Three-phase double-arc plasma for spectrochemical analysis of environmental samples

A new instrument, which uses a three-phase current to support a double-arc argon plasma torch for evaporation, atomization and excitation of solid or powder samples, is described. The sampling arc is ignited between the first and second electrode while the excitation arc is ignited between the second and third electrode. Aerosol generated from the sample (first electrode) is swept by argon gas, through a hole in the second electrode (carbon tubing electrode), into the excitation plasma. A tangential stream of argon gas is introduced through an inlet orifice as a coolant gas for the second electrode. This gas stream forces the excitation arc discharge to rotate reproducibly around the electrode surface. Discharge rotation increases the stability of the excitation plasma. Spectroscopic measurements are made directly in the current-carrying region of the excitation arc. An evaluation of each parameter influencing the device performance was performed. Analytical calibration curves were obtained for Fe, Al, K, and Pb. Finally, the present technique was applied for the analysis of environmental samples. The present method appears to have significant, low cost analytical utility for environmental measurements.     

Three-phase plasma arc atomic-emission spectrometric analysis of environmental samples using an ultrasonic nebulizer

Combination of an ultrasonic nebulizer and plasma excitation sources for spectrochemical analysis offers desirable features of low detection limits, high sample throughput, wide dynamic range of operation, acceptable precision and accuracy, and simultaneous quantitative analytical capabilities. Moreover, the ultrasonic nebulizer does not require sample preconcentration. Recently we have developed a three-phase plasma arc (TPPA) for atomic emission spectrochemical analysis. In the present work, to increase the analytical utility of the three-phase plasma system, an ultrasonic nebulizer was used for sample introduction. The effects of the argon gas flow rate, current, excitation temperature have been studied. The analytical calibration curves are obtained for Ca, Cr, Fe, Mg and Mn, and detection limits have been calculated. The present technique is used to determine the concentration of the elements Ca, Cr, Fe, Mg and Mn in airborne samples.     

Optical emission diagnostics in cascade arc plasma polymerization and surface modification processes

Optical Emission Spectroscopy (OES) was used to identify reactive species and their excitation states in low-temperature cascade arc plasmas of N₂, CF₄, C₂F₄, CH₄, and CH₃OH. In a cascade arc plasma, the plasma gas (argon or helium) was excited in the cascade arc generator and injected into a reactor in vacuum. A reactive gas was injected into the cascade arc torch (CAT) that was expanding in the reactor. What kind of species of a reactive gas, for example, nitrogen, are created in the reactor is dependent on the electronic energy levels of the plasma gas in the cascade arc plasma jet. OES revealed that no ion of nitrogen was found when argon was used as the plasma gas of which metastable species had energy less than the ionization energy of nitrogen. When helium was used, ions of nitrogen were found. While OES is a powerful tool to identify the products of the cascade arc generation (activation process), it is less useful to identify the reactive species that are responsible for surface modification of polymers and also for plasma polymerization. The plasma surface modification and plasma polymerization are deactivation processes that cannot be identified by photoemission, which is also a deactivation process. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1583–1592, 1998     

An Optical Emission Study on Expanding Low-Temperature Cascade Arc Plasmas

The optical emission spectra from expanding low-temperature cascade arc plasmas were studied. The objective of this study was to examine the distinctive features of low-temperature cascade arc plasmas in comparison with a radio frequency (RF) plasma source. The principal results obtained in this study were: (1) in an expanding cascade arc plasma jet, active heavy particles (mainly excited argon or helium neutral species under our operating conditions), rather than electrons, are responsible for the excitation of reactive species when a reactive gas is injected into the plasma jet, (2) the excitation of reactive species was found to be controlled by the electronic energy levels of these excited argon or helium neutrals, (3) changing the operating parameters affected only the emission intensities of excited species, and no effect on the emission nature of plasmas was observed.     

