Vladimir N. Epov

Species-Specific Stable Isotope Fractionation of Mercury during Hg(II) Methylation by an Anaerobic Bacteria (Desulfobulbus propionicus) under Dark Conditions

This work reports the first results on the stable isotope fractionation of Hg during methylation by anaerobic bacteria under dark conditions. The GC-MC-ICPMS methodology employed is capable of simultaneously measuring the species-specific isotopic composition of different Hg species within the same sample. We have studied Hg isotopic fractionation caused by methylation of Hg(II) standard reference material NIST-3133 in the presence of the pure bacterial strain Desulfobulbus propionicus MUD10 (DSM 6523) under fermentative conditions. We have measured the isotopic composition of Hg(II) and monomethyl mercury (MMHg) in these cultures as a function of time and calculated delta-values for both species versus the starting material (NIST-3133) as a delta-zero standard. Two different strategies for the incubation were applied: single sampling cultures and a continuous sampling culture. The results obtained have shown that under the conditions employed in this work the methylation of Hg(II) causes mass-dependent fractionation of the Hg isotopes for both Hg(II) substrate and produced MMHg. Such a process occurred under the exponential growth of the bacteria which preferentially methylate the lighter isotopes of Hg. After 96 h for the continuous culture and 140 h for the single sampling cultures, we observed a change in the fractionation trend in the samples at a similar cell density value (ca. 6.0 x 10(7) cells mL(-1)) which suggests the increasing contribution of a simultaneous process balancing methylation extent such as demethylation. Assuming that Rayleigh type fractionation conditions are met before such suppression, we have obtained a alpha(202/198) fractionation factor of 1.0026 +/- 0.0004 for the single sampling cultures.

Simultaneous Determination of Species-Specific Isotopic Composition of Hg by Gas Chromatography Coupled to Multicollector ICPMS

This work presents the simultaneous online determination of the isotopic composition of different Hg species in a single sample by the hyphenation of gas chromatography (GC) with multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS). With the use of commercially available instrumentation, precise and accurate species-specific Hg isotope delta values (per mil deviation of the Hg isotope ratio in the sample relative to a reference standard) have been obtained online from consecutive GC transient signals. The use of isothermal temperature programs to extend the elution of the Hg species, the proper selection of the peak integration window, as well as the preconcentration of real samples are critical to provide optimal counting statistics. Also, isotope ratio drift during transient signal elution was overcome by introducing a mixed Hg(II) and MeHg standard bracketing scheme and expressing all results using the delta-notation relative to SRM NIST-3133. Using the proposed methodology, we have obtained an external 2SD precision of 0.56 per thousand for delta (202)Hg that is more than 10 times smaller than the overall Hg stable isotope variation thus far observed in terrestrial samples. The measurement of species-specific Hg isotopic composition relative to SRM NIST-3133 has been validated versus two other analytical techniques, i.e., conventional nebulization (CN) of Hg(II) solution and cold vapor (CV) generation of Hg (0) vapor. A good agreement between the species-specific delta values obtained by the different techniques has been obtained in secondary fractionated reference standard (UM-Almaden) and environmental matrixes, i.e., BCR-CRM 464 (tuna fish) and IAEA-085 (human hair). The results show mass-dependent and mass-independent fractionation in environmental samples, i.e., mass-independent fractionation of odd isotopes (199)Hg and (201)Hg in tuna fish was observed. This methodology provides new possibilities for the future study of species-specific stable isotope geochemistry of Hg and other trace metals.

Tracing Sources and Bioaccumulation of Mercury in Fish of Lake Baikal- Angara River Using Hg Isotopic Composition

This study presents the determination and comparison of isotopic compositions of Hg in sediments, plankton, roach, and perch of two freshwater systems in the Lake Baikal-Angara River aquatic ecosystem: the man-made Bratsk Water Reservoir contaminated by Hg from a chlor-alkali factory and the noncontaminated Lake Baikal. Isotopic ratios of biota exhibit both significant mass-independent fractionation (MIF) (Δ(199)Hg from 0.20 to 1.87‰) and mass-dependent fractionation (MDF) (δ(202)Hg from -0.97 to -0.16‰), whereas sediments exhibit high MDF (δ(202)Hg from -1.99 to -0.83‰) but no MIF. δ(15)N and δ(13)C are correlated with methylmercury in organisms from both sites, indicating bioaccumulation and biomagnification through food webs of both regions. Combining this with isotopic composition of samples shows that δ(202)Hg increases with the trophic level of organisms and also with methylmercury in fish from Lake Baikal. This study demonstrates that MIF in fish samples from Bratsk Water Reservoir allow to trace anthropogenic Hg, since fish with the highest levels of Hg in muscle have the same isotopic composition as the sediment in which anthropogenic Hg was deposited. Less contaminated fish do not exhibit this anthropogenic signature accumulating relatively lower Hg amount from the contaminated sediments. This work reveals that Hg isotopic composition can be used to track the contribution of anthropogenic sources in fish from a contaminated lake.

