Mass-independent isotope fractionation of heavy elements measured by MC-ICPMS : a unique probe in environmental sciences

This article overviews recent developments in the use of multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) in studies of mass-independent isotope chemistry of heavy elements. Origins of mass-independent isotope effects and their relevance to isotope ratio measurements by MC-ICPMS are briefly described. The extent to which these effects can affect instrumental mass bias in MC-ICPMS is critically discussed on the basis of the experimental observations. Furthermore, key findings reported in studies of mass-independent isotope fractionation (MIF) of mercury in the field of environmental sciences are reviewed. MIF of heavy elements is not only of interest from a fundamental point of view, but also provides scientists with a new and effective means of studying the biogeochemistry of these elements

Molybdenum isotope enrichment by anion-exchange chromatography

Stable enriched isotopes with an abundance as close to 100% as possible are most desirable parent materials for preparing synthetic isotope mixtures for calibration of mass spectrometers. In this study, we have examined the extent of Mo isotope fractionation during anion-exchange chromatography and assessed the potential of this separation technique for further isotope enrichment of commercially available isotopically enriched molybdenum. The 60 cm wide molybdenum adsorption band was eluted from a 1.8 m long (3 times 60 cm) chromatographic column, filled with strongly basic anion-exchange resin. It was observed that heavier Mo isotopes were preferentially eluted from the resin, resulting in an enrichment of heavier Mo isotopes at the front of the adsorption band and a corresponding depletion at the rear. An equilibrium isotope effect between dissolved and resin-bound chemical forms of molybdenum appears to be the cause of the Mo isotope fractionation observed. The height equivalent to a theoretical plate (HETP) was calculated to be 0.25 mm, while the separation factor (alpha = R-resin/R-solution) was found to be 0.99998 per atomic mass unit. These results show that molybdenum can, in principle, be further enriched isotopically by anion-exchange chromatography in a laboratory environment. However, achieving the highest degree of enrichment was confirmed to be very laborious. It has been estimated that in order to reach an increase in the abundance of Mo-98 isotope from an original value of 98.2% to 99.9%, the molybdenum adsorption band would need to migrate over a distance of at least 400 m through the anion-exchange resin. Another interesting observation made in this study was that no mass-independent isotope fractionation was observed for molybdenum as a result of chemical exchange reactions on the anion-exchange resin

Use of single-collector and multi-collector ICP-mass spectrometry for isotopic analysis

This article is intended as a tutorial review on the use of single-collector and multi-collector ICPMS for isotope ratio determination. The monitoring and quantification of both induced and natural differences in the isotopic composition of target elements is covered. The capabilities of various types of ICPMS instruments for isotope ratio measurements are addressed and issues, such as the occurrence of mass discrimination and detector dead time effects and appropriate ways of correcting for the biases they give rise to are discussed. Applications relying on induced changes include elemental assay via isotope dilution, tracer experiments with stable isotopes, aiming at a more profound insight into physical processes or (bio)chemical reactions, and nuclear applications. Attention is also paid to the origin of natural variations in the isotopic composition, with focus onto the mechanisms behind the isotopic variation for those elements for which isotopic analysis can be realized using ICP-mass spectrometry, i.e. the occurrence of radiogenic nuclides formed as a result of the decay of naturally occurring and long-lived radionuclides and mass fractionation as a result of thermodynamic and kinetic isotope fractionation effects. Geochronological dating via theRb-Sr, U, Th-Pb, and Pb-Pb methods is briefly explained and also the use of Sr and Pb isotopic analysis for provenance determination studies is covered. Subsequently, applications based on isotopic analysis of elements showing a much narrower range of variation as a result of isotope fractionation are described. Next to provenance studies, such applications include the use of isotope ratios in geochemical, environmental and biomedical studies. Although it is not the intention to comprehensively review the literature, several examples of published applications are used to illustrate the capabilities of both single-collector and multi-collector ICPMS in this context. Thereby, attention is devoted both to widely accepted applications and to more 'exotic' applications, aiming at an extension of the application range

Isotopic fractionation of Sn during methylation and demethylation reactions in aqueous solution

