A spatial study of electron density and analyte emission in a d.c. argon plasma

Spatially resolved electron density measurements have been performed on a three-electrode d.c. plasma using a linear photodiode array based spectrometer. The electron density values measured are between 1× 10¹⁵ and 1 × 10¹⁶ cm^?3 depending on spatial position. The spatial distribution of Ca I (422.7 nm) and Ca II (393.4 nm) emission has also been measured and the Ca II-Ca I emission intensity ratio evaluated. Using the n_e values measured, an analagous LTE ratio has been calculated and this has been compared to the experimental values. Measured ratios are found to be from 28 to 100 times less than LTE ratios. Some possible sources leading to these infrathermal ratios are discussed.     

Some analytical characteristics of an argon—nitrogen d.c. plasma arc for emission spectrometry

A low-power d.c. plasma arc device was used to estimate the analytical characteristics of an Ar—N₂ plasma arc compared to those of an argon plasma arc. When the flow rate of added nitrogen was varied from 0 to 1 l min^-1, the Cd I 228.802-nm line showed a maximum signal-to-background ratio at a nitrogen flow rate of approximately 0.3 1 min^-1 which corresponds to 0.23% of the total argon flow rate. Ratios of the signal intensities with the Ar—0.23%N₂ and argon plasma arcs are given for the spectral lines of seventeen elements. Relatively higher ratios were found for the atom lines of the group VIII through IIIA elements in the periodic table. Better precision and lower detection limits were attained for aluminium and cadmium with the Ar—0.23%N₂ plasma arc than with the argon plasma arc.     

Some analytical characteristics of a three electrode d.c. argon plasma source for optical emission spectrometry

An investigation of a commercially available three electrode d.c. plasma source burning in an inverted Y configuration was carried out. It is shown that (i) for atoms the position of maximum line intensity in the plasma is determined by the norm temperature of the spectral line, (ii) the source is stable over long periods of time, (iii) high concentrations of alkali metals in the sample can disturb the plasma resulting in an enhancement of both atomic and ionic spectral lines, (iv) detection limits are generally less than one order of magnitude higher than those obtained with an inductively coupled plasma (ICP), and (v) precision of analysis is better than 2% expressed as relative standard deviation.     

Applications of d.c. argon plasma emission spectroscopy to saline waters: a study of enhancement effects

A commercially available d.c. argon plasma emission spectrometer was used to determine transition metals (3d and 4d) and also Be in salt and brackish water. The effects of salinity on the enhancement of emission intensities of the analyte lines were studied using an empirical approach combined with statistical analysis. One set of experiments deals with the effects of trace metal concentration and salinity on the relative emission intensities of 14 elements using a completely randomized experimental design, i.e. the sequence in which the 48 treatment combinations (12 levels of salinity and 4 levels of metal concentration) were measured was determined by randomization, with the results evaluated by an analysis of variance program. The second set deals with the effect of relative enhancement to salinity on selected ion and atom emission line pairs for various elements at 100 and 50% salinity relative to fresh water (0% salinity). These results are analyzed by a stepwise linear multiple regression analysis program using selected parameters of theoretical interest. It was discovered that the differences in relative enhancement for ion—atom emission line pairs for 100% salinity could be predicted basically with two variables. A coefficient of determination of 95% was achieved by employing the energy of transition for the atomic line and the number of unpaired d-electrons in the lower atomic state.     

Determination of metal traces in wine by argon stabilized d.c. arc

A spetroscopic method for the determination of metal traces (Mn, Cu, Fe) in wine has been developed based on argon stabilized d.c. arc plasma at atmospheric pressure. The experimental conditions were optimized using lateral distributions of spectral line intensities of the trace elements in aqueous and ethanol-aqueous solutions. The method was applied to the analysis of 6 wines from 3 Serbian wine-growing regions. Direct and standard addition methods were tested. The precision of the method is characterized by a relative standard deviation of 0.50– 3.00%. The accuracy of the method was assessed by flame AAS.     

