DETERMINATION METHOD FOR 12CO2-13CO2 ISOTOPE RATIO IN EXHALATION

PROBLEM TO BE SOLVED: To provide a determination method for a CO2 - CO2 isotope ratio in an exhalation by the absorption of infrared rays in which an error caused by N2 gas is suppressed to a minimum. SOLUTION: In the determination method for the CO2 - CO2 isotope ratio in the exhalation by the absorption of the infrared rays, when a plurality of sample gasses are created in a known-sample creation process, a standard gas composed of an N2 gas component whose concentration is equal to that of the N2 gas in air, CO2 gas component and a CO2 gas component whose isotope ratios are known and a gas component which does not influence the absorbance of CO2 and the absorbance of CO2 is prepared, the standard gas is diluted with the air, a plurality of sample gases whose N2 gas concentrations are identical, whose isotope ratios are identical and whose CO2 gas concentrations are different are created, a plurality of sample gases are created from a plurality of standard gases whose known isotope ratios are different from each other, and a sample gas used to create a working curve is obtained.