Isotopic Speciation Research Articles

Speciation Analysis of Metals and Metalloids by Surface Enhanced Raman Spectroscopy.

The presence of metalloids and heavy metals in the environment is of critical concern due to their toxicological impacts. However, not all metallic species have the same risk level. Specifically, the physical, chemical, and isotopic speciation of the metal(loids) dictate their metabolism, toxicity, and environmental fate. As such, speciation analysis is critical for environmental monitoring and risk assessment. In the past two decades, surface-enhanced Raman spectroscopy (SERS) has seen significant developments regarding trace metal(loid) sensing due to its ultrahigh sensitivity, readiness for in situ real-time applications, and cost-effectiveness. However, the speciation of metal(loid)s has not been accounted for in the design and application of SERS sensors. In this Perspective, we examine the potential of SERS for metal(loid) speciation analysis and highlight the advantages, progress, opportunities, and challenges of this application.

High Diazotrophic Diversity but Low N2 Fixation Activity in the Northern Benguela Upwelling System Confirming the Enigma of Nitrogen Fixation in Oxygen Minimum Zone Waters

Oxygen minimum zones (OMZs) have been suggested as a suitable niche for the oxygen-sensitive process of biological fixation of dinitrogen (N2) gas. However, most N2 fixation rates reported from such waters are low. This low N2 fixation activity has been proposed to result from the unusual community of N2 fixers, in which cyanobacteria were typically underrepresented. The Northern Benguela Upwelling System (North BUS) is part of one of the most productive marine ecosystems and hosts a well-developed OMZ. Although previous observations indicated low to absent N2 fixation rates, the community composition of diazotrophs needed to understand the North BUS has not been described. Here, we present a first detailed analysis of the diazotrophic diversity in the North BUS OMZ and the Angola tropical zone (ATZ), based on genetic data and isotope speciation. Consistent with a previous study, we detected a slight N deficit in the OMZ, but isotope data did not indicate any active or past N2 fixation. The diazotroph community in the North BUS was dominated by non-cyanobacterial microbes clustering with members of gamma-proteobacteria, as is typical for other OMZ regions. However, we found a strikingly high diversity of Cluster III diazotrophs not yet described in other OMZs. In contrast to previous observations, we could also identify cyanobacteria of the clades Trichodesmium sp., UCYN-A and Cyanothece sp., in surface waters connected to or above the OMZ, which were potentially active as shown by the presence of genes and transcripts of the key functional marker gene for N2 fixation, nifH. While the detection of diazotrophs and the absence of active N2 fixation (based on isotopic speciation) are consistent with other OMZ observations, the detected regional variation in the diversity and presence of cyanobacteria indicate that we still are far from understanding the role of diazotrophs in OMZs, which, however, is relevant for understanding the N cycle in OMZ waters, as well for predicting the future development of OMZ biogeochemistry in a changing ocean.

Using CO2 Reactions to Achieve Mass-spectrometric Discrimination in Simultaneous Plutonium-isotope Speciation with Inductively Coupled Plasma–Tandem Mass Spectrometry

Abstract Simultaneous isotope speciation of various Pu isotopes is difficult to achieve with conventional inductively coupled plasma–mass spectrometry (ICP–MS) owing to isobaric interferences. In analyzing ICP tandem MS (ICP–MS/MS), CO2 gas reactions in a dynamic reaction cell (DRC) almost eliminated the background noise intensity produced by isobaric interference from isotopes originating from actinides such as Am, Cm, and U at the locations (m/z) of significant Pu isotopes (239Pu, 240Pu, 241Pu, 242Pu, and 244Pu).

Quantifying Molybdenum Isotopic Speciation in Sulfidic Water: Implications for the Paleoredox Proxy

