Jinmei Wang

The Abel inversion was employed to investigate the radial emissivity distribution of solution cathode glow discharge (SCGD) of plasma.

In this study, we demonstrate a unique method for the detection of heavy metals such as chromium (Cr), copper (Cu), lead (Pb), and nickel (Ni) at trace levels in aqueous solutions via laser-induced breakdown spectroscopy (LIBS) enriched using chelating resin. Reduction in sample time preparation was shortened by tailoring the sample processing time with the aid of an enrichment method. The calibration curves and LODs of CH-90 chelating resin for enriching Cr, Cu, Pb, and Ni cations were established under the optimal conditions. The linear correlation coefficients were all above 0.98, and the detection limits were 0.148 mg/L, 0.150 mg/L, 0.149 mg/L, and 0.240 mg/L, corresponding to Cr, Cu, Pb, and Ni, respectively. The quantitative evaluation of the obtained results signifies that the proposed method is highly sensitive for detecting trace elements and offers a good correspondence of the acquired linear correlation with its calibration model. Results comparison with past studies suggests that the proposed method is able to achieve lower LODs for elements under investigation.

A novel atmospheric-pressure plasma source has been developed to determine metal ions in an aqueous solution by means of optical emission spectroscopy. In contrast to the conventional solution cathode glow discharge, this plasma source is sustained by a milli-second pulsed power supply. The electrical characteristics and spectral characteristics of the proposed plasma source are discussed. The effects of operating parameters, including pulse excitation and maintenance discharge voltage, pulse width, pulse modulation duty ratio, solution flow rate, and detector delay time, are investigated. Na, K, and Ca were observed to have a cumulative spectral emission time of 250 μs. Delayed acquisition times (in the case of maximum spectral intensity) are 500, 800, 800, 900, and 900 μs, respectively for Cu, Co, Cd, Mn, and Ag. The detection limits of trace elements such as Na, K, Cu, Co, Cd, Mn, and Ag were 0.080, 0.050, 3.4, 64, 69, 9.1, and 2.0 μg L⁻¹, respectively. Compared to DC-excited solution cathode glow discharge, the detection performance of the above elements is observed to be increased by 2.9, 1.5, 18, 5.5, 25, 1.6, and 2.1 times, respectively. The proposed excitation source provides a promising method for detecting metal ions in aqueous solution samples.

The direct determination of analytes in aqueous solution using laser-induced breakdown spectroscopy (LIBS) has several difficulties, including low spectral signal, poor stability, and rapid quenching of the plasma. In this study, chelate resin and anion exchange resin were used to adsorb the chromium ions Cr(III) and Cr(VI) from the liquid onto a solid substrate, thus avoiding the drawbacks of liquid analysis with LIBS. Cr(III) and Cr(VI) were separated by selective adsorption on these two distinct resins. The following analytical parameters were optimized: flow rate of sample solution for preconcentration on the resins, laser pulse energy, and integral delay time of the ICCD detector. Calibration curves displayed excellent linearity, with correlation coefficients of 0.994 for Cr(III) and 0.993 for Cr(VI). The analytical limits of detection (LODs) were 0.088 mg L⁻¹ for Cr(III) and 0.27 mg L⁻¹ for Cr(VI), and were lower than those in most previous studies. The relative uncertainty of Cr(III) in single and mixed solutions ranged from 2.50% to 4.25% and 4.75% to 14.0%, respectively, and that of Cr(VI) ranged from 2.88% to 7.67% and 11.5% to 13.9%, respectively. Recovery from natural spring water was 92.50–99.86% for Cr(III) and 97.70–98.75% for Cr(VI). Recovery from sewage wastewater using this method was 99.80% for Cr(III) and 107.6% for Cr(VI). These results show that this method can be successfully applied to the determination and separation of chromium ions in aqueous solution.

In this study, a refrigerating anode method was developed to improve solution cathode glow discharge-atomic emission spectrometry (SCGD-AES) for the determination of elements. Spectral characteristics of metal anodes at different temperatures were studied in the range from 10 to 30 °C. The spectral signals of metals were enhanced by reducing the metal anode temperature during the plasma discharge. Moreover, the method of the refrigerating anode improved the stability of spectral emission of metal elements. Compared with the unrefrigerated system under the same experimental conditions, the detection limits of Ag, Cd, Cu, Mn and Zn were 0.6, 9.1, 5.3, 8.7 and 16 μg L⁻¹, respectively, which represented the improvements by 6.5, 3.5, 3.2, 3.9 and 3.1 times, respectively. The measurement results of Ag, Cd, Cu, Mn and Zn in a certified reference solution (BWB2446-2016) and an actual water sample from the Yangtze river by SCGD-AES with the refrigerating anode were in agreement with the reference values measured by ICP-OES. The water sample recovery from the Yangtze river ranged from 101.5 to 110.0%, suggesting that the measurement results exhibited high accuracy and reliability. All the results indicated that SCGD-AES coupled with a refrigerating anode could improve its analytical ability and provide an analytical method for in situ, real-time and on-line determination of metal elements in water samples.

A hollow anode-solution cathode glow discharge (HA-SCGD) was designed as a plasma excitation source for optical emission spectrometry. The operation parameters such as the discharge current, argon flow rate and solution flow rate were investigated. The spatial distribution profiles of normalized results of spectral bands and spectral lines were investigated and analyzed. The detection limits of Na, Li, K, Ca, Mg, Pb, Sr, Mn, Rb, Cs and Zn were 0.22, 0.69, 1.33, 240.37, 21.55, 529, 619.43, 209.51, 2.94, 7.13 and 206.95 µg L⁻¹, respectively. The repeatability, expressed as the relative standard deviation for these elements, was determined to be within the range from 0.53 to 2.70%. The Ca, Fe, K, Mg and Na concentrations in a certified reference material were quantitatively analyzed, and the results obtained by the HA-SCGD system agreed well with the reference values. The results show that the HA-SCGD system accurately and stably determined metals in aqueous solution.