| Heavy metals are serious pollutants because of their toxicity, persistence, and non-degradability in the environment. For example, mercury and cadmium are well known to be two of the most hazardous elements to human health. Mercury is considered a global pollutant as the element and many of its compounds are highly toxic and readily released into the environment because of their high volatility and mobility. Mercury pollution is recognized as a primary environmental issue and public health problem. Cadmium is another toxic metal with a biological half-life in the range of10-30years, mainly accumulates in the environment throughout the food chain. The emission of cadmium into the environment will damage the human body. Many international guidelines have limited maximum allowable contents of Hg and Cd in water and food. Owing to the extremely low concentration of Hg and Cd in natural water, environmental and biological samples, very sensitive analytical methods are often required for accurate determination.The most used technique worldwide for the determination of mercury and cadmium is chemical vapor generation, because of its simplicity, high sensitivity, and its ability to separate the analyte from complex matrixes. Hg(II) or Cd(II) can be effectively converted into Hg(0) or Cd(0) by chemical reductants such as NaBH4, and subsequently swept by a carrier gas into the detection system for determination. However, the drawbacks of chemical vapor generation, including chemical interferences from transition metals, unstable reductant solution, vigorous chemical reactions that can result in liquid transport to the atomization cell and significant waste production. Development of new vapor generation systems that may replace or obviate the use of chemical reagents remains an attractive research topic.Up to now, several novel vapor generation methods including electrochemical hydride generation (ECHG), photochemical vapor generation (photo-CVG), and ultrasound-induced chemical vapor generation (SI-CVG) are developed. ECHG is one of the first successful methods utilizing electrons as a suitable alternative to SnCl2or NaBH4for the reduction process. But this method requires careful selection of the cathode material, has poor reproducibility, and serious memory effects arising from reduced transition metals deposited on the cathode surface. Photo-CVG is a promising new technique for vapor generation with which volatile species of mercury, hydrideforming elements, transition and noble metals could be generated through UV radiation to their aqueous solutions in the presence of low molecular weight organic acid. This Photo-CVG approach processes advantages over conventional chemical vapor generation such as simpler reaction, cost-effectiveness and greener analytical chemistry. SI-CVG is another emerging vapor generation method. It is accomplished with formic acid as the sole chemical reagent. This reagent partly decomposes on ultrasound irradiation to yield reducing gases and radicals which promotes vapor generation. A drawback inherent with SI-CVG is that the presence of oxidizing substances in the sample which precludes the reduction process of the analyte by ultrasound. In addition, it is used only in batch system, and is difficult to couple with flow injection.Plasma chemical vapor generation is an emerging green technique for production of volatile species which offers several advantages. First, it eliminates the need of both strong acid/oxidation reagents (for pre-oxidation) and expensive reducing agents, thus significantly reduces the analysis time and possible sample contamination; second, the interferences from transition and noble metal ions can be minimized or even diminished; third, simplicity, speed, cost and environment protection. Solution cathode glow discharge (SCGD) hsa been used for plasma vapor generation. Without needing for a chemical agent, mercury species are converted to volatile Hg vapor in the solution cathode glow discharge and then transported to the detecting system for measurements. The method is rapidly in vapor generation, sensitive and simple in operation, ease of coupling to injection flow system. It has been used for mercury, iodine and osmium for vapor generation.Dielectric barrier discharge is a non-thermal plasma which includes two electrodes and at least one dielectric barrier is present. The discharge can occur between the two electrodes when supplying high ac voltage to the electrodes. DBD has many attractive advantages including atmospheric pressure working condition, simplicity in fabrication, low power consumption, high dissociation ability and low cost for construction and maintenance. All these make it widely used in industry and very promising for its applications in analytical spectroscopy. In this paper, a novel plasma vapor generation technique based on dielectric barrier discharge plasma was developed. Mercury (including organic mercury) or cadmium can be easily converted into its volatile species. The proposed method was successfully used for quantification of ultra-trace level mercury and cadmium in samples. The main contents of the present dissertation are as follows:1. A novel vapor generation method for mercury based on atmospheric pressure dielectric barrier discharge (DBD) plasma coupling to atomic fluorescence spectrometry (AFS) has been developed. It is a green method that obviates the need of strong oxidization procedure for degradation of organic mercury compounds (thiomersal) to inorganic mercury and reduction agents to generate Hg vapor. The evaporation and atomization of thiomersal was achieved rapidly in one step and then the generated mercury vapor was transported to AFS for analysis. It eliminates the use of any other chemical reagents, so avoids the interference caused by the unstable reducing agents, thus significantly reduces the pollution and also the cost. The effect of analytical figures such as discharge gas type, gas flow rate, and power were optimized. To evaluate the matrix effects on the performance of the proposed method, the influences of several coexisting ions on the fluorescence signal was investigated. The analytical figures of merit of the proposed method were evaluated under optimized conditions. We use this method 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 with our results.2. Direct determination of cadmium by atomic fluorescence spectrometry with dielectric barrier discharge plasma induced vapor generation has been developed. The dielectric barrier discharge generator in this chapter is similar to that mentioned above, flow injection syetem is involved in this technique. Solution is pumped to the DBD reactor with the aid of the peristaltic pump, which form a thin film in the reactor. An extra current of hydrogen is adding to the discharge gas to act as reaction gas. In the hydrogen-containing DBD plasma, Cd2+are readily converted to volatile species by reaction with hydrogen, and then transported to AFS for measurements.The formation of Cd vapor do not need the adding of reduction reagents, Cd(II) can convert to vapor in one step by the DBD plasma. The proposed method is simple, can be easily fabricated, and has low average power consumption (≤18W) and sample analysis is fast and can be completed within1min, and it can be served as a novel and greener vapor generation method for Cd. In this study, we evaluated the effects of several experimental parameters on the vapor generation of Cd. Possible interference by concomitant elements was also investigated. The analytical figures of the technique were evaluated under the optimal experimental condition. The approach was applied to analysis of Cd in reference material of water sample and a rice sample, determined values by the present method agreed well with the reference values.A novel plasma vapor generation technique for mercury and cadmium was developed based on DBD plasma. Thiomersal can be readily converted to Hg vapor in the DBD plasma without the need of oxidation/reduction agents. And it is the first time for this method using for Cd vapor generation. Without adding other chemical agents, the Cd2+is directly converted into volatile Cd vapor through a reaction with the molecular hydrogen gas in the DBD plasma; such a direct reduction process is not readily achievable in conventional chemistry. Compared to other CVGs, DBD plasma-CVG offers several advantages.1) It is a green analytical technique since it eliminates the use of unstable sodium tetrahydroborate reducing reagent and high-purity acids, which in turn reduces possible contamination from reagents and minimizes the generation of hazardous waste.2) The vapor-generation efficiency is high.3) The analysis is fast and cost-effective. Also, it is highly stable and simple in operation. Ongoing work is being performed to further investigate the underlying mechanism of the plasma-CVG process, and to extend this technique to other elements. |
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