End-on Observation Research Articles

Local effects of atomizing analyte droplets on the plasma parameters of the inductively coupled plasma

Monodisperse droplets from aqueous analyte solutions with selected diameters in the range 35–67 µm are introduced into an inductively coupled plasma with frequencies between 1 to 10 droplets per second. The effect of desolvation and atomization in the ICP is studied end-on by optical emission spectroscopy employing simultaneously up to three calibrated monochromators with fast photomultipliers. The onsets of desolvation and analyte atomization and the extremely fast diffusion of hydrogen in the ICP and its effect on the plasma are studied by simultaneous measurements of hydrogen, analyte and Ar lines. The local cooling by analyte atomization as well as the recovering of the plasma excitation temperature after completed atomization is measured quantitatively in dependence on time applying the Boltzmann plot method to simultaneously recorded line intensities of atomized analyte atoms which act as plasma probes. Furthermore, it is shown that relatively small differences in analyte mass cause significant temperature changes during atomization and, as consequence, strong variations of the emission intensity of analyte lines during atomization if measured by end-on observation.

Arc Instabilities in a Plasma Spray Torch

The control over coating quality in plasma spraying is partly dependent on the arc and jet instabilities of the plasma torch. Different forms of instabilities have been observed with different effects on the coating quality. We report on an investigation of these instabilities based on high-speed end-on observation of the arc. The framing rate of 40,500 frames per second has allowed the visualization of the anode attachment movement and the determination of the thickness of the cold-gas boundary layer surrounding the arc. The images have been synchronized with voltage traces. Data have been obtained for a range of arc currents, and mass flow rates for different gas injectors and for anodes displaying different amounts of wear. The analysis of the data has led to quantitative correlations between the cold-gas boundary layer thickness and the instability mode for the range of operating parameters. The arc instabilities can be seen to enhance the plasma jet instabilities and the cold-gas entrainment. These results are particularly useful for guiding plasma torch design and operation in minimizing the influence of plasma jet instabilities on coating properties.

Studies of a helium-operated gas-sampling Grimm-type glow discharge for the atomic emission spectrometric determination of chlorine in halogenated hydrocarbon vapors

Chlorine originating from halogenated hydrocarbons introduced through a fused-silica capillary was detected by atomic emission spectrometry from a helium-operated gas-sampling Grimm-type glow discharge. During the introduction of the halogenated hydrocarbon vapor, the operating voltage of the discharge was found to increase considerably. The device showed excellent discharge and signal stability over periods of several hours. It was found that the noise characteristics of the discharge were virtually white and that they did not change upon introduction of dichloromethane. Spatially resolved measurements performed with an automated x– y translation stage and end-on observation demonstrated that the emission-intensity distribution of both chlorine and helium is dependent on the geometry of the entrance aperture in the cathode block. For conical and hollow-cathode-like entrance arrangements, emission intensities were concentrated at the entrance of the capillary. However, for a flat cathode sampling plate, the intensity profiles exhibited a minimum at the entrance orifice. Background intensities were found to change during the introduction of halogenated hydrocarbon vapors, necessitating both careful background acquisition and correction. After discharge conditions were optimized (at 25 mA and 7 Torr), the detection limit for chlorine utilizing the Cl II 479.5 nm line was found to be 20 ng s −1. Vapor from aliphatic as well as aromatic hydrocarbons were introduced into the discharge. To a first approximation, signals were found to be proportional to the amount of both chlorine and carbon released into the discharge, enabling the ratio of carbon to chlorine to be determined. The device therefore deserves further study regarding its potential as an element-specific detector for gas chromatography.

Argon and copper optical emission spectra in a Grimm glow discharge source: mathematical simulations and comparison with experiment

The spectral line intensities of 605 argon atom lines and 103 copper atom and ion lines were calculated for a Grimm-type glow discharge source under typical conditions of glow discharge optical emission spectrometry. The calculations were based on the Einstein transition probabilities for radiative decay and on the level populations of excited levels of the argon atoms and copper atoms and ions, which were computed previously in a comprehensive self-consistent modeling network for the behavior of the various species present in a glow discharge (argon atoms and ions, sputtered atoms and ions, in various levels, and electrons). To the authors’ knowledge, the optical emission spectra of argon atoms and of copper atoms and ions have been calculated in this explicit way for the first time. The spectra were integrated over the entire cell axis, to simulate end-on observation. Moreover, the spatial dependence of some spectral lines along the cell axis was elucidated. Finally, the effects of voltage and pressure on the calculated spectral line intensities were investigated and comparisons were made with experimental data. The agreement between theory and experiment was satisfactory for argon; for copper there was still some discrepancy, indicating that the model, in spite of its comprehensive nature, still has some shortcomings.

