Rationale

The development of compound-specific chlorine isotope analysis (Cl-CSIA) is hindered by the lack of international organochlorine reference materials with isotopic compositions expressed in the δ³⁷Cl notation. Thus, a reliable off-line analytical method is needed, allowing direct comparison of the δ³⁷Cl values of molecularly different organic compounds with that of ocean-water chloride, to refer measurement results to a Standard Mean Ocean Chloride (SMOC) scale.

Methods

The analytical method included sealed-tube combustion of organochlorines, precipitation and subsequent conversion of formed inorganic chlorides into methyl chloride (CH₃Cl) for the determination of δ³⁷Cl values by Dual-Inlet Isotope Ratio Mass Spectrometry (DI-IRMS). A sample preparation step most sensitive to the sample size – dissolution of the inorganic copper chlorides formed by combustion of organochlorines – was identified.

Results

Recovery of 94 ± 5% of chlorine was reached by applying determined optimal conditions for the dissolution, implying good external precision of δ³⁷Cl values (–0.18 ± 0.03‰, 1σ, n = 3). Validation of the optimized method by the analysis of the produced and initial CH₃Cl samples with known δ³⁷Cl values vs SMOC resulted in a difference of 0.11 ± 0.04‰ (1σ, n = 3), confirming the external precision and accuracy of the entire method.

Conclusions

The efficiency of the sample preparation method for CH₃Cl-DI-IRMS analysis is independent both of the chemical structure of the chlorinated compound and of the amount of chlorine in the sample. This method has the potential to be applied to a broad range of chlorinated organic compounds, e.g. reference material for the calibration of methods for Cl-CSIA against SMOC.