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

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.

Expanding the calibration range of compound-specific chlorine isotope analysis by the preparation of a 37 Cl-enriched tetrachloroethylene

RATIONALE

The recent development of reliable GC/qMS methods for δ³⁷ Cl compound-specific stable isotope analysis (CSIA) paves the way for dual carbon-chlorine isotope analysis of chlorinated ethenes and thus allows deeper insights into underlying transformation processes/mechanisms. A two-point calibration is indispensable for the precise and correct conversion of raw data to the international δ³⁷ Cl_SMOC scale. The currently available calibration standards for tetrachloroethylene (PCE) span only a very narrow range from -2.52‰ (EIL2) to +0.29‰ (EIL1), which is considerably smaller than observed δ³⁷ Cl isotope enrichment in (bio-)transformation studies (up to 12‰).

METHODS

We describe the preparation and evaluation of a new ³⁷ Cl-enriched PCE standard to avoid bias in δ³⁷ Cl CSIA arising from extrapolation beyond the calibration range. The preparation comprised: (i) partial PCE reduction by zero-valent zinc in a system of PCE, ethanol (initial volume ratio 3/5) and trace amounts of water followed by (ii) liquid-liquid extraction and (iii) a subsequent fractional distillation to purify the ³⁷ Cl-enriched PCE.

RESULTS

The obtained PCE (PCE_enriched ) showed a purity of 98.8% (mole fraction) and a δ³⁷ Cl_SMOC value of +10.8 ± 0.5‰. The evaluation of an experimental dataset with and without extrapolation showed no significant variation.

CONCLUSIONS

The new PCE standard (PCE_enriched ) expands the calibration range to 13.3‰ (previously 2.8‰) and thus prevents potential bias introduced by extrapolation beyond the calibration range.

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.

Deciphering the Variability of Stable Isotope (C, Cl) Fractionation of Tetrachloroethene Biotransformation by Desulfitobacterium strains Carrying Different Reductive Dehalogenases Enzymes

Kinetic isotope effects have been used successfully to prove and characterize organic contaminant transformation on various scales including field and laboratory studies. For tetrachloroethene (PCE) biotransformation, however, causes for the substantial variability of reported isotope enrichment factors (ε) are still not deciphered (ε_C = -0.4 to -19.0‰). Factors such as different reaction mechanisms and masking of isotope fractionation by either limited intracellular mass transfer or rate-limitations within the enzymatic multistep reaction are under discussion. This study evaluated the contribution of these factors to the magnitude of carbon and chlorine isotope fractionation of Desulfitobacterium strains harboring three different PCE-transforming enzymes (PCE-RdhA). Despite variable single element isotope fractionation (ε_C = -5.0 to -19.7‰; ε_Cl = -1.9 to -6.3‰), similar slopes of dual element isotope plots (Λ_C/Cl values of 2.4 ± 0.1 to 3.6 ± 0.1) suggest a common reaction mechanism for different PCE-RdhAs. Cell envelope properties of the Desulfitobacterium strains allowed to exclude masking effects due to PCE mass transfer limitation. Our results thus revealed that different rate-limiting steps (e.g., substrate channel diffusion) in the enzymatic multistep reactions of individual PCE-RdhAs rather than different reaction mechanisms determine the extent of PCE isotope fractionation in the Desulfitobacterium genus.

37Cl-compound specific isotope analysis and assessment of functional genes for monitoring monochlorobenzene (MCB) biodegradation under aerobic conditions

A laboratory approach was adopted in this study to explore the potential of ³⁷Cl-CSIA in combination with ¹³C-CSIA and Biological Molecular Tools (BMTs) to estimate the occurrence of monochloroenzene (MCB) aerobic biodegradation. A new analytical method for ³⁷Cl-CSIA of MCB was developed in this study. This methodology using a GC-IRMS allowed to determine δ³⁷Cl values within an internal error of ±0.3‰. Samples from a heavily MCB contaminated site were collected and MCB aerobic biodegradation microcosms with indigenous cultures in natural and enhanced conditions were set up. The microcosms data show a negligible fractionation for ¹³C associated to MCB mass decrease of >95% over the incubation time. Conversely, an enrichment factor of -0.6±0.1‰ was estimated for ³⁷Cl, which is a reflection of a secondary isotope effect. Moreover, the dual isotope approach showed a pattern for aerobic degradation which differ from the theoretical trend for reductive dehalogenation. Quantitative Polymerase Chain Reaction (qPCR) results showed a significant increase in todC gene copy number with respect to its initial levels for both natural attenuation and biostimulated microcosms, suggesting its involvement in the MCB aerobic degradation, whereas phe gene copy number increased only in the biostimulated ones. Indeed, ³⁷Cl fractionation in combination with the dual carbon‑chlorine isotope approach and the todC gene copy number represent valuable indicators for a qualitative assessment of MCB aerobic biodegradation in the field.

Compound-specific hydrogen isotope analysis of fluorine-, chlorine-, bromine- and iodine-bearing organics using gas chromatography-chromium-based high-temperature conversion (Cr/HTC) isotope ratio mass spectrometry

RATIONALE

The conventional high-temperature conversion (HTC) approach towards hydrogen compound-specific isotope analysis (CSIA) of halogen-bearing (F, Cl, Br, I) organics suffers from incomplete H₂ yields and associated hydrogen isotope fractionation due to generation of HF, HCl, HBr, and HI byproducts. Moreover, the traditional off-line combustion of highly halogenated compounds results in incomplete recovery of water as an intermediary compound for hydrogen isotope ratio mass spectrometry (IRMS), and hence also leads to isotope fractionation. This study presents an optimized chromium-based high-temperature conversion (Cr/HTC) approach for hydrogen CSIA of various fluorinated, chlorinated, brominated and iodinated organic compounds. The Cr/HTC approach is fast, economical, and not affected by low H₂ yields and associated isotope fractionation.

METHODS

The performance of the modified gas chromatography/chromium-based high-temperature conversion (GC-Cr/HTC) system was monitored and optimized using an ion trap mass spectrometer. Quantitative conversion of organic hydrogen into H₂ analyte gas was achieved for all halogen-bearing compounds. The corresponding accuracy of CSIA was validated using (i) manual dual-inlet (DI)-IRMS after off-line conversion into H₂ , and (ii) elemental analyzer (EA)-Cr/HTC-IRMS (on-line conversion).

RESULTS

The overall hydrogen isotope analysis of F-, Cl-, Br- and I-bearing organics via GC-Cr/HTC-IRMS achieved a precision σ ≤ 3 mUr and an accuracy within ±5 mUr along the VSMOW-SLAP scale compared with the measured isotope compositions resulting from both validation methods, off-line and on-line. The same analytical performance as for single-compound GC-Cr/HTC-IRMS was achieved compound-specifically for mixtures of halogenated organics following GC separation to baseline resolution.

CONCLUSIONS

GC-Cr/HTC technology can be implemented in existing analytical equipment using commercially available materials to provide a versatile tool for hydrogen CSIA of halogenated and non-halogenated organics. Copyright © 2017 John Wiley & Sons, Ltd.