Determination of Particle Densities and Line Profiles in Plasmas by Resonance Interferometry: A Feasibility Study using Computer Simulated Refractivity Data

Abstract The principles of resonance interferometry are described with regard to two applications: High accuracy particle density determination within plasmas and interferometrical determination of spectral line profiles. The usability of this technique is investigated numerically, and physical limits are given for the regions in which resonance interferometry may be employed successfully. The discussion and the results are helt general for making it possible to decide whether or not to apply this method for an actual problem. An example (an object being longitudinally homogeneous with respect to the direction of light: end-on observation) shows how to use the presented results for calculating the detection limits of the method for a given object geometry.

Easily ionizable element (EIE) interferences in inductively coupled plasma atomic emission spectrometry: II. Minimization of EIE effects by choice of observation volume

Measurements by inductively coupled plasma atomic emission spectrometry (ICP-AES) are expected to be immune to interferences caused by easily ionizable elements. This article demonstrates how observation width, viewing height and viewing volume can be chosen to minimize such interferences and to optimize emission-signal collection in either side-on or end-on configurations. At this time, results must be viewed as a confirmation of previous findings rather than a practical means of performing routine plasma optimizations. Volume data were computed from high-resolution two-dimensional laterally and radially resolved images of Ca I and Ca II emission in the presence and absence of a lithium concomitant. For side-on observation of calcium atomic emission, the customary arrangement, the optimal balance between signal-to-background ( S/B) ratio and interference effects arises from use of a 0.5 mm observation width. The optimal position for measurement and the ideal height of the observation volume were dependent on applied r.f. power. More extended contiguous regions of high S/B ratio became available with the proper spatial averaging, which is inherent in viewing an extended volume. Side-on observation of Ca II emission in the presence of a Li concomitant was best with a 5 mm viewing width at low r.f. power (500 and 1000 W) and a narrow width (< 1 mm) at higher power (1500 W). End-on observation of Ca atomic emission was optimal for a 1000 W plasma viewed within 1.5 mm of the central axis with an observation width of less than 6 mm. For end-on Ca II viewing, a 1500 W plasma minimized the lithium interference while maximizing the S/B ratio when the midpoint of the viewing position was within 1 mm of the central channel and the observation width was less than 2.5 mm.

Thermospray sample introduction into a horizontal low-flow inductively coupled plasma with end-on observation

Thermospray sample introduction into a horizontal water-cooled low-flow inductively coupled plasma (ICP), with end-on observation, has been used successfully to improve detection limits four-fold, relative to sample introduction with a V-groove nebuliser in the sample plasma. This improvement in the detection limits has been observed for 17 elements and is in agreement with the results reported in the literature for the thermospray sample introduction into a conventional high-flow ICP. The linearity of the analytical graphs is 4–5 orders of magnitude, i.e., similar to nebuliser sample introduction. Interferences are not as low as would have been expected for end-on observation of the horizontal water-cooled low-flow plasma, nevertheless they are not inferior to a conventional ICP.

End-on observation of a horizontal low-flow inductively coupled plasma

End-on observation of a horizontal water-cooled low-flow plasma has been compared to side-on observation of a conventional high flow plasma. The analytical performance is similar to the conventional plasma. The detection limits are close to the detection limits measured with a Perkin-Elmer Plasma II conventional ICP. The linearity of calibration curves is also similar to the conventional ICP. However, a remarkable reduction of interferences has been observed for end-on observation of the low-flow plasma.

