Evaluating Chlorine Isotope Effects from Isotope Ratios and Mass Spectra of Polychlorinated Molecules

A Novel Multi-ion Evaluation Scheme to Determine Stable Chlorine Isotope Ratios (37Cl/35Cl) of Chlordecone by LC-QTOF

Organochlorinated pesticides are highly persistent organic pollutants having important adverse effects in the environment. To study their fate, compound-specific isotope analysis (CSIA) may be used to investigate their degradation pathways and mechanisms but is currently limited to 13C isotope ratios. The assessment of 37Cl isotope ratios from mass spectra is complicated by the large number of isotopologues of polychlorinated compounds. For method development, chlordecone (C10Cl10O2H2; hydrate form), an organochlorine insecticide that led to severe contamination of soils and aquatic ecosystems of the French West Indies, was taken as a model analyte. Chlorine isotope analysis of chlordecone hydrate was evaluated using high-resolution liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS), enabling smooth ionization to detect the molecular ion. First, a new evaluation scheme is presented to correct for multiple isotope presence in polychlorinated compounds. The scheme is based on probability calculations of the most frequent isotopologues, distributions by binomial probability functions, and corrections for the presence of nonchlorine heavy isotopes. Second, mobile-phase modifiers, ionization energy (sampling cone tension) and scan time were optimized for accurate chlorine isotope ratios. Chlordecone standard samples were measured up to 10-fold and bracketed with a second chlordecone external standard. δ37Cl values were obtained after conversion to the SMOC scale by a two-point calibration. The robustness of the analysis method and evaluation scheme were tested and gave satisfactory results with standard errors (σm) of ±0.34‰ for precision and ±0.89‰ for long-term accuracy of chlorine isotope ratios of chlordecone hydrate. This work opens perspectives for applications of the C-Cl CSIA approach to investigate the fate of highly toxic and low reactive polychlorinated compounds in the environment.

Source identification of PCBs in Antarctic air by compound-specific isotope analysis of chlorine (CSIA-Cl) using HRGC/HRMS

Occurrence of persistent organic pollutants (POPs) in the Polar Regions has received great concern in the past several decades due to their long-term adverse effect on biological health in such a fragile environment. However, there is still argument over their source and fate in these pristine areas. Here we attempted to use a novel approach (compound-specific isotope analysis of chlorine, CSIA-Cl) to identify the source of POPs in Antarctic air by comparison with the source area. The results showed that the relative isotope-ratio variation of Cl (δ³⁷Cl') values showed a large variation from - 137 to 9.04 ‰ in the gas-phase samples, and a significantly negative correlation (p < 0.01) was obtained against the logKoa values of PCBs. There were no significant correlations (p > 0.05) observed between the δ³⁷Cl' values and meteorological parameters except for PCB-28 which showed temperature dependence. By contrast, the δ³⁷Cl' values in the urban (Beijing) air ranged from - 12.8 to 2.03 ‰. The larger variation of δ³⁷Cl' in Antarctic air indicated evidently influence of long-range atmospheric transport (LRAT) on isotopologue fractionation of PCBs. This study may shed light on the application of CSIA-Cl for source identification of chlorinated POPs on a large scale.

Dual C-Br Isotope Fractionation Indicates Distinct Reductive Dehalogenation Mechanisms of 1,2-Dibromoethane in Dehalococcoides- and Dehalogenimonas-Containing Cultures

Brominated organic compounds such as 1,2-dibromoethane (1,2-DBA) are highly toxic groundwater contaminants. Multi-element compound-specific isotope analysis bears the potential to elucidate the biodegradation pathways of 1,2-DBA in the environment, which is crucial information to assess its fate in contaminated sites. This study investigates for the first time dual C-Br isotope fractionation during in vivo biodegradation of 1,2-DBA by two anaerobic enrichment cultures containing organohalide-respiring bacteria (i.e., either Dehalococcoides or Dehalogenimonas). Different ε_bulk^C values (-1.8 ± 0.2 and -19.2 ± 3.5‰, respectively) were obtained, whereas their respective ε_bulk^Br values were lower and similar to each other (-1.22 ± 0.08 and -1.2 ± 0.5‰), leading to distinctly different trends (Λ_C-Br = Δδ¹³C/Δδ⁸¹Br ≈ ε_bulk^C/ε_bulk^Br) in a dual C-Br isotope plot (1.4 ± 0.2 and 12 ± 4, respectively). These results suggest the occurrence of different underlying reaction mechanisms during enzymatic 1,2-DBA transformation, that is, concerted dihaloelimination and nucleophilic substitution (S_N2-reaction). The strongly pathway-dependent Λ_C-Br values illustrate the potential of this approach to elucidate the reaction mechanism of 1,2-DBA in the field and to select appropriate ε_bulk^C values for quantification of biodegradation. The results of this study provide valuable information for future biodegradation studies of 1,2-DBA in contaminated sites.

