Determination of arsenic by continuous hydride generation with direct introduction into an O2-argon microwave plasma torch atomic emission spectrometer

The determination of arsenic was studied with a simple and economic method. A continuous hydride generation system is interfaced to a microwave plasma torch atomic emission spectrometer (MPT-AES). Arsenic hydride is transferred directly and continuously by the carrier gas into the plasma torch without separation of hydrogen. When oxygen is introduced into the outer tube of the plasma torch, the plasma is more stable and has a higher tolerance to hydrogen. The detection limit (3σ) is 5.2 μg/L when the forward power is 100 W with argon as support gas. Application to the standard sample coal fly ash showed a comparable result to the certified quantity. Received: 3 April 2000 / Revised: 21 May 2000 / /Accepted: 25 May 2000     

Simulation of Gas-Dynamic and Thermal Processes in Vortex-Stabilized,Inductively Coupled Argon–Hydrogen Plasma

The flow of vortex-stabilized argon–hydrogen plasma in a radiofrequency induction (RFI) plasma torch has been investigated using modern methods of computational fluid dynamics. Optimal values of the torch power and energy release in plasma have been found at various argon to hydrogen ratios in the plasma gas mixture. The heat and kinetic fields determined by calculation for a plasma-chemical reactor can be of use in designing an RFI plasma torch in part concerning the determination of the optimum zone for feeding the reactants to the reactor.     

Influence of the Shroud Gas Injection Configuration on the Characteristics of a DC Non-transferred Arc Plasma Torch

The characteristics of the plasma jet emanating from a dc non-transferred plasma torch is affected by many factors including arc current, type of gas, gas flow rate, gas injection configuration and torch geometry. The present work focuses on experimental investigation of the influence of shroud gas injection configuration on the I–V characteristics and electro-thermal efficiency of a dc non-transferred plasma torch operated in nitrogen at atmospheric pressure. The plasma gas is injected into the torch axially and shroud gas is injected through three different nozzles such as normal, sheath and twisted nozzles. The effects of flow rates of plasma/axial gas and arc current on I–V characteristics and electro-thermal efficiency of the torch holding different nozzles are investigated. The I–V characteristics and electro-thermal efficiency of the torch are found to be strongly influenced by the shroud gas injection configuration. The effect of arc current on arc voltage decreases with increasing the axial gas flow rate. At higher axial gas flow rate (>?45 lpm), the I–V characteristics of the plasma torch are similar irrespective of the nozzle used. The variation of electro-thermal efficiency with arc current is almost similar to that of arc voltage with arc current. As expected, the electro-thermal efficiency is increased when the axial gas flow rate is increased and at higher axial gas flow rate, it is not influenced by the arc current and shroud gas configuration. The plasma torch with normal nozzle may be better in the range of operating conditions used in this study.     

氢化物发生与ICP/AES联用后等离子体的性质研究

本文通过测量电子密度ｎｅ,电子温度Ｔｅ,激发温度Ｔｅｘｃ及谱线强度,将氢化物进样同水溶液常规气动雾化进样对比,研究了氢化物进入ＩＣＰ后,对等离子体性质的影响。结果表明,由于氢化物的引入,ＩＣＰ的激性能得到了改善。     

Direct Measurement of the Gas Entrainment Into a Turbulent Thermal Plasma Jet

An experimental study is conducted to investigate the entrainment characteristics of a turbulent thermal plasma jet issuing from a DC arc plasma torch operating at atmospheric pressure. The mass flow rate of the ambient gas entrained into the turbulent plasma jet is directly measured by use of the so-called “porous-wall chamber” technique. It is shown that a large amount of ambient gas is entrained into the turbulent plasma jet. With the increase of the gas mass flow rate at the plasma jet inlet or the plasma torch exit, the mass flow rate of entrained ambient gas almost linearly increases but its ratio to the jet-inlet mass flow rate decreases. The mass flow rate of the entrained gas increases with the increase of the arc current or jet length. It is also found that using different ways to inject the plasma-forming gas into the plasma torch affects the entrainment rate of the turbulent plasma jet. The entrainment rate expression established previously by Ricou and Spalding (J. Fluid Mech. 11: 21, 1961) for the turbulent isothermal jets has been used to correlate the experimental data of the entrainment rates of the turbulent thermal plasma jet, and the entrainment coefficient in the entrainment rate expression is found to be in range from 0.40 to 0.47 for the turbulent thermal plasma jet under study.     

