Continuous flow stable isotope methods for study of δ¹³C fractionation during halomethane production and degradation

Gas chromatography/mass spectrometry/isotope ratio mass spectrometry (GC/MS/IRMS) methods for δ¹³C measurement of the halomethanes CH₃Cl, CH₃Br, CH₃I and methanethiol (CH₃SH) during studies of their biological production, biological degradation, and abiotic reactions are presented. Optimisation of gas chromatographic parameters allowed the identification and quantification of CO₂, O₂, CH₃Cl, CH₃Br, CH₃I and CH₃SH from a single sample, and also the concurrent measurement of δ¹³C for each of the halomethanes and methanethiol. Precision of δ¹³C measurements for halomethane standards decreased (±0.3, ±0.5 and ±1.3‰) with increasing mass (CH₃Cl, CH₃Br, CH₃I, respectively). Given that carbon isotope effects during biological production, biological degradation and some chemical (abiotic) reactions can be as much as 100‰, stable isotope analysis offers a precise method to study the global sources and sinks of these halogenated compounds that are of considerable importance to our understanding of stratospheric ozone destruction.