Detecting selenium

In 1823 Johann Karl Ludwig Zincken (1790—1862) detected selenium in some ores from Zorge and Tilkerode in the eastern part of the Harz, and in 1825 Heinrich Rose analyzed them quantitatively. By heating them in a current of chlorine gas, Rose converted all the metals to chlorides and separated the selenium chloride, which was the only volatile chloride present, from the non-volatile chlorides of the metals (34). He found these minerals to be selenides of lead, copper, cobalt, and mercury. 

In 1826 Carl Kersten of Gottingen detected selenium in the capillary cuprite or so-called copper bloom from Rheinbreitenbach on the Rhine, which Councilor Hausmann had presented to him (39). He also found this element to be present in the earthy ferruginous cuprite (tile ore) from the same locality (39). 

A combination of IPC and inductively coupled plasma (ICP) MS was extensively explored for the speciation of phosphorus, arsenic, selenium, cadmium, mercury, and chromium compounds [108-118] because it provides specific and sensitive element detection. Selenium IPC speciation was joined to atomic fluorescent spectrometry via an interface in which all selenium species were reduced by thiourea before conventional hydride generation [119], Coupling IPC separation of monomethyl and mercuric Hg in biotic samples by formation of their thiourea complexes with cold vapor generation and atomic fluorescence detection was successfully validated [120]. The coupling of IPC with atomic absorption spectrometry was also used for online speciation of Cr(III) and Cr(VI) [121] and arsenic compounds employing hydride generation [122]. 

IPC-ICP-MS was extensively explored for the speciation of phosphorus, arsenic, selenium, cadmium, mercury, and chromium compounds [4-17] because it provides specific and sensitive element detection. Selenium IPC speciation took also advantage of coupling with atomic fluorescent spectrometry via an interface in which all selenium species are reduced by thiourea before conventional hydride generation... 

A separation of arsenite, arsenate and monomethylarsonate (MMA) is shown in Fig. 6.23. The detection limit for arsenic was 10 pg/L and the minimum detectable quantity was 100 pg. It was also possible to separate and detect selenium(IV) and (VI) under similar conditions. 

A recent survey conducted by the U.S. Food and Drug Administration (FDA) which analyzed foods consumed in the United States during the period of 1991-1999, detected selenium in 3,654 out of 6,671 food samples analyzed (FDA 2000). The results of this survey are summarized in Table 6-4. 

AAS techniques are commonly used for determination of selenium in environmental samples. Hydride generation AAS is more sensitive than flame or graphite furnace AAS for determination of selenium in materials of variable composition. Water samples, including freshwater, river water, sea water, and surface waters, and industrial wastes, muds, sediments, and soil samples have been analyzed by AAS techniques to detect selenium at parts-per-trillion levels (Bern 1981). Selenium(VI) and selenium(IV) can be distinguished in water samples with GFAAS by selective extraction procedures. HGAAS can also be used to distinguish between selenium(VI) and selenium(IV) in environmental samples because selenium(VI) does not readily form the hydride without reduction (Koirtyohann and Morris 1986). selenium(VI) is calculated on the basis of the total selenium minus selenium(IV) (Bern 1981). 

It is possible to detect selenium levels as low as 1 ng per cubic meter of air using neutron activation analysis. Standardized methods for selenium determination in different environmental samples such as water, soil, sludge, and industrial waste are available in the above-mentioned literature. 

Vydyanath and co-workers [8.80] have reported the development of selenium-doped Si for 3-5 pm detection selenium acts as a donor. The ionization energy of selenium was found from photoconductivity measurements to be 0.3 eV, corresponding to = 4.1 pm, They found the maximum solubility of selenium in Si to be slightly under 10 atoms/cm. ... 

The complexity of analyte matrixes and the low level of selenium compoimds even in em-iched samples make speciation difficult, but the combination of separation processes with selenium-specific detection is a powerful approach. High sensitivity is vital, and MS with an atmospheric pressure ionization source, such as the ICP, has proved successful for HPLC detection. Selenium presents problems due to moderate ionization efficiency and isobaric interferences, although these can be partly overcome with high-resolution mass spectrometers or dynamic reaction cell (DRC) technology. Significant isotopic overlap from " °Ar2 on the most abundant isotope Se (49.6%) may necessitate measurement of the less abundant isotopes Se (8.6%) or Se (7.6%) of total selenium. 