Parametric study of the decomposition of NH3 for an induction plasma reactor design

The present paper reports a study of the gas mixing and chemical transformation in an induction plasma reactor under atmospheric pressure, and its dependence on the plasma operating conditions. For this purpose, the thermal dissociation of ammonia into nitrogen and hydrogen was chosen because of the relative simplicity of the reactions involved and its use in a number of studies on plasma synthesis of ultrafine nitride ceramic powders using ammonia as nitriding agent. A hot-wall reactor configuration is investigated in which ammonia is injected radially through multiple orifices into the gases at the exit nozzle of an induction plasma torch. Concentration mapping in the mixing zone was carried out, using a VG-Micromass-PC 300 D quadrupole mass spectrometer, for different plasma power levels, in the range 13–24 kW. A 3-point injection mode is used with the injection ports oriented upstream at 45° to the torch axis. This allows uniform mixing of the injected gas in the plasma jet. A systematic study of the effects of plate power and ammonia and plasma gas flow rates on the mixing and dissociation of NH₃ in the reactor is reported. The results are analyzed and discussed from the viewpoint of their use for optimizing the design of induction plasma reactors, to he applied to the vapor-phase synthesis of ultrafine silicon nitride powders.     

Evaluation of hydrogen line emission and argon plasma electron concentrations resulting from the gaseous sample injection involved in hydride generation-ICP-atomic emission spectrometric analysis

The simultaneous injection of volatile hydride species and hydrogen gas, originating in reagent decomposition, was monitored during the operation of a continuous hydride generation manifold employed for the determination of trace arsenic by HG-ICP-AES. Line and background intensities as well as the FWHM of the hydrogen Hgamma and Hdelta lines were measured, and electron number densities (ne) estimated from Stark broadening of the line profiles. Results were compared with those obtained by conventional pneumatic injection of aqueous solutions. Overlapping with atomic nitrogen lines at 410 nm and 411 nm tends to distort the Hdelta line profile for the hydrogen-seeded plasma, rendering unreliable results. The N I lines seem to be quenched by the presence of water aerosol. More consistent results were obtained with the Hgamma line. When no solutions are pumped through the hydride generation manifold ("dry" plasma), the measured ne value was (1.57 +/- 0.22) x 10(15)cm(-3). Conversely, when the reducing reagent flow was replaced by pure water (corresponding to the injection of water vapor in equilibrium that is swept by the argon carrier gas passing through the phase separator), the electron concentration is 25% higher. In that case the ne value agrees between the experimental error with that obtained for a plasma in which a water aerosol is introduced at a flow rate of 1 mL/min. An enhancement of 52% relative is observed in ne when the system is operated under optimized conditions for arsine generation, employing sodium tetrahydroborate in acidic medium as reducing agent (i.e. hydrogen seeded plasma). It was also observed that the continuum emission near 410 nm for the hydrogen containing plasma correlates with the measured electron number density, suggesting that the background enhancement under hydride generation conditions may respond to the ion-electron recombination mechanism.     

Parametric study of an atmospheric pressure microwave-induced plasma of the mini MIP torch — II. Two-dimensional spatially resolved excitation temperature measurements

Two-dimensional mapping of the excitation temperatures have been carried out in the microwave-induced plasma torch with tangential argon flow. It is detected that the presence of wet aerosols in a nebulizer gas increases axial excitation temperature while desolvation acts in the opposite direction. The influence of low hydrogen concentration in the support gas and potassium in the nebulizer gas on the temperature distribution is studied in detail. The comparison of excitation temperature and electron number density distributions clearly indicates non-equilibrium plasma conditions.     

A versatile new torch for inductively coupled plasma spectrometry

A modified torch for optical emission spectrometry with an inductively coupled plasma source is described. The demountable torch incorporates a flared intermediate tube, a capillary injector tube and interchangeable jets at the gas inlets. The optimised performance of the torch is compared with that of a conventional torch. The new torch can be operated over a wide range of gas flows and shows considerable promise in work with an argon-cooled plasma. The ability to operate at high or low gas flow rates, and the possibility of interchanging tubes and jets easily illustrate the versatility of the new design.     

Improvement of plasma spraying efficiency and coating quality

A commercial torch has been modified to introduce an additional anti-vortex and shroud gas flow to counter the detrimental effects brought about by the vortex plasma gas flow which is used to stabilize the cathode arc attachment and to increase the anode life. Deposition efficiency and coating quality are used as criteria to judge the modified versus the nonmodified torch. High-speed videography and computerized image analysis systems are used to determine the particle trajectories, velocities, and the plasma jet geometry. The results show that the additional anti-vortex and shroud gas flow to the torch can keep the particles closer to the torch axis and reduce the amount of entrainment of cold air into the plasma jet. The consequence is that deposition efficiency and coating quality are substantially improved.     

Thermal Plasma Flow Modeling: A Simple Model for Gas Heating and Acceleration

A simple model of gas heating and acceleration in a plasma torch is proposed. With simple conditions, easy to get from the torch operating parameters, this model leads to apprehend the gas behavior interacting with the surrounding atmosphere outside the torch, in steady or unsteady state. To better apprehend the air introduction into the plasma flow, different values of the Schmidt number have been investigated according to a classic k–ε turbulence model. Comparisons between velocity, temperature and Nitrogen percentage measurements and calculations were made in steady state.     

