Method of measuring the ratio of isotopologue concentrations in the gas phase

A method of measuring a ratio of concentrations of two isotopologues of a chemical species in a first gas mixture using a first optical absorption spectroscopy based gas analyzer, the method comprising the steps of:

• i) measuring an optical absorbance of the first gas mixture at a first wavelength in a first spectral range containing a first rotational-vibrational line of a first rotational-vibrational band of a first isotopologue and an optical absorbance of the first gas mixture at a second wavelength in a second spectral range containing a second rotational-vibrational line of a second rotational-vibrational band of a second isotopologue at a first gas temperature and at a first gas pressure;
• ii) determining a first ratio of two said optical absorbances at the first gas temperature;
• iii) comparing the first ratio to a second ratio of the optical absorbance at the first wavelength and the optical absorbance at the second wavelength of a second gas mixture at a second gas temperature and a second gas pressure, where the second gas mixture having a known ratio of concentrations of two said isotopologues, wherein the second ratio was determined using a second optical absorption spectroscopy based gas analyzer;
• iv) calculating the ratio of concentrations of the said isotopologues in the first gas mixture.

A method for defining a primary optical transfer standard of the ratio of concentrations of two gaseous isotopologues of a chemical species, the method comprising the steps of:

• i) converting a first isotopic standard medium into a gas phase medium by producing a first gaseous isotopologue and a second gaseous isotopologue containing the type of atoms defined for the isotopic standard by first medium, unless the first medium is already in the gas phase;
• ii) transferring gaseous isotopologues produced in the previous step into an optical absorption based gas analyzer, iii) stabilizing the gas temperature of the gas transferred into the said gas analyzer at a first temperature, iv) measuring rotational-vibrational absorption spectra of the said gas in a first spectral range containing a first rotational-vibrational line of a first rotational-vibrational band of the first isotopologue and in a second spectral range containing of a second rotational-vibrational line of a second rotational-vibrational band of the second isotopologue at the first gas temperature;
• v) based on the said spectra determining the line intensity of the first rotational-vibrational line of the first rotational-vibrational band of the first isotopologue and the line intensity of the second rotational-vibrational line of the second rotational-vibrational band of the second isotopologue at the first gas temperature;
• vi) defining a first ratio of the line intensity of the first rotational-vibrational line and the line intensity of the second rotational-vibrational line at the first gas temperature;
• vii) defining the primary optical transfer standard of the ratio of concentrations of two gaseous isotopologues as the first ratio of the line intensity of the first rotational-vibrational line of the first rotational-vibrational band of the first isotopologue and the line intensity of the second rotational-vibrational line of the second rotational-vibrational band of the second isotopologue at the first gas temperature.