A capacitively coupled microwave plasma atomic emission spectrometer for the determination of trace metals in microsamples

A capacitively coupled microwave plasma operating at 450 to 850 W is used for atomic emission spectroscopy. The laboratory-constructed system contains a tungsten cup electrode capable of holding a volume of up to 30 μl. Microwaves are used to dry the sample, while at higher powers the plasma is ignited for sample vaporization and excitation. The entire analysis can be carried out in less than 5 min. A mixture of helium and hydrogen is used as the plasma gas. A spherical or cylindrical shaped plasma can be formed depending upon the gas flow rate and the microwave power selected. The effects of experimental parameters, such as gas flow rate, atomization power, electrode position and plasma shape are examined. Detection limits for Cd, Mg and Zn are in the low picogram range for a 10 μl sample; the relative standard deviation is less than 10%.     

Effect of Cathode Nozzle Geometry and Process Parameters on the Energy Distribution for an Argon Transferred Arc

The influence of two nozzle geometries and three process parameters (arc current, arc length and plasma sheath gas flow rate) on the energy distribution for an argon transferred arc is investigated. Measurements are reported for a straight bore cylindrical and for a convergent nozzle, with arc currents of 100 A and 200 A and electrode gaps of 10 mm and 20 mm. These correspond to typical operating parameters generally used in plasma transferred arc cutting and welding operations. The experimental set up consisted of three principal components: the cathode-torch assembly, the external, water-cooled anode, and the reactor chamber. For each set of measurements the power delivered to each system component was measured through calorimetric means, as function of the arc’s operating conditions. The results obtained from this study show that the shape of the cathode torch nozzle has an important influence on arc behaviour and on the energy distribution between the different system components. A convergent nozzle results in higher arc voltages, and consequently, in higher powers being generated in the discharge for the same applied arc current, when compared to the case of a straight bore nozzle. This effect is attributed to the fluidynamic constriction of the arc root attachment, and the consequential increase in the arc voltage and thus, in the Joule heating. The experimental data so obtained is compared with the predictions of a numerical model for the electric arc, based on the solution of the Navier–Stokes and Maxwell equations, using the commercial code FLUENT©. The original code was enhanced with dedicated subroutines to account for the strong temperature dependence of the thermodynamic and transport properties under plasma conditions. The computational domain includes the heat conduction within the solid electrodes and the arc-electrode interactions, in order to be able to calculate the heat distribution in the overall system. The level of agreement achieved between the experimental data and the model predictions confirms the suitability of the proposed, “relatively simple” model as a tool to use for the design and optimization of transferred arc processes and related devices. This conclusion was further supported by spectroscopic measurements of the temperature profiles present in the arc column and image analysis of the intensity distribution within the arc, under the same operating conditions.     

Influence of Gas Entry Point on Plasma Chemistry,Ion Energy and Deposited Alumina Thin Films in Filtered Cathodic Arc

The effect of gas entry point on the plasma chemistry, ion energy distributions and resulting alumina thin film growth have been investigated for a d.c. cathodic arc with an aluminum cathode operated in an oxygen/argon atmosphere. Ions of aluminum, oxygen and argon, as well as ions originating from the residual gas are investigated, and measurements for gas entry at both the cathode and close to the substrate are compared. The latter was shown to result in higher ion flux, lower levels of ionised residual gas, and lower ion energies, as compared to gas inlet at the cathode. These plasma conditions that apply when gas entry at the substrate is used result in a higher film deposition rate, less residual gas incorporation, and more stoichiometric alumina films. The results show that the choice of gas entry point is a crucial parameter in thin film growth using reactive PVD processes such as reactive cathodic arc deposition.     

Spectroscopic study of a magnetically rotating arc between opper electrodes in contaminated argon

The effects of N₂ and CO contaminants in atmospheric-pressure argon on an arc rotating between two concentric copper electrodes has been studied using optical spectroscopy of copper lines. The axial temperature of the magnetically driven arc in Ar + %N₂ was determined to be around 10,000 K for arc currents of SO to 200 A. The diffusion process of the copper vapor from the cathode was also studied. A copper density maximum 1 mm from the cathode along the arc column was found in Ar + %N₂. Removal of the contaminated cathode surface layers by the arc when contaminant injection in the plasma gas was stopped was found to be a slow process with a time scale depending on the type of the gas contaminant. The presence of gas contaminant in the electrode material controls the cathode erosion mechanism and the overall arc behavior in the transition between a contaminated to a pure argon arc.     

