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Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references (leaves 76-87). Also available in electronic version.

An atmospheric pressure radio frequency capacitively coupled plasma (CCP) has been developed and characterized for applications in atomic emission spectrometry (AES), atomic absorption spectrometry (AAS) and gas chromatography (GC). The CCP torch was initially designed as an atom reservoir for carrying out elemental analysis using atomic absorption. Functionally, the device consists of two parts, the CCP discharge tube and the tantalum strip electrothermal vaporization sample introduction system. The torch design provides for very effective energy transfer from the power supply to the plasma by capacitive coupling. Therefore, the plasma can be generated at atmospheric pressure with a flexible geometry. The plasma can be operated at very low rf input powers (30-600 W) enabling optimal conditions for atom resonance line absorption measurements. Absorption by the analyte takes place within the plasma discharge which is characterized by a long path length (20 cm) and low support gas flow rate (0.2 L/Min). Both of these characteristics ensure a relatively long residence time. The device exhibits linear calibration plots and provides sensitivities in the range of 3.5-40 pg. A preliminary measurement gave a Fe I excitation temperature of approximately 4000 K for the discharge. At this temperature, potential chemical interferences are likely to be minimal. Chemical interferences for Fe, Al, As, Ca, Co, Cd, Li, Mo and Sr were negligible in the determination of silver. Chloride interference, which is prevalent in GF-AAS, was not found. The amount of Ag found in a SMR#1643b (NIST) water sample was 9.5 ± 0.5 ng/g which fell in the certified range of 9.8 ± 0.8 ng/g. Spikes of 30 ng/g and 60 ng/g of silver were added to the SRM and recoveries were found to be in a range from 105 % to 96.2 %. The RSD obtained for 7 replicates of 270 pg silver was 4.6 %. The results for the CCP AES are even more promising. The interferences of thirteen elements are negligible in the determination of silver. The chloride interference was not found. The detection limits for Ag, Cd, Li, Sb and B are 0.7, 0.7, 2, 80 and 400 pg respectively. The amount of silver found in a SRM#1643b (NIST) water sample was 9.3 ± 0.5 ng/g which also fell in the certified range of 9.8 ±0.8 ng/g. Spikes of 30 ng/g and 60 ng/g of silver were added into the SRM#1643b (NIST) samples; the recoveries were found to range from 97 % to 104 %. The RSD obtained for 7 analyses of 270 pg silver were 1.5 % for CCP-AES. It was also found that the signal to noise ratios (S/N) are higher in the AES mode than those in the AAS mode in the same CCP atomizer. In order to exploit advantages inherent in both GF-AAS and I CP-AES, an atmospheric pressure capacitively coupled plasma sustained inside a graphite furnace was developed. This source combines the high efficiency of atomization in furnaces and the high efficiency of the excitation in atmospheric pressure plasmas. In general, plasma sources are able to effectively excite high-lying excited states for most metals and non-metals and can also ionize vaporized elements. Therefore the possibility exists of using non-resonance lines to avoid the effects of self-absorption at high analyte concentrations. Ion lines may also be used in cases where they provide better sensitivity or freedom from spectral interferences. This source also offers the ability to independently optimize vaporization and excitation. However, the most important aspect of this new source is that it can be used for simultaneous, multielement determinations of small sized samples in a graphite furnace atomizer, a design which has been proven to be effective over many years of use. Preliminary quantitative characteristics of this new atmospheric pressure plasma emission source have been studied. The detection limit for Ag of 0.3 pg is lower than the value of 0.4 pg reported for GF-AAS. Variants of the CCP, including a gas chromatography (GC) detector, combinations of laser ablation - CCP, rf sputtering - CCP direct solid analysis, and its application as an intense spectral lamp have been developed and are reported in this dissertation. / Science, Faculty of / Chemistry, Department of / Graduate

