scholarly journals Can Adsorption on Graphene be Used for Isotopic Enrichment? A DFT Perspective

Molecules ◽  
2018 ◽  
Vol 23 (11) ◽  
pp. 2981 ◽  
Author(s):  
Mateusz Pokora ◽  
Piotr Paneth
Keyword(s):  
Aromatic Compounds ◽  
Model Compound ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Adsorbed Molecule ◽  
Polar Compounds ◽  
Isotopic Enrichment ◽  
Reliable Prediction ◽  
Computational Procedures ◽  
Nonpolar Molecules

We have explored the theoretical applicability of adsorption on graphene for the isotopic enrichment of aromatic compounds. Our results indicate that for nonpolar molecules, like benzene, the model compound used in these studies shows a reasonable isotopic fractionation that is obtained only for the deuterated species. For heavier elements, isotopic enrichment might be possible with more polar compounds, e.g., nitro- or chloro-substituted aromatics. For benzene, it is also not possible to use isotopic fractionation to differentiate between different orientations of the adsorbed molecule over the graphene surface. Our results also allowed for the identification of theory levels and computational procedures that can be used for the reliable prediction of the isotope effects on adsorption on graphene. In particular, the use of partial Hessian is an attractive approach that yields acceptable values at an enormous increase of speed.

Paleohydrology of a Canadian Shield lake inferred from 18O in sediment cellulose

1989 ◽  
Vol 26 (9) ◽  
pp. 1850-1859 ◽  
Author(s):  
T. W. D. Edwards ◽  
J. H. McAndrews
Keyword(s):  
Lake Water ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Cellulose Synthesis ◽  
Isotopic Enrichment ◽  
Oxygen And Carbon Isotope ◽  
The Past ◽  
Two Factors ◽  
Isotope Analyses ◽  
Isotope Content

Oxygen- and carbon-isotope analyses on cellulose in the postglacial sediment of Weslemkoon Lake, southern Ontario, show that the cellulose came mainly from aquatic plants or algae, rather than from terrestrial sources. If a wholly aquatic source is assumed, the oxygen-isotope content permits inferences of lake-water δ18O values over the past 10 000 years by accounting for the isotopic fractionation that occurs during cellulose synthesis. Chronological control is provided by pollen analysis and six 14C dates. Our reconstruction shows lake-water δ18O fluctuated from about 5‰ lower than present in the early postglacial to 5‰ or more above present values during the mid-postglacial. These broad, secular shifts reflect a combination of fluctuating mean annual δ18O of local precipitation, evaporative isotopic enrichment of surface waters, and snowmelt-bypass effects. The first two factors reflect the changing paleotemperature and paleohydrology, respectively, whereas the third factor is a more speculative interpretation of isotope effects during snowmelt delivery to the lake. The snowmelt-bypass mechanism is supported by parallel changes in the overall abundance and seasonal distribution of precipitation. This effect is probably responsible for pronounced isotopic enrichment of the water throughout the moist climate of the past 6000 years.

scholarly journals 2H/1H variation in microbial lipids is controlled by NADPH metabolism

2019 ◽  
Vol 116 (25) ◽  
pp. 12173-12182 ◽  
Author(s):  
Reto S. Wijker ◽  
Alex L. Sessions ◽  
Tobias Fuhrer ◽  
Michelle Phan
Keyword(s):  
Metabolic Flux ◽  
Isotope Effects ◽  
Full Range ◽  
Isotopic Fractionation ◽  
Flux Analysis ◽  
Organic Substrates ◽  
Nadph Metabolism ◽  
Highly Correlated ◽  
Geologic Record ◽  
Ancient Ecosystems

The hydrogen-isotopic compositions (2H/1H ratios) of lipids in microbial heterotrophs are known to vary enormously, by at least 40% (400‰) relative. This is particularly surprising, given that most C-bound H in their lipids appear to derive from the growth medium water, rather than from organic substrates, implying that the isotopic fractionation between lipids and water is itself highly variable. Changes in the lipid/water fractionation are also strongly correlated with the type of energy metabolism operating in the host. Because lipids are well preserved in the geologic record, there is thus significant potential for using lipid 2H/1H ratios to decipher the metabolism of uncultured microorganisms in both modern and ancient ecosystems. But despite over a decade of research, the precise mechanisms underlying this isotopic variability remain unclear. Differences in the kinetic isotope effects (KIEs) accompanying NADP+ reduction by dehydrogenases and transhydrogenases have been hypothesized as a plausible mechanism. However, this relationship has been difficult to prove because multiple oxidoreductases affect the NADPH pool simultaneously. Here, we cultured five diverse aerobic heterotrophs, plus five Escherichia coli mutants, and used metabolic flux analysis to show that 2H/1H fractionations are highly correlated with fluxes through NADP+-reducing and NADPH-balancing reactions. Mass-balance calculations indicate that the full range of 2H/1H variability in the investigated organisms can be quantitatively explained by varying fluxes, i.e., with constant KIEs for each involved oxidoreductase across all species. This proves that lipid 2H/1H ratios of heterotrophic microbes are quantitatively related to central metabolism and provides a foundation for interpreting 2H/1H ratios of environmental lipids and sedimentary hydrocarbons.