Laboratory experiments, modeling the methylation of inorganic Sn(II) by methylcobalamin and the decomposition of methyltin under irradiation with UV light in aqueous solution, have been performed. Methyltin has been separated from inorganic Sri using anion-exchange chromatography and subjected to Sri isotope ratio measurements via solution nebulization multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The methylation of Sn(II) in the dark was accompanied by mass-dependent Sri isotopic fractionation, which resulted in preferential partitioning of the lighter Sri isotopes into the organic phase, with a shift of similar to 0.57 +/- 0.12%D in terms of delta(124)/Sn-116 between methylated and inorganic Sn. The methylation of Sn(II) by methylcobalamin under UV irradiation resulted in the accelerated formation of methyltin in the beginning of the process, but was followed by the photolytic degradation of methyltin until its complete mineralization. The photolytic degradation of methyltin in the presence of methylcobalamin and inorganic Sn(II) was slower than that of pure solutions of commercially obtained monomethyltin. This is attributed to the methylating action of methyl radicals produced from photolytically decomposing methylcobalamin. Both synthesis and decomposition of methyltin under UV irradiation Were accompanied by both mass-dependent and mass-independent Sri isotopic fractionation, with the latter due to the magnetic isotope effect. As a result of this, the lighter Sri isotopes preferentially partition into reaction products, while the odd isotopes, Sn-117 and Sn-119, are selectively enriched relative to the other isotopes in the starting molecules. The extent of the observed variations in the isotopic composition of Sri is larger than that documented previously for geological and archeological samples. These results indicate that Sri isotopic fractionation between various chemical forms of Sri in the natural aquatic systems may be significant and can provide new insights into the biogeochemical cycling of the element

Experimental study of mass-independence of Hg isotope fractionation during photodecomposition of dissolved methylmercury

Experiments modelling photolytic decomposition of methylmercury chloride in aqueous solutions of different chemical composition have been performed. Ion-exchange chromatographic separation using Chelex (R) 100 resin was used in order to separate methylmercury from inorganic mercury prior to the isotope ratio measurements by solution nebulization multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The performance of the chromatographic separation has been evaluated in terms of recovery of both methylmercury and inorganic Hg using synthetic solutions. Both mass-dependent and mass-independent fractionation of Hg isotopes concomitant with the decomposition process have been observed. Mass-independent Hg isotope fractionation (MIF) resulted in selective enrichment of Hg-199 and Hg-201 relative to the other isotopes in the methylmercury molecules and has been attributed to the magnetic isotope effect. The highest extent of MIF of Hg isotopes, expressed as Delta Hg-199 and Delta Hg-201 values, has been observed in acidified solution with low concentration of total dissolved solids (TDS). Progressive decrease in Delta Hg-199 and Delta Hg-201 values in acidified solution with higher concentration of TDS, alkaline solutions of both low and high concentration of TDS, and in a solution of ascorbic acid has been attributed to suppression of the radical pair reaction mechanism, responsible for the occurrence of the magnetic isotope effect, by substances acting as radical scavengers, such as OH- or ascorbic acid. The data obtained in this study demonstrate the significance of spin chemistry effects in the isotope fractionation of mercury

Modern mass spectrometry for studying mass-independent fractionation of heavy stable isotopes in environmental and biological sciences

This is the first review to focus specifically on the application of modern mass spectrometry techniques for studying mass-independent or anomalous isotope fractionation for 'heavy' elements. This review covers a period of ten years, starting from 2000. In the first part of the manuscript, we address theoretical aspects of mass-independent isotope fractionation, such as nuclear volume and nuclear spin effects. Secondly, most published articles on this topic to date are reviewed. Mass-independent isotope fractionation was determined by ICP-MS, MC-ICP-MS or TIMS for nineteen elements so far: Hg, Sn, Cd, Zn, Te, Sr, Pb, Cr, U, Ti, Ni, Mo, Ru, Ba, Nd, Sm, Gd, Yb and Hf, as reported in about fifty publications. In addition, mass-independent fractionation was observed for some other elements, silicon, germanium and sulfur, by other techniques

Fractionation of iron isotopes in shallow-marine ferromanganese concretions

Godkänd; 2007; 20110704 (andbra)</p

Data report : Iron isotope geochemistry of mid-Cretaceous organic-rich sediments at Demerara Rise (odp leg 207)

Godkänd; 2005; 20120223 (andbra)</p