Improvement of analytical method for chlorine dual‐inlet isotope ratio mass spectrometry of organochlorines

Abstract

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 δ(37) Cl notation. Thus, a reliable off-line analytical method is needed, allowing direct comparison of the δ(37) Cl values of molecularly different organic compounds with that of ocean-water chloride, to refer measurement results to a Standard Mean Ocean Chloride (SMOC) scale. The analytical method included sealed-tube combustion of organochlorines, precipitation and subsequent conversion of formed inorganic chlorides into methyl chloride (CH3 Cl) for the determination of δ(37) Cl 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. Recovery of 94 ± 5% of chlorine was reached by applying determined optimal conditions for the dissolution, implying good external precision of δ(37) Cl values (-0.18 ± 0.03‰, 1σ, n = 3). Validation of the optimized method by the analysis of the produced and initial CH3 Cl samples with known δ(37) Cl 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. The efficiency of the sample preparation method for CH3 Cl-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.