Molybdenum (Mo) isotopic signature and concentrations can be a geologic indicator of changes in molecular oxygen concentrations in Earth’s oceans and atmosphere. However, in natural waters that are weakly to strongly sulfidic, few studies have been able to determine the exact mechanisms controlling partitioning and speciation related to isotopic fractionation of Mo. To understand the isotopic contribution of each Mo species to the overall bulk Mo isotope signature, we report a method that measures both the concentration and the isotopic composition of each Mo-bearing species during thiolation using synthesized sulfidic water. We accomplish this by using reversed-phase ion chromatography to isolate individual Mo species and then measure the isotopic ratio of each species with a multicollector inductively coupled plasma mass spectrometer. Our findings show that progressively thiolated species contribute to an increasingly negative isotopic value (i.e., −3.16 to +3.14) compared to the precursor stock solution. Additionally, these results corroborate Tossell’s theoretical predictions, where lower Mo isotope values correlate to increasing sulfide presence. Furthermore, the formation of thiolated species and evolution of Mo isotopic signatures in our experiments do not occur via a stepwise formation or a quantitative conversion model for molybdate to tetrathiomolybdate. Overall, our study improves our understanding of Mo systematics, shows the potential of measuring specific Mo species, and thereby strengthens the application of Mo paleoproxy through consideration of all Mo species.

Mass spectrometry – based imaging techniques for iodine-127 and iodine-129 detection and localization in the brown alga Laminaria digitata

129I is one of the main radioisotopes of iodine derived from the nuclear fuel cycle that can be found sustainably in the environment due to its long half-life. In coastal marine environment, brown macroalgae, such laminariales (or kelps), are known to naturally feature highest rates of iodine accumulation, and to be an important source of biogenic volatile iodinated compounds released to the atmosphere. These seaweeds are therefore likely to be significantly marked by but also potential vectors of radioactive iodine. In order to better understand the chemical and isotopic speciation of iodine in brown algal tissues, we combined mass spectrometry-based imaging approaches in natural samples of Laminaria digitata young sporophytes, collected at two different locations along the south coast of the English Channel (Roscoff and Goury). Laser desorption ionization (LDI) and desorption electrospray-ionization techniques (DESI), coupled with mass spectrometry, confirmed the predominance of inorganic I− species on the surface of fresh algae, and a peripheral iodine localization when applied on micro-sections. Moreover, radioactive isotope 129I was not detected on plantlet surface or in stipe sections of algal samples collected near Roscoff but was detected in L. digitata samples collected at Goury, near La Hague, where controlled liquid radioactive discharges from the ORANO La Hague reprocessing plant occur. At the subcellular scale, cryo-fixed micro-sections of algal blade samples from both sites were further analyzed by secondary ion mass spectrometry (nano-SIMS), leading to similar results. Even if the signal detected for 129I was much weaker than for 127I in samples from Goury, the chemical imaging revealed some differences in extracellular distribution between radioactive and stable iodine isotopes. Altogether LDI and nano-SIMS are complementary and powerful techniques for the detection and localization of iodine isotopes in algal samples, and for a better understanding of radioactive and stable iodine uptake mechanisms in the marine environment.

Stable calcium isotope speciation and calcium oxalate production within beech tree (Fagus sylvatica L.) organs

In this study, we linked Ca speciation with isotope composition in plants. To do this, we performed leachate experiments to access the soluble Ca, structurally bound Ca and insoluble Ca (i.e., water and weak acid resistant) within beech tree organs (Fagus sylvatica L.). Ca isotopic measurements were combined with infrared spectroscopy and calcium oxalate biomineralization identification. The results from our study indicate that bark and leaves are the most enriched in monohydrated calcium oxalate crystals (whewellite), which are observable in parenchyma and sclerenchyma tissues, whereas roots and wood are enriched in structurally bound Ca. Our leaching experiments also show decreasing δ44/40Ca isotopic signatures in the order of soluble Ca > structurally bound Ca > insoluble Ca. This finding implies that because leaves degrade faster than wooden organs and because Ca linked to pectate decomposes faster than Ca linked to oxalate crystals, differential Ca isotopic signatures are expected to be observed during litter degradation.