The presence of asymmetric carbons in selenomethionine, other a-selenoamino acids, and related compounds produces different chiral enantiomers with different physiological activities. HPLC separation of enantiomers is possible with a range of chiral stationary phases, d- and 1-Selenomethionine enantiomers have been resolved with an a-cyclodextrin stationary phase and other species with a teicoplanin-based chiral phase. Hybrid chiral methodologies based on GC, HPLC, and capillary electrophoresis, coupled with ICP-MS are feasible. Enantiomers of d,l-selenocystine, d,l-selenomethio-nine, and d,l-selenoethionine were examined in a range of commercial dietary supplements using a chiral crown ether stationary phase and ICP-MS detection. Selenium-em-iched onion, garlic, and yeast were analyzed and some of the selenoamino acid enantiomers were identified. l-Fluoro-2, 4-dinitro-phenyl-5-l-alanine amide was used to derivatize enantiomers of selenoamides for enhanced resolution. 

To detect selenium by the method presented here, it must be transformed into active [S...Se ] ions. If present as selenite, the conversion is easily accomplished by adding excess alkali sulfide The reaction ... 

Under practical conditions this can be measured/visualized by the difference in the detection of wire IQI s. The published results (e.g. [2,3,4]) show a difiference of approximately 1 - 2 wires below 10 mm of steel thickness and 1 wire above in favour of Selenium. 

Both pyrrole and indole react with selenium dioxide in the presence of nitric acid to give a deep violet solution. Very small quantities (ca 4 of pyrrole can be detected by this method. 

It is known that Selenium catalyzes reaction of some dye reduction by Sulphide. On this basis spectrophotometric and test-techniques for Selenium determination are developed. Inefficient reproducibility and low sensitivity are their deficiencies. In the present work, solid-phase reagent on silica gel modified first with quaternary ammonium salt and then by Indigocarmine was proposed for Selenium(IV) test-determination. Optimal conditions for the Selenium determination by method of fixed concentration were found. The detection limit of Se(IV) is 10 ftg/L = 2 ng/sample). Calibration curve is linear in the range 50-400 ftg/L of Se(IV). The proposed method is successfully applied to the Selenium determination in multivitamins and bioadditions. 

The stability of this derivative is attributed to the strong mesomeric effect of the NMca group, which changes the nature of the N=S bond. The selenium analogues RNSe have only been detected as transient species. 

The protein from D. desulfuricans has been characterized by Mbss-bauer and EPR spectroscopy 224). The enzyme has a molecular mass of approximately 150 kDa (three different subunits 88, 29, and 16 kDa) and contains three different types of redox-active centers four c-type hemes, nonheme iron arranged as two [4Fe-4S] centers, and a molybdopterin site (Mo-bound to two MGD). Selenium was also chemically detected. The enzyme specific activity is 78 units per mg of protein. 

The pollutants characteristic of the industry wastewaters are summarized in Table 5.4 through Table 5.11, for both classical and toxic pollutants. The toxic pollutant data have been developed using a verification protocol established by U.S. EPA, with the exception of the following selenium, silver, thallium, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCCD). Table 5.12 presents the minimum detection limit for the toxic pollutants. Any value below the minimum limit is listed in the summary tables as below detection limit (BDL). 

The detection of metal-binding proteins, especially of Cd- and Hg-binding metallothioneins or of the merR protein, induced numerous studies of model compounds of Cd and Hg with more or less simple sulfur- and selenium-containing ligands. 

The 77Se NMR spectra can be used to analyse the composition of a complex mixture of Se-N compounds. For example, 77Se NMR provides a convenient probe for analysing the decomposition of selenium(IV) diimides RN=Se=NR (e.g., R=tBu).30 An elegant application of 77Se NMR spectroscopy, in conjunction with 15N NMR spectroscopy, involved the detection of the thermally unstable eight-membered rings (RSeN)4.31... 

Raman spectroscopy is a useful probe for detecting transannular S - S interactions in bicyclic or cage S-N molecules or ions. The strongly Raman active vibrations occur at frequencies in the range 180-300 cm-1, and for S- -S distances in the range 2.4-2.7 A. On the basis of symmetry considerations, the Raman spectrum of the mixed sulfur-selenium nitride S2Se2N4 was assigned to the 1,5- rather than the 1,3- isomer.37... 

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