Robert A. Fletcher

The evolution of soot precursor particles in a diffusion flame

Abstract The chemical evolution of soot precursor particles on the centerline of the laminar ethene diffusion flame has been analyzed using laser microprobe mass spectrometry (LMMS) as they undergo the transition to carbonaceous aggregates. LMMS is a reliable microanalytical technique for the detection of intermediate and heavy polycyclic aromatic hydrocarbons (PAHs) in particulate material. The analyses show that many of the masses present within the precursor particles coincide with those predicted by Stein and Fahr (1985) to be most thermodynamically stable (stabilomers). The stabilomer PAHs that consist solely of six-membered rings, the benzenoid PAHs, prove to be the most important members of the stabilomer grid. Pericondensed PAHs as large 472 amu, which is attributed to the molecule C 38 H 16 with 12 hexagonal rings, are found to be constituents of the precursor particles. The PAH mass distribution diverges to the larger sizes with increasing height in the flame, and includes many of the species identified by others as gas-phase PAH constituents in hydrocarbon flames. Carbonization on the centerline of the flame occurs abruptly between 35 and 40 mm above the burner where the particle metamorphosis (from single precursor liquid-like particles to fused aggregates) and the decrease in hydrogen mole fraction (from 0.35 to 0.15) simultaneously occur. The presence of stabilomer PAHs reported by others in the particulate combustion product of a variety of fuels—aliphatic and aromatic gases, diesel fuel, crude oil, kerogen, carbon black feed stock, cigarette tobacco, and biomass—suggests that the stabilomer grid represents the common path for the growth of PAHs which contribute to the formation of carbonaceous soot in these diverse instances. This observation can account for the previously noted invariance of the soot product of combustion from diverse fuels and devices.

Laser microprobe analysis of soot precursor particles and carbonaceous soot

Abstract Samples of soot precursor particles and carbonaceous soot from a nonsmoking ethene diffusion flame have been analyzed by means of laser microprobe mass spectrometry (LMMS). The mass spectra of soot precursor particles from the lower flame display many peaks in the 200–300-amu range that are characteristic of polycyclic aromatic hydrocarbons (PAHs). The most prominent of these are the masses 252, 276, and 300, which correspond to the isomers of C 20 H 12 , C 22 H 12 , and C 24 H 12 , respectively. These masses are among those predicted by Stein and Fahr to be the most thermodynamically stable (“stabilomers”) under typical hydrocarbon flame conditions, and they have been previously reported as components of soot collected from a variety of fuels and combustion configurations. Carbonaceous soot from the upper region of the flame yields mass spectra composed of carbon-hydrogen clusters, C x H y with x = 3 to 24 and y usually 0, 1 or 2. Smaller amounts of PAH-like species with masses in the 418 to 444 amu range are also found. These results suggest that the carbonization process observed in this flame is the dehydrogenation of the PAH species formed in the lower flame unaccompanied by polymeric growth. The LMMS technique provides a lower bound value of the hydrogen mole fraction X H of 0.36 ( C H = 1.8 ) for precursor particles from the lower diffusion flame and 0.15 ( C H = 5.6 ) for carbonaceous soot aggregates.

The existence of young soot in the exhaust of inverse diffusion flames

Knowledge of the chemical and physical structure of early soot is useful in the development of sootparticle inception models. This paper examines the hypothesis that soot exiting an inverse diffusion flame is similar in chemical and morphological structure to (1) soot precursor particles and (2) soot that exits underventilated flames. Experiments in volving soot collection from the exhaust of laminar ethylene inverse diffusion flames were performed. Soot samples were analyzed for morphology using transmission electron microscopy, for carbon-to-hydrogen ratio using elemental analysis, for organic fraction using thermaloptical analysis, and for polycyclic aromatic hydrocarbon content using laser microprobe mass spectrometry and gas chromatography/mass spectrometry. Results of these analyses support the validity of the above hypothesis. This finding is significant because exhaust collection from the inverse flame provides an opportunity to gather large samples of young soot without invading the flame with an intrusive probe (a necessary task when collecting precursors low in the center of a normal diffusion flame). Larger samples can then be subjected to more detailed analysis than previously possible. An identification of specific polycyclic aromatic hydrocarbon isomers present in young soot from diffusion flames is reported. The data are available for comparison with polycyclic aromatic hydrocarbon growth, soot inception, and soot growth models.