The favourable physical and mechanical properties of silicon carbide (SiC) nanopowders allow application across many areas, including high-power, high-frequency electronics and high-temperature technologies. Many different synthesis methods for the creation of SiC nanoparticles have been studied, including carbothermic reduction, pulsed laser deposition, sol-gel processes, microwave heating and various plasma techniques. Among the different synthesis methods reported in the literature, very few experiments investigate the microwave-induced plasma synthesis of SiC nanoparticles. The few reported studies show promising results with regard to particle size and production rate. In this work, the synthesis of SiC nanoparticles from methyltrichlorosilane (MTS) is reported using a microwave-induced plasma, operating at atmospheric pressure. The investigation was done experimentally using a 1 500 W power supply, a microwave generator operating at 2.45 GHz, a stub tuner, a waveguide and a sliding short. Quartz tubes were used, in which the plasma was generated and maintained. Hydrogen served as an added reductant for the conversion reaction, and argon served as the MTS carrier gas. The parameters studied were the H2:MTS molar ratio and the total enthalpy, in the ranges 0 to 10 and 70 to 220 MJ/kg respectively. X-ray diffraction studies confirmed the presence of β-SiC and optical emission spectrometry showed the majority of peaks to be that of elementary silicon, carbon and argon, indicative of MTS decomposition in the plasma. Scanning electron microscopy shows average individual particle sizes ranging between 50 and 135 nm, whereas transmission electron microscopy shows average individual particle sizes ranging from 15 to 140 nm. Larger agglomerates are also present, ranging in sizes from 460 to 1 800 nm. Through response surface methodology (RSM), it was shown that the optimum conditions for the production of nanoparticles lie within the range of enthalpy > 180 MJ/kg and H2:MTS ratio of > 5. / Dissertation (MEng)--University of Pretoria, 2015. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

Gas-phase reaction kinetics is at the center of the evolution of reacting systems. Many important reactions in the gas phase proceed through rovibrationally excited complexes AB* that are formed either from the association of two particles A + B or from the activation of thermalized species AB by collisions between AB and inert colliders. The fate of these rovibrationally excited complexes is governed by the competition among energy-transferring collisions, unimolecular decomposition, and bimolecular reactions -- yielding a strong dependence of all emergent rate constants on temperature, pressure, and composition of the surrounding mixture. In nearly all realistic environments critical to combustion and planetary atmospheres, multiple species are present in significant quantities and thus contribute to the evolution of the rovibrationally excited complexes. A substantial fraction of these species is inert in the sense that they merely participate in energy-transferring collisions, and a portion of them are instead reactive, which participate in reactive collisions and induce reactions with the rovibrationally excited complexes rather than merely transferring energy. Most of the inert species have distinct collisional energy transfer characteristics and thus contribute differently to the energy-transferring collisions -- making the multi-component pressure dependence in mixtures different from the pressure dependence in the constituent components when pure. Accounting for such “mixture effects,” in practice, one employs a “mixture rule” to interpolate kinetic data in mixtures from individual pure bath gas components. While mixture effects for reactions proceeding through a single potential well and a single reaction channel have been extensively investigated, mixture effects and mixture rules for multi-well and/or multi-channel reactions are significantly less characterized despite their ubiquitousness in gas-phase reaction systems. This work presents an investigation of and seeks reliable representations of bath gas mixture effects on multi-channel (both single-well and multi-well) reactions and their impacts on combustion predictions. The performance of different mixture rules for representing multi-component pressure dependence of rate constants for various systems is evaluated through comparisons against ab initio master equation calculations for the mixture. The comparisons revealed that the classic linear mixture rule, the most commonly applied mixture rule, yields substantial deviations (exceeding a factor of 50) for typical combustion mixtures. The comparisons, together with results from combustion simulations, suggest that recently proposed mixture rules based on the reduced pressure provide a considerably more accurate representation of mixture effects for various systems. These new mixture rules are therefore recommended for use in fundamental and applied chemical kinetics investigations. The importance of reactive collisions between the rovibrationally excited complexes AB*, formed from the association of A + B, and reactive colliders C was largely ignored historically. Recent studies have demonstrated that reactive collisions of AB* with C often occur on the same timescale as energy-transferring collisions. And these reactive collisions can induce non-Boltzmann kinetic sequences that proceed through AB* and propagate across multiple coupled potential energy surfaces. The non-Boltzmann kinetic sequences can be represented by the chemically termolecular reactions A + B + C → products in phenomenological kinetic models. While these non-Boltzmann kinetic sequences consume the same set of species as their equivalent thermal sequential pathways, they are kinetically and dynamically distinct and can have substantial impacts on the global reactivity in combustion and atmospheric systems beyond those imposed by thermal sequential pathways. Evaluating the kinetics of non-Boltzmann kinetic sequences requires that rovibrational excitation of reacting complexes from one potential energy surface be carried over to the following and appropriate treatments for the augmentation and dissipation of the energy distributions due to reactions. This work presents a theoretical and computational methodology that couples multiple master equations and derives rate constants for all emergent phenomenological reactions for non-Boltzmann kinetic sequences spanning across the coupled master equations. Results from implementing the methodology for a couple of systems demonstrate that reactive collisions can both increase the overall rate of conversion of reactants to products and alter the branching ratios among final products. Combustion simulations indicate that reactive collisions can have significant impacts on the overall system reactivity. Therefore, suitable rate laws and appropriate treatment are needed for the distinct effects of reactive collisions to be represented in phenomenological kinetic models.