Tracking chlorinated contaminants in the subsurface using compound-specific chlorine isotope analysis: A review of principles, current challenges and applications

Many chlorinated hydrocarbons have gained notoriety as persistent organic pollutants in the environment. Engineered and natural remediation efforts require a monitoring tool to track the progress of degradation processes. Compound-specific isotope analysis (CSIA) is a robust method to evaluate the origin and fate of contaminants in the environment and does not rely on concentration measurements. While carbon CSIA has established itself in the routine assessment of contaminated sites, studies incorporating chlorine isotopes have only recently become more common. Although some aspects of chlorine isotope analysis are more challenging than carbon isotope analysis, having additional isotopic data yields valuable information for contaminated site management. This review provides an overview of chlorine isotope fractionation of chlorinated contaminants in the subsurface by different processes and presents analytical techniques and unresolved challenges in chlorine isotope analysis. A summary of successful field applications illustrates the potential of using chlorine isotope data. Finally, approaches in modelling chlorine isotope fractionation due to degradation, diffusion, and sorption processes are discussed.

Dual carbon - chlorine isotope fractionation during dichloroelimination of 1,1,2-trichloroethane by an enrichment culture containing Dehalogenimonas sp

Chlorinated ethanes are frequent groundwater contaminants but compound specific isotope analysis (CSIA) has been scarcely applied to investigate their degradation pathways. In this study, dual carbon and chlorine isotope fractionation was used to investigate for the first time the anoxic biodegradation of 1,1,2-trichloroethane (1,1,2-TCA) using a Dehalogenimonas-containing culture. The isotopic fractionation values obtained for the biodegradation of 1,1,2-TCA were ɛ_C = -6.9 ± 0.4‰ and ɛ_Cl = -2.7 ± 0.3‰. The detection of vinyl chloride (VC) as unique byproduct and a closed carbon isotopic mass balance corroborated that dichloroelimination was the degradation pathway used by this strain. Combining the values of δ¹³C and δ³⁷Cl resulted in a dual element C-Cl isotope slope of Λ = 2.5 ± 0.2‰. Investigation of the apparent kinetic isotope effects (AKIEs) expected for cleavage of a CCl bond showed an important masking of the intrinsic isotope fractionation. Theoretical calculation of Λ suggested that dichloroelimination of 1,1,2-TCA was taking place via simultaneous cleavage of two CCl bonds (concerted reaction mechanism). The isotope data obtained in this study can be useful to monitor natural attenuation of 1,1,2-TCA via dichloroelimination and provide insights into the source and fate of VC in contaminated groundwaters.

Carbon-bromine bond cleavage - A perspective from bromine and carbon kinetic isotope effects on model debromination reactions

In this work, we explore the effect of solvent on ¹³C and ⁸¹Br kinetic isotope effects (KIEs) during elimination of bromine substituent from brominated organic compounds promoted by hydroxyl anion. In the present study, we investigate HBr elimination from 2-bromoethylbenzene in three different polar media (water, ethanol, and acetonitrile) as well as bromide ion elimination from 1,2-dibromoethane upon nucleophilic substitution by the hydroxyl ion in aqueous solution using carbon and bromine isotope analysis as mechanistic tools. We reconsider the hypothesis that the magnitude of leaving group halide KIE should visibly depend on the solvent and bond-breaking in a protic solvent should be accompanied by hydrogen bonding which would result in less zero-point energy loss than in an aprotic solvent. Modeling the elimination reaction using the available popular theoretical methods along with different approaches for including environment effects we demonstrate in the presented study no interpretable effect of the solvent on the transition state structure and hence on the theoretically predicted KIEs. The comparison of the magnitudes of carbon and bromine kinetic isotope effects for two different mechanistic pathways (elimination vs substitution) is also discussed.

Characterization of Substrate, Cosubstrate, and Product Isotope Effects Associated With Enzymatic Oxygenations of Organic Compounds Based on Compound-Specific Isotope Analysis

Enzymatic oxygenations are among the most important biodegradation and detoxification reactions of organic pollutants. In the environment, however, such natural attenuation processes are extremely difficult to monitor. Changes of stable isotope ratios of aromatic pollutants at natural isotopic abundances serve as proxies for isotope effects associated with oxygenation reactions. Such isotope fractionations offer new avenues for revealing the pathway and extent of pollutant transformation and provide new insights into the mechanisms of catalysis by Rieske non-heme ferrous iron oxygenases. Based on compound-specific C, H, N, and O isotope analysis, we present a comprehensive methodology with which isotope effects can be derived from the isotope fractionation measured in substrates, the cosubstrate O₂, and organic oxygenation products. We use dioxygenation of nitrobenzene and 2-nitrotoluene by nitrobenzene dioxygenase as illustrative examples to introduce different mathematical procedures for deriving apparent substrate and product isotope effects. We present two experimental approaches to control reactant and product turnover for isotope fractionation analysis in experimental systems containing purified enzymes, E. coli clones, and pure strains of environmental microorganisms. Finally, we present instrumental procedures and sample treatment instructions for analysis of C, H, and N isotope analysis in organic compounds and O isotope analysis in aqueous O₂ by gas and liquid chromatography coupled to isotope ratio mass spectrometry.