An equilibrium model for the radial intensity distribution of analyte lines in the ICP discharge

Simple expressions are derived in terms of exponential functions for the relative intensity distributions of analyte atom and ion lines in the chemical ICP torch for the limiting cases of vanishing and full ionization. The expressions are based upon a concave, parabolic temperature profile, a Gaussian distribution of analyte particle density and upon the existence of thermal equilibrium. The electron density distribution over the radius is assumed to be constant. For given temperature and particle profiles the shape of the line profile is determined by the excitation energy; beyond a critical value it develops off-axis peaks whose separation increases with further increase in excitation energy. The ionization energy of an element in general also affects the distributions of atom and ion line intensities. Its influence vanishes only for atom lines under zero ionization and for ion lines under full ionization. Numerical values of the profile parameters for specific operating conditions are determined by fitting expressions derived from the model to experimental values of peak separation and profile half width for atomic and ionic lines that were obtained through end-on observation and thus represent averages over the torch length. To obtain a theoretical basis for the effect of axial position on the profile shape, the model is made two dimensional by assuming plausible variations of the profile parameters with the axial coordinate. Profiles calculated on this basis show a gradual transition from double peak to single peak with increasing observation height and agree qualitatively with experiments. Finally, it is shown that off-axis profiles of a line as they have been obtained in laser fluorescence experiments can, for a given off-axis distance, be readily calculated from a radial profile of that line.

Untersuchungen des thermischen gleichgewichts im wandstabilisierten ar-niederstrombogen mit N 2- und H 2-zusätzen

On a wall-stabilized Ar arc (4 mm dia., end-on observation, 4 A, 1 atm) radial temperature and electron density distributions were found by measuring intensities of band lines and atomic lines and of the Ar continuum; the gas was mixed with small quantities of N 2 and H 2. Plasma dates of the gas mixtures were computed. Temperatures, electron densities and field strengths could be explained by considering the laws of electrical and heath conduction. The electron temperature of the pure Ar arc (centre) was (1500 ± 500)°K higher than the gas temperature (extrapolation). This difference was decreasing at increasing admixtures of molecule gases; at a few percents, LTE was reached.

Shock-Tube Study of the Acetylene–Oxygen Reaction. II. Chemi-ionization during the Induction Period

The exponential growth of CH*(A 2Δ → X 2Π) emission and chemi-ion production were simultaneously monitored behind reflected shock waves during the induction period of the C2H2+O2 reaction. Experiments were performed in the temperature range 1479–2282°K using three C2H2+O2+Ar gas mixtures whose C2H2:O2:Ar proportions were 5:5:990, 5:10:985, and 5:30:965. End-on observation was used to monitor CH* emission, and a new saturation-current technique was developed and used to monitor the rate of chemi-ion production. In each experiment the exponential growth constant for ion production was twice that for CH* emission. The results establish that chemi-ionization during the C2H2+O2 induction period is second order in reaction intermediates, which rules out the reaction CH*+C2H2→C3H3++e and is consistent with reaction CH+O→CHO++e as the major chemi-ionization reaction during this early portion of the reaction. An ion–molecule reaction mechanism together with the data obtained in this study is used to demonstrate that the predominance of C3H3+ during the induction period can be explained by such a mechanism.

Experimentelle Untersuchung der Rayleigh-Taylor-Instabilität eines rotierenden Wasserstoff-Lichtbogens im axialen Magnetfeld

Abstract A hydrogen arc with a hollow anode in an axial magnetic field is subjected to Rayleigh-Taylor instabilities as a result of rotation due to the radial current components at the anode. These instabilities take the form of periodic fluctuations in the light intensity emitted from the boundary regions of the arc. The modes m = 4 to m = 7 which occur in succession with increasing magnetic field are identified by measuring the signal phases in end-on observation of the arc. The rotational velocity of the arc plasma is determined spectroscopically and comparison is made with the non-dissipative MHD theory, which yields growth rates ∼ .The deviations from this theory that were observed in this experiment are ascribed to dissipative effects.

Temperaturmessungen am Argon-Hochleistungsbogen

Spectroscopic measurements of radial temperature distributions in the cylindric cascade arc of 5 mm diameter in argon at atmospheric pressure and 5 atm in the current range from 200 to 500 amperes are reported. The applied method is the determination of the absolute intensity of both appropriate atom and ion lines and of the continuum, where the influence of self-absorption was carefully examined by variation of the arc lenth in end-on observation. The measured axial temperatures were between 17,800 ‡K and 20,800 ‡K at atmospheric pressure and between 13,700 ‡K and 14,100 ‡K at 5 atm pressure.