The range of plasma spectroscopy tends to increase with the introduction of more efficient plasma excitation sources. In this thesis the use of one such plasma excitation source, the microwave induced plasma is evaluated as an atom cell for atomic spectrometry. The modes of spectrometry evaluated are atomic emission and atomic fluoresence. Analytical merits of the microwave induced plasma using detection limits and studies of interelement effects (i.e. vaporization, ionizationâ and scatter interferences) are also presented. / Master of Science

The major thrusts of this work have been: 1) To develop a high-efficiency low-power TM-010 microwave cavity for nitrogen support gas at atmospheric pressure, 2) To discover and physically characterize potential laser and emission spectroscopic applications of this atom source, with a particular emphasis on laser-induced fluorescence. The result is the most efficient microwave-induced plasma cavity for nitrogen at one atmosphere that exists to date, giving stable and analytically useful molecular plasmas with only 50 Watts applied power. It is called the “High-Efficiency Molecular Microwave Plasma" (HEMMP) cavity. The HEMMP possesses excellent vaporization and atomization properties. It can handle aqueous sample flows of around 1 mL/min, introduced as an aerosol from a nebulizer. A detection system and sampling system were designed and an analytical instrument was built around the HEMMP cavity. Details of construction, operating conditions and operation of the instrument are described. Applications investigated include laser-induced fluorescence (LIF), atomic emission spectroscopy (AES), and laser-enhanced ionization (LEI) [also known as the opto-galvanic effect (OGE)]. The major emphasis of the application work has been physical characterization of the low-power nitrogen plasma as an atom source for LIF. This is the first time that either laser-induced fluorescence or laser-enhanced ionization have been observed and extensively characterized in any microwave-induced plasma (MIP). This is also the first time that atomic emission has been studied in a low-power N₂-MIP. LIF, AES, and LEI signal intensities were studied as a function of applied microwave power, support gas flow rate, signal observation height, and support gas composition using nitrogen and argon mixtures. Results for LIF yielded detection limits in the very low parts per billion range, and for AES in the low parts per billion range. Limit of detection (LOD) and background noise studies were done for all 3 techniques. Signal intensities were measured as a function of laser light intensity for LIF and LEI. Laser saturation was not observed with 300 mW power from the CW dye laser. The effects of electrode geometry and applied electrode voltage on LEI signals were also studied. Extensive background spectral studies were done for the nitrogen plasma. Analytical feasibility has been demonstrated for AES, LIF, and LEI in the low-power nitrogen MIP. The results presented provide the background physical investigations required for a full-scale development of these techniques for chemical analysis. / Ph. D.