Measurement and Prediction of Chlorine Kinetic Isotope Effects in Enzymatic Systems

Approaches to determine chlorine kinetic isotope effects (Cl-KIEs) on enzymatic dehalogenations are discussed and illustrated by representative examples. Three aspects are considered. First methodology for experimental measurement of Cl-KIEs, with stress being on FAB-IRMS technique developed in our laboratory, is described. Subsequently, we concentrate our discussion on the consequences of reaction complexity in the interpretation of experimental values, a problem especially important in cases of polychlorinated reactants. The most fruitful studies of enzymatic dehalogenations by Cl-KIEs require their theoretical evaluation, hence the computational focus of the second part of this chapter.

Reductive Outer-Sphere Single Electron Transfer Is an Exception Rather than the Rule in Natural and Engineered Chlorinated Ethene Dehalogenation

Chlorinated ethenes (CEs) such as perchloroethylene, trichloroethylene and dichloroethylene are notorious groundwater contaminants. Although reductive dehalogenation is key to their environmental and engineered degradation, underlying reaction mechanisms remain elusive. Outer-sphere reductive single electron transfer (OS-SET) has been proposed for such different processes as Vitamin B₁₂-dependent biodegradation and zerovalent metal-mediated dehalogenation. Compound-specific isotope effect (¹³C/¹²C, ³⁷Cl/³⁵Cl) analysis offers a new opportunity to test these hypotheses. Defined OS-SET model reactants (CO₂ radical anions, S^2--doped graphene oxide in water) caused strong carbon (ε_C = -7.9‰ to -11.9‰), but negligible chlorine isotope effects (ε_Cl = -0.12‰ to 0.04‰) in CEs. Greater chlorine isotope effects were observed in CHCl₃ (ε_C = -7.7‰, ε_Cl = -2.6‰), and in CEs when the exergonicity of C-Cl bond cleavage was reduced in an organic solvent (reaction with arene radical anions in glyme). Together, this points to dissociative OS-SET (SET to a σ* orbital concerted with C-Cl breakage) in alkanes compared to stepwise OS-SET (SET to a π* orbital followed by C-Cl cleavage) in ethenes. The nonexistent chlorine isotope effects of chlorinated ethenes in all aqueous OS-SET experiments contrast strongly with pronounced Cl isotope fractionation in all natural and engineered reductive dehalogenations reported to date suggesting that OS-SET is an exception rather than the rule in environmental transformations of chlorinated ethenes.

Distinct Dual C-Cl Isotope Fractionation Patterns during Anaerobic Biodegradation of 1,2-Dichloroethane: Potential To Characterize Microbial Degradation in the Field

This study investigates, for the first time, dual C-Cl isotope fractionation during anaerobic biodegradation of 1,2-dichloroethane (1,2-DCA) via dihaloelimination by Dehalococcoides and Dehalogenimonas-containing enrichment cultures. Isotopic fractionation of 1,2-DCA (ε_bulk^C and ε_bulk^Cl) for Dehalococcoides (-33.0 ± 0.4‰ and -5.1 ± 0.1‰) and Dehalogenimonas-containing microcosms (-23 ± 2‰ and -12.0 ± 0.8‰) resulted in distinctly different dual element C-Cl isotope correlations (Λ = Δδ¹³C/Δδ³⁷Cl ≈ ε_bulk^C/ε_bulk^Cl), 6.8 ± 0.2 and 1.89 ± 0.02, respectively. Determined isotope effects and detected products suggest that the difference on the obtained Λ values for biodihaloelimination could be associated with a different mode of concerted bond cleavage rather than two different reaction pathways (i.e., stepwise vs concerted). Λ values of 1,2-DCA were, for the first time, determined in two field sites under reducing conditions (2.1 ± 0.1 and 2.2 ± 2.9). They were similar to the one obtained for the Dehalogenimonas-containing microcosms (1.89 ± 0.02) and very different from those reported for aerobic degradation pathways in a previous laboratory study (7.6 ± 0.1 and 0.78 ± 0.03). Thus, this study illustrates the potential of a dual isotope analysis to differentiate between aerobic and anaerobic biodegradation pathways of 1,2-DCA in the field and suggests that this approach might also be used to characterize dihaloelimination of 1,2-DCA by different bacteria, which needs to be confirmed in future studies.

Calibration bias of experimentally determined chlorine isotope enrichment factors: the need for a two-point calibration in compound-specific chlorine isotope analysis

RATIONALE

The recent development of compound-specific online chlorine isotope analysis (³⁷ Cl-CSIA) methods has fostered dual chlorine-carbon isotope studies to gain better insights into sources and environmental transformation reactions of chlorinated ethenes. One-point and two-point calibration schemes are currently used to convert raw data to the international δ³⁷ Cl_SMOC scale, but a critical evaluation of best practices to arrive at reliable δ³⁷ Cl_SMOC signatures and enrichment factors was missing and is presented here.

