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Emission studies on a glow discharge in atmospheric pressure air using water as a cathode

A stable discharge was produced in atmospheric air using water as a cathode. Spectral lines of elements dissolved in tap water could be observed. It is demonstrated that the appearance of these spectral lines is a consequence of cathode sputtering of water during the discharge. The intensity of the lines was found to depend strongly on the acidity of the water. This kind of discharge gives a possibility for continuous analysis of water and waste water solutions.

Direct solution analysis by glow discharge: electrolyte-cathode discharge spectrometry

A new glow discharge atomic emission source was developed for the direct determination of metals in aqueous solutions by applying an atmospheric glow discharge in the air gap (2–6 mm) between an electrolyte solution cathode and a W-rod anode. Cathode sputtering of the solution surface and subsequent excitations occur when the pH of the solution is below 2.5. The spectrum emitted contains the basic atomic lines of the dissolved metals from K 769.9 to Zn 213.8 nm, ion lines of Mg and Ca and strong OH, NH and N2 bands. Boiling of the cathode pole can be avoided by use of a flow-through technique. The electron temperature was found to be around 5000 K. The calibration curves of line intensities versus metal concentrations were linear in the 1–50 ppm range and showed strong positive dependence on the discharge current and on the hydrogen ion concentration of the solution. Pulse modulated operation with time-resolved signal processing promises a substantial increase in the signal-to-background ratio. The flow-through electrolyte acts as a continuously renewed cathode pole, thereby enabling the continuous direct multi-metal assay of solutions. This report is the first discussion on the analytical characteristics of electrolyte-cathode discharge (ELCAD). spectrometry as a new technique for metal monitoring in aqueous solutions.

Development of open-air type electrolyte-as-cathode glow discharge-atomic emission spectrometry for determination of trace metals in water

Abstract The open-air type electrolyte cathode atomic glow discharge (ELCAD) has been developed and studied for fundamental and analytical applications for determination of trace heavy metals in water. The normal closed-type discharge cell shows some problems such as unstable plasma due to changes in the pressure inside the cell during the discharge, and water vapor condensing onto the window. Applying approximately 1500 V to the several-millimeter gap between the electrolyte solution cathode and a Pt rod anode in atmospheric air pressure produced a stable plasma and significantly improved sensitivity. The emission spectrum of de-ionized water containing 100 mg/l Cu was measured and some emission lines were found from Cu I (324.7 nm, 327.4 nm and 510.5 nm) and Cu II (224.7 nm and 229.4 nm). The LODs of Cr, Cu, Fe, Mn, Ni, Pb, and Zn are in the ranges from 0.01 mg/l to 0.6 mg/l. The LODs of Cu, Mn, Pb and Zn improve by approximately one order of magnitude compared to the previous closed-type ELCAD.

Direct sample introduction of wines in graphite furnace atomic absorption spectrometry for the simultaneous determination of arsenic, cadmium, copper and lead content

A multi-element graphite furnace atomic absorption spectrometry (GFAAS) method was elaborated for the simultaneous determination of As, Cd, Cu, and Pb in wine samples of various sugar contents using the transversally heated graphite atomizer (THGA) with end-capped tubes and integrated graphite platforms (IGPs). For comparative GFAAS analyses, direct injection (i.e., dispensing the sample onto the IGP) and digestion-based (i.e., adding oxidizing agents, such as HNO(3) and/or H(2)O(2) to the sample solutions) methods were optimized with the application of chemical modifiers. The mixture of 5 microg Pd (applied as nitrate) plus 3 microg Mg(NO(3))(2) chemical modifier was proven to be optimal for the present set of analytes and matrix, it allowing the optimal 600 degrees C pyrolysis and 2200 degrees C atomization temperatures, respectively. The IGP of the THGA was pre-heated at 70 degrees C to prevent the sputtering and/or foaming of sample solutions with a high organic content, dispensed together with the modifier solution, which method also improved the reproducibility of the determinations. With the digestion-based method, the recovery ranged between 87 and 122%, while with the direct injection method it was between 96 and 102% for Cd, Cu, and Pb, whereas a lower, compromise recovery of 45-85% was realized for As. The detection limits (LODs) were found to be 5.0, 0.03, 1.2, and 0.8 microg l(-1) for As, Cd, Cu, and Pb, respectively. The characteristic mass (m(0)) data were 24 pg As, 1.3 pg Cd, 13 pg Cu, and 35 pg Pb. The upper limits of the linear calibration range were 100, 2, 100, and 200 microg l(-1) for As, Cd, Cu, and Pb, respectively. The precisions were not worse than 4.8, 3.1, 3.7, and 2.3% for As, Cd, Cu, and Pb, respectively. For arsenic, a higher amount of the modifier (e.g., 20 microg Pd plus 12 microg Mg(NO(3))(2)) could be recommended to overcome the interference from the presence of sulphate and phosphate in wines. Although this method increased the sensitivity for As (m(0)=20 pg), it also enhanced the background noise, thus only a slight improvement in the LOD of As (3.9 microg l(-1)) was realized. For the 35 red and white wine samples studied, the highest metal contents were observed for Cu ranging from 20 to 640 microg l(-1) (average: 148 microg l(-1)), followed by Pb from 6 to 90 microg l(-1) (average: 32.3 microg l(-1)), and Cd from 0.05 to 16.5 microg l(-1) (average: 1.06 microg l(-1)), whereas the As content was below the LOD. This wide fluctuation in the trace metal content could be associated with the origin of wines from various regions (i.e., different trace metal level and/or quality of soil, and/or anthropogenic impact), and with diverse materials (e.g., additives and containers) involved in the wine production processes. The Cu content of wine samples was significantly correlated with Pb, whereas its weak anti-correlation was found with Cd. Interestingly, the level of Pb was anti-correlated with the year of production of the wines. This is likely due to the gradual decrease in the Pb content of soils of vineyards by time, which certainly causes less Pb-uptake of the grape plant, thus a decrease in the Pb content of wines as well.

