach combines isomer-resolving capabilities with high sensitivity and a large dynamic range5,6. The first part of the video describes experiments involving burner-stabilized, reduced-pressure (20-80 mbar) laminar premixed flames. A small hydrocarbon fuel was used for the selected flame to demonstrate the general experimental approach. It is shown how species&#8217; profiles are acquired as a function of distance from the burner surface and how the tunability of the VUV photon energy is used advantageously to identify many combustion intermediates based on their ionization energies. For example, this technique has been used to study gas-phase aspects of the soot-formation processes, and the video shows how the resonance-stabilized radicals, such as C3H3, C3H5, and i-C4H5, are identified as important intermediates7. The work has been focused on soot formation processes, and, from the chemical point of view, this process is very intriguing because chemical structures containing millions of carbon atoms are assembled from a fuel molecule possessing only a few carbon atoms in just milliseconds. The second part of the video highlights a new experiment, in which an opposed-flow diffusion flame and synchrotron-based aerosol mass spectrometry are used to study the chemical composition of the combustion-generated soot particles4. The experimental results indicate that the widely accepted H-abstraction-C2H2-addition (HACA) mechanism is not the sole molecular growth process responsible for the formation of the observed large polycyclic aromatic hydrocarbons (PAHs).

A typical mass spectrum of flame-sampled gases from the low-pressure premixed burner is shown in Figure 5. The identities of the species contributing to the signal are revealed by the flame-sampled photoionization efficiency (PIE) curves for each mass-to-charge (m/z) ratio and their comparison to known isomer-specific ionization energies and PIE curves. Typical examples of flame-sampled PIE curves are shown in Figure 6 for m/z=39 (C3H3) and 41 (C3H5). The data are taken from a stoichiometric propyne flame22. The signal is unambiguously identified by their characteristic ionization thresholds to originate from the resonantly stabilized propargyl and allyl radicals. For many m/z values, multiple isomers are routinely identified by observing multiple thresholds. Many examples have been discussed extensively in the literature, for example, m/z=40 (allene and propyne), 44 (ethenol and acetaldehyde), 54 (1,3-butadiene, 1-butyne, and 2-butyne), or 78 (fulvene and benzene)23-27.
<img alt="Figure 5" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig5highres.jpg" width="500" /> 
Figure 5.&#160;Time-of-flight mass spectrum. Recorded with photons of 9.9 eV at 1.25 mm distance from the low-pressure premixed burner in a stoichiometric propyne-O2 flame. All peaks can be easily assigned to various combustion intermediates as outlined in the next step.
<img alt="Figure 6" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig6highres.jpg" width="500" /> 
Figure 6.&#160;Flame-sampled photoionization efficiency curves for m/z=39 and 41. The resonantly stabilized radicals propargyl and allyl can unambiguously be identified based on the observed ionization thresholds.
Once the isomeric composition is known, mass spectra are taken at various photon energies and from within different positions in the flame, as described above in the Protocol Section, to allow for the determination of isomer-specific mole fraction profiles of the individual species as a function of distance from the burner surface. Representative mole-fraction profiles of fulvene and benzene in a stoichiometric propyne flame from the low-pressure premixed burner are shown in Figure 7&#160;22.&#160;For each flame, typically a total of 40 to 50 individual mole-fraction profiles are determined for species ranging from m/z=1 (H atom) to m/z=78 (benzene and/or fulvene) or even higher, depending on the scientific goals. These mole fraction profiles are then used to assess the predictive capabilities of combustion-chemistry models and to validate them.
<img alt="Figure 7" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig7highres.jpg" width="500" /> 
Figure 7.&#160;Experimental mole fraction profiles.&#160;Profiles of fulvene and benzene in a stoichiometric propyne flame from the low-pressure premixed burner.
A typical aerosol mass spectrum is shown in Figure 8. It was taken from within a propane opposed-flow diffusion flame. Ion signal was observed for species with m/z ratios ranging from 150 to 600, with a peak around m/z=226. It is beyond the current experimental capabilities to identify all of the species observed in the mass spectrum or to unravel their possible formation pathways. Taking such mass spectra as a function of distance from the fuel outlet as described above (and shown in the video) provides spatially resolved profiles. A representative example is shown in the inlet of Figure 8 for the m/z=256 (C20H16) species. Similar profiles can be obtained for any of the other species as well, which consequently could be used as validation targets for any combustion-chemistry model.
<img alt="Figure 8" fo:content-width="5in" src="/files/ftp_upload/51369/51369fig8highres.jpg" width="500" /> 
Figure 8.&#160;Flame-sampled aerosol mass spectrum from a propane-O2 opposed-flow diffusion flame. The inlet shows a representative spatially resolved profile for the C20H16 species at m/z=256.

