Process and device for measuring isotope ratios

A process for measuring the isotope ratio, specifically of stable isotopes, of chemical substances in a gas to be examined, the process featuring the following steps:

• alternately charging a sample cell with a reference gas and a measuring gas containing the gas to be examined, and
• determining the isotope content of the substance contained in the alternately present gases,
• characterized by the following further process steps:
• addition of a metrologically neutral gas to the gas to be examined, generating a measuring gas by mixing these two gases;
• variation of the mixing ratio of the measuring gas by changing the share of neutral gas to be added, in such a way that the determined contents of the substance of the one isotope 1 in the reference gas R.sub.1 and in the measuring gas M.sub.1 are essentially equal;
• and
• computation of the isotope ratio V.sub.P of the substance from the alternately determined content of the substance on the other isotope 2 in the reference gas R.sub.2 and in the measuring gas M.sub.2, under inclusion of the known isotope ratio V.sub.R of the reference gas according to the equation:
• ##EQU4##