Low power cross-flow atmospheric pressure Ar + He plasma jet : Spectroscopic diagnostic and excitation capabilities

A cross-flow atmospheric plasma jet with distilled water or analyte solution nebulization has been investigated. The plasma gas flows perpendicularly to the RF powered electrode (11.21 MHz) and a grounded electrode was added for plasma stabilization. The working parameters of the plasma generator can be controlled in order to maximize either the plasma power (75 W) or the voltage on the RF powered electrode (plasma power, 40 W). The plasma gas, pure argon (0.4 l min⁻¹) or a mixture of argon (0.3–0.4 l min⁻¹) and helium (0–0.2 l min⁻¹), was also used for liquid nebulization. Optical emission of the plasma, collected in the normal viewing mode, was used for plasma diagnostics and for evaluating its excitation capabilities. The influence of helium content in the mixed-gas plasma on the plasma characteristics and on the emission axial profiles of the plasma gas constituents and of the analytes originate from the wet aerosol was studied. The addition of helium to the argon plasma, generally determines decreases in the emission of the plasma gas constituents (with the exception of molecular nitrogen), in the rotational temperature and in the electron number density and increases in the excitation temperatures and in the emission of easily excitable analytes. Based on the determined electron number densities, it was concluded that in the plasma zone which presents interest from analytical point of view the plasma is not very far from the partial thermodynamic equilibrium. In function of the helium content in the plasma gas and of the axial distance from the powered electrode the excitation temperatures are in the range of 2420–3340 K for argon, 2500–5450 K for oxygen and 900–2610 K for ionic calcium and the electron number densities are in the range of 1.2 10¹²–1.25 10¹³ cm⁻³. Some elements with excitation energy lower than 6 eV were excited in the plasma. The plasma excitation capability depends on the working conditions of the plasma generator (maximum power or maximum voltage on the RF powered electrode) and on the helium content in the mixed-gas plasma. The estimated detection limits for the studied elements (Na, Li, K, Ca, Cu, Ag, Cd, Hg and Zn) are in the range of 7 ng ml⁻¹ to 28 μg ml⁻¹.     

Solution nebulization of aqueous samples into the tubular-electrode torch capacitatively-coupled microwave plasma

This work shows the feasibility of using nebulization for introduction of aqueous samples into the tubular-torch capacitatively-coupled microwave plasma (CMP). Previously, solid electrodes were used with this type of plasma, in which analyte carrier and plasma support gases are premixed and swept around the electrode tip. With the new design, the analyte carrier gas passes through the centre of the hollow tubular electrode and mixes with the plasma support gas at the tip of the electrode where the plasma is formed. Sample solutions are nebulized with a Meinhard nebulizer and a laboratory-constructed spray chamber and desolvation system. The tubular torch is made of tantalum. Plasma gases investigated include argon, helium and nitrogen. Typical operating powers are 300-350 W. Elements studied include Ag, Al, Ba, Ca, Cd, Cr, Cs, Cu, K, Li, Na, Pb, Pd, Sr and Zn.     

Surface Modification of Poly-ε-Caprolactone with an Atmospheric Pressure Plasma Jet

In this work, poly-ε-caprolactone samples are modified by an atmospheric pressure plasma jet in pure argon and argon/water vapour mixtures. In a first part of the paper, the chemical species present in the plasma jet are identified by optical emission spectroscopy and it was found that plasmas generated in argon/0.05 % water vapour mixtures show the highest emission intensity of OH (A–X) at 308 nm. In a subsequent section, plasma jet surface treatments in argon and argon/water vapour mixtures have been investigated using contact angle measurements and X-ray photoelectron spectroscopy. The polymer samples modified with the plasma jet show a significant decrease in water contact angle due to the incorporation of oxygen-containing groups, such as C–O, C=O and O–C=O. The most efficient oxygen inclusion was however found when 0.05 % of water vapour is added to the argon feeding gas, which correlates with the highest intensity of OH (X) radicals. By optimizing the OH (X) radical yield in the plasma jet, the highest polymer modification efficiency can thus be obtained.     