METHODS

Aqueous solutions of neat perchloroethylene and trichloroethylene (TCE) and aqueous samples from a TCE biodegradation experiment with pure cultures of Desulfitobacterium hafniense Y51 were analysed for their chlorine isotope ratios using GC/qMS and GC/IRMS. The δ³⁷ Cl_SMOC values were obtained using one-point and two-point calibration schemes. Chlorine isotope enrichment factors, ε_Cl , were calculated using both approaches and the corresponding bias of δ³⁷ Cl_SMOC values introduced by the different types of calibration was determined.

RESULTS

Different calibration methods resulted in significant differences (up to 30%) in both δ³⁷ Cl signatures and ε_Cl values.

CONCLUSIONS

Our results demonstrate that a two-point calibration together with comprehensive information on reference materials is indispensable and should become standard practice for reliable ³⁷ Cl-CSIA of organic compounds. Copyright © 2016 John Wiley & Sons, Ltd.

Resiliency of Stable Isotope Fractionation (δ(13)C and δ(37)Cl) of Trichloroethene to Bacterial Growth Physiology and Expression of Key Enzymes

Quantification of in situ (bio)degradation using compound-specific isotope analysis requires a known and constant isotope enrichment factor (ε). Because reported isotope enrichment factors for microbial dehalogenation of chlorinated ethenes vary considerably we studied the potential effects of metabolic adaptation to TCE respiration on isotope fractionation (δ(13)C and δ(37)Cl) using a model organism (Desulfitobacterium hafniesne Y51), which only has one reductive dehalogenase (PceA). Cells grown on TCE for the first time showed exponential growth until 10(9) cells/mL. During exponential growth, the cell-normalized amount of PceA enzyme increased steadily in the presence of TCE (up to 21 pceA transcripts per cell) but not with alternative substrates (<1 pceA transcript per cell). Cultures initially transferred or subcultivated on TCE showed very similar isotope fractionation, both for carbon (εcarbon: -8.6‰ ± 0.3‰ or -8.8‰ ± 0.2‰) and chlorine (εchlorine: -2.7‰ ± 0.3‰) with little variation (0.7‰) for the different experimental conditions. Thus, TCE isotope fractionation by D. hafniense strain Y51 was affected by neither growth phase, pceA transcription, or translation, nor by PceA content per cell, suggesting that transport limitations did not affect isotope fractionation. Previously reported variable ε values for other organohalide-respiring bacteria might thus be attributed to different expression levels of their multiple reductive dehalogenases.

Carbon and chlorine isotope analysis to identify abiotic degradation pathways of 1,1,1-trichloroethane

This study investigates dual C-Cl isotope fractionation during 1,1,1-TCA transformation by heat-activated persulfate (PS), hydrolysis/dehydrohalogenation (HY/DH) and Fe(0). Compound-specific chlorine isotope analysis of 1,1,1-TCA was performed for the first time, and transformation-associated isotope fractionation ε bulk C and ε bulk Cl values were -4.0 ± 0.2‰ and no chlorine isotope fractionation with PS, -1.6 ± 0.2‰ and -4.7 ± 0.1‰ for HY/DH, -7.8 ± 0.4‰ and -5.2 ± 0.2‰ with Fe(0). Distinctly different dual isotope slopes (Δδ13C/Δδ37Cl): ∞ with PS, 0.33 ± 0.04 for HY/DH and 1.5 ± 0.1 with Fe(0) highlight the potential of this approach to identify abiotic degradation pathways of 1,1,1-TCA in the field. The trend observed with PS agreed with a C-H bond oxidation mechanism in the first reaction step. For HY/DH and Fe(0) pathways, different slopes were obtained although both pathways involve cleavage of a C-Cl bond in their initial reaction step. In contrast to the expected larger primary carbon isotope effects relative to chlorine for C-Cl bond cleavage, ε bulk C < ε bulk Cl was observed for HY/DH and in a similar range for reduction by Fe(0), suggesting the contribution of secondary chlorine isotope effects. Therefore, different magnitude of secondary chlorine isotope effects could at least be partly responsible for the distinct slopes between HY/DH and Fe(0) pathways. Following this dual isotope approach, abiotic transformation processes can unambiguously be identified and quantified.

C and Cl isotope fractionation of 1,2-dichloroethane displays unique δ¹³C/δ³⁷Cl patterns for pathway identification and reveals surprising C-Cl bond involvement in microbial oxidation

