Compound-Specific δ^(34)S Analysis of Volatile Organics by Coupled GC/Multicollector-ICPMS

We have developed a highly sensitive and robust method for the analysis of δ^(34)S in individual organic compounds by coupled gas chromatography (GC) and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The system requires minimal alteration of commercial hardware and is amenable to virtually all sample introduction methods. Isobaric interference from O_2^+ is minimized by employing dry plasma conditions and is cleanly resolved at all masses using medium resolution on the Thermo Neptune MC-ICPMS. Correction for mass bias is accomplished using standard−sample bracketing with peaks of SF6 reference gas. The precision of measured δ^(34)S values approaches 0.1‰ for analytes containing >40 pmol S and is better than 0.5‰ for those containing as little as 6 pmol S. This is within a factor of 2 of theoretical shot-noise limits. External accuracy is better than 0.3‰. Integrating only the center of chromatographic peaks, rather than the entire peak, offers significant gain in precision and chromatographic resolution with minimal effect on accuracy but requires further study for verification as a routine method. Coelution of organic compounds that do not contain S can cause degraded analytical precision. Analyses of crude oil samples show wide variability in δ^(34)S and demonstrate the robustness and precision of the method in complex environmental samples

Sulfur isotope fractionation during incorporation of sulfur nucleophiles into organic compounds

34S enrichment is shown to occur during sulfurization reactions and for the first time conclusively attributed to an isotope equilibrium effect rather than selective addition of 34S enriched nucleophiles

Organic sulfur as part of the sulfur cycle from early diagenesis through catagenesis

During the diagenetic stage the sulfur–organic matter (S-OM) chemical environment is dominated by the aquatic media. The pH variations of the interstitial water and the redox potential prevailing control the various sulfur species hence their chemical activity. Some of the more polar organic matter being hydrophilic is exposed to the reduced active sulfides (e.g. S_x^(-2), HS^-,RS^-) that are very active nucleophiles. In the last four years we have extended the research to decipher the mechanisms controlling these reactions. The more hydrophobic organic matter can be exposed to similar reactions in the presence of phase transfer catalysts (PTC). However, under these conditions the actual environment for the reaction is the organic phase. Since the diagenetic stage is considered to pertain to ambient to mild increased temperatures range all laboratory simulations indicate equilibration controlled mechanisms for both sulfur introduction into the (OM) as well as nucleophilicity controlled exchange. The only kinetic controlled reactions were either due to thermal stress or easily removed (substituted) groups most of which are rare in young sedimentary OM. We consider for these transformations the sulfate as un-reactive sulfur specie

Compound-Specific δ S Analysis of Volatile Organics by Coupled GC/Multicollector-ICPMS

We have developed a highly sensitive and robust method for the analysis of δ 34 S in individual organic compounds by coupled gas chromatography (GC) and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The system requires minimal alteration of commercial hardware and is amenable to virtually all sample introduction methods. Isobaric interference from O 2 + is minimized by employing dry plasma conditions and is cleanly resolved at all masses using medium resolution on the Thermo Neptune MC-ICPMS. Correction for mass bias is accomplished using standard-sample bracketing with peaks of SF 6 reference gas. The precision of measured δ 34 S values approaches 0.1‰ for analytes containing >40 pmol S and is better than 0.5‰ for those containing as little as 6 pmol S. This is within a factor of 2 of theoretical shot-noise limits. External accuracy is better than 0.3‰. Integrating only the center of chromatographic peaks, rather than the entire peak, offers significant gain in precision and chromatographic resolution with minimal effect on accuracy but requires further study for verification as a routine method. Coelution of organic compounds that do not contain S can cause degraded analytical precision. Analyses of crude oil samples show wide variability in δ 34 S and demonstrate the robustness and precision of the method in complex environmental samples. Sulfur is an important constituent of many natural and anthropogenic organic compounds. Its reactivity and facile redox chemistry make it an important intermediary in a variety of natural biogeochemical processes, 1,2 as well as a key component of some environmental and atmospheric contaminants. 3 Specific redox transitions of sulfur are linked to large isotopic fractionations, and the record of those fractionations is potentially well-preserved by individual organosulfur compounds. Thus, measurements of the sulfur-isotopic composition of specific organic molecules are potentially very useful for tracing both the origins of those compounds and the processes that have affected them. Potential research applications for such measurements include tracking sources of atmospheric trace gases, 3,4 studying the sulfurization of natural organic matter, 5 producing paleoenvironmental and paleoatmospheric proxy records, 2,6 identifying the provenance of industrial products and contaminants, 7,8 and investigation of both biogenic and thermogenic alterations and sources in crude oils. 9 The conventional approach to sulfur-isotopic analysis of organic materials is based on combustion to SO 2 in an elemental analyzer (EA), followed by measurement of 34 S/ 32 S ratios in a gas-source isotope ratio mass spectrometer (IRMS). 10 Conversion to SF 6 with analysis by IRMS has also been used to examine 33 S and 36 S isotopes. 11 Both types of analysis are necessarily restricted to bulk materials, or to compounds or fractions that can be purified in milligram quantities. There is thus no simple and robust analytical route to sulfur-isotopic analysis of individual molecular species. Efforts to develop compound-specific 34 S analysis by coupling gas chromatography (GC) and IRMS via a combustion reactor have been ongoing for over a decade, but remain largely unsuccessful. The main difficulty in this approach is the need for continuous oxidation/reduction and separation of the combustion products (e.g., H 2 O, CO 2 ) from SO 2 . A potentially more tractable approach involves the coupling of GC separation to a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). This avoids most of the problems associated with using a combustion interface to couple GC and IRMS, because organic species are atomized and ionized in the plasma source