Determination of metal traces in wine by argon stabilized d.c. arc

A spetroscopic method for the determination of metal traces (Mn, Cu, Fe) in wine has been developed based on argon stabilized d.c. arc plasma at atmospheric pressure. The experimental conditions were optimized using lateral distributions of spectral line intensities of the trace elements in aqueous and ethanol-aqueous solutions. The method was applied to the analysis of 6 wines from 3 Serbian wine-growing regions. Direct and standard addition methods were tested. The precision of the method is characterized by a relative standard deviation of 0.50– 3.00%. The accuracy of the method was assessed by flame AAS. Received: 8 October 1998 / Revised: 17 March 1999 / Accepted: 20 March 1999     

Determination of trace elements in natural waters by the d.c. argon plasma,multielement atomic emission spectrometer (DCP-MAES) technique

The capabilities and limitations of a commercial direct current argon plasma, multielement atomic emission spectrometric (DCP-MAES) technique for the determination of 18 common elements in water have been evaluated. The results demonstrate that the unit offers acceptable capabilities with respect to selectivity, sensitivity, accuracy, speed, and economy for the determination of many of the elements investigated. Interferences in the determinations of those elements subject to stray light due to the presence of calcium and (or) magnesium can often be compensated for by use of a simple linear correction procedure. A comparison of two-electrode and three-electrode d.c. argon plasma systems has shown that the latter offer advantages such as improved stability and lower background. A comparison with results published for a system based on excitation in an inductively coupled plasma has indicated that the present system offers comparable analytical capabilities for several of the elements investigated.     

Characterization of d.c. plasma torch voltage fluctuations

The arc root fluctuations at the anode-nozzle of a d.c. plasma spray torch with a special configuration of the electrodes allowing to work with the same gas flowrate with nozzle diameters ranging from 6 to 10 mm were systematically studied. The plasma gas was Ar/H2 (25 vol % H2), the current was varied between 200 and 600 A and the plasma gas flowrate between 24 and 80 slm. After 30–60 mn working the nozzle wall started to be sufficiently eroded to have a stagnant arc spot which lived until arcing created another one. It was shown that the life time of the upstream arc spots were 30–40 % longer than the downstream ones which could play an important role in the electrode erosion. Dimensional analysis allowed to find a relationship between the nozzle diameter D, the arc current I and gas flow rate G and the mean spot lifetime which is closely connected with the difference between D and the electrical diameter of the arc column. The comparison of voltage signal and light emission at a point of the plasma jet close to the nozzle exit on its axis allowed to determine the mean electrical field within the plasma column and the mean position of the arc root. The comparison with the electrode erosion area for well defined conditions showed a good correlation with the calculated arc root position.     

Thermal decomposition of carbon dioxide in an argon plasma jet

The decomposition of carbon dioxide to carbon monoxide and oxygen in an argon plasma is studied. The overall kinetic and energy-related parameters of the process, as well as the quenching rates of the products, are evaluated on the basis of measured radial and longitudinal profiles of both temperature and degree of decomposition in the high-temperature reaction zone.     

Analysis of mineral suspensions by d.c. plasma emission spectrometry

Dilute aqueous suspensions of sub-sieve fractions of various minerals are analyzed for their major constituents by d.c. plasma emission spectrometry. In favourable cases (e.g., determination of zinc in sphalerite or smithsonite powders below 5 μm), the suspensions may be analyzed quantitatively simply by comparison with standard solutions, and with a relative error less than 10%. In other cases (e.g., determination of beryllium in beryl powders), the required element is so poorly atomized that the analysis is impossible, even when very fine-grained suspensions are used. As expected, the atomization efficiency for a particular element decreases with increasing particle size, and with increasing free energy of formation of its oxide at high temperatures. The region extending along the vertical branch of the plasma is a convenient excitation zone for suspensions containing elements that are difficult to atomize but the high background in this region limits the analytical possibilities.     

A stable d.c. capillary arc plasma for solution analysis

The analytical capability of a d.c. capillary arc plasma, operating on argon, is described. Solutions containing Cd, Pb, Hg, I, As and Zn are introduced into the plasma from a tantalum-filament atomizer. Signals are observed from both the arc and the tail-flame. The problem of sample rejection by the hot plasma is illustrated and discussed.     

Transport velocities of particles in the plasma of a d.c. arc in air

Axial transport velocities of materials injected into a d.c. arc plasma by a liquid jet method have been determined on the assumption that the motion is influenced by diffusion (due to gradients in the number density of particles), axial electric field and convection. The velocities were measured using both high speed cinematography and cross-correlated photomultiplier signal techniques. In the interpretation of the measurements, with both methods use had to be made of the motion of the constant concentration boundary of the injected material in the frames of reference relative to both the arc electrodes and the point of maximum material density. Particular attention was given to convection effects.     

