Selenium stable isotopes

Herbel MJ, Johnson TM, Oremland RS, Bullen TD (1998) Selenium stable isotope fractionation during bacterial dissimilatory reduction of selenium oxyanions (Abstract). EOS, Transact. A.G.U. 79, Suppl. F356... 

Johnson TM, Bullen TD (2003) Selenium isotope fractionation during reduction by Fe(ll)-Fe(lll) hydroxide-sulfate (green rust). Geochim Cosmochim Acta 67 413-419 Johnson TM, Bullen TD, Zawislanski PT (2000) Selenium stable isotope ratios as indicators of sources and cycling of selenium Results from the northern reach of San Francisco Bay. Environ Sci Tech 34 ... 

HerbelM. J., Johnson T. M., Tanji K. K., Gao S. D., and Bullen T. D. (2002b) Selenium stable isotope ratios in California agricultural drainage water management systems. J. Environ. Qual. 31, 1146-1156. 

Selenium stable isotopes Selenium is recognized as an essential trace element for humans. Se status is determined by dietary Se intake and its bioavail-ability. Se bioavailability can be estimated by assessing absorption and retention of Se stable isotopes. These studies require analytical techniques that allow precise and accurate determination of stable isotope ratios at low levels of total selenium. 

Naturally selenium contains six stable isotopes. Fifteen other isotopes have been characterized. The element is a member of the sulfur family and resembles sulfur both in its various forms and in its compounds. 

Abundances of lUPAC (the International Union of Pure and Applied Chemistry). Their most recent recommendations are tabulated on the inside front fly sheet. From this it is clear that there is still a wide variation in the reliability of the data. The most accurately quoted value is that for fluorine which is known to better than I part in 38 million the least accurate is for boron (1 part in 1500, i.e. 7 parts in [O ). Apart from boron all values are reliable to better than 5 parts in [O and the majority arc reliable to better than I part in 10. For some elements (such as boron) the rather large uncertainty arises not because of experimental error, since the use of mass-spcctrometric measurements has yielded results of very high precision, but because the natural variation in the relative abundance of the 2 isotopes °B and "B results in a range of values of at least 0.003 about the quoted value of 10.811. By contrast, there is no known variation in isotopic abundances for elements such as selenium and osmium, but calibrated mass-spcctrometric data are not available, and the existence of 6 and 7 stable isotopes respectively for these elements makes high precision difficult to obtain they are thus prime candidates for improvement. 

Selenium, Te and Po are the three heaviest members of Group 16 and, like their congenors O and S, have two p electrons less than the next following noble gases. Selenium is normally said to have 6 stable isotopes though the heaviest of these ( Se, 8.73% abundant) is actually an extremely long-lived fi emitter. 

Eighteen isotopes of sulfur, 17 of selenium, 21 of tellurium, and 27 of polonium have been registered of these, 4 sulfur, 6 selenium, and 8 tellurium isotopes are stable, while there is no stable isotope of polonium. None of the naturally occurring isotopes of Se is radioactive its radioisotopes are by-products of the nuclear reactor and neutron activation technology. The naturally occurring, stable isotopes of S, Se, and Te are included in Table 1.2. 

Table 1.2 Naturally occurring, stable isotopes of sulfur, selenium, and tellurium...

Ellis AS, Johnson,TM, Bullen TD (submitted) Using chromium stable isotope ratios to quantify Cr(VI) reduction lack of sorption effects. Environ Sci Technol submitted 9/2003 Ellis AS, Johnson TM, Bullen TD, Herbel MJ (2003) Stable isotope fractionation of selenium by natural microbial consortia. Chem Geol 195 119-129... 

Johnson TM (in press) A review of mass-dependent fractionation of selenium isotopes and implications for other heavy stable isotopes. Chem Geol... 

ISOTOPES There are a total of 35 isotopes of selenium. Five of these are stable, and a sixth isotope has such a long half-life that it is also considered stable Se-82 = 0.83x10+20 years. This sixth isotope constitutes 8.73% of selenium s abundance in the Earth s crust, and the other five stable isotopes make up the rest of selenium s abundance on Earth. 

