Abstract: An isotope ratio monitoring gas chromatograph-mass spectrometer (irm-GC/MS) unit for generating a chromatogram having improved resolution for peaks representative of individual carbon dioxide isotopes measured when a complex GC amenable organic sample (e.g. petroleum hydrocarbons) is combusted in the presence of a metal catalyst. The present irm-GC/MS unit is similar to known irm-GC/MS units of this type except it uses a capillary combustion tube in the furnace which is connected to the capillary column and has approximately the same inside diameter as that of the capillary column (e.g. 0.25 mm I.D.) whereby any "dead volume" attributable to the combustion tube is substantially eliminated.