Measurement of stable isotope ratios of halogenated organic compounds by liquid chromatographic, high-temperature combustion isotope ratio mass spectrometry.

Halogenated compounds are pollutants of international concern because of their persistence and resistance to degradation in the environment. However, their robust C-Cl and C-F bonds have made precision and accurate measurements of carbon stable isotope ratios (δ13C) using liquid chromatography-isotope ratio mass spectrometry (LC-IRMS) challenging. We developed a high-temperature combustion interface that we operated at high temperature (up to 600°C) and high pressure (5.2 MPa) to establish an LC-IRMS analytical method that enables high-precision, accurate δ13C analysis of selected halogenated compounds among 19 samples of 15 types of compounds. The results were highly reproducible to within Δδ13C = ±0.5‰ (n = 3), where Δδ13C is the difference between the δ13C obtained with an LC-IRMS and elemental analyzer (EA)-IRMS. Small Δδ13C values indicate accurate results. Fluorinated organic compounds showed significant deviations of Δδ13C (-17 to 75‰), likely due to incomplete oxidation of fluorinated species under the present reaction conditions, which is related to the high stability of C-F bonds. These results highlighted the limitations of current oxidation approaches for fluorinated compounds and indicate the need for further optimization. The interface developed for the LC-IRMS system enabled reliable δ13C measurements for highly polar and non-volatile compounds such as trichloroacetic acid that have been difficult to analyze using conventional LC-IRMS approaches. In contrast, the results obtained for fluorinated compounds revealed clear limitations of the current oxidation system. Future optimization of oxidant composition and reaction conditions is expected to improve oxidation efficiency and expand applicability of LC-IRMS to a wide range of halogenated organic compounds, including fluorinated compounds.