Thermo Electron Corporation, Bremen, Germany Rapid Communications in Mass Spectrometry (2004), V18, pp2260–2266, doi: 10.1002/rcm.1620 A new concept for isotope ratio monitoring liquid chromatography/mass spectrometry Michael Krummen, Andreas W. Hilkert, Dieter Juchelka, Alexander Duhr, Hans-Jürgen Schlüter and Reinhold Pesch A new interface for the on-line coupling of a liquid chromatograph to a stable isotope ratio mass spectrometer has been developed and tested. The interface is usable for 13C/12C determination of organic compounds, allowing measurement of small changes in 13C abundance in individual analyte species. All of the carbon in each analyte is quantitatively converted into CO2 while the analyte is still dissolved in the aqueous liquid phase. This is accomplished by an oxidizing agent such as ammonium peroxodisulfate. The CO2 is separated from the liquid phase and transferred to the mass spectrometer. It is shown that the whole integrated process does not introduce isotope fractionation. The measured carbon isotope ratios are accurate and reproducible. The sensitivity of the complete system allows isotope ratio determination down to 400?ng of compound on-column.