Mirjana S. Pavlović

Emission Spectrometric Analysis of Fresh Waters with U-Shaped D.C. Arc with Aerosol Supply

Abstract The application of a U-shaped d.c. arc with aerosol supply for analysis of fresh waters has been described. Limits of detection obtained with d.c. plasma compare well with those reported for ICP. A number of spectral lines and molecular bands were examined and the most suitable are selected to ensure the best limits of detection or wide dynamic ranges. Limits of detection in ng ml−1, given in the parenthesis, following the element symbol, are: Al(0.9), B(0.6), Ba(0.2), Be(0.15), Ca(0.05), Cd(3.5), Co(1.1), Cr(0.2), Fe(2.4), Ga(0.5), Hg(9.0), K(0.3), Li(0.008), Mg(0.4), Mn(0.3), Mo(0.7), Na(0.04), Ni(2.0), P(45), Pb(12), Si(9.0), Sr(0.04), Zn(1.3).

The role of demixing effect in analyte emission enhancement by easily ionized elements in d.c. plasma

Abstract The effect of easily ionized elements (EIE) in d.c. plasma (DCP) has been modeled and experimentally investigated. The introduction of the EIEs in the U-shaped DCP decreases a ‘potential barrier’ due to an internal radial electric field, which allows the partially ionized analyte better entry into the hot plasma zone and produces enhancement of the analyte spectral emission. Compared to the equilibrium concentration, the increased electron concentration in the fringe region due to large radial gradients is accounted for in the model. By comparing model predictions to experimental data, the validity of the model is tested. The main features of the effect, the large enhancement at small addition of EIE and almost equal enhancement of ion and atom line intensities, are fairly well described by the model. The results are believed to be applicable to the inverted Y-shaped DCP. The effect in the inductively coupled plasma (ICP) has been briefly discussed from the point of view of the proposed model.

Demixing effect induced by the radial electric field in the cylindrically shaped d.c. plasma. Modeling and experimental investigation

Sample introduction and a demixing effect in the plasma of the d.c. arc in argon with aerosol supply are studied using spectroscopic techniques. The observed deviations from the uniform distribution of the partially ionized plasma gas components are correlated to the radial electric field in the arc column plasma. A mathematical model is proposed for the quantitative treatment of the radial field effect. The radial electric field is assessed from the radial distribution of the electron pressure. The proposed model is used in two ways: (a) starting from the close-to-real arc temperature distribution, other parameters are calculated assuming that plasma is in local thermodynamic equilibrium. The calculations are performed for mixtures of two gases: majority gas with high ionization energy and minority gas with low ionization energy at low concentration. The results obtained show that the partial pressure of an easily ionizable component decreases from the periphery towards the arc column axis. The size of the decrease ranges from a negligible value up to four orders of magnitude, depending on the ionization energy of the minority gas. (b) For the experimental verification of the proposed model, all the variables governing the sample entry into the plasma, including the initial partial pressure of the minority gas, are determined by independent spectroscopic techniques. Vapors of Ba, Ca, Fe and Zn are utilized as a minority gas. These values are then used to calculate the radial distribution of the minority gas partial pressure according to the proposed model. Good agreement between the experimental and the calculated values is observed.

The effect of potassium addition on plasma parameters in argon dc plasma arc

The effect of potassium addition on the radial distribution of temperature and electron number density in a U-shaped direct current (dc) argon plasma operating at different arc currents has been studied by optical emission spectroscopic techniques and the power interruption method. Spatially resolved electron number densities (ne) have been determined from measured radial profiles of Balmer-Hβ spectral line. The obtained electron number densities have been used for thermodynamic temperature (TLTE) evaluation with the assumption that the arc plasma is in a state of local thermodynamic equilibrium. The excitation temperatures (Texc) have been determined from the absolute integral emissivity of the argon line at 430.01 nm. For heavy particle temperature (Th) evaluations we have used a power interruption method. The obtained results have shown that an addition of KCl decreases both electron number density and temperature of the arc column. The magnitude of such an influence on plasma parameters increases with an increase in the KCl concentration and decreases with an increase in the arc current.

Excitation and ionization characteristics of a d.c. argon plasma evaluated by means of emission and absorption of iron lines, and broadening of the hydrogen Hβ line-experimental facts

Spectroscopic diagnostic techniques were used to investigate the plasma of a 7.5 A d.c. arc in argon with aerosol supply. An analytically usable portion of the arc column was 50 mm long, cylinder shaped, and suitable for observation in the axial direction, i.e. for “end-on” observation. This made possible the evaluation of the spatially resolved local plasma parameters without the use of Abel inversion. Solutions containing 1 gl of iron were introduced with a pneumatic nebulizer. Absorption and emission of Fe I and Fe II lines were used to evaluate populations of ground and excited states. From the Boltzmann-Saha plots, apparent temperatures and electron number densities were determined as functions of the radius. The electron number densities were also determined from the broadening of the Hβ line. Stratification in the radial direction limited the applicability of the spectroscopic techniques. At about 2.9 mm off the arc axis, excitation of Fe II lines by charge transfer was observed. Different values of the electron number density were deduced from emission, combined emission and absorption, and Hβ profile measurements.

A Power Interruption Technique for Determining the Difference Between Electron and Gas Temperatures in the Argon d.c. Arc Supplied with Aqueous Aerosol

The difference between electron and gas temperatures in a low current U‐shaped d.c. arc with aqueous aerosol supply was studied using a power interruption technique. Monitoring of the temporal evolution of recombination continuum after power switch‐off revealed that it was necessary to do a correction of the measured values due to a change in electron concentration. It was shown that the plasma studied was a two temperature type and that the temperature difference ranges from 1850 K to 600 K for arc currents from 3 to 10.5 A, respectively. A comparison with literature results for pure argon arc plasma shows that aerosol introduction reduces the temperature difference.

Properties of plasma induced by pulsed CO2 laser on a copper target under different ambient conditions

Optical emission spectroscopy was applied for investigation of copper plasma induced by a nanosecond transversely excited atmospheric CO2 laser, operating at 10.6 μm. The effect of the background gas (air, Ar, He and N2) and pressure (1–25 mbar) on plasma formation was examined. The plasma shielding effect was more pronounced for background gases with lower ionization potential than for He. The increase of He pressure from 1 to 25 mbar resulted in fivefold increase of Cu atomic line intensity.

A Spectroscopic Investigation of Spatial Symmetry of Radiation in the U-shaped DC Argon Plasma with Aerosol Supply

ABSTRACT Radial distributions of plasma parameters, temperature, and electron number density, together with radial distribution of analyte absorption and emission, were investigated in order to obtain insight into the radial symmetry of low-current (≤ 10 A), atmospheric pressure, argon stabilized arc with tangential introduction of the aerosol. For plasma diagnostics, several methods were used: measurements of Hβ line profile, absolute intensity measurements of the argon 430.01-nm line, and of the spectrally adjacent continuum and power interruption technique. It was found that the inevitable asymmetry in aerosol introduction has negligible influence on basic plasma parameters but influences considerably the spatial distribution of the analyte particle spectral absorption and emission.