Description of a flowing cascade arc plasma

The plasma in a cascaded arc in argon with flow is studied both experimentally and theoretically. The plasma pressure has been measured as a function of axial position in the are channel with a Baratron pressure transducer. The electron density and the electron temperature have been determined as a function of axial position using H_β-Stark broadening and line-continuum emissivity ratio, respectively. Comparison of the gas pressure measurements with an equilibrium model suggests that the /low is laminar. A one-dimensional nonequilibrium model based on the electron- and heavy-particle number balances and the heavy-particle energy balance is presented. The measured axial profiles of the electron density agree well with the model predictions, especially in the most upstream part of the arc channel. The plasma is strongly ionizing. Temperature equilibration takes about 20 mm of arc length, depending on the argon flow.     

Temperature Distribution in a Pilot Plasma Tundish: Comparison Between Plasma Torch and Graphite Electrode Systems

Arc, bath, and refractory wall temperatures are measured in a pilot transferred-arc plasma furnace by atomic emission spectroscopy (AES) and multiwavelength pyrometry. Argon plasma torch and graphite electrode with nitrogen as plasma gas are both examined and compared using the steel bath as anode. With argon, a two-slope characteristic curve is measured for arc temperature, which ranges from 9000 to 25,000 K. Another trend is observed with nitrogen for temperatures in the range 8000–12,000 K. In this latter case, the bath temperature is very sensitive to arc length: more than 100 K increase results in arc length rise from 50 to 150 mm. Experimental data shows the variation of heat transfer efficiency between the two configurations, which is supported by results about surface emissivity in the spectral domain 1–15 m.     

Probe measurements in thermal plasma jets

Measurements of composition, temperature, and velocity in atmospheric argon plasma jets are reported, using enthalpy probes. The plasma jets are generated by a commercial type plasma gun and the measurements are expected to be of particular interest for industrial applications such as plasma spraying. Emphasis has been on the central and downstream regions of the plasma flame. The entrainment of air into the jet was found to be very high, even close to the axis of the jet. Gas samples analyzed with a gas chromatograph showed demixing of the air, i.e., nitrogen is more abundant in the jet than at room temperature. The high air entrainment has a strong cooling effect on the plasma, resulting in a rapid temperature drop along the axis. The influence of the argon flow rate and of the arc current on the jet's conditions was parametrically studied. Matching of the quantities measured in the jet with the torch input confirmed the validity of the results, and the relevance of enthalpy probe diagnostics in thermal plasma jets.     

Comparative study of microwave-induced plasma atomic emission spectrometry and atomic fluorescence spectrometry as gas-chromatographic detectors for the determination of methylmercury in biological samples

In order to attain a lower detection limit with the HS GC MIP analytical method (Head-Space Gas Chromatography with Microwave-Induced Plasma detection) recently developed for the analysis of methylmercury in biological samples, the quarter-wave Evenson-type cavity used until now was replaced by a TM₀₁₀ Beenakker-type cavity, which was used with both argon and helium as carrier gas. With an argon plasma, an eightfold increase in detection limit was gained compared with the argon plasma sustained by the Evenson cavity, while only a four-fold increase was gained with the helium plasma. In a second step of the study, the MIP detector was replaced by an AFS (atomic fluorescence) detector (CVAFS Model-2, Brooks Rand Ltd, Seattle, USA). With this AFS detector a detection limit of 1 ng methyl mercury per g biological tissue could be reached; i.e. measurements were 40 times more sensitive than those using the Evenson cavity. This detector has some other advantages compared with MIP detection: it is less expensive and easier to manipulate, while the same precision and accuracy are obtained. The use of AFS as detector in the headspace gas chromatographic system is therefore an important improvement for the analysis of methyl-mercury in biological samples.     