Isotopic composition and speciation of sedimentary nitrogen and carbon in the Okinawa Trough over the past 30 ka

AbstractTotal organic carbon to total nitrogen ratios (C/N) and their isotopic compositions (δ13CTOC and δ15NTN) are oft‐applied proxies to discern terrigenous from marine‐sourced organics and to unravel ancient environmental information. In high depositional Asian marginal seas, the inclusion of N‐bearing minerals in the bulk sediment dilutes or masks signals sought after in δ15N and δ13C analyses, thus leading to incorrect and even contradictory interpretations. We used KOH‐KOBr to separate operationally defined total organic matter into oxidizable (labile) and residual fractions for content and isotope measurements. In a sediment core in the Okinawa Trough, significant amounts of carbon and nitrogen existed in the residual phase, in which the C/N ratio was ~9, resembling most documented sedimentary bulk C/N ratios in the China marginal seas. The residual carbon, which was included in operationally defined total organic carbon (TOC), displayed a δ13C range (−22.7 to −18.9‰, mean −20.7‰) similar to the oft‐used marine end‐member. After removing the interference from the residual fraction, we found that the temporal pattern of δ13CLOC (LOC denotes labile organic carbon) was more variable. The residual nitrogen content was associated with illite, suggesting a terrestrial origin. Additionally, δ15N in the residual fraction likely reflected the climatic controls (e.g., precipitation) on lithogenic source materials. Further studies are required to explore the controlling factors for carbon and nitrogen isotopic speciation and to retrieve the information locked in the residual fraction.

An experimental approach to investigate mercury species transformations under redox oscillations in coastal sediments

This work describes a laboratory experiment designed to unravel mercury species reactivity in superficial coastal sediments oscillating between oxic and anoxic conditions. The experimental set-up has been applied to a sediment slurry from the Arcachon Bay (France) to follow the evolution of both naturally occurring (i.e. endogenous) and isotopically enriched added mercury species (i.e. exogenous, 199IHg and 201MMHg) at environmental levels. The transformation and partition between the different phases (aqueous, solid and gaseous) of the endogenous and exogenous mercury species (inorganic Hg (IHg), monomethyl Hg (MMHg), elemental Hg (Hg°) and dimethyl Hg (DMHg)) have been investigated by isotopic speciation methods throughout the experiment. The results demonstrate that the experimental approach is able to promote sediment redox oscillations and to simultaneously follow the biogeochemical fate of naturally occurring or added mercury species. Experimentally driven redox transition events were found to significantly enhance the aqueous Hg species concentrations, while the MMHg burden is not greatly affected. Indeed, during the anoxic-oxic transition, while aqueous endogenous IHg and MMHg exhibited a two-fold increase, aqueous exogenous IHg and MMHg increased 7 and 4 times, respectively. Transient increases of the net IHg methylation were recorded during the redox transitions with the largest increase of the MMHg contents (factor 1.8) observed during the oxic-anoxic transition. High resolution in situ redox experiments have not been performed up to now, therefore the developed experimental set-up provides novel insights in both the influence of redox conditions on Hg methylation/demethylation and adsorption/desorption processes and kinetics in superficial sediments.

Study of the partitioning of uranium and thorium in Moroccan black shale

Moroccan black shale was analysed for uranium and thorium isotopes using alpha-spectrometry. A sequential leaching procedure was used to define the speciation of isotopes of the actinides in the material. As in most other organic rich rocks, uranium is highly enriched in the black shale. This actinide is associated predominantly with humic acids. Thorium (232Th) is a less mobile radionuclide in this environment as was expected from its chemical properties, and in agreement with the most widely adopted views in the published literature. It is partitioned between silicate minerals, pyrite and kerogen.

CO2 dynamics in the Amargosa Desert: Fluxes and isotopic speciation in a deep unsaturated zone

Natural unsaturated‐zone gas profiles at the U.S. Geological Survey's Amargosa Desert Research Site, near Beatty, Nevada, reveal the presence of two physically and isotopically distinct CO2 sources, one shallow and one deep. The shallow source derives from seasonally variable autotrophic and heterotrophic respiration in the root zone. Scanning electron micrograph results indicate that at least part of the deep CO2 source is associated with calcite precipitation at the 110‐m‐deep water table. We use a geochemical gas‐diffusion model to explore processes of CO2 production and behavior in the unsaturated zone. The individual isotopic species 12CO2, 13CO2, and 14CO2 are treated as separate chemical components that diffuse and react independently. Steady state model solutions, constrained by the measured δ13C (in CO2), and δ14C (in CO2) profiles, indicate that the shallow CO2 source from root and microbial respiration composes ∼97% of the annual average total CO2 production at this arid site. Despite the small contribution from deep CO2 production amounting to ∼0.1 mol m−2 yr−1, upward diffusion from depth strongly influences the distribution of CO2 and carbon isotopes in the deep unsaturated zone. In addition to diffusion from deep CO2 production, 14C exchange with a sorbed CO2 phase is indicated by the modeled δ14C profiles, confirming previous work. The new model of carbon‐isotopic profiles provides a quantitative approach for evaluating fluxes of carbon under natural conditions in deep unsaturated zones.

