Electrolyte Cathode Atmospheric Glow Discharge Research Articles

Evaluation of analytical performance for the simultaneous detection of trace Cu, Co and Ni by using liquid cathode glow discharge-atomic emission spectrometry

In this paper, a novel liquid cathode glow discharge (LCGD) was established as a micro-plasma excitation source of atomic emission spectrometry (AES) for simultaneous detection of trace Cu, Co and Ni in aqueous solution. In order to evaluate the analytical performance, the operating parameters such as discharge voltage, supporting electrolyte, solution pH and flow rate were thoroughly investigated. The results showed that the optimal conditions are 640 V discharge voltage, pH = 1 HNO3 as supporting electrolyte and 4.5 mL min−1 flow rate. The R2 of Cu, Co and Ni are 0.9977, 0.9991 and 0.9977, respectively. The relative standard deviation (RSD) is 1.4% for Cu, 3.2% for Co and 0.8% for Ni. Under this condition, the power of LCGD is below 55 W and the plasma is quite stable. The limits of detections (LODs) for Cu, Co and Ni are 0.380, 0.080, and 0.740 mg L−1, respectively, which are basically consistent with the closed-type electrolyte cathode atmospheric glow discharge (ELCAD). Compared with ICP-AES, the LCGD-AES has small size, low power consumption, no inert gas requirement and low cost in set-up. It may be developed as a portable instrument for in-site and real-time monitoring of metals in solution samples with further improvement.

Determination of calcium and zinc in gluconates oral solution and blood samples by liquid cathode glow discharge-atomic emission spectrometry

A novel flowing liquid cathode glow discharge (LCGD) was developed as an excitation source of the atomic emission spectrometry (AES) for the determination of Ca and Zn in digested calcium and zinc gluconates oral solution and blood samples, in which the glow discharge is produced between the electrolyte (as cathode) overflowing from a quartz capillary and the needle-like Pt anode. The electron temperature and electron density of LCGD were calculated at different discharge voltages. The discharge stability and parameters affecting the LCGD were investigated in detail. In addition, the measured results of real samples using LCGD-AES were verified by ICP-AES. The results showed that the optimized analytical conditions are pH = 1 HNO3 as supporting electrolyte, 4.5mLmin–1 solution flow rate. The power consumption of LCGD is 43.5–66.0W. The R2 and the RSD ranged from 630 to 680V are 0.9942–0.9995 and 0.49%–2.43%, respectively. The limits of detections (LODs) for Zn and Ca are 0.014–0.033 and 0.011–0.097mgL–1, respectively, which are in good agreement with the closed-type electrolyte cathode atmospheric glow discharge (ELCAD). The obtained results of Ca and Zn in real samples by LCGD-AES are basically consistent with the ICP-AES and reference value. The results suggested that LCGD-AES can provide an alternative analytical method for the detection of metal elements in biological and medical samples.

Detection of some industrially relevant elements in water by electrolyte cathode atmospheric glow discharge optical emission spectrometry

An electrolyte cathode atmospheric glow discharge optical emission spectrometry (ELCAD-OES) method was developed for the detection of the industrially relevant In, Rh and Te in water samples. Acid/additive type, sample pH and flow rate were optimized. The UV–Vis spectrum was scanned for analytical lines, free from spectral overlap interferences, and sensitive enough for quantifying the analytes at mg L−1 or lower levels. In several cases, the background spectrum of the ELCAD hindered the use of conventional, resonant analytical lines in the UV due to overlaps with bands of molecular species (e.g., OH, NO, N2). Te and Rh showed lower emission intensities than In (determined at In I 451.1nm), even using the most sensitive, interference-free transitions (i.e., Te I 214.3nm, Te I 238.6nm and Rh I 437.5nm). The emission intensities were highly sample pH dependent, i.e., analytical signals could only be detected at pH levels lower than 2. Conversely, the use of acidity lower than pH1 caused lower plasma volume, due to its contraction into the sample introduction capillary, and discharge instability in terms of its frequent self-extinction. The detection limits for In, Rh and Te were 0.01, 0.5 and 2.4mgL−1, respectively. Calibration curves were linear up to 100–150mgL−1. The precision for In, Rh and Te in aqueous standards, expressed as relative standard deviation (RSD), was not higher than 4.6%, 6.4% and 7.4%, respectively. Samples with high salt content (e.g., well water) caused positive matrix effects (i.e., 2.0- to 3.6-fold signal enhancements), but also ~1.5 times higher RSDs.