This study investigates dual element isotope fractionation during aerobic biodegradation of 1,2-dichloroethane (1,2-DCA) via oxidative cleavage of a C-H bond (Pseudomonas sp. strain DCA1) versus C-Cl bond cleavage by S(N)2 reaction (Xanthobacter autotrophicus GJ10 and Ancylobacter aquaticus AD20). Compound-specific chlorine isotope analysis of 1,2-DCA was performed for the first time, and isotope fractionation (ε(bulk)(Cl)) was determined by measurements of the same samples in three different laboratories using two gas chromatography-isotope ratio mass spectrometry systems and one gas chromatography-quadrupole mass spectrometry system. Strongly pathway-dependent slopes (Δδ13C/Δδ37Cl), 0.78 ± 0.03 (oxidation) and 7.7 ± 0.2 (S(N)2), delineate the potential of the dual isotope approach to identify 1,2-DCA degradation pathways in the field. In contrast to different ε(bulk)(C) values [-3.5 ± 0.1‰ (oxidation) and -31.9 ± 0.7 and -32.0 ± 0.9‰ (S(N)2)], the obtained ε(bulk)(Cl) values were surprisingly similar for the two pathways: -3.8 ± 0.2‰ (oxidation) and -4.2 ± 0.1 and -4.4 ± 0.2‰ (S(N)2). Apparent kinetic isotope effects (AKIEs) of 1.0070 ± 0.0002 (13C-AKIE, oxidation), 1.068 ± 0.001 (13C-AKIE, S(N)2), and 1.0087 ± 0.0002 (37Cl-AKIE, S(N)2) fell within expected ranges. In contrast, an unexpectedly large secondary 37Cl-AKIE of 1.0038 ± 0.0002 reveals a hitherto unrecognized involvement of C-Cl bonds in microbial C-H bond oxidation. Our two-dimensional isotope fractionation patterns allow for the first time reliable 1,2-DCA degradation pathway identification in the field, which unlocks the full potential of isotope applications for this important groundwater contaminant.

Diffusive fractionation of BTEX and chlorinated ethenes in aqueous solution: quantification of spatial isotope gradients

Laboratory experiments were performed to investigate and quantify the extent of diffusive isotope fractionation of organic contaminants in aqueous solution. We selected petroleum hydrocarbons (toluene and ethylbenzene, in 1:2 mixtures of labeled (perdeuterated) and nonlabeled isotopologues) and chlorinated solvents (trichloroethene, TCE, and cis-dichloroethene, cis-DCE, at their natural isotopic abundance) as model compounds. The experimental approach using gel diffusion tubes allowed us to resolve concentration and isotopic gradients induced by isotopologue-specific diffusion and to determine aqueous diffusion coefficients in agreement with the values calculated using published empirical correlations. The experimental results were quantitatively evaluated with numerical simulations to determine the aqueous diffusion coefficients, D, and the exponent of the inverse power-law relation between D and the molecular mass of the isotopologues. The results show remarkable diffusive isotope fractionation for all the investigated organic compounds; however, the extent of fractionation was found to be smaller for the chlorinated ethenes and remarkably deviating from an inverse square root relationship between the isotopologues diffusion coefficients and their molecular mass. The outcomes of this study are relevant for the interpretation of isotopic signatures of organic contaminants in environmental systems and for the quantitative application of compound specific isotope analysis (CSIA) that needs to take into account the fractionation effects of both physical and transformation processes.

Chlorine isotope effects from isotope ratio mass spectrometry suggest intramolecular C-Cl bond competition in trichloroethene (TCE) reductive dehalogenation

Chlorinated ethenes are prevalent groundwater contaminants. To better constrain (bio)chemical reaction mechanisms of reductive dechlorination, the position-specificity of reductive trichloroethene (TCE) dehalogenation was investigated. Selective biotransformation reactions (i) of tetrachloroethene (PCE) to TCE in cultures of Desulfitobacterium sp. strain Viet1; and (ii) of TCE to cis-1,2-dichloroethene (cis-DCE) in cultures of Geobacter lovleyi strain SZ were investigated. Compound-average carbon isotope effects were −19.0‰ ± 0.9‰ (PCE) and −12.2‰ ± 1.0‰ (TCE) (95% confidence intervals). Using instrumental advances in chlorine isotope analysis by continuous flow isotope ratio mass spectrometry, compound-average chorine isotope effects were measured for PCE (−5.0‰ ± 0.1‰) and TCE (−3.6‰ ± 0.2‰). In addition, position-specific kinetic chlorine isotope effects were determined from fits of reactant and product isotope ratios. In PCE biodegradation, primary chlorine isotope effects were substantially larger (by −16.3‰ ± 1.4‰ (standard error)) than secondary. In TCE biodegradation, in contrast, the product cis-DCE reflected an average isotope effect of −2.4‰ ± 0.3‰ and the product chloride an isotope effect of −6.5‰ ± 2.5‰, in the original positions of TCE from which the products were formed (95% confidence intervals). A greater difference would be expected for a position-specific reaction (chloride would exclusively reflect a primary isotope effect). These results therefore suggest that both vicinal chlorine substituents of TCE were reactive (intramolecular competition). This finding puts new constraints on mechanistic scenarios and favours either nucleophilic addition by Co(I) or single electron transfer as reductive dehalogenation mechanisms.

