| abstract |
The invention provides a method and apparatus for the mass spectrometric determination of the isotopic composition of an element comprised in a sample by gas chromatography-isotopic ratio mass spectrometry. As a sample elutes from the column 5, it enters a sample conversion means 17 which converts the element to an analysis gas (a gas comprising the element in elemental form or in the form of a simple compound, eg N₂ or CO₂) which is admitted into the mass spectrometer 35 only during a second time interval corresponding to the elution of the peak containing the sample from column 5. One or more calibration samples of a reference gas are admitted into the spectrometer 35 during first time intervals which do not overlap the second time interval, thereby minimizing the contribution of the analysis gas to the reference gas mass analysis, and v.v. Various devices (7,12,30,41,49) are provided to minimize disturbance to the flow of gas into the spectrometer 35 during operation of the method. |