A Prototype of High-Precision Carbon Isotopic Ratio Sensing System for CO₂ Dissolved in Water

Abstract

In natural gas hydrates' (NGHs) exploration, the carbon dioxide isotope abundance of dissolved gas in seawater should be accurately measured. However, in practical application, many challenges, such as the interference by organic compounds or the influence of the scatter distribution of NGHs, should be solved. In this study, a prototype of a midinfrared (MIR) CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> isotope abundance sensing system utilizing a simple linear optical structure is investigated to achieve a better precise measurements performance than current instruments. A self-designed temperature controlling system with a precision of 0.1 °C for the multipass gas cell (MPGC) is fabricated to guarantee the carbon isotope measuring precision. For <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> CO2 measurement, the minimum detection limit (MDL) is 48.4 ppbv with an integral time of 75 s. In addition, the precision of carbon isotope is low to 0.083 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> / <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> with an integration time of 121 s. A gas-liquid separator (GLS) combined with the sensing system is utilized to measure the carbon isotope ratio of the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The results validate the ability of the sensing system to measure continuously dissolved gases from water.