A long pathlength spectrophotometric pCO2 sensor using a gas-permeable liquid-core waveguide

The first long pathlength fiber optic-based sensor system to measure pCO2 in natural waters and the atmosphere is described. The sensor is based on a liquid-core (an indicator—HCO3−/CO32− buffer solution) waveguide made of a low refractive index amorphous fluoropolymer tubing, the wall of which serves as a gas-permeable membrane to sense pCO2 changes. The system detects the indicator absorbance changes when the liquid-core reaches CO2 equilibrium with the surrounding sample. Theoretical calculations demonstrate that due to indicator buffer effects, increasing the optical pathlength is a more efficient way to obtain higher sensitivity than increasing the indicator concentration. Using an 18-cm cell with low indicator concentrations (10 μM), this system achieves a precision and an accuracy of ±2–3 μatm in the pCO2 range of 200–500 μatm. The sensor also features a response time (99%) of only 2 min for low-level (<1000 μatm) pCO2 measurements as a result of high CO2 permeability of the amorphous fluoropolymer membrane. Field tests indicate that this new sensor is capable of handling both atmospheric and aquatic pCO2 monitoring.