Compound Specific Stable Chlorine Isotopic Analysis of Volatile Aliphatic Compounds Using Gas Chromatography...

Stable chlorine isotope analysis is increasingly used to characterize sources, transformation pathways, and sinks of organic aliphatic compounds, many of them being priority pollutants in groundwater and the atmosphere. A wider use of chlorine isotopes in environmental studies is still inhibited by limitations of the different analytical techniques such as high sample needs, offline preparation, confinement to few compounds and mediocre precision, respectively. Here we present a method for the δ³⁷Cl determination in volatile aliphatic compounds using gas chromatography coupled with multiple-collector inductively coupled plasma mass spectrometry (GC-MC-ICPMS), which overcomes these limitations. The method was evaluated by using a suite of five previously offline characterized in-house standards and eight chlorinated methanes, ethanes, and ethenes. Other than in previous approaches using ICP methods for chlorine isotopes, isobaric interference of the ³⁶ArH dimer with ³⁷Cl was minimized by employing dry plasma conditions. Samples containing 2-3 nmol Cl injected on-column were sufficient to achieve a precision (σ) of 0.1 mUr (1 milliurey = 0.001 = 1‰) or better. Long-term reproducibility and accuracy was always better than 0.3 mUr if organics were analyzed in compound mixtures. Standardization is carried out by using a two-point calibration approach. Drift, even though very small in this study, is corrected by referencing versus an internal standard. The presented method offers a direct, universal, and compound-specific procedure to measure the δ³⁷Cl of a wide array of organic compounds overcoming limitations of previous techniques with the benefits of high sensitivity and accuracy comparable to the best existing approaches.