Stable chlorine isotope analysis of chlorinated acetic acids using gas chromatography/quadrupole mass spectrometry

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.

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.

Compound-specific bromine isotope ratio analysis using gas chromatography/quadrupole mass spectrometry

RATIONALE

Brominated organic compounds (BOCs) are common persistent toxic pollutants. Compound-specific stable bromine isotope ratio analysis is one of the potential approaches for investigating BOC transformations in the environment. In the present study, we demonstrate that precise bromine isotope analysis of BOCs can be successfully performed by gas chromatography/quadrupole mass spectrometry (GC/qMS) systems that are widely available in analytical laboratories.

METHODS

Optimization and validation of the GC/qMS method were performed by analysis of bromoform, 3-bromophenol and 4-bromotoluene. In addition, comparison of the results obtained by GC/qMS and GC/multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) for 1,2-dibromoethane and 3-bromophenol samples with different bromine isotope composition was carried out to evaluate the analytical performance of the developed method.

RESULTS

Precisions in the range 0.2-0.3‰ were attained for sample amounts in the range of tens to thousands pmol. Good correlation between the results obtained by GC/qMS and GC/MC-ICPMS for laboratory standard materials (1,2-dibromoethane and 3-bromophenol) (regression coefficient R(2)  > 0.98) was achieved.

CONCLUSIONS

The GC/qMS method for bromine isotope analysis shows a good performance and can be applied routinely for studying transformations of BOCs. Due to the observed dependence of the measured isotope ratios on the amount of the analyte and the calculation scheme applied, normalization of the results versus appropriate standards is required for source attribution applications. Copyright © 2016 John Wiley & Sons, Ltd.