Watch this Scientific Journal Video about Flame Experiments at the Advanced Light Source: New Insights into Soot Formation Processes at JoVE.com

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

Gas sampling from laboratory-scale flames with online analysis of all species by mass spectrometry is a powerful method to investigate the complex mixture of chemical compounds occurring during combustion processes. Coupled with tunable soft ionization via synchrotron-generated vacuum-ultraviolet radiation, this technique provides isomer-resolved information and potentially fragment-free mass spectra.

The following experimental protocols and the accompanying video are concerned with the flame experiments that are performed at the Chemical Dynamics ...

flame-experiments-at-advanced-light-source-new-insights-into-soot

Research

JoVE Journal

Engineering

12.8K Views.  Sandia National Laboratories. Gas sampling from laboratory-scale flames with online analysis of all species by mass spectrometry is a powerful method to investigate the complex mixture of chemical compounds occurring during combustion processes. Coupled with tunable soft ionization via synchrotron-generated vacuum-ultraviolet radiation, this technique provides isomer-resolved information and potentially fragment-free mass spectra. 

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

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 &#181;m spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO).
The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation&#160;and are analyzed to infer electron density and temperature.

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement ...

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. 
Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties.
The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex interactions occurring at the interface between gas-phase flames and condensed-phase fuels. The current research provides a technique to explore the dynamic relationship between a combustible condensed fuel surface and gas-phase flames in laminar boundary layers. Experiments have previously been conducted in both forced and free convective environments over both solid and liquid fuels. A unique methodology, based on the Reynolds Analogy, was used to estimate local mass burning rates and flame heat fluxes for these laminar boundary layer diffusion flames utilizing local temperature gradients at the fuel surface. Local mass burning rates and convective and radiative heat feedback from the flames were measured in both the pyrolysis and plume regions by using temperature gradients mapped near the wall by a two-axis traverse system. These experiments are time-consuming and can be challenging to design as the condensed fuel surface burns steadily for only a limited period of time following ignition. The temperature profiles near the fuel surface need to be mapped during steady burning of a condensed fuel surface at a very high spatial resolution in order to capture reasonable estimates of local temperature gradients. Careful corrections for radiative heat losses from the thermocouples are also essential for accurate measurements. For these reasons, the whole experimental setup needs to be automated with a computer-controlled traverse mechanism, eliminating most errors due to positioning of a micro-thermocouple. An outline of steps to reproducibly capture near-wall temperature gradients and use them to assess local burning rates and heat fluxes is provided.

We describe the use of micro-thermocouples to estimate local temperature gradients in steady laminar boundary layer diffusion flames. By extension of the Reynolds Analogy, local temperature gradients can be further used to estimate the local mass burning rates and heat fluxes in such flames with high accuracy.

Modeling the realistic burning behavior of condensed-phase fuels has remained out of reach, in part because of an inability to resolve the complex ...

Combustion Characterization and Model Fuel Development for Micro-tubular Flame-assisted Fuel Cells

Combustion based power generation has been accomplished for many years through a number of heat engine systems. Recently, a move towards small scale power generation and micro combustion as well as development in fuel cell research has created new means of power generation that combine solid oxide fuel cells with open flames and combustion exhaust. Instead of relying upon the heat of combustion, these solid oxide fuel cell systems rely on reforming of the fuel via combustion to generate syngas for electrochemical power generation. Procedures were developed to assess the combustion by-products under a wide range of conditions. While theoretical and computational procedures have been developed for assessing fuel-rich combustion exhaust in these applications, experimental techniques have also emerged. The experimental procedures often rely upon a gas chromatograph or mass spectrometer analysis of the flame and exhaust to assess the combustion process as a fuel reformer and means of heat generation. The experimental techniques developed in these areas have been applied anew for the development of the micro-tubular flame-assisted fuel cell. The protocol discussed in this work builds on past techniques to specify a procedure for characterizing fuel-rich combustion exhaust and developing a model fuel-rich combustion exhaust for use in flame-assisted fuel cell testing. The development of the procedure and its applications and limitations are discussed.

