A selective unsaturated hydrocarbon subtraction technique for stable carbon isotopic analysis of atmospheric methyl chloride, methyl bromide, and C2-C5 saturated hydrocarbons using continuous-flow isotope ratio mass spectrometry

Using continuous-flow isotope ratio mass spectrometry, we have developed a new analytical system which enables us to determine the stable carbon isotopic composition of CH3Cl, CH3Br, and C-2-C-5 saturated hydrocarbons in gas samples even if they contain substantial amounts of unsaturated hydrocarbons, using an I2O5 reagent for their selective subtraction. The analytical precision of the delta(13)C determinations is better than 0.5parts per thousand for >300 pmolC injections and better than 5parts per thousand for 20 pmolC injections. Using the system, delta(13)C values for CH3Cl and CH3Br were found in burning exhaust that contain a substantial quantity of unsaturated hydrocarbons. CH3Cl and CH3Br measured in exhaust from burning rice plants exhibit highly C-13-depleted values of -56.6 +/- 1.3parts per thousand and -48.6 +/- 3.9parts per thousand, respectively, while saturated hydrocarbons exhibit delta(13)C values (-26.4 to -28.9parts per thousand) that are comparable with the total delta(13)C value of the parent material (rice plant; -28.0parts per thousand). Using the system, we can determine the delta(13)C values of methyl halides and hydrocarbons in many kinds of gas samples. Copyright (C) 2005 John Wiley Sons, Ltd.