Reductive dechlorination of TCE by chemical model systems in comparison to dehalogenating bacteria: insights from dual element isotope analysis (13C/12C, 37Cl/35Cl)

Chloroethenes like trichloroethene (TCE) are prevalent environmental contaminants, which may be degraded through reductive dechlorination. Chemical models such as cobalamine (vitamin B12) and its simplified analogue cobaloxime have served to mimic microbial reductive dechlorination. To test whether in vitro and in vivo mechanisms agree, we combined carbon and chlorine isotope measurements of TCE. Degradation-associated enrichment factors ε(carbon) and ε(chlorine) (i.e., molecular-average isotope effects) were -12.2‰ ± 0.5‰ and -3.6‰ ± 0.1‰ with Geobacter lovleyi strain SZ; -9.1‰ ± 0.6‰ and -2.7‰ ± 0.6‰ with Desulfitobacterium hafniense Y51; -16.1‰ ± 0.9‰ and -4.0‰ ± 0.2‰ with the enzymatic cofactor cobalamin; -21.3‰ ± 0.5‰ and -3.5‰ ± 0.1‰ with cobaloxime. Dual element isotope slopes m = Δδ(13)C/ Δδ(37)Cl ≈ ε(carbon)/ε(chlorine) of TCE showed strong agreement between biotransformations (3.4 to 3.8) and cobalamin (3.9), but differed markedly for cobaloxime (6.1). These results (i) suggest a similar biodegradation mechanism despite different microbial strains, (ii) indicate that transformation with isolated cobalamin resembles in vivo transformation and (iii) suggest a different mechanism with cobaloxime. This model reactant should therefore be used with caution. Our results demonstrate the power of two-dimensional isotope analyses to characterize and distinguish between reaction mechanisms in whole cell experiments and in vitro model systems.

Carbon and chlorine isotope fractionation during microbial degradation of tetra- and trichloroethene

Two-dimensional compound-specific isotope analysis (2D-CSIA), combining stable carbon and chlorine isotopes, holds potential for monitoring of natural attenuation of chlorinated ethenes (CEs) in contaminated soil and groundwater. However, interpretation of 2D-CSIA data sets is challenged by a shortage of experimental Cl isotope enrichment factors. Here, isotope enrichments factors for C and Cl (i.e., εC and εCl) were determined for biodegradation of tetrachloroethene (PCE) and trichloroethene (TCE) using microbial enrichment cultures from a heavily CE-contaminated aquifer. The obtained values were εC = -5.6 ± 0.7‰ (95% CI) and εCl = -2.0 ± 0.5‰ for PCE degradation and εC = -8.8 ± 0.2‰ and εCl = -3.5 ± 0.5‰ for TCE degradation. Combining the values for both εC and εCl yielded mechanism-diagnostic εCl/εC ratios of 0.35 ± 0.11 and 0.37 ± 0.11 for the degradation of PCE and TCE, respectively. Application of the obtained εC and εCl values to a previously investigated field site gave similar estimates for the fraction of degraded contaminant as in the previous study, but with a reduced uncertainty in assessment of the natural attenuation. Furthermore, 16S rRNA gene clone library analyses were performed on three samples from the PCE degradation experiments. A species closely related to Desulfitobacterium aromaticivorans UKTL dominated the reductive dechlorination process. This study contributes to the development of 2D-CSIA as a tool for evaluating remediation strategies of CEs at contaminated sites.

Integrated carbon and chlorine isotope modeling: applications to chlorinated aliphatic hydrocarbons dechlorination

We propose a self-consistent method to predict the evolution of carbon and chlorine isotope ratios during degradation of chlorinated hydrocarbons. The method treats explicitly the cleavage of isotopically different C-Cl bonds and thus considers, simultaneously, combined carbon-chlorine isotopologues. To illustrate the proposed modeling approach we focus on the reductive dehalogenation of chlorinated ethenes. We compare our method with the currently available approach, in which carbon and chlorine isotopologues are treated separately. The new approach provides an accurate description of dual-isotope effects regardless of the extent of the isotope fractionation and physical characteristics of the experimental system. We successfully applied the new approach to published experimental results on dehalogenation of chlorinated ethenes both in well-mixed systems and in situations where mass-transfer limitations control the overall rate of biodegradation. The advantages of our self-consistent dual isotope modeling approach proved to be most evident when isotope fractionation factors of carbon and chlorine differed significantly and for systems with mass-transfer limitations, where both physical and (bio)chemical transformation processes affect the observed isotopic values.

Chlorine and carbon isotopes fractionation during volatilization and diffusive transport of trichloroethene in the unsaturated zone

To apply compound-specific isotope methods to the evaluation of the origin and fate of organic contaminants in the unsaturated subsurface, the effect of physicochemical processes on isotope ratios needs to be known. The main objective of this study is to quantify chlorine and carbon isotope fractionation during NAPL-vapor equilibration, air-water partitioning, and diffusion of trichloroethene (TCE) and combinations of these effects during vaporization in porous media. Isotope fractionation is larger during NAPL-vapor equilibration than air-water partitioning. During NAPL-vapor equilibration, carbon, and chlorine isotope ratios evolve in opposite directions although both elements are present in the same bond, with a normal isotope effect for chlorine (ε(Cl) = -0.39 ± 0.03‰) and an inverse effect for carbon (ε(C) = +0.75 ± 0.04‰). During diffusion-controlled vaporization in a sand column, no significant carbon isotope fractionation is observed (ε(C) = +0.10 ± 0.05‰), whereas fairly strong chlorine isotope fractionation occurs (ε(Cl) = -1.39 ± 0.06‰) considering the molecular weight of TCE. In case of carbon, the inverse isotope fractionation associated with NAPL-vapor equilibration and normal diffusion isotope fractionation cancel, whereas for chlorine both processes are accompanied by normal isotope fractionation and hence they cumulate. A source of contamination that aged might thus show a shift toward heavier chlorine isotope ratios.