The spatial distribution of the temperatures and the emitted spectrum in the electrolyte cathode atmospheric glow discharge

In an electrolyte cathode atmospheric glow discharge (ELCAD), the gas temperature (Tgas) obtained from the intensity ratio of the OH 306–309 nm unresolved rotational band heads and the electron temperature (Tel) received from the intensity ratio of the Cu I 510.5 and 515.3 nm were investigated. In the near anode region, Tgas ≈ Tel ≈ 6000 K; in the positive column, Tgas ≈ 4000 K, Tel ≈ 5500 K; in the near cathode region, Tgas ≈ 8000 K, Tel ≈ 7500 K were found. These temperatures agree with our earlier results and are in accordance with the literature; that is, at atmospheric pressure, the gas and the electron temperatures approximate well with each other. The spatial intensity distribution of the atomic lines of metals dissolved in the electrolyte was measured and the intensity maximum occurred in the negative glow due to the high electron temperature and thus to the high excitation rate of this region. (Some figures in this article are in colour only in the electronic version)

Similarity laws for glow discharges with cathodes of metal and an electrolyte

The pressure dependence of the cathodic current density in the case of an electrolyte and a metal cathode glow discharge operating in air and helium atmospheres was investigated. In all cases, at pressures above 100-200 mbar, violation of the similarity law was observed, which was attributed to the occurrence of the dissociative recombination of molecular ions of the applied gases (O, and ). Taking into consideration this process, the relation was deducted, which was found to be in accordance with experimental data. In the case of an electrolyte cathode this relation was found to be valid also at low pressures.

Pressure Dependence of the Atmospheric Electrolyte Cathode Glow Discharge Spectrum

The intensity of the spectral lines emitted by the electrolyte cathode glow discharge plasma was investigated as a function of the air pressure at different discharge currents and pH values of the cathode solution. The observed increase in the metal line intensity is explained by three-body collisional recombination of positive metal ions in the cathode dark space (M++e+e→M+e) followed by diffusion of neutral metal atoms into the negative glow, where electron impact excitation of the neutral atoms takes place. On the basis of these two processes, the dependence of the intensity on pressure was calculated by an approximating equation, giving an excellent agreement with the experimental results in the pressure range 530–1200 mbar. Furthermore, the model predicts the appearance of an intensity maximum as a function of the air pressure. This maximum was observed in preliminary experiments in the pressure range 1200–2300 mbar.

The investigation of an abnormal electrolyte cathode atmospheric glow discharge (ELCAD)

In a capillary electrolyte cathode atmospheric glow discharge (ELCAD) plasma, the current density and the length of the cathode dark space (LCDS) were measured and the gas temperature (Tg) in the cathode surface-dark space boundary layer was estimated. Using a CCD camera, the current density (the diameter of the cathode spot) and the LCDS were determined from the light intensity distribution provided by the FITSVIEW image processing software. The current density was significantly higher compared with that measured in a normal ELCAD glow and increased with increasing current. The LCDS was also estimated by means of the similarity law relating to the ELCAD plasma. The estimated and measured values of the LCDS are in good agreement and smaller than that observed in a normal ELCAD. In the boundary layer a higher Tg was estimated because of the increased current density.

The influence of chlorine on the intensity of metal atomic lines emitted by an electrolyte cathode atmospheric glow discharge

The effect of different matrix anions in the solution on the intensity of metal atomic lines was investigated. A significant increase in intensity was found for chloride anions compared with nitrate and sulfate anions. This effect was even greater when the appropriate acids were applied. A further enhancement of the metal line intensities could be observed when HCl was used in the solution phase and simultaneously elemental chlorine was mixed with atmospheric air at levels up to 6-10 vol.%. This double effect was especially high for the Cu, Ni and Pb resonant atomic lines at higher chlorine-to-air ratios in the gas phase, and the W-anode tip was destroyed by chemical burning. The application of volatile organic chlorine compounds (carbon tetrachloride and chloroform) in the gas phase, even without any acidification, also caused an enhancement of the metal line intensities. The experimental results can be attributed to the different rates of the ion-ion (positive metal ion-negative chloride ion) and the positive metal ion-electron recombination processes taking place in the cathode dark space of the discharge plasma, yielding neutral metal atoms for excitation. This study is important for the on-line measurement of heavy metals in liquids.

Investigations on the element dependency of sputtering process in the electrolyte cathode atmospheric discharge

Using a coupling of ICP with a closed electrolyte cathode discharge (ELCAD) cell as nebulizer unit it was possible to observe a well defined element dependency of the cathode sputtering process on the electrolyte surface. Investigating the ICP signal ratio referred to the pneumatic nebulization, the ELCAD sputtering produces about three times higher mass transport for Al, Cr, Pb and Cd than for Mg and Cu. B, Ba and Ca have an even lower signal while Hg shows a “super-sputtering” effect, having a 17 times higher signal with ELCAD than with pneumatic nebulization. Assuming that positively charged particles are forced back to the cathode by the 107 V m−1 field in the cathode dark space these observations fit to the model of stepwise charge neutralization by hydrolysis in the vapor-phase, which process is known in mass spectrometry. The sputtering process in the ELCAD can be described with a four-zone kinematic model of the cathode dark space.