Approach to Measure Isotopic Ratios in Species Using Multicollector-ICPMS Coupled with Chromatography

A new approach was demonstrated for the isotope ratio measurement in different elemental species of Hg using transient signal obtained by chromatography coupled with multicollector-inductively coupled plasma mass spectrometry (MC-ICPMS). The method based on the slope of linear regression by transient intensities of different isotopes shows improved accuracy and reproducibility (0.2-0.5 per thousand as 2 standard deviation (SD)). Internal precision (RSD) of the method is very close to the theoretical value given by the counting statistic and is better by a factor of 6 in comparison with previous conventional methods of calculation. We demonstrated that internal RSD (uncertainty) depends on regression coefficients of the linear function (R(2)). The typical internal precision of isotopic ratio measurements (0.003-0.02%) was achieved for delta(202)Hg when injecting as low as 90 pg of Hg species. With the new methodology, it is possible to (i) measure the isotopic composition when a sample and a bracketing standard have significantly different concentrations, (ii) measure the isotopic composition of different species in samples versus single species in a bracketing standard, and (iii) measure the isotopic ratios for low abundant isotopes. We demonstrated application of this method for different environmental samples and processes.

Higher Mass-Independent Isotope Fractionation of Methylmercury in the Pelagic Food Web of Lake Baikal (Russia)

Mercury undergoes several transformations that influence its stable isotope composition during a number of environmental and biological processes. Measurements of Hg isotopic mass-dependent (MDF) and mass-independent fractionation (MIF) in food webs may therefore help to identify major sources and processes leading to significant bioaccumulation of methylmercury (MeHg). In this work, δ(13)C, δ(15)N, concentration of Hg species (MeHg, inorganic Hg), and stable isotopic composition of Hg were determined at different trophic levels of the remote and pristine Lake Baikal ecosystem. Muscle of seals and different fish as well as amphipods, zooplankton, and phytoplankton were specifically investigated. MDF during trophic transfer of MeHg leading to enrichment of heavier isotopes in the predators was clearly established by δ(202)Hg measurements in the pelagic prey-predator system (carnivorous sculpins and top-predator seals). Despite the low concentrations of Hg in the ecosystem, the pelagic food web reveals very high MIF Δ(199)Hg (3.15-6.65‰) in comparison to coastal fish (0.26-1.65‰) and most previous studies in aquatic organisms. Trophic transfer does not influence MIF signature since similar Δ(199)Hg was observed in sculpins (4.59 ± 0.55‰) and seal muscles (4.62 ± 0.60‰). The MIF is suggested to be mainly controlled by specific physical and biogeochemical characteristics of the water column. The higher level of MIF in pelagic fish of Lake Baikal is mainly due to the bioaccumulation of residual MeHg that is efficiently turned over and photodemethylated in deep oligotrophic and stationary (i.e., long residence time) freshwater columns.

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.

Is it really organic? - Multi-isotopic analysis as a tool to discriminate between organic and conventional plants

Novel procedures for analytical authentication of organic plant products are urgently needed. Here we present the first study encompassing stable isotopes of hydrogen, carbon, nitrogen, oxygen, magnesium and sulphur as well as compound-specific nitrogen and oxygen isotope analysis of nitrate for discrimination of organically and conventionally grown plants. The study was based on wheat, barley, faba bean and potato produced in rigorously controlled long-term field trials comprising 144 experimental plots. Nitrogen isotope analysis revealed the use of animal manure, but was unable to discriminate between plants that were fertilised with synthetic nitrogen fertilisers or green manures from atmospheric nitrogen fixing legumes. This limitation was bypassed using oxygen isotope analysis of nitrate in potato tubers, while hydrogen isotope analysis allowed complete discrimination of organic and conventional wheat and barley grains. It is concluded, that multi-isotopic analysis has the potential to disclose fraudulent substitutions of organic with conventionally cultivated plants.

Species‐specific stable isotope analysis by the hyphenation of chromatographic techniques with MC‐ICPMS

This work reviews the basis and all the existing publications on the hyphenation of chromatography-based techniques to MC-ICPMS for isotopic studies that were published until the end of 2010. A brief historical retrospective of the measurement of isotope ratios from transient signals by ICPMS with different sample introduction techniques is also included. The most important experimental parameters and data reduction strategies affecting the accurate and precise measurement of compound-specific isotope ratios by either HPLC or GC coupled to MC-ICPMS are discussed. All the applications are reported and critically reviewed in terms of analytical characteristics, performances, optimization, advantages and disadvantages and future applicability to the environmental, geochemical, or bioinorganic studies.