Organochlorine isotopic pattern-enhanced detection and quantification of triclosan and its metabolites in human serum by ultra-high-performance liquid chromatography/quadrupole time-of-flight/mass spectrometry

Organochlorines possess special isotopic patterns that obey the chlorine rule. In the case of triclosan (TCS), which contains three chlorine atoms, the isotopic patterns are composed of seven obvious peaks with the calculated masses ranging from 286.9435 to 292.9350 in negative ion mode and with specific isotopic abundance ratios of 100:13.1:97.1:12.6:31.8:4.1:3.6. In this study, mass differences between the calculated and observed m/z values for all isotopic peaks of TCS were less than 3.5 ppm in the analyses of the serum samples by ultra-high-performance liquid chromatography/quadrupole time-of-flight/mass spectrometry (UHPLC-Q-TOF/MS). Combining the characteristics described above, four metabolites were identified as sulfonated TCS, glucuronidated TCS and hydroxylated sulfonated TCS. Several novel MS techniques were applied to improve the sensitivity of quantification of TCS. The limit of detection for TCS in a 250 μL serum sample was 0.05 ng/mL, which was over twenty times lower than values obtained by the LC/triple quadrupole-MS/MS method reported in the literature. The concentration of total TCS (free and conjugated) was quantified to range from 0.15 to 217 ng/mL, whereas free TCS ranged from 0.15 to 10 ng/mL. To the best of our knowledge, this is the first report on the identification of TCS and metabolites in human serum, and it also provides the most sensitive LC/MS approach for the quantification of TCS.

Chlorine isotope analysis of organic contaminants using GC-qMS: method optimization and comparison of different evaluation schemes

Compound-specific online chlorine isotope analysis of chlorinated hydrocarbons was evaluated and validated using gas chromatography coupled to a regular quadrupole mass spectrometer (GC-qMS). This technique avoids tedious off-line sample pretreatments, but requires mathematical data analysis to derive chlorine isotope ratios from mass spectra. We compared existing evaluation schemes to calculate chlorine isotope ratios with those that we modified or newly proposed. We also tested systematically important experimental procedures such as external vs. internal referencing schemes, and instrumental settings including split ratio, ionization energy, and dwell times. To this end, headspace samples of tetrachloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cDCE) at aqueous concentrations in the range of 20-500 μg/L (amount on-column range: 3.2-115 pmol) were analyzed using GC-qMS. The results (³⁷Cl/³⁵Cl ratios) showed satisfying to good precisions with relative standard deviations (n = 5) between 0.4‰ and 2.1‰. However, we found that the achievable precision considerably varies depending on the applied data evaluation scheme, the instrumental settings, and the analyte. A systematic evaluation of these factors allowed us to optimize the GC-qMS technique to determine chlorine isotope ratios of chlorinated organic contaminants.

Assessing chlorinated ethene degradation in a large scale contaminant plume by dual carbon-chlorine isotope analysis and quantitative PCR

The fate of chlorinated ethenes in a large contaminant plume originating from a tetrachloroethene (PCE) source in a sandy aquifer in Denmark was investigated using novel methods including compound-specific carbon and chlorine isotope analysis and quantitative real-time polymerase chain reaction (qPCR) methods targeting Dehaloccocoides sp. and vcrA genes. Redox conditions were characterized as well based on concentrations of dissolved redox sensitive compounds and sulfur isotopes in SO(4)(2-). In the first 400 m downgradient of the source, the plume was confined to the upper 20 m of the aquifer. Further downgradient it widened in vertical direction due to diverging groundwater flow reaching a depth of up to 50 m. As the plume dipped downward and moved away from the source, O(2) and NO(3)(-) decreased to below detection levels, while dissolved Fe(2+) and SO(4)(2-) increased above detectable concentrations, likely due to pyrite oxidation as confirmed by the depleted sulfur isotope signature of SO(4)(2-). In the same zone, PCE and trichloroethene (TCE) disappeared and cis-1,2-dichloroethene (cDCE) became the dominant chlorinated ethene. PCE and TCE were likely transformed by reductive dechlorination rather than abiotic reduction by pyrite as indicated by the formation of cDCE and stable carbon isotope data. TCE and cDCE showed carbon isotope trends typical for reductive dechlorination with an initial depletion of (13)C in the daughter products followed by an enrichment of (13)C as degradation proceeded. At 1000 m downgradient of the source, cDCE was the dominant chlorinated ethene and had reached the source δ(13)C value confirming that cDCE was not affected by abiotic or biotic degradation. Further downgradient (up to 1900 m), cDCE became enriched in (13)C by up to 8 ‰ demonstrating its further transformation while vinylchloride (VC) concentrations remained low (<1 μg/L) and ethene was not observed. The correlated shift of carbon and chlorine isotope ratios of cDCE by 8 and 3.9 ‰, respectively, the detection of Dehaloccocides sp genes, and strongly reducing conditions in this zone provide strong evidence for reductive dechlorination of cDCE. The significant enrichment of (13)C in VC indicates that VC was transformed further, although the mechanism could not be determined. The transformation of cDCE was the rate limiting step as no accumulation of VC occurred. In summary, the study demonstrates that carbon-chlorine isotope analysis and qPCR combined with traditional approaches can be used to gain detailed insight into the processes that control the fate of chlorinated ethenes in large scale plumes.

