Isotope effects with sulfur

An important criticism of the use of combustion trains is that combustion is not site specific, that is all atoms in the analyte end up in the gas transferred to the IRMS. For studies of carbon isotope effects this is invariably C02. The question is especially important for carbon isotope analysis because analyte molecules of interest usually contain several different kinds of carbon atoms and therefore combustion methods average or dilute the IE s of interest. Should site specific isotope ratios be required another method of sample preparation (usually much more tedious) is necessary. Combustion methods, however, are frequently used to study nitrogen and sulfur IE s because many organic molecules are singly substituted with these atoms. Obviously, oxygen isotope effects cannot be determined using combustion trains because external oxygen is employed. Rather some type of pyrolytic sample preparation is required. 

Bruchert V, Knoblauch C, Jorgensen BB (2001) Controls on stable sulfur isotope fractionation during bacterial sulfate reduction in Arctic sediments. Geochim Cosmochim Acta 65 763-776 Bryan BA, Shearer G, Skeeters JL, Kohl DH (1983) Variable expression of the nitrogen isotope effect associated with denitrification of nitrate. J Biol Chem 258 8613-8617 Canfield DE (2001) Biogeochemistry of sulfur isotopes. Rev Mineral Geochem 43 607-636 Chau YK, Riley JP (1965) The determination of selenium in sea water, silicates, and marine organisms. Anal Chim Acta 33 36-49... 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

The unexpected formation of the blue crystalline radical cation (97) from the reaction of triazinium salt (98) with tetracyanoethylene has been reported and the product identified by its EPR spectrum and by X-ray crystallography (Scheme 42).199 Carboxylic acids react with the photochemically produced excited state of N-t-a-phenynitrone (PBN) to furnish acyloxy spin adducts RCOOPBN. The reaction was assumed to proceed via ET oxidation of PBN to give the PBN radical cation followed by reaction with RCO2H.200 The mechanism of the protodiazoniation of 4-nitrobenzenediazonium fluoroborate to nitrobenzene in DMF has been studied.201 Trapping experiments were consistent with kinetic isotope effects calculated for the DMF radical cation. The effect of the coupling of radicals with different sulfur radical cations in diazadithiafulvalenes has been investigated.202... 

Another procedure for sulfur isotope measurements has been developed where samples are converted to solid arsenic sulfide, AS0S3 (s), and measured by thermal ionization mass spectrometry (TIMS) (22). This technique offers several advantages over the gaseous methods in that both memory and isotope effects are eliminated, and the chemical procedure is simpler. A precision of 1 0/00, and the capability of making measurements on small samples, makes the TIMS technique competitive with gas phase MS techniques. 

The effects of anthropogenic sulfur dioxide on the remote marine atmosphere may be evident from rainwater studies by Chukhrov et al. (60) in which the isotopic composition of sulfur in rain was studied systematically at great distances from the continent. Rainwater sulfate ranged from +12.1 to +15.0 0/00 over the Atlantic and from +9.5 to +16.2 0/00 over the Pacific, with a one month average value of +13.3 0/00 for the two oceans. Their study included measurements of rainwater sulfate from a wide variety of continental areas and found that most inland 634S values were significantly lower than those over the oceans. The oceanic rainfall sulfate was most likely a mixture of the isotopically lower continental sulfate and the more enriched marine sulfate. 

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