Novel application of the electrolyte cathode atmospheric glow discharge: Atomic absorption spectrometry studies

The analytical characteristics of a capillary design of the electrolyte cathode atmospheric glow discharge (ELCAD), operated with a W-rod anode at a discharge current of 70mA and a discharge voltage of 950V, were exploited through spatially resolved atomic absorption spectrometry (AAS) experiments. For this purpose, the ELCAD cell, placed on a platform adjustable with micrometer screws, was inserted into the optical path of a commercial line-source AAS instrument. A flow injection system was developed and applied to introduce 3mL of aqueous standards of a set of environmentally relevant metals (Ca, Cd, Cr, Cu, Na, Pb and Zn) into the plasma. The analyte atom distribution along the vertical axis of the conically-shaped ELCAD plasma (height: 3.5mm) is element specific. All the absorbance maxima are observed in the near cathode region (e.g., in the range of 0.5–1.0mm from the cathode), while the AA signal smoothly fades towards the anode. Several spectrochemical buffers (citric acid, EDTA, chlorides of Ca, Cs, La, Li, and Na) were studied for improving the sensitivity of the AAS determinations for Cr. A significant increase in the sensitivity (20%) was found only with the addition of 0.55% (m/v) La solution. The limit of detection data for Cd, Cu, Na and Zn are 3.4, 4.2, 9.2 and 0.9mgL−1, respectively. The AAS calibration curves for Cd, Cu, Na and Zn are linear up to 75, 200, 100 and 25mgL−1, respectively.

Electrolyte Cathode Atmospheric Glow Discharges for Direct Solution Analysis

The electrolyte cathode atmospheric glow discharge (ELCAD) invented in 1992 is a new optical emission source with upcoming application in the field of environmental protection as an outstanding instrument for monitoring the toxic heavy metal content of waters and wastewaters. The main operating parameters, mechanisms (secondary electron emission from the electrolyte cathode, self‐sustaining processes in the cathode dark space, dependence of the emitted line intensities on the discharge parameters, temperatures), and the analytical performance of this special discharge are presented through a critical review using the papers related to the ELCAD published from 1993 to 2006.

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 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.

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.

Operating mechanism of the electrolyte cathode atmospheric glow discharge.

Cathode fall ( U(cf)), cathodic current density and atomic emission intensities originating from metal salts in the electrolyte cathode were measured as a function of different discharge parameters. Emission intensities in function of cathode fall indicate a potential barrier in the sputtered mass flux. This means that the primary particles of the cathode sputtering are of positive charge and the cathode fall including its internal variables is the most important factor. The measured current density and the U(cf) as a function of pressure are in accordance with the low pressure data in the literature. The observed decrease of the U(cf) with decreasing pH was explained by a model in that the secondary electron emission coefficient of the cathode (gamma) is controlled through a reaction net of competing reactions of different electron scavengers involving the hydroxonium ions of the cathode solution. The model revealed two different electron emission processes of the electrolyte cathode, an emission coupled with hydrated electrons is dominating below pH 2.5 while a proton-independent emission of poor efficiency is working above pH 3. Our model fits to the reported yields of the ultimate products both in the solution and in the gas phase and offers a calculation of gamma and U(cf) in the function of the cathode acidity. The model provides two other independent gamma calculation methods based on product analysis data.