Lecithin:cholesterol acyltransferase (LCAT) was purified 30,000-fold from hog plasma in a homogeneous state as indicated by polyacrylamide gel electrophoresis. The purified enzyme had an apparent molecular weight of 66,000 and was found to contain about 21.4 percent (w/w) carbohydrate. The properties of hog LCAT including amino acid composition were compared with human LCAT. High density lipoprotein (HDL) was isolated from the hog plasma by an immunoaffinity column chromatography. The isolated HDL showed nearly identical lipid-protein composition although it contained additional protein components when it was compared to HDL isolated by a traditional method involving ultracentrifugation.

A Global Enhanced Vibrational Kinetic (GEVKM) model is presented and applied to the simulation of a new High Current Negative Hydrogen Ion Source (HCNHIS) developed by Busek Co. Inc. and Worcester Polytechnic Institute. The HCNHIS consists of a high-pressure radio-frequency discharge (RFD) chamber in which the main production of high-lying vibrational states of the hydrogen molecules occurs, a bypass system, and a low-pressure negative hydrogen ion production (NIP) region where negative ions are generated by the dissociative attachment of low energy electrons to rovibrationally excited hydrogen molecules. The GEVKM is developed from moment equations for multi-temperature chemically reacting plasmas and for a cylindrical geometry of an inductively coupled discharge chamber. The species included into the model are ground state hydrogen atoms H and molecules H2, 14 vibrationally excited hydrogen molecules H2(v), v=1-14, electronically excited hydrogen atoms H(2), H(3), ground state positive ions H+, H2+, H3+, ground state negative ions H-, and electrons e. The space-averaged steady-state continuity equations coupled with the electron energy equation, the total energy equation and heat transfer to the chamber walls, are solved simultaneously in order to obtain the space-averaged number densities of the plasma components, the electron and heavy particle temperatures as well as the wall temperature. The GEVKM is supplemented by a comprehensive set of surface and volumetric chemical processes governing vibrational and ionization kinetics of hydrogen plasmas. The GEVKM is verified and validated in the low pressure, in the intermediate to high-pressure (1-100 Torr) and high absorbed power density (8.26-22 W/cm3) regimes by comparisons with the numerical simulations and experimental measurements. The GEVKM is applied to the simulation of the RFD chamber of the HCNHIS. The GEVKM predictions of negative hydrogen ions number densities and electron temperatures in the RFD chamber of the HCNHIS are used to estimate the negative hydrogen ion current using the Bohm flux approximation. The estimated negative current compare well with the Faraday Cup measurements and provide additional validation of the model. The GEVKM is used in a parametric investigation of the RFD chamber of HCNHIS-2 with hydrogen inlet flow rates 5-1000 sccm and absorbed powers 200-1000 W.

Radio frequency (rf) atmospheric pressure glow discharges (APGDs) have received growing attention for their exciting scope of new science and their immense potential for widespread applications. While geometrically similar to conventional low-pressure discharges used in the semiconductor industry for decades, rf APGDs present new physics that require investigation. This thesis presents an experimental and computational study of helium rfAPGDs aimed at making a contribution to the current understanding of these discharges and enabling their optimization for different applications. The timely interest and significance of this work is highlighted by the publication of different parts of this thesis in 10 peer-reviewed international journals. Starting with the electron trapping in rf APGDs, the thesis looks into the electron heating mechanism responsible for sustaining the discharges, the influence of the rf excitation frequency on the discharge characteristics, the role of impurities in the discharge chemistry as well as the evolution of the discharge as the size is reduced down to microplasma dimensions. The findings of this research are based on the synergistic use of electrical measurements, optical diagnostics and self-developed computational models. With the knowledge gained from this thesis, rf-APGDs can be readily used for a wide-range of applications including biological decontaminations, nanostructure fabrication and portable gas analyzers.