Zhifu Liu

Plasma Jet Desorption Atomization-Atomic Fluorescence Spectrometry and Its Application to Mercury Speciation by Coupling with Thin Layer Chromatography

A novel plasma jet desorption atomization (PJDA) source was developed for atomic fluorescence spectrometry (AFS) and coupled on line with thin layer chromatography (TLC) for mercury speciation. An argon dielectric barrier discharge plasma jet, which is generated inside a 300 μm quartz capillary, interacts directly with the sample being analyzed and is found to desorb and atomize surface mercury species rapidly. The effectiveness of this PJDA surface sampling technique was demonstrated by measuring AFS signals of inorganic Hg(2+), methylmercury (MeHg), and phenylmercury (PhHg) deposited directly on TLC plate. The detection limits of the proposed PJDA-AFS method for inorganic Hg(2+), MeHg, and PhHg were 0.51, 0.29, and 0.34 pg, respectively, and repeatability was 4.7%, 2.2%, and 4.3% for 10 pg Hg(2+), MeHg, and PhHg. The proposed PJDA-AFS was also successfully coupled to TLC for mercury speciation. Under optimized conditions, the measurements of mercury dithizonate (Hg-D), methylmercury dithizonate (MeHg-D), and phenylmercury dithizonate (PhHg-D) could be achieved within 3 min with detection limits as low as 8.7 pg. The combination of TLC with PJDA-AFS provides a simple, cost-effective, relatively high-throughput way for mercury speciation.

Non-chromatographic determination of inorganic and total mercury by atomic absorption spectrometry based on a dielectric barrier discharge atomizer

A novel, fast and simple non-chromatographic approach for determination of inorganic and total mercury has been developed by cold vapor atomic absorption spectrometry based on dielectric barrier discharge (DBD) atomizer. The determination of inorganic mercury and total mercury can be achieved in a fast sequential fashion (1 min sample−1). In the proposed method, with 0.01% (m/v) NaBH4 used as reductant, inorganic mercury is reduced to elemental mercury, whereas the methylmercury forms an intermediate volatile methylmercury hydride (CH3HgH). A low temperature DBD atomizer was employed for the atomization of CH3HgH. Only inorganic mercury can be measured in the absence of the DBD plasma. However, in addition to inorganic mercury, CH3HgH can be atomized and total mercury is determined in the presence of the plasma. The methylmercury concentration can then be obtained from the difference. The effects of several experimental parameters, such as NaBH4 concentration, HCl concentration, discharge gas and gas flow rate on the analytical performance were investigated. The method provided good reproducibility (<3% RSD) and the detection limits of Hg2+ and CH3Hg+ were found to be 0.35 ng mL−1 and 0.54 ng mL−1, respectively. The proposed method was validated by the analysis of a certified reference material (tuna fish). In addition, it was successfully applied to the analysis of fish samples. The method is excellent for mercury speciation measurements as it provides short analysis time and good reproducibility.

Dielectric Barrier Discharge for High Efficiency Plasma-Chemical Vapor Generation of Cadmium

A novel approach for Cd vapor generation was developed on the basis of a plasma-assisted chemical process. The generated Cd vapor was subsequently measured by atomic fluorescence spectrometry. Dissolved Cd species were readily converted into volatile species by reaction with hydrogen in a coaxial thin-film dielectric barrier discharge (DBD) plasma reactor. Both atomic and molecular Cd species were produced when a solution containing Cd(2+) was exposed to hydrogen-containing DBD plasma. Fast and efficient vapor generation of Cd was achieved simply in plain (neutral) water medium. Optimal conditions for the DBD-plasma Cd vapor generator were identified. The performance of this thin-film DBD plasma-chemical vapor generation (CVG) was evaluated through comparison with that arising from the conventional HCl-KBH4 system. The vapor generation efficiency of the proposed method (He-DBD) was found to be superior to the conventional CVG approach. Under the optimized conditions, the detection limits of Cd were found to be from 0.03 ng mL(-1) (Ar-DBD) to 0.008 ng mL(-1) (He-DBD) with a heated quartz tube atomizer (QTA); good repeatability (relative standard deviation (RSD) = 1.4%, n = 5) was obtained for a 1 ng mL(-1) standard. The new thin-film DBD plasma-CVG provides several additional advantages including simple setup, easy coupling with flow injection, low power consumption (≤18 W), cost-effectiveness, and long operation lifetime. The accuracy of the proposed method was validated through analysis of cadmium in reference material of simulated natural water sample GBW(E)080402 and rice reference material GBW10045. The concentration of cadmium determined by the present method agreed well with the reference values.