On the operation of inductively coupled plasmas as a function of pressure

The effect of pressure on the operating conditions within a 27-MHz inductively coupled plasma is investigated. The results show a direct relationship between torch pressure and the excitation temperature of an analyte species introduced into the plasma. The excitation temperature of the plasma support gas is shown to increase with torch pressure. A shift towards local thermal equilibrium is also observed with increasing pressure. The possibility of using alternate plasma support gases such as He is also investigated.     

Influence of the Gas Injection Angle on the Jet Characteristics of a Non-transferred DC Plasma Torch

The non-transferred direct current (DC) plasma torch has been widely used in various industrial applications due to its special jet characteristics. The jet characteristics are determined by different factors, including the working parameters, the torch construction, the gas injection angle (GIA) etc. As there is little study on the influence of the GIA on the jet characteristics, experimental study on the GIA’s effects on the jet characteristics has been carried out on a specially designed non-transferred DC plasma torch, whose GIA can be changed by replacing a gas injection component. The arc voltages and thermal efficiencies of the plasma torch, the specific enthalpies and jet lengths of the plasma jets at different working conditions were obtained and analyzed. It has been found that the GIA greatly affects the arc voltage, the thermal efficiency, the specific enthalpy and the jet length. Based on these findings, plasma torch with appropriate GIA could be used to help generating the plasma jet with desired characteristics.     

Analytical characterisation of a capacitively coupled plasma torch with a central tube electrode

A new type of radiofrequency capacitively coupled plasma torch is presented. The torch electrode geometry is coaxial with a tubular central electrode and one or two outer ring electrodes. The argon plasma is generated at 275 W radiofrequency power and 27.12 MHz and it has a very good stability and a low gas consumption of 0.4 l min(-1). The nebulized sample is introduced through the tubular electrode into the core of the annular shaped plasma thus achieving a better atomisation and a lower background. The limits of detection for 20 elements are in the range of ng ml(-1) and the dynamic range between 2.5 and 3.5. The best results are obtained with the torch with two outer ring electrodes.     

Mixing study of the induction plasma reactor: Part II. Radial injection mode

A study was undertaken on the mixing pattern in am induction plasma torch and reactor system. The results presented in this part of the paper relate to the radial injection mode, in which an auxiliary gas is injected into the main plasma stream through a set of 2, 4, or 8 injection ports located in the torch nozzle at the level of the torch exit flange. A much faster mixing of the gases occurred in this mode compared to the axial injection mode investigated in Part I of this paper. As in the case of axial injection, the present study demonstrates that gas mixing, in the presence of the discharge, is considerably more difficult than under ambient temperature conditions. Lower turbulence levels exist in the plasma reactor. due to the considerably higher viscosity of the gases under plasma conditions. Results obtained with a three-point injection flange, in which the injection ports were oriented at 45° to the torch and reactor axes toward the upstream .side, mere most interesting since they achieved essentially the same degree of mixing as was obtained with the radial injection ports without the need to locate the injection ports at the exit nozzle of the plasma torch. This arrangement provides for added flexibility in reactor design.     

Modeling of the Subsonic–Supersonic Flow and Heat Transfer in a DC Arc Plasma Torch

Two-dimensional modeling results are presented concerning the subsonic–supersonic flow and heat transfer within a DC plasma torch used for low-pressure (or soft vacuum) plasma spraying. The so-called fictitious anode method is used in the modeling in order to avoid inclusion of the complex three-dimensional effects near the anode arc root and also to avoid the forced heating of all the incoming cold gas stream by the arc. A nonorthogonal boundary-conforming grid, nonstaggered variable arrangement and the all-speed SIMPLE algorithm are employed for the solution of the governing equations, including gas viscous effects, temperature-dependent properties, and compressible effects. Good agreement of the predictions with available results for a few benchmarked compressible flow problems shows that the new version of the FAST-2D program can be employed for the present plasma flow modeling. Temperature, axial velocity, Mach number, and static pressure contours, and streamlines within the DC arc plasma torch are presented to show the flow and heat transfer characteristics. The flow transits gradually from upstream subsonic regime into downstream supersonic regime with the subsonic–supersonic transition within the cylindrical segment of the torch nozzle. Additional numerical tests show that gas viscosity and Lorentz force have only a slight effect on the plasma flow.     

