A long pathlength liquid-core waveguide sensor for real-time pCO2 measurements at sea
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An improved spectrophotometric pCO(2) sensor based on a long pathlength liquid-core waveguide is described for pCO(2) 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 muatm in the pCO(2) range of 200-500 muatm) due to the long pathlength. The temperature dependence of the sensor is also discussed. The sensor’s measurements agreed well with the “showerhead equilibrator plus infrared detector” method during an underway survey of sea surface pCO(2) along a transect off the Georgia coast in December 2000. Besides underway mapping, the sensor shows broad applicability for measuring pCO(2) in different environments. (C) 2003 Elsevier B.V. All rights reserved.