Capillary Absorption Spectrometer for In-Situ, Underwater Gas Analysis

A Capillary Absorption Spectrometer (CAS) is proposed for in-situ, underwater gas concentration and isotope measurements. The concept utilizes mid-infrared laser absorption spectroscopy within a hollow fiber optic capillary that both confines a gas sample and acts as an optical waveguide. A tunable laser beam propagates through the hollow fiber to a detector with near unity overlap between the beam and the analyte. The CAS can perform fast (~ 1 s) and precise (isotope ratio ~ 1 %) measurements within the ultra-small sample volume (V ~ 0.1 mL) of the hollow fiber, enabling 3 to 4 orders of magnitude more sensitivity than related cavity-ring-down and multi-pass systems. Compared to these other systems, the sensor proposed here will have higher throughput analysis of dissolved gasses within a smaller size, weight, and power package. In this project, the concept will be extended and specifically proven for methane isotope analysis (13CCH4 and 2HCH4), building on the team’s ongoing development of CO2, SO2, and H2O isotope sensors for NASA planetary applications. In addition, improvements over typical sample inlet methods (e.g., membranes) will be developed for deep-ocean monitoring. Specifically, degassing of discrete water samples will be enabled due to the ultra-small sample volume of the CAS.