Stable isotope fractionation to investigate natural transformation mechanisms of organic contaminants: principles, prospects and limitations

Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) has made it possible to analyze natural stable isotope ratios (e.g., (13)C/(12)C, (15)N/(14)N, (2)H/(1)H) of individual organic contaminants in environmental samples. They may be used as fingerprints to infer contamination sources, and may demonstrate, and even quantify, the occurrence of natural contaminant transformation by the enrichment of heavy isotopes that arises from degradation-induced isotope fractionation. This review highlights an additional powerful feature of stable isotope fractionation: the study of environmental transformation mechanisms. Isotope effects reflect the energy difference of isotopologues (i.e., molecules carrying a light versus a heavy isotope in a particular molecular position) when moving from reactant to transition state. Measuring isotope fractionation, therefore, essentially allows a glimpse at transition states! It is shown how such position-specific isotope effects are "diluted out" in the compound average measured by GC-IRMS, and how a careful evaluation in mechanistic scenarios and by dual isotope plots can recover the underlying mechanistic information. The mathematical framework for multistep isotope fractionation in environmental transformations is reviewed. Case studies demonstrate how isotope fractionation changes in the presence of mass transfer, enzymatic commitment to catalysis, multiple chemical reaction steps or limited bioavailability, and how this gives information about the individual process steps. Finally, it is discussed how isotope ratios of individual products evolve in sequential or parallel transformations, and what mechanistic insight they contain. A concluding session gives an outlook on current developments, future research directions and the potential for bridging the gap between laboratory and real world systems.

Modeling chlorine isotope trends during sequential transformation of chlorinated ethenes

There is increasing interest in combined carbon-chlorine compound-specific isotope analysis (CSIA)to differentiate between contaminant sources and to assess transformation processes. However, the significant abundance of polychlorinated molecules with several heavy chlorine isotopes complicates the evaluation of chlorine isotope trends. Therefore, the goal of this study was to develop a conceptual and mathematical framework that describes the expected chlorine isotope fractionation patterns during multistep transformation of chlorinated compounds. Reductive dechlorination of chlorinated ethenes served as an example. The study demonstrates that chlorine isotope trends can be simulated by reproducing the average behavior of light and heavy isotopes or by explicitly simulating molecules with different numbers of heavy isotopes (isotopologues). The calculations reveal that initial chlorine isotope ratios of products equal the isotope ratios of their parent compounds in the absence of secondary isotope effects, while steadily increasing during transformation. The slopes in dual isotope plots are linear for reactant and product during a one-step reaction. They become nonlinear for products that are degraded further but converge to characteristic slopes. Consideration of different scenarios reveals that combined carbon-chlorine isotope analysis bears high potential to differentiate between contaminant sources, to elucidate reaction mechanisms in laboratory studies, and to identify transformation processes in the field.

Carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride and cis-1,2-dichloroethene

The study investigated carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride (VC) and cis-1,2-dichloroethene (cDCE). The experimental data followed a Rayleigh trend. For aerobic oxidation, the average carbon isotope enrichment factors were -7.2 per thousand and -8.5% for VC and cDCE, respectively, while average chlorine isotope enrichment factors were only -0.3 per thousand for both compounds. These values are consistent with an initial transformation by epoxidation for which a significant primary carbon isotope effect and only a small secondary chlorine isotope effect is expected. For reductive dechlorination, larger carbon isotope enrichment factors of -25.2 per thousand for VC and -18.5 per thousand for cDCE were observed consistent with previous studies. Although the average chlorine isotope enrichmentfactors were larger than those of aerobic oxidation (-1.8 per thousand for VC, -1.5 per thousand for cDCE), they were not as large as typically expected for a primary chlorine isotope effect suggesting that no cleavage of C-Cl bonds takes place during the initial rate-limiting step. The ratio of isotope enrichment factors for chlorine and carbon were substantially different for the two reaction mechanisms suggesting that the reaction mechanisms can be differentiated at the field scale using a dual isotope approach.