High-Volume Dichotomous Virtual Impactor for the Fractionation and Collection of Particles According to Aerodynamic Size

A prototype dichotomous virtual impactor (DVI) using a single acceleration nozzle, operating at approximately 500 1/min, and having an aerodynamic particle outpoint diameter of about 2–3 μm has been constructed and tested. Under these conditions the flow through the acceleration nozzle is calculated to be turbulent. This sampler was calibrated with a monodisperse aerosol, and the measured particle size-dependent collection efficiencies demonstrate that the sampler size fractionates atmospheric particulate matter as efficiently as the low-volume dichotomous virtual impactors. Analysis of test data indicates that the high-volume sampler can be described by classical impaction theory. These data also indicate that over the range of Reynolds numbers from 24,000 to 81,000 there is little, if any, dependence of inferred acceleration nozzle turbulence on the performance characteristics of the sampling system. A comparison of the concentration of atmospheric particulate matter, sulfate, and calcium on the fine fi...

Carbonization Rate of Soot Precursor Particles

Abstract The presence of soot precursor particles in hydrocarbon diffusion flames is made clearly evident by means of contemporary sampling techniques in combination with transmission electron microscopy (TEM). These particles are the immediate predecessors of mature, partially dehydrogenated carbonaceous soot aggregates, and recent analytical tests indicate they contain a mixture of polycyclic aromatic hydrocarbons. In this work, the conversion rate of precursor panicles to carbonaceous soot aggregates is investigated. In one method the time interval for the morphological conversion of precursor particles to aggregates is observed in diluted diffusion flames. Temperature profiles are measured by rapid insertion thermocouple thermometry. This information is supplemented by the observation of the sudden decrease of the hydrogen mole fraction of soot precursor particles as measured by laser microprobe mass spectrometry. The Arrhenius rate constants for the carbonization of precursor particles are derived fr...

A method to determine collection efficiency of particles by swipe sampling

A methodology was developed to evaluate particle collection efficiencies from swipe sampling of trace residues. Swipe sampling is used for many applications where trace residues must be collected, including the evaluation of radioactive particle contamination and the analysis of explosives and contraband at screening checkpoints using ion mobility spectrometry (IMS). Collection efficiencies were evaluated for micrometer-sized polystyrene latex (PSL) spheres with respect to the particle size and mode of deposition, collection trap, surface type and swiping force. Test surfaces containing particles were prepared under controlled conditions and swiped with a reproducible technique that allows for the evaluation of frictional forces. Collection efficiencies were determined by optical imaging and particle counting. Of the two IMS collection traps studied, the polytetrafluoroethylene (PTFE) trap has significantly lower collection efficiencies. This is likely to be due to a combination of texture and composition. The larger (42 µm diameter) particles are collected more efficiently than the smaller (9 µm diameter) particles. Particles in a matrix similar to latent fingerprints are collected more efficiently than dry particles. Applying greater force during swiping does not greatly improve collection efficiencies. This fact, coupled with the observation that many particles are detached but not collected, implies that improvements in collection efficiency are dependent on improvements in adhesion of the particles to the collection surface, rather than larger forces to detach the particles.

Application of Inkjet Printing Technology to Produce Test Materials of 1,3,5-Trinitro-1,3,5 Triazcyclohexane for Trace Explosive Analysis

The feasibility of the use of piezoelectric drop-on-demand inkjet printing to prepare test materials for trace explosive analysis is demonstrated. RDX (1,3,5-trinitro-1,3,5 triazcyclohexane) was formulated into inkjet printable solutions and jetted onto substrates suitable for calibration of the ion mobility spectrometry (IMS) instruments currently deployed worldwide for contraband screening. Gravimetric analysis, gas chromatography/mass spectrometry (GC/MS), and ultraviolet-visible (UV-vis) absorption spectroscopy were used to verify inkjet printer solution concentrations and the quantity of explosive dispensed onto test materials. Reproducibility of the inkjet printing process for mass deposition of the explosive RDX (1,3,5-trinitro-1,3,5 triazcyclohexane) was determined to be better than 2% for a single day of printing and better than 3% day-to-day.