A protocol for creating a model fuel-rich combustion exhaust is developed through combustion characterization and is applied for micro-tubular flame-assisted fuel cell testing and research.

Combustion based power generation has been accomplished for many years through a number of heat engine systems. Recently, a move towards small scale power ...

Reaction Kinetics and Combustion Dynamics of I4O9 and Aluminum Mixtures

Tetraiodine nonoxide (I4O9) has been synthesized using a dry approach that combines elemental oxygen and iodine without the introduction of hydrated species. The synthesis approach inhibits the topochemical effect promoting rapid hydration when exposed to the relative humidity of ambient air. This stable, amorphous, nano-particle material was analyzed using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) and showed an exothermic energy release at low temperature (i.e., 180 &#176;C) for the transformation of I4O9 into I2O5. This additional exothermic energy release contributes to an increase in overall reactivity of I4O9 when dry mixed with nano-aluminum (Al) powder, resulting in a minimum of 150% increase in flame speed compared to Al + I2O5. This study shows that as an oxidizer, I4O9 has more reactive potential than other forms of iodine(V) oxide when combined with Al, especially if I4O9 can be passivated to inhibit absorption of water from its surrounding environment.

Reaction Kinetics and Combustion Dynamics of I4O9 and Aluminum Mixtures

A protocol for measuring flame speeds of a reactive mixture composed of tetraiodine nonoxide (I4O9) and aluminum (Al) is presented. A method for resolving reaction kinetics using differential scanning calorimetry (DSC) is also presented. It was found that I4O9 is 150% more reactive than other iodine(V) oxides.

Tetraiodine nonoxide (I4O9) has been synthesized using a dry approach that combines elemental oxygen and iodine without the introduction of hydrated ...

Split Point Analysis and Uncertainty Quantification of Thermal-Optical Organic/Elemental Carbon Measurements

Researchers from myriad fields frequently seek to quantify and classify concentrations of carbonaceous aerosols as organic carbon (OC) or elemental carbon (EC). This is commonly accomplished using thermal-optical OC/EC analyzers (TOAs), which enable measurement via controlled thermal pyrolysis and oxidation under specific temperature protocols and within constrained atmospheres. Several commercial TOAs exist, including a semi-continuous instrument that enables on-line analyses in the field. This instrument employs an in-test calibration procedure that requires relatively frequent calibration. This article details a calibration protocol for this semi-continuous TOA and presents an open-source software tool for data analysis and rigorous Monte Carlo quantification of uncertainties. Notably, the software tool includes novel means to correct for instrument drift and identify and quantify the uncertainty in the OC/EC split point. This is a significant improvement on the uncertainty estimation in the manufacturer&#8217;s software, which ignores split point uncertainty and otherwise uses fixed equations for relative and absolute errors (generally leading to under-estimated uncertainties and often yielding non-physical results as demonstrated in several example data sets). The demonstrated calibration protocol and new software tool enabling accurate quantification of combined uncertainties from calibration, repeatability, and OC/EC split point are shared with the intent of assisting other researchers in achieving better measurements of OC, EC, and total carbon mass in aerosol samples.

This article presents a protocol and software tool for the quantification of uncertainties in the calibration and data analysis of a semi-continuous thermal-optical organic/elemental carbon analyzer.

Researchers from myriad fields frequently seek to quantify and classify concentrations of carbonaceous aerosols as organic carbon (OC) or elemental carbon ...

