Continuous flow stable isotope methods for study of delta C-13 fractionation during halomethane production and degradation

Gas chromatography/mass spectrometry/isotope ratio mass spectrometry (GC/MS /IRMS) methods for delta C-13 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 quantific ation of CO2, O-2, CH3Cl, CH3Br, CH3I and CH3SH from a single sample, and a lso the concurrent measurement of delta C-13 for each of the halomethanes a nd methanethiol. Precision of delta C-13 measurements for halomethane stand ards decreased (+/-0.3, +/-0.5 and +/-1.3 parts per thousand) with increasi ng mass (CH3Cl, CH3Br, CH3I, respectively). Given that carbon isotope effec ts during biological production, biological degradation and some chemical ( abiotic) reactions can be as much as 100 parts per thousand, 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 unde rstanding of stratospheric ozone destruction. Copyright (C) 2001 John Wiley & Sons, Ltd.