A long pathlength liquid-core waveguide sensor for real-time pCO2 measurements at sea

An improved spectrophotometric pCO2 sensor based on a long pathlength liquid-core waveguide is described for pCO2 underway measurements. A low refractive index (RI) amorphous fluoropolymer (Teflon AF 2400) tubing, the heart of the sensor, served as both a CO2-permeable membrane equilibrator and a long pathlength liquid-core waveguide spectrophotometric cell. By using absorbance ratios at three wavelengths and carefully preparing and storing indicator solution, good reproducibility and long-term stability were achieved. The sensor behaved closely to theoretical prediction. Two pronounced features of the sensor were fast response (approximately 2 min to reach 99% of full response) due to high permeability of the Teflon AF, and high precision (about ±2–3 μatm in the pCO2 range of 200–500 μatm) due to the long pathlength. The temperature dependence of the sensor is also discussed. The sensorʹs measurements agreed well with the “showerhead equilibrator plus infrared detector” method during an underway survey of sea surface pCO2 along a transect off the Georgia coast in December 2000. Besides underway mapping, the sensor shows broad applicability for measuring pCO2 in different environments.