A critical review on isotopic fractionation correction methods for accurate isotope amount ratio measurements by MC-ICP-MS
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Abstract:
Proper correction of mass-dependent and mass-independent isotopic fractionation is crucial to obtain accurate isotope amount ratios by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS).Keywords:
Mass-independent fractionation
Equilibrium fractionation
Mass-independent fractionation
Kinetic isotope effect
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The overarching aim of this study is to define mercury (Hg) isotopic features of plants which have different photosynthetic pathways (C3, C4 and CAM) and to understand if different parts of the plants have different Hg isotopic fractionation behavior. For this, carbon isotopic values of terrestrial plants were analyzed which were used to determine the photosynthetic pathways of plants. Plants were sub-sampled into leaves, stems and roots and their Hg isotopic values were analyzed.
Results showed that C3 and C4 plants exhibit mass dependent (even Hg isotopes) and mass independent Hg isotope fractionation (odd Hg isotopes). Both C3 and C4 plants are enriched in light isotopes, but the degree of mass fractionation is approximately three times greater in C3 plants, than in C4 plants. Hg in both C3 and C4 plants exhibit negative MIF isotope effect which reported as depletion “and no clear MIF effect. These findings suggest a connection between the Hg isotopic composition and the photosynthetic pathway.
In addition, the leaves are slightly more fractionated than the roots. Differences in the degree of MIF between roots and leaves suggest that they obtain Hg from different sources.
Mass-independent fractionation
Mercury
Isotope Analysis
Equilibrium fractionation
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To utilize stable Hg isotopes as a tracer for Hg cycling and pollution sources in the environment, it is imperative that fractionation factors for important biogeochemical processes involving Hg are determined. Here, we report experimental results on Hg isotope fractionation during precipitation of metacinnabar (β-HgS) and montroydite (HgO). In both systems, we observed mass-dependent enrichments of light Hg isotopes in the precipitates relative to the dissolved Hg. Precipitation of β-HgS appeared to follow equilibrium isotope fractionation with an enrichment factor ε202Hgprecipitate–supernatant of −0.63‰. Precipitation of HgO resulted in kinetic isotope fractionation, which was described by a Rayleigh model with an enrichment factor of −0.32‰. Small mass-independent fractionation was observed in the HgS system, presumably related to nuclear volume fractionation. We propose that Hg isotope fractionation in the HgS system occurred in solution during the transition of O- to S-coordination of Hg(II), consistent with theoretical predictions. In the HgO system, fractionation was presumably caused by the faster precipitation of light Hg isotopes, and no isotopic exchange between solid and solution was observed on the timescale investigated. The results of this work emphasize the importance of Hg solution speciation and suggest that bonding partners of Hg in solution complexes may control the overall isotope fractionation. The determined fractionation factor and mechanistic insights will have implications for the interpretation of Hg isotope signatures and their use as an environmental tracer.
Equilibrium fractionation
Mass-independent fractionation
Enrichment factor
Mercury
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Equilibrium fractionation
Mass-independent fractionation
Isotopes of magnesium
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Mass-independent fractionation
Equilibrium fractionation
Isotope Analysis
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Minerals and rocks exhibit various isotope compositions depending on their origins and histories. In interpreting their isotopic variations, the equilibrium isotope fractionation factor is a key because it depends on the environment parameters such as temperature. Recent studies have shown that the effect of pressure on the isotope fractionation, which was considered negligible compared to temperature, is significant under the conditions of the Earth's interior. In this article we review recent advances in experimental studies to determine the isotope fractionation of iron and hydrogen at high pressure over several GPa, discussing their issues and future perspectives.
