Photochemical reduction of Hg(II) by various low-molecular-weight organic compounds (LMWOC) was investigated to evaluate the effect of specific functional groups that are typically encountered in natural dissolved organic matters (DOM) on the photoreactivity and isotope fractionation of Hg. LMWOC with reduced sulfur functional groups (e.g., cysteine, glutathione) resulted in slower photochemical reduction of Hg(II) than those without reduced sulfur groups (e.g., serine, oxalic acid). Reduction rate constants were specifically determined for two contrasting LMWOC: dl-serine (0.640 h−1) and l-cysteine (0.047 h−1). Different mass independent isotope effects of Hg were induced by the two types of LMWOC. S-containing ligands specifically enriched magnetic isotopes (199Hg and 201Hg) in the product (Hg(0)) while sulfurless ligands enriched 199Hg and 201Hg in the reactant (Hg(II)), suggesting that opposite magnetic isotope effects were produced by different types of ligands. The nuclear field shift effect was als...

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Isotope fractionation of mercury during its photochemical reduction by low-molecular-weight organic compounds.

Separation, preconcentration and inductively coupled plasma-mass spectrometric (ICP-MS) determination of thorium(IV), titanium(IV), iron(III), lead(II) and chromium(III) on 2-nitroso-1-naphthol impregnated MCI GEL CHP20P resin

A novel vapor-generation technique is described for mercury determination in aqueous solutions. Without need for a chemical reducing agent, dissolved mercury species are converted to volatile Hg vapor in a solution cathode glow discharge. The generated Hg vapor is then transported to an inductively coupled plasma for determination by atomic emission spectrometry. Mercury vapor is readily generated from a background electrolyte containing 0.1 M HNO 3. Vapor generation efficiency was found to be higher by a factor of 2-3 in the presence of low molecular weight organic acids (formic or acetic acids) or alcohols (ethanol). Optimal conditions for discharge-induced vapor generation and reduced interference from concomitant inorganic ions were also identified. However, the presence of chloride ion reduces the efficiency of Hg-vapor generation. In the continuous sample introduction mode, the detection limit was found to be 0.7 microg L (-1), and repeatability was 1.2% RSD ( n = 11) for a 20 microg L (-1) standard. In comparison with other vapor generation methods, it offers several advantages: First, it is applicable to both inorganic and organic Hg determination; organic mercury (thiomersal) can be directly transformed into volatile Hg species without the need for prior oxidation. Second, the vapor-generation efficiency is high; the efficiency (with formic acid as a promoter) is superior to that of conventional SnCl 2-HCl reduction. Third, the vapor generation is extremely rapid and therefore is easy to couple with flow injection. The method is sensitive and simple in operation, requires no auxiliary reagents, and serves as a useful alternative to conventional vapor generation for ultratrace Hg determination.

Use of a solution cathode glow discharge for cold vapor generation of mercury with determination by ICP-atomic emission spectrometry.

Recently, cloud point extraction (CPE) has been an attractive subject as an alternative to liquid-liquid extraction. The technique is based on the property of most non-ionic surfactants in aqueous solutions to form micelles and become turbid when heated to the cloud point temperature. This review covers a selection of the literature published on applications of CPE in determination of metal ions over the period between 2004 and 2008.

Separation and preconcentration by a cloud point extraction procedure for determination of metals: an overview.

Determination of cadmium and copper in water and food samples by dispersive liquid–liquid microextraction combined with UV–vis spectrophotometry

With UV irradiation, Hg(2+) in aqueous solution can be converted into Hg(0) cold vapor by low molecular weight alcohols, aldehydes, or carboxylic acids, e.g., methanol, formaldehyde, acetaldehyde, glycol, 1,2-propanediol, glycerol, acetic acid, oxalic acid, or malonic acid. It was found that the presence of nano-TiO(2) more or less improved the efficiency of the photo-induced chemical/cold vapor generation (photo-CVG) with most of the organic reductants. The nano-TiO(2)-enhanced photo-CVG systems can be coupled to various analytical atomic spectrometric techniques for the determination of ultratrace mercury. In this work, we evaluated the application of this method to the atomic fluorescence spectrometric (AFS) determination of mercury in cold vapor mode. Under the optimized experimental conditions, the instrumental limits of detection (based on three times the standard deviation of 11 measurements of a blank solution) were around 0.02-0.04 microg L(-1), with linear dynamic ranges up to 15 microg L(-1). The interference of transition metals and the mechanism of the photo-CVG are briefly discussed. Real sample analysis using the photo-CVG-AFS method revealed that it was promising for water and geological analysis of ultralow levels of mercury.