Hypogenic karst at the Arabian platform margins: Implications for far-field groundwater systems

We analyzed maze caves and the associated hydrogeology in the northern Negev–Judean Desert in Israel to provide insight on fluid migration and porosity development, with relevance to groundwater and petroleum reservoirs on the Arabian Platform flanks. The caves occur specifically in the arid region of the southern Levant, with no equivalent in the moister climate areas further to the north. The karstified bedrock consists of Upper Cretaceous epicontinental carbonates. Caves were formed mainly above deep faults associated with the Syrian arc fold system. Hypogenic flow is shown to have formed the maze caves particularly under the confinement of thick chalk and marl cap rock. Speleogenesis occurred during the Oligocene–early Miocene when the Afro-Arabian dome was rising and became erosionally truncated. Calcite deposits depleted in 18O point to a connection between the caves and recharge over far-field Nubian Sandstone outcrops, north of the Precambrian basement outcrops on the eastern side of the Red Sea. During the early–middle Miocene, the Dead Sea rift began dissecting the region, forming a deep endorheic depression at the eastern margin of the study area and disconnecting the far-field groundwater flow. This was followed by subsiding groundwater levels and associated dewatering of the caves. Fault escarpments and canyon downcutting then dissected the caves, forming the present entrances. The caves are currently mostly dry, with scarce speleothem occurrences. Gypsum crusts with δ34SSO4 values lower than other sulfate deposits point to bacterial sulfur reduction, hydrogen sulfide, and sulfuric acid being involved in the speleogenesis

Theoretical study on the reactivity of sulfate species with hydrocarbons

The abiotic, thermochemically controlled reduction of sulfate to hydrogen sulfide coupled with the oxidation of hydrocarbons, is termed thermochemical sulfate reduction (TSR), and is an important alteration process that affects petroleum accumulations in nature. Although TSR is commonly observed in high-temperature carbonate reservoirs, it has proven difficult to simulate in the laboratory under conditions resembling nature. The present study was designed to evaluate the relative reactivities of various sulfate species in order to provide greater insight into the mechanism of TSR and potentially to fill the gap between laboratory experimental data and geological observations. Accordingly, quantum mechanics density functional theory (DFT) was used to determine the activation energy required to reach a potential transition state for various aqueous systems involving simple hydrocarbons and different sulfate species. The entire reaction process that results in the reduction of sulfate to sulfide is far too complex to be modeled entirely; therefore, we examined what is believed to be the rate limiting step, namely, the reduction of sulfate S(VI) to sulfite S(IV). The results of the study show that water-solvated sulfate anions View the MathML source are very stable due to their symmetrical molecular structure and spherical electronic distributions. Consequently, in the absence of catalysis, the reactivity of SO4 2- is expected to be extremely low. However, both the protonation of sulfate to form bisulfate anions (HSO4-) and the formation of metal-sulfate contact ion-pairs could effectively destabilize the sulfate molecular structure, thereby making it more reactive. Previous reports of experimental simulations of TSR generally have involved the use of acidic solutions that contain elevated concentrations of HSO4- relative to SO4 2-. However, in formation waters typically encountered in petroleum reservoirs, the concentration of HSO4- is likely to be significantly lower than the levels used in the laboratory, with most of the dissolved sulfate occurring as SO4 2-, aqueous calcium sulfate ([CaSO4](aq)), and aqueous magnesium sulfate ([MgSO4](aq)). Our calculations indicate that TSR reactions that occur in natural environments are most likely to involve bisulfate ions (HSO4-) and/or magnesium sulfate contact ion-pairs ([MgSO4]CIP) rather than ‘free’ sulfate ions (SO4 2-) or solvated sulfate ion-pairs, and that water chemistry likely plays a significant role in controlling the rate of TSR

Sulfur isotope variability of DMS(P) production by Antarctic sea ice microalgal communities

info:eu-repo/semantics/nonPublishe

Sulfur Isotope Variability of DMS(P) Production by Antarctic Sea Ice Microbial Communities

info:eu-repo/semantics/nonPublishe