Bubble enclosures in polar ice cores represent the only direct paleoatmospheric archive. To constrain past methane sources the isotopic composition (13C and D) of CH4 in high temporal resolution and with high precision is needed. High precision isotope analyses will be achieved by an online gas chromatography isotope ratio monitoring mass spectrometry technique (GC-irmMS) based on developments by Merritt et al. (1995). The method including air extraction from the ice, preconcentration of methane, separation of trace gases, pyrolysis (P) of methane and internal isotopic calibration is presented.The features of a newly built pyrolysis furnace and its power supply are presented. First results concerning precision in dD of methane standards suggest that only 500g of LGM ice will be required for analysis.