Stable isotope measurement method and apparatus by spectroscopy

A stable isotope measurement method for spectrometrically analyzing an isotopic gas by introducing a test gas sample containing a plurality of component gases into a cell, measuring intensities of light transmitted therethrough at wavelengths suitable for the respective component gases, and processing data of the light intensities to determine a concentration ratio between the component gases, the test gas sample being human breath, the component gases being carbon dioxide 12CO2 and carbon dioxide 13CO2, where the wavelength suitable for 12CO2 is about 4280 nm, and the wavelength suitable for 13CO2 is about 4412 nm, the method comprising:

• a first step of introducing the test gas sample into the cell and determining absorbances of light transmitted therethrough at the wavelengths suitable for the respective component gases;
• a second step of determining a concentration ratio between the component gases in the test gas sample on the basis of a calibration curve prepared through measurement on gas samples each containing the component gases in known concentrations;
• and
• a third step of measuring a concentration of water vapor contained in the test gas sample and correcting the concentration ratio between the component gases in the test gas sample in accordance with the measured water vapor concentration on the basis of a correction curve prepared through measurement on gas samples each containing water vapor in a known concentration.

A stable isotope measurement apparatus for spectrometrically analyzing an isotopic gas by introducing a test gas sample containing a plurality of component gases into a cell, measuring intensities of light transmitted therethrough at wavelengths suitable for the respective component gases, and processing data of the measured light intensities by data processing means to determine a concentration ratio between the component gases, wherein

• the test sample is human breath,
• the component gases are carbon dioxide 12CO2 and carbon dioxide 13CO2, comprising:
• a first filter for passing light the wavelength of which suitable for 12CO2 is about 4280 nm, and a second filter for passing light the wavelength of which suitable for 13CO2 is about 4412 nm,
• absorbance calculation means for determining absorbances of light transmitted through the test gas sample introduced into the cell on the basis of the light intensities measured at the wavelengths suitable for the respective component gases;
• concentration calculation means for calculating a concentration ratio between the component gases in the test gas sample on the basis of a calibration curve prepared through measurement on gas samples each containing the component gases in known concentrations;
• water vapor concentration measuring means for measuring a concentration of water vapor contained in the test gas sample;
• and
• correction means for correcting the concentration ratio between the component gases in the test gas sample in accordance with the measured water vapor concentration on the basis of a correction curve prepared through measurement on gas samples each containing water vapor in a known concentration.