Environment

Combustion Chemistry of Fuels: Quantitative Speciation Data Obtained from an Atmospheric High-temperature Flow Reactor with Coupled Molecular-beam Mass Spectrometer

This manuscript describes a high-temperature flow reactor experiment coupled to the powerful molecular beam mass spectrometry (MBMS) technique. This flexible tool offers a detailed observation of chemical gas-phase kinetics in reacting flows under well-controlled conditions. The vast range of operating conditions available in a laminar flow reactor enables access to extraordinary combustion applications that are typically not achievable by flame experiments. These include rich conditions at high temperatures relevant for gasification processes, the peroxy chemistry governing the low temperature oxidation regime or investigations of complex technical fuels. The presented setup allows measurements of quantitative speciation data for reaction model validation of combustion, gasification and pyrolysis processes, while enabling a systematic general understanding of the reaction chemistry. Validation of kinetic reaction models is generally performed by investigating combustion processes of pure compounds. The flow reactor has been enhanced to be suitable for technical fuels (e.g. multi-component mixtures like Jet A-1) to allow for phenomenological analysis of occurring combustion intermediates like soot precursors or pollutants. The controlled and comparable boundary conditions provided by the experimental design allow for predictions of pollutant formation tendencies. Cold reactants are fed premixed into the reactor that are highly diluted (in around 99 vol% in Ar) in order to suppress self-sustaining combustion reactions. The laminar flowing reactant mixture passes through a known temperature field, while the gas composition is determined at the reactors exhaust as a function of the oven temperature. The flow reactor is operated at atmospheric pressures with temperatures up to 1,800 K. The measurements themselves are performed by decreasing the temperature monotonically at a rate of -200 K/h. With the sensitive MBMS technique, detailed speciation data is acquired and quantified for almost all chemical species in the reactive process, including radical species.

An investigation of the oxidative combustion chemistry of novel biofuels, fuel components, or jet fuels by comparison of quantitative speciation data is presented. The data can be used for kinetic model validation and enables fuel assessment strategies.This manuscript describes the atmospheric high-temperature flow reactor and demonstrates its capabilities.

This manuscript describes a high-temperature flow reactor experiment coupled to the powerful molecular beam mass spectrometry (MBMS) technique. This ...

Chemistry

Wind Tunnel Experiments to Study Chaparral Crown Fires

The present protocol presents a laboratory technique designed to study chaparral crown fire ignition and spread. Experiments were conducted in a low velocity fire wind tunnel where two distinct layers of fuel were constructed to represent surface and crown fuels in chaparral. Chamise, a common chaparral shrub, comprised the live crown layer. The dead fuel surface layer was constructed with excelsior (shredded wood). We developed a methodology to measure mass loss, temperature, and flame height for both fuel layers. Thermocouples placed in each layer estimated temperature. A video camera captured the visible flame. Post-processing of digital imagery yielded flame characteristics including height and flame tilt. A custom crown mass loss instrument developed in-house measured the evolution of the mass of the crown layer during the burn. Mass loss and temperature trends obtained using the technique matched theory and other empirical studies. In this study, we present detailed experimental procedures and information about the instrumentation used. The representative results for the fuel mass loss rate and temperature filed within the fuel bed are also included and discussed.

This protocol describes wind tunnel experiments designed to study the transition of a fire from the ground to the canopy of chaparral shrubs.

The present protocol presents a laboratory technique designed to study chaparral crown fire ignition and spread. Experiments were conducted in a low ...

Experimental Procedure for Laboratory Studies of In Situ Burning : Flammability and Burning Efficiency of Crude Oil

A new method for the simultaneous study of the flammability and burning efficiency of fresh and weathered crude oil through two experimental laboratory setups is presented. The experiments are easily repeatable compared to operational scale experiments (pool diameter &#8805;2 m), while still featuring quite realistic in situ burning conditions of crude oil on water. Experimental conditions include a flowing water sub-layer that cools the oil slick and an external heat flux (up to 50 kW/m2) that simulates the higher heat feedback to the fuel surface in operational scale crude oil pool fires. These conditions enable a controlled laboratory study of the burning efficiency of crude oil pool fires that are equivalent to operational scale experiments. The method also provides quantitative data on the requirements for igniting crude oils in terms of the critical heat flux, ignition delay time as a function of the incident heat flux, the surface temperature upon ignition, and the thermal inertia. This type of data can be used to determine the required strength and duration of an ignition source to ignite a certain type of fresh or weathered crude oil. The main limitation of the method is that the cooling effect of the flowing water sub-layer on the burning crude oil as a function of the external heat flux has not been fully quantified. Experimental results clearly showed that the flowing water sub-layer does improve how representative this setup is of in situ burning conditions, but to what extent this representation is accurate is currently uncertain. The method nevertheless features the most realistic in situ burning laboratory conditions currently available for simultaneously studying the flammability and burning efficiency of crude oil on water.