Atomic and molecular emission spectroscopy on an expanding argon/methane plasma

A supersonically expanding cascaded arc plasma in argon is analyzed axperimentally by emission spectroscopy. The thermal cascaded arc plasma is allowed to expand through a conically shaped nozzle in the arc anode into the vacuum vessel. In the nozzle monomers (C _n H _v ) are injected. The monomers are dissociated and ionized by the argon carrier plasma, and transported toward a substrate by means of the expansion. Emission spectroscopy is used to obtain temperatures and particle densities. By varying external parameters (e.g., arc power, gas flow rates) plasma parameters can be linked with (e.g. parameters (e.g., refractive index).     

Operating characteristics and energy distribution in transferred plasma arc systems

A specially designed plasma chamber was constructed to study the operating characteristics of a dc plasma-transferred arc of argon, struck between a fluid convective cathode and a water-cooled anode. The arc voltage increased markedly with arc length and with an increase in the inlet velocity of the argon flow past the cathode tip, and much less with an increase in current. Radiation from the plasma column to the chamber walls and transfer of energy to the anode were the two principal modes of transfer of the arc energy. The former was dominant in the case of long arcs and at high inlet argon velocities. At the anode, the major contribution was from electron transfer, which occurred on a very small area of the anode (～5 mm in diameter). Convective heat transfer from the plasma was somewhat less. In all cases, the arc energy contributions to cathode cooling and to the exit gas enthalpy were small. From total heat flux and radiative heat transfer measurements, it was estimated that the plasma temperature just above the anode was in the range 10,000–12,000 K. Preliminary experiments with an anode consisting of molten copper showed that the arc root was no longer fixed but moved around continuously. The arc was othwewise quite stable, and its operating characteristics differed little from those reported for solid anodes, in spite of the greater extent of metal vaporization.     

Simulation of droplet heating and desolvation in an inductively coupled plasma — Part I

The total desolvation rate of sample droplets in an argon inductively coupled plasma (Ar ICP) is investigated through the development of a two-phase continuum flow computer model. The desolvation model is supplemented by equations used to determine the trajectories of particles through the plasma. The model is used to calculate the behavior of aerosol droplets from a direct injection high efficiency nebulizer (DIHEN), a micronebulizer used to inject microliter quantities of samples that are toxic, expensive, or of limited volume. We use the combination of desolvation and transport models to present the first predicted spatial distribution of droplet concentrations and evaporation rates in an ICP flow. These data are compared with the behavior of a DIHEN spray in an environment with no net argon gas flow to determine the importance of gas flow rates to overall droplet concentration profiles in the ICP. In addition, two separate techniques (Stokes’ equation and the direct simulation Monte Carlo treatment) for determining droplet trajectories are contrasted.     

Thermal pinch effect in the argon d.c. plasma

Creation and consequences of the marked thermal pinch in the analytical d.c. plasma (DCP) are investigated. The flowing plasma constricts due to interaction between forced convective cooling and electric current regulation of the low power arc. Distinctive visual, spectroscopic and operational features of the DCP are associated with the thermal pinch and the transport properties of hot argon. Response of modelled electron density to changes in cooling gas flow rates are compared to experimental data. Implications for non-local thermodynamic equilibrium (LTE) spectrochemical excitation are noted.     

The effect of low concentrations of a polyatomic gas in argon on erosion on copper cathodes in a magnetically rotated arc

Experimental results are presented for electrode erosion on copper cathodes in magnetically rotated arcs in argon, dry air, nitrogen, ammonia, and carbon monoxide as well mixtures of the above with argon. Water-saturated argon was also used. Erosion rates were determined by weight loss after chemical cleaning, and the runs were sufficiently long (between 5 to 60 min) to represent steady-state operation. Arc currents of 100 A and gas pressures of 1.1 atm. were used. Pure argon gave the highest erosion rates and the lowest arc velocities. Small concentrations of any of the diatomic gases in argon greatly increased the arc velocity and decreased the erosion rates. The results suggest that erosion is primarily a thermal phenomenon but that the surface chemistry can greatly influence erosion rates by modifying arc behavior.