Why magnesium isotope fractionation is absent from basaltic melts under thermal gradients in natural settings

2019 ◽  
Vol 157 (7) ◽  
pp. 1144-1148
Author(s):  
Yingkui Xu ◽  
Dan Zhu ◽  
Xiongyao Li ◽  
Jianzhong Liu
Keyword(s):  
Isotope Fractionation ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Chemical Diffusion ◽  
Kinetic Isotope Effects ◽  
Thermal Gradients ◽  
Magnesium Isotopes ◽  
Magnesium Isotope ◽  
Natural Systems ◽  
Kinetic Isotope

AbstractLaboratory experiments have shown that thermal gradients in silicate melts can lead to isotopic fractionation; this is known as the Richter effect. However, it is perplexing that the Richter effect has not been documented in natural samples as thermal gradients commonly exist within natural igneous systems. To resolve this discrepancy, theoretical analysis and calculations were undertaken. We found that the Richter effect, commonly seen in experiments with wholly molten silicates, cannot be applied to natural systems because natural igneous samples are more likely to be formed out of partially molten magma and the presence of minerals adds complexity to the behaviour of the isotope. In this study, we consider two related diffusion-rate kinetic isotope effects that originate from chemical diffusion, which are absent from experiments with wholly molten samples. We performed detailed calculations for magnesium isotopes, and the results indicated that the Richter effect for magnesium isotopes is buffered by kinetic isotope effects and the total value of magnesium isotope fractionation can be zero or even undetectable. Our study provides a new understanding of isotopic behaviour during the processes of cooling and solidification in natural magmatic systems.

scholarly journals Quantifying N<sub>2</sub>O reduction to N<sub>2</sub> based on N<sub>2</sub>O isotopocules – validation with independent methods (helium incubation and <sup>15</sup>N gas flux method)

2017 ◽  
Vol 14 (3) ◽  
pp. 711-732 ◽  
Author(s):  
Dominika Lewicka-Szczebak ◽  
Jürgen Augustin ◽  
Anette Giesemann ◽  
Reinhard Well
Keyword(s):  
Isotopic Fractionation ◽  
Isotopic Signature ◽  
Residual Fraction ◽  
Simultaneous Estimation ◽  
Site Preference ◽  
Isotopic Enrichment ◽  
Flux Method ◽  
Gas Flux ◽  
Isotopic Analyses

Abstract. Stable isotopic analyses of soil-emitted N2O (δ15Nbulk, δ18O and δ15Nsp = 15N site preference within the linear N2O molecule) may help to quantify N2O reduction to N2, an important but rarely quantified process in the soil nitrogen cycle. The N2O residual fraction (remaining unreduced N2O, rN2O) can be theoretically calculated from the measured isotopic enrichment of the residual N2O. However, various N2O-producing pathways may also influence the N2O isotopic signatures, and hence complicate the application of this isotopic fractionation approach. Here this approach was tested based on laboratory soil incubations with two different soil types, applying two reference methods for quantification of rN2O: helium incubation with direct measurement of N2 flux and the 15N gas flux method. This allowed a comparison of the measured rN2O values with the ones calculated based on isotopic enrichment of residual N2O. The results indicate that the performance of the N2O isotopic fractionation approach is related to the accompanying N2O and N2 source processes and the most critical is the determination of the initial isotopic signature of N2O before reduction (δ0). We show that δ0 can be well determined experimentally if stable in time and then successfully applied for determination of rN2O based on δ15Nsp values. Much more problematic to deal with are temporal changes of δ0 values leading to failure of the approach based on δ15Nsp values only. For this case, we propose here a dual N2O isotopocule mapping approach, where calculations are based on the relation between δ18O and δ15Nsp values. This allows for the simultaneous estimation of the N2O-producing pathways' contribution and the rN2O value.

scholarly journals Iron-Manganese System for Preparation of Radiocarbon Ams Targets: Characterization of Procedural Chemical-Isotopic Blanks and Fractionation

Radiocarbon ◽  
1997 ◽  
Vol 39 (3) ◽  
pp. 269-283 ◽  
Author(s):  
R. Michael Verkouteren ◽  
Donna B. Klinedinst ◽  
Lloyd A. Currie
Keyword(s):  
Sample Size ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Chemical Yield ◽  
Standard Reference Materials ◽  
Methane Formation ◽  
Fractionation Factor ◽  
Beam Geometry ◽  
C Fibers ◽  
Counting Statistics

We report a practical system to mass-produce accelerator mass spectrometry (AMS) targets with 10–100 μg carbon samples. Carbon dioxide is reduced quantitatively to graphite on iron fibers via manganese metal, and the Fe-C fibers are melted into a bead suitable for AMS. Pretreatment, reduction and melting processes occur in sealed quartz tubes, allowing parallel processing for otherwise time-intensive procedures.Chemical and isotopic (13C, 14C) blanks, target yields and isotopic fractionation were investigated with respect to levels of sample size, amounts of Fe and Mn, pretreatment and reduction time, and hydrogen pressure. With 7-day pretreatments, carbon blanks exhibited a lognormal mass distribution of 1.44 μg (central mean) with a dispersion of 0.50 μg (standard deviation). Reductions of 10 μg carbon onto targets were complete in 3–6 h with all targets, after correction for the blank, reflecting the 13C signature of the starting material. The 100 μg carbon samples required at least 15 h for reduction; shorter durations resulted in isotopic fractionation as a function of chemical yield. The trend in the 13C data suggested the presence of kinetic isotope effects during the reduction. The observed CO2-graphite 13C fractionation factor was 3–4% smaller than the equilibrium value in the simple Rayleigh model. The presence of hydrogen promoted methane formation in yields up to 25%.Fe-C beaded targets were made from NIST Standard Reference Materials and compared with graphitic standards. Although the 12C ion currents from the beads were one to two orders of magnitude lower than currents from the graphite, measurements of the beaded standards were reproducible and internally consistent. Measurement reproducibility was limited mainly by Poisson counting statistics and blank variability, translating to 14C uncertainties of 5–1% for 10–100 μg carbon samples, respectively. A bias of 5–7% (relative) was observed between the beaded and graphitic targets, possibly due to variations in sputtering fractionation dependent on sample size, chemical form and beam geometry.

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