Experimental Procedure for Laboratory Studies of In Situ Burning : Flammability and Burning Efficiency of Crude Oil

Here, we present a protocol to simultaneously study the flammability and burning efficiency of fresh and weathered crude oil under conditions that simulate in situ burning operations on the sea.

A new method for the simultaneous study of the flammability and burning efficiency of fresh and weathered crude oil through two experimental laboratory ...

Flame Photometry: Overview

Flame photometry, also known as flame emission spectrometry, is a technique used for the qualitative and quantitative analysis of elements present in a sample using a flame as the source of excitation energy. The concept of flame photometry was realized in the early 1860s by Kirchhoff and Bunsen, who discovered that specific elements emit characteristic radiation when excited in flames. The first instrument developed for this purpose was used to measure sodium (Na) in plant ash using a Bunsen flame. However, the challenge lay in finding the most effective way to introduce the sample into the flame. It wasn&#39;t until 1929, when Lundegardh introduced a nebulizer, that a significant breakthrough was made, allowing a reproducible introduction of the sample into the flame.
In flame emission spectrometry, the nebulizer converts a liquid sample into a fine mist or aerosol. This is achieved by passing a high-pressure gas stream over the end of a capillary tube containing the sample and aspirating it into a spray chamber. The aerosol produced is then passed to the burner, where flame heat desolvates it, forming dry particles that volatilize and produce free atoms for analysis.
Early instruments utilized quartz prism spectrographs and photographic recording to disperse and capture atomic emission lines. However, advancements in optical filters and electrical photodetectors replaced these components, improving precision and convenience.
Flame photometry involves introducing the sample solution into a nebulizer, which converts it into a fine mist or aerosol. The atomized sample then enters the flame, along with air or oxygen and a fuel gas such as propane or acetylene. The flame provides the thermal excitation necessary to energize the atoms in the sample. As these excited atoms relax, their emitted radiation is detected by a photocell or photomultiplier.
Flame photometry is particularly effective in measuring sodium, potassium, lithium, and calcium elements. The flame used in flame photometry is typically a propane-air flame with a temperature range of 1900-2000 &#176;C, although alternative flames like butane-air or natural gas-air can also be used. Flame photometry has some limitations that can be overcome by using higher temperatures and more reducing flames, such as air-acetylene, along with higher-resolution spectrometric detection. However, these approaches are not cost-competitive compared to the more broadly applicable flame atomic absorption spectrometry or AAS technique.

Flame photometry, also known as flame emission spectrometry, is a technique used for the qualitative and quantitative analysis of elements present in a ...

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

Summary

Explore More Videos

Chapters in this video

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C₂, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Experimental Methodology for Estimation of Local Heat Fluxes and Burning Rates in Steady Laminar Boundary Layer Diffusion Flames

Combustion Characterization and Model Fuel Development for Micro-tubular Flame-assisted Fuel Cells

Reaction Kinetics and Combustion Dynamics of I₄O₉and Aluminum Mixtures

Split Point Analysis and Uncertainty Quantification of Thermal-Optical Organic/Elemental Carbon Measurements

Combustion Chemistry of Fuels: Quantitative Speciation Data Obtained from an Atmospheric High-temperature Flow Reactor with Coupled Molecular-beam Mass Spectrometer

Wind Tunnel Experiments to Study Chaparral Crown Fires

Experimental Procedure for Laboratory Studies of In Situ Burning : Flammability and Burning Efficiency of Crude Oil

Flame Photometry: Overview