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Vers un instrument commercial pour la mesure des rapports isotopiques par Optical Feedback Cavity Enhanced Absorption Spectroscopy

Abstract : The study of stable isotopes has led to many important advances in science, such as, for example, the determination of the timing of the oxygenation of Earth’s atmosphere using sulfur isotopes and the quantification of the exchanges of carbon between the atmosphere and other reservoirs (biosphere, oceans) from the analysis of the 13C/12C isotope ratio. The conventional method to measure such isotopic ratios is by means of isotope ratio mass spectrometry (IRMS). About ten years ago, commercial instruments appeared that enabled the measurement of isotopic ratios using infrared spectroscopy of small molecules in the gas phase of interest for environmental studies (notably water and carbon dioxide). However, for some applications these instruments are not well adapted, for example, because they require too much material, or too expensive, or not able to measure isotopes in question. This is in particular the case for sulfur isotopes. During this work two prototype instruments were developed for the analysis of isotopic ratios by means of Optical Feedback Cavity Enhanced Absorption Spectroscopy (OFCEAS). The first targets the 13C/12C isotopic ratio in atmospheric air samples containing between 200 and 500 ppmv of CO2.The lowest concentration of this range is typical of paleo-air trapped in bubbles in ice cores, whereas the higher concentrations are encountered in modern air samples. For this instrument we have shown, through a detailed and systematic study, the effect of pressure, temperature, and optical interference fringes on the measurement of the isotopic ratio. Mitigating these sources of errors has led to a performance characterized by a precision of 0.2‰ in 2 minutes on measurements at a concentration of about 280 ppmv and a precision of 0.05‰ when alternatingly referencing the sample measurement to that of a standard material in a measurement cycle with duration of 2h for one sample. The second instrument is the first of its kind being able to analyze the 33S/32S and 34S/32S ratios through near-infrared spectroscopy on H2S gas. Sulfur compounds, such as sulfate, are easily converted to H2S, but this is not the case for SF6 or SO2, which are the common gases used in the conventional IRMS method. The instrument was demonstrated by performing static measurements on three isotopically different samples of H2S – nitrogen mixtures. These local standard materials were synthesized by us, and shown to have relative enrichment levels of 0‰, 42.6‰ and 83.6‰ for 34S/32S, and 0 ‰, 2.81‰, and 5.39‰ for 33S/32S by IRMS analyses. Our laser spectrometer measurements agree to within 1‰ with the calibrated IRMS values. The precision obtained after averaging three 12-minute measurements is 0.08‰ for both isotopic ratios.
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Marine Favier. Vers un instrument commercial pour la mesure des rapports isotopiques par Optical Feedback Cavity Enhanced Absorption Spectroscopy. Optique [physics.optics]. Université Grenoble - Alpes, 2017. Français. ⟨tel-02464740⟩

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