Silver is a white, ductile metal occurring naturally in its pure form and in ores (USEPA 1980). Silver has the highest electrical and thermal conductivity of all metals. Some silver compounds are extremely photosensitive and are stable in air and water, except for tarnishing readily when exposed to sulfur compounds (Heyl et al. 1973). Metallic silver is insoluble in water, but many silver salts, such as silver nitrate, are soluble in water to more than 1220 g/L (Table 7.3). In natural environments, silver occurs primarily in the form of the sulfide or is intimately associated with other metal sulfides, especially fhose of lead, copper, iron, and gold, which are all essentially insoluble (USEPA 1980 USPHS 1990). Silver readily forms compounds with antimony, arsenic, selenium, and tellurium (Smith and Carson 1977). Silver has two stable isotopes ( ° Ag and ° Ag) and 20 radioisotopes none of the radioisotopes of silver occurs naturally, and the radioisotope with the longest physical half-life (253 days) is "° Ag. Several compounds of silver are potential explosion hazards silver oxalate decomposes explosively when heated silver acetylide (Ag2C2) is sensitive to detonation on contact and silver azide (AgN3) detonates spontaneously under certain conditions (Smith and Carson 1977). 

Unlike its lower homologues, sulfur, selenium, and tellurium, polonium has no long-lived or stable isotopes. It has, in fact, one of the most unstable nuclei of naturally occurring elements, the only readily accessible isotope being that of mass 210 this decays by alpha emission with a half-life of 138.4 days and occurs in nature as the penultimate member of the radium decay series, the last three stages being... 

P. B. Moser-Veillon, A. R. Mangels, K. Y. Patterson, C. Veillon, Utilization of two different chemical forms of selenium during lactation using stable isotope tracers an example of speciation in nutrition, Analyst, 117 (1992), 559D562. 

Numerous trace elements are known to be nutritionally essential in man In order to assess the essentiality, dietary availability, and metabolic fate of these, means of labeling for subsequent identification are needed In animal studies, radioisotopes are often used for this purpose, but their use in human studies is generally contraindicated due to the radiation hazards An alternate method is to use stable isotopes of the elements, which overcomes this limitation A method will be described for conveniently measuring the stable isotopes of selenium, permitting their use as metabolic tags in tracer studies Using one stable isotope as the tracer and another as internal standard, one can quantitatively identify in a sample the tracer, natural (unenriched) selenium present with it, and total selenium Some of the kinds of information obtainable from metabolic tracer studies will be discussed ... 

Stable Isotopes of selenium (as well as those of other elements) can provide a means of addressing these questions by their employment In metabolic tracer studies The same Information can be obtained as when employing radiotracers In animal studies, but without the associated radiation hazards In human studies For example, stable Isotopes of selenium can be biologically Incorporated Into test foods and these used to monitor selenium bloavallablllty (6,7) ... 

Described herein Is a convenient, accurate, sensitive and rapid method for measuring stable Isotopes of selenium In biological materials A rapid sample preparation technique Is used which does not require perchloric acid, with Its associated dangers Based on Isotope dilution techniques and Isotope ratio measurements, the method employs one enriched stable Isotope as the Internal standard and another as the metabolic tag This permits the quantitative measurement of the enriched tracer, unenrlched (natural) selenium present with It, and total selenium In the samples Some examples of the type of Information that can be obtained with these techniques will be described ... 

Total Selenium As mentioned earlier, stable isotope dilution is a powerful tool in trace element analysis. Let us first look at how it can be used to determine the total selenium content of a sample. In the following section we will develop the method further for stable isotopes in metabolic tracer studies. 

As it occurs in nature, selenium conslts of six stable isotopes with the relative abundances shown in Table I. 

When Se-NPD is introduced into the mass spectrometer, the most intense group of ions is that of the parent ion, Se-NPD+. This is illustrated in Figure 1 for natural (unenriched) Se-NPD. Six peaks are observed, corresponding to Se-NPD containing each of the six Se stable isotopes. By adding a known amount of enriched se (spike) to the samples, and monitoring the observed isotope ratio of, say, Se/ 2se, we can readily calculate the amount of natural selenium that had to be present in the sample. 

Let us now consider some of the types of information one can obtain using stable isotopes in metabolic studies. In essence, one can obtain the same information as with radioisotopes, but with less convenient measurement. Stable Isotopes also have the advantage of an "infinite" half-life, permitting long term studies. For an element like selenium with six stable Isotopes, more than one can be used in the same experiment at the same time. 

Selenium is in group 16 of the periodic table and although it has chemical and physical properties intermediate between metals and nonmetals (Table 1), it is usually described as a nonmetal. The chemical behavior of selenium has some similarities to that of sulfur. Formally, selenium can exist in the —II, 0, IV, and VI oxidation states (Table 2). Selenium has six natural stable isotopes, the most important being Se and °Se. Although Se is generally also regarded as a stable isotope, it is a )8-emitter with a very long half-life (1.4 X 10 ° yr). Both arsenic and selenium tend to be covalently bonded in all of their compounds. 

CAS 13494-80-9. Te. A nonmetallic element with many properties similar to selenium and sulfur. Atomic number 52, group VIA of the period table, aw 127.60, valences of 2, 4, 6 eight stable isotopes. 

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