Calibration of a Condensation Particle Counter Using a NIST Traceable Method

This work presents a calibration of a commercial condensation particle counter using National Institute of Standards and Technology (NIST) traceable methods. By the nature of the metrology involved, this work also compares the measurement results of three independent techniques for measuring aerosol concentration: continuous flow condensation particle counter (CPC); aerosol electrometer (AE); and the aerosol concentration derived from microscopic particle counting. Because of the transient nature of aerosol, there are no concentration artifact standards such as exist for particle diameter standards. We employ a mobility classifier to produce a nearly monodisperse, 80 nm, polystyrene latex aerosol. The test aerosol is used as a challenge for the CPC and the AE, and is subsequently filter sampled for electron microscopy. Our test stand design incorporates a continuous CPC aerosol concentration monitor to verify the aerosol stability. The CPC determines particle concentration by single particle counting at a constant sample flow rate. The AE has been calibrated to a NIST traceable current standard. The subsequent aerosol concentration measurement is obtained by determining the electrical current produced by a charged aerosol transported to the detector by a controlled aerosol flow rate. We have NIST traceability for flow rates for all methods and a methodology to calibrate the AE to NIST traceable electrical standards. The latter provides a calibration and a determination of the uncertainty in the aerosol derived current measurement. A bias in the measurements due to multiple charged particles was observed and overcome by using an electrospray aerosol generator to produce the challenge particles. This generator was able to produce aerosol concentrations over the range of 100 particles/cm 3 to 15 000 particles/cm 3 with lower number of dimer particles (≈1%). In our work, independent measurement of aerosol concentration is obtained by quantitatively collecting samples of the airborne polystyrene latex spheres on a small pore filter material and determining the number of particles collected by electron microscopy. Electron micrograph images obtained using a field-emission scanning electron microscope are analyzed using particle counting. We found the relative uncertainty in the aerosol electrometer measurements to be in excess of 100% for particle concentrations of approximately 120 particles/cm 3 and approximately 5% for concentrations above 6000 particles/cm 3 . The uncertainty found by the microscopy method was approximately 3%.

Fabrication of Polymer Microsphere Particle Standards Containing Trace Explosives Using an Oil/Water Emulsion Solvent Extraction Piezoelectric Printing Process

We present a methodology for fabricating polymer microspheres using inkjet printing of a biodegradable polymer containing either high explosives or high explosive simulant. Poly(DL-lactide/glycolide) 85:15 (PLGA) microsphere production is based on an oil/water emulsion solvent extraction process. The inkjet printing process allows for precise control of the microsphere diameter and the chemical composition. The microspheres can be used as calibrants or verification standards for explosives trace detection instruments. Gas chromatography/mass spectrometry analysis demonstrated that the composition of the microspheres was consistent with predicted concentrations based on the amount of analyte incorporated into the polymer solution and the inkjet operating parameters. We have demonstrated that the microspheres can be fabricated with a mass fraction of 70% of an analyte compound.

Measurements of Air Jet Removal Efficiencies of Spherical Particles from Cloth and Planar Surfaces

Experiments were conducted to determine particle removal efficiencies from surfaces due to air jet impingement. We utilize monodisperse fluorescent polymer spheres ranging from 1 μm to 45 μm diameter distributed on polycarbonate surfaces and muslin cloth. Particle removal efficiencies are determined from cloth that simulates clothing or fabrics, surfaces important for trace explosives detection. Optical images of the sample surface are taken using a fluorescence microscope (488 nm radiation) before and after being challenged by a pulsed air or nitrogen jet. The jet is oriented at 45 degrees with respect to the particle laden substrate and is precisely aligned onto the center of the optical axis of the microscope. Particle removal rates are determined by automated particle counting implemented by image processing and analysis. We found that the cloth surface has a release rate comparable to the rate for polycarbonate surface under similar jet conditions. As expected, there is a particle size dependence on removal efficiency, with larger particles being more easily removed.