Equilibrium fractionation
Mass-independent fractionation
Hydrogen isotope
Kinetic isotope effect
Isotope Geochemistry
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Mass-independent fractionation
Equilibrium fractionation
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Isotope analysis is a potentially sensitive method to trace in situ degradation of organic contaminants. In a recent paper, Morasch et al. (3) investigated the mechanism of isotope fractionation during toluene biodegradation using deuterium-labeled toluene. The authors overlooked that the Rayleigh equation that is normally used to evaluate isotope fractionation at natural abundance level (2) is not applicable to studies with labeled substrate, particularly if large isotope fractionation occurs. For several of their experiments they obtained negative hydrogen isotope fractionation factors (see Table 1 in reference 3), which contradict the definition of the fractionation factor (see below). Since labeled compound will likely be used in further investigations to study isotope fractionation, it is important to demonstrate why the commonly used Rayleigh equation is usually not applicable in such studies and to provide an alternative method to quantify isotope fractionation.
The magnitude of isotope fractionation is normally characterized by the fractionation factor, which is defined as follows for kinetic isotope fractionation:
(1)
where H and L are the concentrations of the substrate with heavy and light isotopes, respectively, at a given time and dHp and dLp are increments of product with heavy or light isotopes, respectively, that appear in an infinitely short time (instantaneous product). In some studies, the fractionation factor is defined by the inverse ratio (2). Since all terms in equation 1 are positive, α has to be positive. For mass balance reasons,
(2)
Combining equations 1 and 2 and rearrangement leads to
Integration of equation 3 from L0 to L and H0 to H gives
(4)
Dividing both sides by L/L0 yields
(5)
where R and R0 are the isotope ratios (H/L) at a given time t and at time zero, respectively. The fraction of substrate that has not reacted yet, f, at time t is given by
(6)
Equations 5 and 6 are analogous to those given by Bigeleisen and Wolfsberg (1), except that here they were derived without any specific assumption about the reaction kinetics and using a different definition of α and f.
The crucial point is that L/L0 in equation 5 can only be approximated by f if either (i) the concentrations of the heavy isotopes, H and H0, are small, as common for studies at natural abundance level, or (ii) 1 + R ≈ 1 + R0. In the first case, the first expression for f in equation 6 approaches L/L0; in the second case, the second expression can be approximated by L/L0. If one of these two conditions is fulfilled, equation 5 can be simplified to
(7)
which corresponds to the Rayleigh equation as used by the authors of the study (3). However, in the experiments with labeled compound presented in the study, condition i is not fulfilled since the compound with deuterium accounts for 50% of the total toluene concentration. Condition ii is not fulfilled either. For example, for the experiment illustrated in Fig. 1 in reference 3, R0 is 1 and R varies between 1 and about 12 and thus, the assumption that 1 + R ≈ 1 + R0 holds true is not valid. In other experiments, even higher R values of up to about 54 were observed (see Fig. 2 in reference 3).
By combining equations 5 and 6, an accurate equation is obtained that relates R, R0, f, and α:
(8)
This equation can be used to determine α by plotting ln(R/R0) versus ln{f/[(1 + R)/(1 + R0)]}. Applying this approach to the data of the experiment with Desulfobacterium cetonicum (as given in Fig. 1 in reference 3), an α value of approximately 2.7 is obtained instead of −5.09. The value of 2.7 is only an approximation, since the data for the calculation were estimated from Fig. 1 in reference 3. The calculated value is in the typical range for primary hydrogen isotope effects. Using the correct equation, the introduction of an uncommon parameter to characterize isotope fractionation becomes unnecessary and the data can be discussed in a framework consistent with a large number of studies on isotope fractionation during enzymatic reactions.
Equilibrium fractionation
Mass-independent fractionation
Kinetic isotope effect
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Mass-independent fractionation
Equilibrium fractionation
Isotope Analysis
Kinetic isotope effect
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Laboratory experiments demonstrate that iron isotopes can be chemically fractionated in the absence of biology. Isotopic variations comparable to those seen during microbially mediated reduction of ferrihydrite are observed. Fractionation may occur in aqueous solution during equilibration between inorganic iron complexes. These findings provide insight into the mechanisms of iron isotope fractionation and suggest that nonbiological processes may contribute to iron isotope variations observed in sediments.
Ferrihydrite
Equilibrium fractionation
Mass-independent fractionation
Iron Isotopes
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