Device for quantifying the relative contents of two isotopes of at least one specific gaseous constituent contained in a gaseous sample from a fluid, comprising a quantifying device configured to quantify the relative contents of the two isotopes to be analysed of the or each specific constituent, the quantifying device comprising: - an optical measurement unit comprising: - an optical measurement cell comprising: - at least two mirrors (137A, 137B) which delimit a measurement cavity; and - a mechanism configured to introduce a laser incident optical signal having a variable wavelength into the measurement cavity, and to generate a plurality of reflections of the optical signal in at least two separate points (174A, 174B) on each mirror (137A, 137B) during its travel in the cavity; and - a measuring device configured to measure a transmitted optical signal resulting from an interaction between the optical signal and each isotope in the measurement cavity; and - a calculating device configured to calculate the relative contents on the basis of the transmitted optical signal; characterised in that the device comprises a forming device configured to form a gaseous flow from the sample, comprising a separation device having a column configured to separate the gaseous sample by selective retention of the or each specific gaseous constituent to obtain a gas flow in which the or each specific gaseous constituent is separated over time at an outlet of said column, the optical measurement unit being directly connected to the outlet of the separation column, and in that the device comprises a transportation means configured to transport the gaseous flow directly to the measurement cavity. - an optical measurement unit comprising: - an optical measurement cell comprising: - at least two mirrors (137A, 137B) which delimit a measurement cavity; and - a mechanism configured to introduce a laser incident optical signal having a variable wavelength into the measurement cavity, and to generate a plurality of reflections of the optical signal in at least two separate points (174A, 174B) on each mirror (137A, 137B) during its travel in the cavity; and - a measuring device configured to measure a transmitted optical signal resulting from an interaction between the optical signal and each isotope in the measurement cavity; and - a calculating device configured to calculate the relative contents on the basis of the transmitted optical signal; - an optical measurement cell comprising: - at least two mirrors (137A, 137B) which delimit a measurement cavity; and - a mechanism configured to introduce a laser incident optical signal having a variable wavelength into the measurement cavity, and to generate a plurality of reflections of the optical signal in at least two separate points (174A, 174B) on each mirror (137A, 137B) during its travel in the cavity; and - at least two mirrors (137A, 137B) which delimit a measurement cavity; and - a mechanism configured to introduce a laser incident optical signal having a variable wavelength into the measurement cavity, and to generate a plurality of reflections of the optical signal in at least two separate points (174A, 174B) on each mirror (137A, 137B) during its travel in the cavity; and - a measuring device configured to measure a transmitted optical signal resulting from an interaction between the optical signal and each isotope in the measurement cavity; and - a calculating device configured to calculate the relative contents on the basis of the transmitted optical signal;