The sample size for conventional double collector isotope mass spectrometry has been reduced more than three orders of magnitude by using a ‘‘pushing’’ gas for precise stabilization of sample pressure. The sample is stored in a long microcontainer (50 cm, 0.5 mm i.d.) to moderate the effect of diffusive mixing of sample with the pushing gas. If the analysis is sufficiently rapid (e.g., a run of 1 min for the unknown sample between two runs on the standard), then almost total sample consumption is possible. This method retains the high precision and simplicity of dynamic flow, yet possesses the high sensitivity of static mass spectrometry.