Abstract
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.
Highlights
The electrolyte cathode atmospheric glow discharge (ELCAD) technique was invented for the direct measurement of metals (Zn, Cd, Cu, Ni, Cr, Pb, alkali and earth metals, Fe, Mn, etc.) in aqueous samples [1]
The main reasons of this are the following: (1) It is not yet widely understood that ELCAD plasmas operate in saturated H2O vapor due to the very intense sputtering of the aqueous cathode
The gas temperature determination based on the emitted spectrum of N2 molecule being only in the outer sheath cannot give the correct, real gas temperature data of the plasma
Summary
The electrolyte cathode atmospheric glow discharge (ELCAD) technique was invented for the direct measurement of metals (Zn, Cd, Cu, Ni, Cr, Pb, alkali and earth metals, Fe, Mn, etc.) in aqueous samples [1]. The emitted intensity of the atomic lines of metals dissolved in the cathode solution, which have a maximum in the negative glow region, are determined by the pressure, the current and the solution pH, fall on the cathode [4,5,6]. TG relates to the gas particle density, the collision number between the electrons and the gas particles, the mean free path of electrons and the electron energy gained in this distance [8,9,10] In this way, the TG and the ne are two basic parameters of the ELCAD plasma determining the operation and the excitation mechanisms, the emitted intensities as well. Saha-equation [27,28] related to the weakly ionized glow discharges with low charge densities
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