Online technique for isotope and mixing ratios of CH 4, N 2O, Xe and mixing ratios of organic trace gases on a single ice core sample

Schmitt, J.; Seth, B.; Bock, M.; Fischer, H.

Firn and polar ice cores enclosing trace gas species offer a unique archive to study changes in the past atmosphere and in terrestrial/marine source regions. Here we present a new online technique for ice core and air samples to measure a suite of isotope ratios and mixing ratios of trace gas species on a single sample. Isotope ratios are determined on methane, nitrous oxide and xenon with reproducibilities for ice core samples of 0.15‰ for δ 13C–CH 4, 0.22‰ for δ 15N–N 2O, 0.34‰ for δ 18O–N 2O, and 0.05‰ per mass difference for δ 136Xe for typical concentrations of glacial ice. Mixing ratios are determined on methane, nitrous oxide, xenon, ethane, propane, methyl chloride and dichlorodifluoromethane with reproducibilities of 7 ppb for CH 4, 3 ppb for N 2O, 70 ppt for C 2H 6, 70 ppt for C 3H 8, 20 ppt for CH 3Cl, and 2 ppt for CCl 2F 2. However, the blank contribution for C 2H 6 and C 3H 8 is large in view of the measured values for Antarctic ice samples. The system consists of a vacuum extraction device, a preconcentration unit and a gas chromatograph coupled to an isotope ratio mass spectrometer. CH 4 is combusted to CO 2 prior to detection while we bypass the oven for all other species. The highly automated system uses only ~ 160 g of ice, equivalent to ~ 16 mL air, which is less than previous methods. The measurement of this large suite of parameters on a single ice sample is new and key to understanding phase relationships of parameters which are usually not measured together. A multi-parameter data set is also key to understand in situ production processes of organic species in the ice, a critical issue observed in many organic trace gases. Novel is the determination of xenon isotope ratios using doubly charged Xe ions. The attained precision for δ 136Xe is suitable to correct the isotopic ratios and mixing ratios for gravitational firn diffusion effects, with the benefit that this information is derived from the same sample. Lastly, anomalies in the Xe mixing ratio, δXe/air, can be used to detect melt layers.

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Schmitt, J. / Seth, B. / Bock, M. / et al: Online technique for isotope and mixing ratios of CH4, N2O, Xe and mixing ratios of organic trace gases on a single ice core sample. 2014. Copernicus Publications.

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