Isotope Effects, Mass Discrimination and Isotopic Fractionation

2010 ◽  
pp. 10-15
Keyword(s):  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Mass Discrimination

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 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.

scholarly journals Isotopic signatures of production and uptake of H<sub>2</sub> by soil

2015 ◽  
Vol 15 (22) ◽  
pp. 13003-13021 ◽  
Author(s):  
Q. Chen ◽  
M. E. Popa ◽  
A. M. Batenburg ◽  
T. Röckmann
Keyword(s):  
Isotope Effects ◽  
Deposition Velocity ◽  
Isotopic Fractionation ◽  
Isotopic Signature ◽  
Deuterium Content ◽  
Forest Site ◽  
Fractionation Factor ◽  
Isotopic Signatures ◽  
The Difference ◽  
Isotope Equilibrium

Abstract. Molecular hydrogen (H2) is the second most abundant reduced trace gas (after methane) in the atmosphere, but its biogeochemical cycle is not well understood. Our study focuses on the soil production and uptake of H2 and the associated isotope effects. Air samples from a grass field and a forest site in the Netherlands were collected using soil chambers. The results show that uptake and emission of H2 occurred simultaneously at all sampling sites, with strongest emission at the grassland sites where clover (N2 fixing legume) was present. The H2 mole fraction and deuterium content were measured in the laboratory to determine the isotopic fractionation factor during H2 soil uptake (αsoil) and the isotopic signature of H2 that is simultaneously emitted from the soil (δDsoil). By considering all net-uptake experiments, an overall fractionation factor for deposition of αsoil = kHD / kHH = 0.945 ± 0.004 (95 % CI) was obtained. The difference in mean αsoil between the forest soil 0.937 ± 0.008 and the grassland 0.951 ± 0.026 is not statistically significant. For two experiments, the removal of soil cover increased the deposition velocity (vd) and αsoil simultaneously, but a general positive correlation between vd and αsoil was not found in this study. When the data are evaluated with a model of simultaneous production and uptake, the isotopic composition of H2 that is emitted at the grassland site is calculated as δDsoil = (−530 ± 40) ‰. This is less deuterium depleted than what is expected from isotope equilibrium between H2O and H2.

DIFFUSION PHENOMENA AND ISOTOPE EFFECTS IN THE EXTRACTION OF FISSION-PRODUCT XENON AND KRYPTON FROM IRRADIATED U3O8

1960 ◽  
Vol 38 (7) ◽  
pp. 945-954 ◽  
Author(s):  
T. J. Kennett ◽  
H. G. Thode
Keyword(s):  
Activation Energy ◽  
Fission Product ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Low Energy ◽  
Energy Of Activation ◽  
Diffusion Phenomena ◽  
Fission Yields

An investigation of the diffusion of fission-product xenon and krypton from irradiated U3O8 powder as a function of temperature has revealed isotopic fractionation and also a double-valued activation energy. The apparent fission yields of Xe131 and Xe132 show abnormal enrichments of up to a factor of 10. These enrichments appear to be related to the precursor half-lives of xenon. When isotopic fractionation exists, the diffusion results exhibit an extremely low energy of activation.

scholarly journals Isotopic signatures of production and uptake of H<sub>2</sub> by soil

2015 ◽  
Vol 15 (17) ◽  
pp. 23457-23506 ◽  
Author(s):  
Q. Chen ◽  
M. E. Popa ◽  
A. M. Batenburg ◽  
T. Röckmann
Keyword(s):  
Isotope Effects ◽  
Deposition Velocity ◽  
Isotopic Fractionation ◽  
Isotopic Signature ◽  
Deuterium Content ◽  
Forest Site ◽  
Fractionation Factor ◽  
Isotopic Signatures ◽  
The Difference ◽  
Isotope Equilibrium

Abstract. Molecular hydrogen (H2) is the second most abundant reduced trace gas (after methane) in the atmosphere, but its biogeochemical cycle is not well understood. Our study focuses on the soil production and uptake of H2 and the associated isotope effects. Air samples from a grass field and a forest site in the Netherlands were collected using soil chambers. The results show that uptake and emission of H2 occurred simultaneously at all sampling sites, with strongest emission at the grassland sites where clover (N2 fixing legume) was present. The H2 mole fraction and deuterium content were measured in the laboratory to determine the isotopic fractionation factor during H2 soil uptake (αsoil) and the isotopic signature of H2 that is simultaneously emitted from the soil (δDsoil). By considering all net-uptake experiments, an overall fractionation factor for deposition of αsoil = kHD/kHH = 0.945 ± 0.004 (95 % CI) was obtained. The difference in mean αsoil between the forest soil 0.937 ± 0.008 and the grassland 0.951 ± 0.025 is not statistically significant. For two experiments, the removal of soil cover increased the deposition velocity (vd) and αsoil simultaneously, but a general positive correlation between vd and αsoil was not found in this study. When the data are evaluated with a model of simultaneous production and uptake, the isotopic composition of H2 that is emitted at the grassland site is calculated as δDsoil = (−530 ± 40) ‰. This is less deuterium-depleted than what is expected from isotope equilibrium between H2O and H2.

Carbon-13 kinetic isotope effects. V. Substituent effects on k12/k13 for alcoholysis of 1-phenyl-1-bromoethane

1969 ◽  
Vol 47 (13) ◽  
pp. 2506-2509 ◽  
Author(s):  
Jan Bron ◽  
J. B. Stothers
Keyword(s):  
Isotope Effect ◽  
Phenyl Ring ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Substituent Effects ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope ◽  
Factor Governing

As a test of our earlier interpretations of the 13C kinetic isotope effects found for alcoholysis of 1-phenyl-1-bromoethane, we have examined the effect of the p-methyl and p-bromo substituents on the 13C fractionations in ethanol and methanol. Isotopic fractionation at the α-carbon is found to be substituent dependent, and the observed trend is consistent with the proposal that stabilization of the cationic center by the phenyl ring is a major factor governing the isotope effect in these systems. The first example of an inverse primary kinetic isotope effect for carbon (k12/k13 < 1) is described.

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