Alternating current driven atmospheric-pressure liquid discharge for the determination of elements with optical emission spectrometry

An atmospheric pressure plasma sustained by an alternating-current power supply was generated using the electrolyte solution as one electrode. The emission spectra of the plasma are related to the metal ions present in the solution and thus can be used for metal ion detection. The main advantage of this source is that it operates at low flow rates in the range of 0.1 to 0.8 mL/min. The effects of the discharge electrolyte anion, the pH and solution flow rate on emission signal were studied. The analytical response curves for Na and Cd demonstrated good linearity at least in the range of 0.5–100 mg L−1. The limit of detection of Na and Cd were determined to be 0.04 and 0.09 mg L−1, respectively. The method was applied for the determination of elements in simulated natural water samples (GBW(E)080408 and GBW(E)080402). The obtained results were in good agreement with the reference values. It provides an alternative analytical method for the determination of elements in water samples.

On line vapor generation of osmium based on solution cathode glow discharge for the determination by ICP-OES

A novel plasma induced vapor generation method is proposed to determine osmium in solutions. Without any chemical oxidizing agents, osmium ion can be readily converted to volatile osmium tetraoxide vapor in the solution cathode glow discharge (SCGD) system. The generated osmium vapor is then transported to inductively coupled plasma for determination by optical emission spectrometry. The influences of background electrolyte, carrier gas flow rate, sample flow rate, ICP power and discharge current were investigated. The analytical performances of this proposed technique were evaluated under optimized conditions. The detection limit of Os was calculated to be 0.51 ng mL(-1). The reproducibility, expressed as the relative standard deviation (n=11) of a 2.0 μg mL(-1) standard solution, was 1.9%. This SCGD induced vapor generation is sensitive and simple, oxidation reagents free, providing an alternative analytical method for measuring Os in geological or environmental water samples.

Dielectric barrier discharge-plasma induced vaporization for the determination of thiomersal in vaccines by atomic fluorescence spectrometry

Direct determination of thiomersal in vaccine by atomic fluorescence spectrometry with dielectric barrier discharge (DBD)-plasma induced vaporization was developed. No sample pretreatments other than dilution were needed in the measurement. The evaporation and atomization of thiomersal in vaccine samples was achieved rapidly in one step. The produced mercury vapor was transported to atomic fluorescence spectrometry for analysis. It is a green method that obviates the need of strong oxidization procedure for degradation of thiomersal to inorganic mercury and reduction agents such as sodium tetrahydroborate(III) or tin(II) chloride to generate Hg vapor. The DBD plasma was generated in a quartz tube (i.d. 5 mm and o.d. 6 mm) by a concentric circles model. The operating parameters such as discharge gas type, gas flow rate, and power were optimized. Possible interference by concomitant elements was also investigated. External calibration was achieved with aqueous standards solution of thiomersal. Under the optimal experimental condition, the detection limit was 0.03 μg L−1 for Hg, equivalent to 0.06 μg L−1 of thiomersal in solution. The analytical precision, represented by the relative standard deviation (RSD) from multiple measurements (n = 5) of 5 μg L−1 thiomersal (as Hg), was 2.5%. This method was also applied to measure thiomersal in five commercial vaccines. In the absence of CRMs for thiomersal in vaccine samples, it was validated by comparing with results from inductively coupled plasma mass spectroscopy (ICP-MS), which showed good correlation.

Generation of Volatile Cadmium and Zinc Species Based on Solution Anode Glow Discharge Induced Plasma Electrochemical Processes

In this study, a novel high efficiency vapor generation strategy was proposed on the basis of solution anode glow discharge for the determination of Cd and Zn by atomic fluorescence spectrometry. In this approach, a glow discharge microplasma was acted as a gaseous cathode to initiate the plasma electrochemical vapor generation of Cd and Zn. Cadmium/zinc ions could be converted into molecular species efficiently at the plasma-liquid interface from a supporting electrolyte (HCl, pH = 3.2). It was found that the overall efficiency of the plasma electrochemical vapor generation (PEVG) system was much higher than the conventional electrochemical hydride generation (EcHG) and HCl-KBH4 system. With no requirement for other reducing reagents, this new approach enabled us to detect Cd and Zn with detection limits as low as 0.003 μg L-1 for Cd and 0.3 μg L-1 for Zn. Good repeatability (relative standard deviation (RSD), n = 5) was 2.4% for Cd (0.1 μg L-1) and 1.7% for Zn (10 μg L-1) standard. The accuracy of the proposed method was successfully validated through analysis of cadmium in reference material of stream sediment (GBW07311), soil (GBW07401), rice (GBW10045), and zinc in a simulated water sample (GSB 07-1184-2000). Replacing a metal electrode with a plasma offers the advantage of eliminating potential interactions between the species in liquid and the electrode, which solves the issues associated with electrode encountered in conventional EcHG. The ability to initiate electrochemical vapor generation reactions at the plasma-liquid interface opens a new approach for chemical vapor generation based on interactions between plasma gas-phase electrons and solutions.