Evaluation of hydrogen line emission and argon plasma electron concentrations resulting from the gaseous sample injection involved in hydride generation-ICP-atomic emission spectrometric analysis

The simultaneous injection of volatile hydride species and hydrogen gas, originating in reagent decomposition, was monitored during the operation of a continuous hydride generation manifold employed for the determination of trace arsenic by HG-ICP-AES. Line and background intensities as well as the FWHM of the hydrogen Hγ and Hδ lines were measured, and electron number densities (n_e) estimated from Stark broadening of the line profiles. Results were compared with those obtained by conventional pneumatic injection of aqueous solutions. Overlapping with atomic nitrogen lines at 410 nm and 411 nm tends to distort the Hδ line profile for the hydrogen-seeded plasma, rendering unreliable results. The N I lines seem to be quenched by the presence of water aerosol. More consistent results were obtained with the Hγ line. When no solutions are pumped through the hydride generation manifold (“dry” plasma), the measured n_e value was (1.57 ± 0.22) × 10¹⁵ cm^–3. Conversely, when the reducing reagent flow was replaced by pure water (corresponding to the injection of water vapor in equilibrium that is swept by the argon carrier gas passing through the phase separator), the electron concentration is 25% higher. In that case the n_e value agrees between the experimental error with that obtained for a plasma in which a water aerosol is introduced at a flow rate of 1 mL/min. An enhancement of 52% relative is observed in n_e when the system is operated under optimized conditions for arsine generation, employing sodium tetrahydroborate in acidic medium as reducing agent (i.e. hydrogen seeded plasma). It was also observed that the continuum emission near 410 nm for the hydrogen containing plasma correlates with the measured electron number density, suggesting that the background enhancement under hydride generation conditions may respond to the ion-electron recombination mechanism.     

Studying the process of ionization of hydrogen in conditions of its forced delivery to a porous nickel oxide electrode

The hydrogen ionization process is studied experimentally on an industrial sintered nickel oxide electrode in models of sealed nickel-metal hydride batteries. It is shown that the hydrogen ionization rates that are reached during overcharge by high current densities in conditions of forced gas delivery into the electrode pores (up to 40 mA cm^?2) exceed the self-discharge rate of a nickel-hydrogen battery by two orders of magnitude. Up to 70% of hydrogen delivered into the compact assembly block undergoes ionization during forced charge of models of sealed nickel-metal hydride batteries with a closed hydrogen cycle. Two independent methods (potentiostatic and manometric) are used to determine the relationship between rates of hydrogen ionization with the degree of the electrode filling with gas and perform estimation of the process intensity at a unit reaction surface. It is established that, in conditions of forced gas delivery, practically all the hydrogen oxidation current is generated at the surface of the nickel oxide electrode beneath thin films of an electrolyte solution at the rate of 4–5 mA cm^?2. It is shown that the hydrogen oxidation rate on a nickel oxide electrode filled in part by gas is independent of the electrode potential, probably because of a tangible contribution made by diffusion limitations to the overall hampering of the process.     

Multiscale Finite Element Modeling of Arc Dynamics in a DC Plasma Torch

The dynamics of the electric arc inside a direct current non-transferred arc plasma torch are simulated using a three-dimensional, transient, equilibrium model. The fluid and electromagnetic equations are solved numerically in a fully coupled approach by a multiscale finite element method. Simulations of a torch operating with argon and argon–hydrogen under different operating conditions are presented. The model is able to predict the operation of the torch in steady and takeover modes without any further assumption on the reattachment process except for the use of an artificially high electrical conductivity near the electrodes, needed because of the equilibrium assumption. The results obtained indicate that the reattachment process in these operating modes may be driven by the movement of the arc rather than by a breakdown-like process. It is also found that, for a torch operating in these modes and using straight gas injection, the arc will tend to re-attach to the opposite side of its original attachment. This phenomenon seems to be produced by a net angular momentum on the arc due to the imbalance between magnetic and fluid drag forces.     

Synthesis of Ozone at Atmospheric Pressure by a Quenched Induction-Coupled Plasma Torch

The technical feasibility of using an induction-coupled plasma (ICP) torch to synthesize ozone at atmospheric pressure is explored. Ozone concentrations up to ~250 ppm were achieved using a thermal plasma reactor system based on an ICP torch operating at 2.5 MHz and ~11 kVA with an argon/oxygen mixture as the plasma-forming gas. The corresponding production rate and yield were ~20 g ozone/hr and ~2g ozone/kWh, respectively. A gaseous oxygen quench formed ozone by rapid mixing of molecular oxygen with atomic oxygen produced by the torch. The ozone concentration in the reaction chamber was measured by Fourier Transform infrared (FTIR) spectroscopy over a wide range of experimental conditions and configurations. The geometry of the quench gas flow, the quench flow velocity, and the quench flow rate played important roles in determining the ozone concentration. The ozone concentration was sensitive to the torch RF power, but was insensitive to the torch gas flow rates. These observations are interpreted within the framework of a simple model of ozone synthesis.