Laser Doppler anemometry under plasma conditions. Part I. Measurements in a d.c. plasma jet

A study is presented on the use of laser Doppler anemometry (LDA) techniques for the measurement of the gas and particle velocities under plasma conditions. Experimental data is presented for a d.c. plasma jet in which alumina particles are injected under different operating conditions. The results reveal that the plasma velocity at the exit of the jet is of the order of 200–300 m/s. The intensity of turbulence is as high as 30 to 40% in the free shear layer and the particle velocity distribution is shown to be asymmetric, with particle dispersion in the plane of injection considerably more important than that in the perpendicular direction. The average particle velocity depends on the composition of the plasma gas, the torch current, and power.     

Measured and modeled enhancement of transition metal emissions in the d.c. plasma jet

Enhancement of sensitive transition metal lines by a sodium matrix is measured in a 3-electrode d.c. plasma jet. Spiking with 0.43 M NaCI causes enhancement by factors of 1.85–2.92 in ionic lines and of 1.22–1.99 in atomic lines for eight of the structurally related analyte elements, but suppresses Zn I and Zn II emissions by about 25 %. Emission response to NaCI of lines within the same spectrum, or between different spectra of like ionization stage, can be simulated to 15 % and 20–25 %, respectively, by approximations linear in energy differences. For ionic lines these differences are the absolute value of the line excitation potential minus the energy of the ion state most readily pumped by Penning ionization by argon. For atomic lines it is the difference between emitting state excitation potential and the first ionization potential. Analyses of the experimental data strongly suggest that: (1) Na acts mainly to pertub radiative transfer rather than collisional redistribution processes; (2) population pumping of excited analyte states is largely driven by Penning ionization; (3) accelerated radiative cooling due to Na is manifested in a lowering of local kinetic temperature; (4) to a first-order of approximation, ambipolar diffusion, analyte-Na collisions of the second kind, and analyte ground state spin, do not influence emission line enhancement by easily ionized elements (EIE). Approximations are developed for predicting transition metal enhancements by arbitrary Na doping concentrations, and means are sketched for extending the method to other analyte group/EIE combinations. Practical implications of the work are noted.     

A comparative study of the excitation conditions in the plasma and the electrical parameters of a d.c. arc when using power-stabilized,current-stabilized and manually controlled d.c. sources

The abilities of three d.c. arc sources—a manually controlled source, a constant current source and a constant power source—to control variations in arc temperature, electron pressure and anode temperature were compared, and the dependency of these factors on the electrical parameters was investigated. It was shown that with well buffered samples there is little advantage to begained in using any one of the three sources. With poorly buffered samples, however, the constant power source showed greater control over the variations in arc temperature, electron pressure and anode temperature. It was also shown that the only electrical. parameter which could be used to obtain an estimate of the arc temperature is the arc power.     

Probe measurements in argon plasma jets operated in ambient argon

Measurements of local enthalpies and velocities have been performed in plasma jets generated by a DC plasma spray torch, using an enthalpy probe. The torch has been operated in an argon confined atmosphere at different currents and argon flow rates.⁽¹⁾ The validity of the measured enthalpy and velocity profiles has been checked by performing energy flux and mass flux balances, which show reasonable agreement between the input quantities, measured independently, and those obtained by integrating over the experimental profiles. The data are compared with those obtained by operating the same torch in ambient air. The results show that temperatures and velocities measured in pure argon are substantially higher than those in air, and consequently, the jets in argon appear wider and substantially longer.     

Determination of arsenic in sediments by chloride formation and d.c. plasma arc emission spectrometry

The sediment sample is heated in an induction furnace in hydrogen chloride—argon; arsenic trichloride evolved is trapped and then swept into a d.c. plasma arc. Calcium sulfate addition enhances the signals and suppresses interference from organic matter. The limit of detection is 15 ng As; the relative standard deviation is 3.4% (n = 10) for 1.5μg As₂0₃.     

The first ionization potential of uranium and thorium measured in a d.c. arc plasma

Tha Saha relationship was used to derive the ionization potentials of uranium and thorium from measurements of temperature or of electron density in a plasma in thermodynamic equilibrium. Introducing into the plasma elements with well defined ionization potentials, such as Ba, Al, V, Cr, Zr, Mo, Cu, Si and some of the rare-earths, as matrices, the temperature and electron density were measured in the central region of the arc plasma. A relation was established between the different ionization potentials and the plasma temperature or the electron density. From this relation the values of 6.3 ± 0.3 and 7.5 ± 0.3 eV were found for the ionization potentials of U and Th respectively.