Isotope fractionation during capillary leaking in an isotope ratio mass spectrometer

Rationale When isotope composition is measured in dual-inlet mode with an isotope ratio mass spectrometer (IRMS), reference gas may be gradually enriched in heavy isotopes due to preferential loss of light ones from the bellows over time. Quantifying the degree of isotopic enrichment of the reference gas is imperative for high-precision isotopic analysis (i.e. at per meg level). Methods O-2 and CO2 leaking experiments were performed with the dual-inlet system of an IRMS (Thermo Fisher (R) MAT 253 Plus). During each experiment, the drop of gas pressure in the bellows with time was recorded and isotope ratios of residual gas at various time intervals were analyzed. Results Isotopic enrichment of residual O-2 gas could be as large as 1 parts per thousand for delta O-18 when a large fraction (>75%) of initial gas was lost. The evolution of isotope compositions of the remaining gas can be well described by a pressure-dependent Rayleigh fractionation equation. When the pressure in the bellows is within 10-50 mBar, the isotope fractionation factor (alpha O-18) for O-2 leaking ranges from 0.99911 to 0.99982 and the characteristic relationship of alpha O-17 and alpha O-18 is from 0.5123 to 0.5124. Conclusions Isotope fractionation associated with capillary leaking from bellows is pressure-dependent. We recommend that the reference gas should be reloaded frequently, especially after a measurement with a low analyzing pressure for the analysis of small amounts of sample.