Metallic trace element reduction analyses

The largest group of elements comprises those isolated from solution in the elemental form as a result of reduction, usually electrochemical. In acid solution, the electrolytic deposition of metal on a solid cathode is limited to noble and semi-noble metals. Trace analysis of copper and its compounds may serve as an example [100]. An anodic dissolution technique may be applied for the isolation of macroscopic amounts of copper. A sample in the form of a bar, plate, or wire is the anode in the electrolytic system. When current is passed through the electrolyte (nitric acid + persulphate), Cu is deposited on the graphite cathode, while most trace elements accumulate in the solution. In the trace analysis of platinum, the matrix has been also separated on a cathode [101]. 

The introduction of EU directives on Waste Electrical and Electronic Equipment and Reduction of Hazardous Substances has highlighted the need for precise and repeatable elemental analysis of heavy metals in the plastics production process. X-ray fluorescence (XRF) spectroscopy has emerged as the most economical and effective analytical tool for achieving this. A set of certified standards, known as TOXEL, is now available to facilitate XRF analyses in PE. Calibration with TOXEL standards is simplified by the fact that XRF is a multi-element technique. Therefore a single set of the new standards can be used to calibrate several heavy elements, covering concentrations from trace level to several hundred ppm. This case study is the analysis of heavy metals in PE using an Epsilon 5 XRF spectrometer. 

All metals at trace concentration, or in trace quantities, can be analyzed by atomic absorption (AA) spectrophotometry in flame or graphite furnace (electrothermal reduction) mode. A rapid, multi-element analysis may use... 

Trace metals have been measured in various tissues by ICP-MS to investigate Alzheimer s disease [249-252]. Various sample preparation and processing approaches have been used, including flow injection analysis and extraction. Al, Si, and Sn levels were reported to be higher than in healthy tissue, whereas zinc and selenium concentrations were lower. In the temporal cortex there were also reductions of cesium and cerium concentrations. The mechanisms responsible and the key elements remain incompletely understood. 

Gold, silver, mercury, and platinum metals, as well as Se and Te, can be precipitated from acid solution in the elemental form by reduction with chemical reagents such as zinc, NH2OH, N2H4, SO2, or formic acid. In the trace analysis of high purity mercury the sample (about 100 g) is dissolved in HNO3 and the solution is warmed in the presence of formic acid. First of all, nitric acid, then mercury, is reduced. The mercury forms a separate liquid phase, and the impurities remain in the aqueous solution [102]. In the trace analysis of silver, the sample is dissolved in nitric acid, then formic acid and mercury are added. The silver liberated on reduction dissolves in the mercury to form an amalgam [102]. 

A wide variety of inorganic materials have been used to precipitate or collect trace metals from solution. The most direct approach is a cementation process, which is one that removes the trace pollutants from solution by reduction with a metal and plating onto that metal surface. Although this process may be slow, the filtration is usually quick, since decantation is often sufficient. Finely divided cadmium extracts copper, selenium, and mercury from nitric and sulfuric acid solutions (66). When copper was used to preconcentrate mercury from water or biological fluids prior to atomic absorption analysis, the detection limit was 1-2 X 10 g (67, 68). Iron (69), zinc (70), and tungsten (71), as metals, have also been used to obtain a deposit of several trace metals from aqueous systems as dilute as 10 ppb for subsequent analysis. Elemental tellurium can be produced in solution by reduction using tin(II) chloride or sulfur dioxide, and coprecipitates silver (72) and selenium (73). Granulated silicon-metal alloys were used to remove metal ions from water and brine by reduction as well (74, 75). 

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