Analytical Methods: Improvements, Advancements and New Horizons

The workshop and exhibits dealing with analytical methods were selected to highlight the current state of the art in elemental analysis. The presentations in the first part of the workshop described approaches and advances important to the analysis of trace minerals. These presentations included: 1) two approaches to mass spectrometry, inductively coupled plasma and accelerator mass spectrometry; 2) use of nuclear magnetic resonance in studies of mineral function; and 3) the use and limitations of fluorescent probes in studies of metal uptake and regulation. In the second part of the workshop, the International Atomic Energy's contributions to nutritional "metrology" were described. Advances in instrumentation over the past decade have led to extraordinary improvements in the precision and sensitivity of mineral analyses. The ability to address isotopic speciation at such low levels sets the stage for numerous novel approaches in the assessment of trace mineral function.

Biomedical aspects of trace element speciation

Speciation affects the bioavailability and toxicity of elements and so is important in toxicology and nutrition. Exploitation of speciation profiles in medical management is widely unexplored. Isotopic speciation in the body can also offer clues to sources of exposure. The redox state of some elements determines toxicity and affects transport across biological barriers. Distinguishing inorganic from organometallic forms of Hg, Pb, and Sn is important to assess exposure to the more toxic organometallic species, whereas the organic forms of the metalloids As and Se reflect metabolism and detoxification. Special questions of speciation arise in therapeutic metal chelation and the use of metal-based drugs, contrast, and imaging agents. Essential elements in blood plasma are distributed among one or more macromolecular species, with generally a small low molecular mass fraction that is difficult to define because of the complex composition of the biological fluid, including amino and other organic acids, and thiols. Albumin and/or transferrin dominate the macromolecular species of many essential and non-essential elements.

A preliminary investigation of the operational and isotopic speciation of uranium in sediments

Uranium was determined in extracts of inter-tidal sediment, obtained from a contaminated harbour, using the three-stage sequential extraction procedure recommended by BCR. The element was found mainly in association with reducible, or reducible and oxidisable, phases and the overall concentration was enhanced (up to 6.7 μg g–1 dry weight) with respect to typical, UK levels. The 238U/235U ratio has been measured in digests of stream sediments obtained from the vicinity of a uranium enrichment plant. Significant enhancement in 235U with respect to the natural 238U/235U ratio (137.88:1) was observed at some locations.

Plasma source mass spectrometry in experimental nutrition.

The development and commercial availability of plasma ion source, specifically inductively coupled plasma, mass spectrometers (ICP-MS) have significantly extended the potential application of stable isotopes for nutritional modeling. The status of research and commercial ICP-MS instruments, and their applications and limitations for stable isotopic studies are reviewed. The consequences of mass spectroscopic resolution and measurement sensitivity obtainable with quadrupole, sector, time-of-flight, and trap instruments on stable isotope analysis are examined. Requirements for reliable isotope measurements with practical biological samples including tissues and fluids are considered. The possibility for stable isotope analysis in chemically separated compounds (speciation) also is explored. On-line compound separations by chromatography or electrophoresis, for example, have been combined instrumentally with ICP-MS. Som possibilities and requirements are described for stable isotope speciation analysis.

Partitioning of Trace Metals in a Macrotidal Estuary. Implications for Contaminant Transport Models

The estuarine geochemical behaviour of trace metals has been studied in a series of experiments using samples from the Dee Estuary (north-east Wales) spiked with radiotracers 109Cd, 51Cr, 137Cs, 59 Fe, 55Mn and 65Zn. The solid-solution phase partitioning, quantified using K D s, is dependent on salinity, particle size/composition (operationally separated in settling experiments), pre-equilibrated speciation of isotopes, and the phase selected to encompass flocculant products. Although quantitatively the results are metal-specific and site-(estuary-) specific, these concepts are generally applicable to predictive estuarine contaminant transport models. To this end, a model for the removal of dissolved Cr in an estuarine turbidity maximum is developed.