The online coupling of high performance liquid chromatography with atomic fluorescence spectrometry based on dielectric barrier discharge induced chemical vapor generation for the speciation of mercury

Based on a novel green dielectric barrier discharge plasma induced chemical vapor generation (DBD plasma-CVG), an on-line hyphenation of high performance liquid chromatography (HPLC) with atomic fluorescence spectrometry (AFS) was developed for the speciation of inorganic mercury (Hg2+), methylmercury (MeHg) and ethylmercury (EtHg). Without the use of other chemical reagents, the decomposition of organic mercury species and the reduction of Hg2+ could be completed in one step with the DBD plasma-CVG system. The operating parameters of DBD plasma-CVG and HPLC were optimized. Separation of mercury species was accomplished in less than 14 min on a ZORBAX SB-C18 separation column with a mobile phase containing 0.01% (v/v) 2-mercaptoethanol, 10% (v/v) methanol and 0.06 mol L−1 NH4Ac at pH 6.8. The limits of detection of the method were found to be 1.6, 0.42 and 0.75 μg L−1 for Hg2+, MeHg and EtHg, respectively. The developed method was validated by determination of the main analytical figures of merit and applied to the analysis of the certified reference material GBW10029 (tuna fish). The on-line interfacing of HPLC with DBD plasma-CVG-AFS for the speciation of mercury is simple, environmentally friendly, and represents an attractive alternative to the conventional tetrahydroborate interface system.

Battery-Operated Atomic Emission Analyzer for Waterborne Arsenic Based on Atmospheric Pressure Glow Discharge Excitation Source

In this paper, a sensitive atomic emission spectrometer (AES) based on a new low power and low argon consumption (<8 W, 100 mL min-1) miniature direct current (dc) atmospheric pressure glow discharge (APGD) plasma (3 mm × 5 mm) excitation source was developed for the determination of arsenic in water samples. In this method, arsenic in water was reduced to AsH3 by hydride generation (HG), which was then transported to the APGD source for excitation and detected by a compact CCD (charge-coupled device) microspectrometer. Different parameters affecting the APGD and the hydride generation reactions were investigated. The detection limit for arsenic with the proposed APGD-AES was 0.25 μg L-1, and the calibration curves were found to be linear up to 3 orders of magnitude. The proposed method was successfully applied to the determination of certified reference material (GBW08605), tap water, pond water, groundwater, and hot spring samples. Measurements from the APGD analyzer showed good agreement with the certified value/values obtained with well-established hydride generation atomic fluorescence spectrometry (HG-AFS). These results suggest that the developed robust, cost-effective, and fast analyzer can be used for field based arsenic determination and may provide an important tool for arsenic contamination and remediation programs.

On-site separation of Cr(VI) and Cr(III) in natural waters by parallel cartridge ion-exchange columns

A simple, fast, portable, and solvent-free method is developed for field separation of Cr(VI) and Cr(III) in natural waters. The method involves passing a water sample through parallel cation- and anion exchange resin cartridge columns at the field site. In a matter of seconds, all the Cr(III) is retained on the cation-exchange column and Cr(VI) passes to the effluent, while the Cr(VI) is quantitatively retained on the anion-exchange resin and Cr(III) passes to the effluent. The two collected solutions are preserved and determined later in the laboratory using inductively coupled plasma mass spectrometry (ICP-MS) or any other elemental analysis technique sufficiently sensitive to measure the Cr concentrations of interest. Cr(VI) (or Cr(III)) can be separated from Cr(III) (or Cr(VI)) at the pH range 1–10 at Cr(III)/Cr(VI) (or Cr(VI)/Cr(III)) concentration ratios as high as 10 000 (or 500). The limits of detection for Cr(III) and Cr(VI) are 0.03 and 0.01 μg L−1, respectively. Repeatability expressed as relative standard deviation is determined to be 7.1% for Cr(VI) and 5.7% for Cr(III), using a drinking water sample containing 1.14 μg L−1 Cr(VI) and 0.72 μg L−1 Cr(III). The parallel cartridge ion-exchange resin columns can be used continuously for 40 drinking water samples without any regeneration or cleaning procedure. The accuracy is validated by analysing Cr(III) and Cr(VI) in three water standard reference materials (GBW080257, GBW080403, and GBW080404). Furthermore, the proposed method is applied to the on-site separation of Cr(III) and Cr(VI) from natural waters.