The increasing effect of greenhouse gases is arousing more and more concern of the people in the world and soil as both a source and a sink of these trace gases has been studied.Accurate determination of stable isotope composition of these trace gases will help people trace their sources and find out their release patterns.Isotopic mass spectrometers with a fully automated interface for pre-GC concentration(PreCon) of trace gases can be used with high precision to determine stable isotope ratios of carbon,nitrogen,and oxygen in N₂O,CH₄,and CO₂ at atmospheric concentrations.A series of tests including sampling,concentrating,and mass spectrometric determination were conducted,and accurate and reliable results were obtained,confirming that the method is feasible and applicable to air quality monitoring and the study of atmospheric chemistry,plant respiration,and soil gas generation.