| Heavy metals are serious pollutants because of their toxicity, persistence, and non-degradability in the environment. It is well known that mercury and cadmium are the extremely toxic elements even at low concentration levels and can be accumulated in the food chain, impacting on human health. Futhermore, the toxicity, bioavailability, and mobility of mercury not only depend on its concentration but also significantly depend on its chemical form. Therefore, the the analysis of mercury and cadmium as well as speciation analysis of mercury are of great importance to evaluate their toxicity to environmrnt and human health.The most often used analytical method to determine Hg and Cd is atomic spectrometry coupled with chemical vapor generation (CVG) technique. The CVG technique improves the transport efficiency of the analytes, and reduces matrix effects, which are benefit for atomic spectrometry. However, the conventional tetrahydroborate-based CVG technique is subjected to interferences from transition metals. In addition, the reagent NaBH4is expensive; its aqueous solution is unstable. Futhermore, the reagent NaBH4is a potential source of contamination. Great efforts have been needed to develop novel, high efficiency and pollution-free vapor generation technique.Generally, the combination of chromatographic separation techniques such as gas chromatography, high performance liquid chromatography, and capillary electrophoresis with atomic spectrometry is a practical approach for mercury speciation analysis. An obvious advantage of chromatographic methods is the ability to distinguish between different mercury species in samples. However, a common disadvantage of these methods is that some complex and tedious pretreatment procedures are often involved in the separation of organic mercury. In addition, the instruments are expensive and the operation cost is high. Hence, the development of the simple, fast and inexpensive speciation technique is still attractive.Recently, microplasmas have attracted attention because of their several unique advantages such as small size, reduced gas and power consumption, low manufacturing cost and simple construction. The radicals, electrons, and energetic charged or excited particles involved in plasma could simulate various chemical reactions, some of which are not typical in conventional chemistry. New measurement methods based on microplasma are of growing interest in the field of analytical spectrometry. For example, microplasma has been used as a low-temperature atom reservoir and excitation source. However, few sample introduction techniques based on microplasma have been developed. Due to the presence of radicals, which can simulate oxidation/reduction reactions, in the plasma, plasma induced vapor generation, which voids the use of chemical reduction/oxidation reagents, is a green sample introduction technique.In the present dissertation, novel sample introduction techniques based on microplasma were developed. The main contents of the present dissertation are as follows:1. A low-temperature dielectric barrier discharge (DBD)-plasma induced vaporization technique was proposed and was used for the determination of mercury. The cylindrical DBD plasma reactor comprises two major parts:an inner quartz tube with a copper electrode in it and an outer quartz tube wrapped with copper foil as another electrode. The DBD plasma was generated concentrically in-between two quartz tube. In this work, Hg2+,methylmercury (MeHg) and ethylmercury (EtHg) were used as model analytes. Sample solution was applied onto the outer surface of the inner tube.The evaporation and atomization of dissolved mercury species in the sample solution can be achieved rapidly in one step, allowing mercury to be directly detected by atomic fluorescence spectrometry. The effects of operating parameters such as plasma power, plasma gas identity, plasma gas flow rate and interferences from concomitant elements have been investigated. The sensitivities of Hg2+, methylmercury (MeHg) and ethylmercury (EtHg) were found to be negligible difference in the presence of formic acid (≥1%v/v). The analytical performances of the present technique were evaluated under optimized conditions. The limits of detection were calculated to be0.02ng mL-1for Hg2+, MeHg and EtHg, and repeatability was6.2%,4.9%and4.3%RSD (n=11) for1ng mL-1of Hg2+, MeHg and EtHg, respectively. It provides a simple mercury detection method for small-volume samples with an absolute limit of detection at femtogram level. The proposed method was applied to direct analysis of trace mercury in simulated water matrix, urine and fish standard reference materials, and the concentration of mercury determined by the present method agreed well with the reference values. The satisfactory results indicate that the proposed method has great potential for the determination of trace or ultra-trace level mercury in various environmental samples.2. 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 a300-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 Hg2+methylmercury (MeHg) and phenylinercury (PhHg) deposited directly on TLC plate. The detection limits of the proposed PJDA-AFS method for inorganic Hg2+, MeHg and PhHg were0.51,0.29and0.34pg, respectively, and repeatability was4.7%,2.2%and4.3%for10pg Hg2+MeHg and PhHg. The proposed PJDA-AFS was also successfully coupled to TLC for mercury speciation. Under optimized conditions, the measurements of Hg2+,MeHg and PhHg could be achieved within3minutes. The detection limits for Hg2+, MeHg and PhHg were3.1,8.7and6.0pg. The combination of TLC with PJDA-AFS provides a simple, cost-effective, relatively high-throughput way for mercury speciation.3. 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 (1min sample-1). In the proposed method, with0.01%(m/v) NaBH4used 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 NaBH4concentration, HCl concentration, discharge gas, 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 be0.35ng mL-1and0.54ng mL-1, respectively. The proposed method was validated by the analysis of 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.4. A novel vapor generation approach was developed for Cd determination based on plasma electrochemical process. In this approach, a glow discharge microplasma, which was generated between the tip of a stainless steel capillary and the liquid (i.e. electrolyte) surface, acted as a gaseous cathode to initiate the vapor generation of Cd. Cadmium ions could be converted into molecular cadmium species (presumably CdH2) efficiently at the plasma-liquid interface from a supporting electrolyte (HCl,pH=3.2). The overall efficiency of the plasma-liquid electrochemical vapor generation (PLEVG) system for Cd vapor generation was estimated to be2.3times higher than the conventional HCl-KBH4-thiourea-Co2+system. The proposed PLEVG method is a green vapor generation method, which requires no hydrogen gas or any reaction chemical reagents; it is hypothesized that the cadmium ion is reduced by electrons in the plasma. The effects of operating parameters such as electrolyte pH, discharge current, carrier gas flow rate, hydrogen flow rate and sample flow rate were investigated. Under the optimized conditions, the detection limit of Cd was found to be0.003μg L-1; good repeatability (relative standard deviation (RSD)=2.4%, n=5) was obtained for a0.1μg L-1Cd2+standard. The proposed method was applied to direct analysis of trace Cd in a series of rice, soil and sediment standard reference materials and the satisfactory results showed that it has great potential for the direct determination of trace or ultra-trace levels of cadmium in environmental and geological samples. |
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