Tamás Cserfalvi

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.

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 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.

A critical review of published data on the gas temperature and the electron density in the electrolyte cathode atmospheric glow discharges

Electrolyte Cathode Discharge (ELCAD) spectrometry, a novel sensitive multielement direct analytical method for metal traces in aqueous solutions, was introduced in 1993 as a new sensing principle. Since then several works have tried to develop an operational mechanism for this exotic atmospheric glow plasma technique, however these attempts cannot be combined into a valid model description. In this review we summarize the conceptual and technical problems we found in this upcoming research field of direct sensors. The TG gas temperature and the ne electron density values published up to now for ELCAD are very confusing. These data were evaluated by three conditions. The first is the gas composition of the ELCAD plasma, since TG was determined from the emitted intensity of the N2 and OH bands. Secondly, since the ELCAD is an atmospheric glow discharge, thus, the obtained TG has to be close to the Te electron temperature. This can be used for the mutual validation of the received temperature data. Thirdly, as a consequence of the second condition, the values of TG and ne have to agree with the Engel-Brown approximation of the Saha-equation related to weakly ionized glow discharge plasmas. Application of non-adequate experimental methods and theoretical treatment leads to unreliable descriptions which cannot be used to optimize the detector performance.

On the pressure dependence of the positive column cross section in high-pressure glow discharges

The cross section and the current density at the two ends of a positive column, measured as functions of pressure, were found to be different in glow discharges operated between copper electrodes in air, nitrogen and helium in the pressure range of 100-760 Torr and at an electrode distance of 7 mm. Taking into consideration the occurrence of the dissociative recombination of positive molecular ions, the observed difference is explained by the different pressure dependences and magnitudes of the cathode and anode falls, which determine the current densities at these places.