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Photo-induced cold vapor generation with low molecular weight alcohol, aldehyde, or carboxylic acid for atomic fluorescence spectrometric determination of mercury.

The surface chemistry of TiO2 as well as the powerful adsorption capability of the nanometre material was used to separate inorganic chromium species in this work. When the pH of the sample solution is lower than the isoelectric point of TiO2, its surface is positively charged and anions of Cr(VI) can be adsorbed; on the contrary, the surface is negatively charged and cations of Cr(III) can be caught. Maximum adsorption rates of 34.5% and 95.4% for Cr(VI) at pH 3 and Cr(III) at pH 6 were obtained, respectively. Possible adsorption mechanisms were discussed considering the surface chemistry of TiO2 and the coordination chemistry of Ti, O and Cr. A simple and automated chromium speciation method was therefore developed using flow injection minicolumn separation and electrothermal atomic absorption spectrometric detection (ET-AAS). The adsorption and elution were incorporated in a flow injection program for selective preconcentration of Cr(III) or Cr(VI). As a result, no extra oxidizing/reducing process was necessary for an automated chromium speciation analysis. Factors influencing adsorption and elution of inorganic chromium species were carefully investigated. Detection limits as low as 0.01 µg L−1 for Cr(VI) and 0.006 µg L−1 for Cr(III) were achieved. The proposed method is highly sensitive and selective for inorganic chromium speciation analysis of drinking water.

Exploring surface chemistry of nano-TiO2 for automated speciation analysis of Cr(III) and Cr(VI) in drinking water using flow injection and ET-AAS detection

A novel displacement-cloud point extraction approach was developed for the selective determination of trace silver in complicated matrices by thermospray flame furnace atomic absorption spectrometry. This method involves two steps of cloud point extraction, i.e., firstly, copper ion reacts with diethyldithiocarbamate (DDTC) to form Cu-DDTC before it is extracted; secondly, after removing the aqueous phase, a sample or standard solution containing silver ion is added and another cloud point extraction procedure is carried out. Because the stability of Ag-DDTC is larger than that of Cu-DDTC, Ag+ can displace Cu2+ from the pre-extracted Cu-DDTC and thus the preconcentration and separation of Ag+ from complicated sample matrix is achieved. Potential interference from co-existing transition metal ions with lower DDTC complex stability was largely eliminated as they cannot displace Cu2+ from Cu-DDTC complex. Up to 5000 μg mL−1Ni(II), 2000 μg mL−1 Cd(II), 1000 μg mL−1Fe(III), 2000 μg mL−1Co(II), 5000 μg mL−1Zn(II) and 5000 μg mL−1Mn(II) caused no significant interference with the determination of silver of 5 μg L−1. Compared with conventional cloud point extraction, the tolerance limit for the co-existing transition metal ions was increased by at least two orders of magnitude. Under the optimal chemical and instrumental conditions, the limit of detection was 0.2 μg L−1 for silver with a sample volume of 10 mL, and a sensitivity enhancement factor of 21 was achieved. The proposed method was successfully applied to interference-free determination of trace silver in soil, marine sediment and ore samples with high contents of co-existing heavy metals.

Selective determination of trace amounts of silver in complicated matrices by displacement-cloud point extraction coupled with thermospray flame furnace atomic absorption spectrometry

A new and sensitive method is described for the on-line preconcentration and determination of ultra-trace mercury by coupling a trapping scheme based on coating gold nanorods onto the inner wall of the graphite furnace for mercury vapor from photochemical vapor generation (photo-CVG) and graphite furnace atomic absorption spectrometric (GF-AAS) detection. The optimal experimental conditions for the photo-CVG, trapping and atomization were carefully investigated, together with the interference from transition metals. The limit of detection of this method was found to be 0.02 μg L−1 and the precision was better than 5% at 0.1 μg L−1. Mercury in four spiked water samples and two certified reference materials was successfully determined by the proposed method.

Guanglei Cheng

Papers

Photo-induced cold vapor generation with low molecular weight alcohol, aldehyde, or carboxylic acid for atomic fluorescence spectrometric determination of mercury.

Exploring surface chemistry of nano-TiO2 for automated speciation analysis of Cr(III) and Cr(VI) in drinking water using flow injection and ET-AAS detection

Selective determination of trace amounts of silver in complicated matrices by displacement-cloud point extraction coupled with thermospray flame furnace atomic absorption spectrometry

Photochemical vapor generation and in situ preconcentration for determination of mercury by graphite furnace atomic absorption spectrometry