Elsevier

Marine Chemistry

Volume 84, Issues 1–2, December 2003, Pages 73-84
Marine Chemistry

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

https://doi.org/10.1016/S0304-4203(03)00112-9Get rights and content

Abstract

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 sensor's 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.

Introduction

Our current understanding of relative sources and sinks to atmospheric CO2 in marine environments is mostly derived from a combination of field data and model analysis Sarmiento and Sundquist, 1992, Feely et al., 1998, Sarmiento and Gruber, 2002. Validation of model analysis relies on field CO2 data, which is presently limited by sparse spatial and temporal coverage. Consequently, the need for reliable and easy-to-use CO2 measuring devices has prompted the development in recent years of a variety of techniques and methods for coastal and oceanic water applications.

“CO2 gas equilibrator plus detector” Wanninkhof and Thoning, 1993, Feely et al., 1998 and spectrophotometric measurement using fiber optic sensors Goyet et al., 1992, DeGrandpre, 1993 are two methods for pCO2 measurement in marine environments. The systematic configuration in “equilibrator plus detector” method consists of a gas–water equilibrator and a CO2 gas analyzer. Showerhead sprinkler, bubbler-filled container, and thin-film equilibrator are three designs used in water–gas equilibrators (Körtzinger et al., 2000). A nondispersive infrared gas analyzer (NDIR), such as Li-Cor® Model 6252 or 6262, or gas chromatograph, is used to measure CO2. Gas-phase samples after equilibrium with sample water are directed into the CO2 analyzer for detection. Most underway observations of sea surface pCO2 have been made using this method, which can achieve a precision as high as 0.3–0.4 μatm pCO2 (Feely et al., 1998). This technique needs a large quantity of water sample to reach the gas–water equilibrium state, which is itself a function of temperature. The whole system is often large and complex and not suitable for long-term mooring and unattended monitoring.

The spectrophotometric method has promising features, which include high sensitivity, good stability and selectivity, and simplicity. Over years, investigators have put much effort into developing spectrophotometric pCO2 sensors to realize these promising features in oceanic pCO2 monitoring. Currently, spectrophotometric pCO2 sensors can achieve a precision around 1 μatm CO2 (DeGrandpre et al., 1995) and a response time as fast as about 2 min (99% of full response) (Wang et al., 2002) for oceanic application. These sensors are mostly compact in size, require low power, and directly measure water-phase samples. They have been successfully deployed in mooring applications DeGrandpre et al., 1995, Merlivat and Brault, 1995, Bates et al., 2000.

Spectrophotometric CO2 measurement generally is based on the same theoretical principle as spectrophotometric pH measurement Robert-Baldo et al., 1985, Byrne and Breland, 1989, Goyet et al., 1992. For spectrophotometric pCO2 sensors, a sulfonephthalein indicator (pH sensitive) is enclosed inside a membrane cell, which functions as both a traditional cuvette of a spectrophotometer and a CO2 equilibrator. pCO2 changes in samples surrounding the cell are reflected in pH changes of the indicator inside the cell when CO2 crosses the membrane and reaches equilibrium by diffusion. pH changes of the indicator are detected with optical fibers connected to a spectrophotometer. The membrane material of the cell is mostly made by PTFE or silicone rubber, which is permeable to CO2 molecules but reduces input light quickly (DeGrandpre et al., 1999). The length of the membrane cell (optical pathlength) is therefore mostly less than 1 cm in order to increase the signal/noise (S/N) ratio of throughput light.

Wang et al. (2002) presented a long pathlength (10–20 cm) pCO2 fiber optic sensor using a gas-permeable liquid-core waveguide made by a new copolymer of 2,2-bis trifluoromethyl-4,5-difluoro-1,3-dioxole with tetrafluoroethylene (trademarked as Teflon AF 2400 by DuPont™). At about same time, Byrne et al. (2002) developed a similar Teflon AF sensor to measure total dissolved inorganic carbon (TC) in natural waters using a similar concept. Teflon AF is superior as a membrane material due to its low refractive index (RI) and high permeability to CO2 molecules. Lower RI relative to indicator solution allows a significant increase in the optical pathlength of the sensor without sacrificing S/N ratio. High sensitivity (2–3 μatm in the pCO2 range of 200–500 μatm) was achieved by using long capillary Teflon AF tubes as measuring cells (Wang et al., 2002). High permeability to CO2 molecules results in a fast response time of 2 min (time to reach 99% of full response). The application using Teflon AF as liquid-core waveguides with long optical pathlength was first introduced by Waterbury et al. (1997) for measuring trace concentrations of Fe(II). Thereafter, similar applications have also been presented to measure chromium(VI) and molybdenum(VI) (Yao and Byrne 1999), pH (Kaltenbacher et al., 2000), and nitrite and nitrate Zhang, 2000, Steimle et al., 2002 in natural waters.

We herein report an underway CO2 system based on spectrophotometric measurement. The major portion of the system is an improved long pathlength Teflon AF pCO2 fiber optic sensor which used a multi-wavelength technique to detect changes of light signal Robert-Baldo et al., 1985, Byrne and Breland, 1989, DeGrandpre et al., 1995. Wang et al. (2002) chose relative absorbance at a single wavelength to be the optical detection signal for the pCO2 sensor. A primary disadvantage of using single-wavelength absorbance was the short-time baseline drift that resulted in short-time stability and required frequent calibration. Multi-wavelength measurements used absorbance at three wavelengths. Two wavelengths assessed the absorbance peaks of acid and base forms of the indicator, while a third wavelength (reference wavelength) measured changes of optical system. Signal changes in the acid and base absorbance peaks resulting from chemical sensing did not affect the third wavelength Byrne and Breland, 1989, DeGrandpre et al., 1999. A calculated pCO2 response was derived by using the absorbance ratios at three wavelengths. By this technique, the sensor's stability was significantly improved. Meanwhile, temperature dependence of the sensor response was “dampened” because of the similarity in the temperature dependence of indicator dissolution constant and seawater pH (Byrne and Breland, 1989).

Section snippets

Theory

Most sulfonephthalein indicators used in spectrophotometric pCO2 sensors produce two-step dissociation in aqueous solution as shown below:H2IKI1H++HIKI22H++I2−where KI1 and KI2 are the first and second dissociation constants of the indicator. Since mostly pKI2pKI1>6, these indicators can be treated as simple monoprotic acids (the second step dissociation, KI2) in theoretical consideration of the sensor's response for typical seawater pCO2 measurement (200–1000 μatm). The indicator solution

Teflon AF sensor and optimization

The design of the sensor was similar to that by Wang et al. (2002) (Fig. 1A). The fiber optic-based spectrophotometer (SpectroPette™, World Precision Instruments, Sarasota, FL) measured the light intensities in the visible spectrum from 400 to 800 nm with a wavelength accuracy of ±1 nm. Bromothymol blue (BTB) was selected as the indicator in this study (see later discussion). The intensities at three selected wavelengths, 434 (λ1), 620 (λ2), and 740 nm, were recorded after the BTB buffer

Sensor's performance

Although the direct detection signal of the sensor was changed from measuring a single-wavelength absorbance (Wang et al., 2002) to light intensities at three wavelengths in this study (absorbance ratios and SCO2 were calculated from the intensities), there was no apparent change in the sensor's response time based on laboratory tests (data not shown). The sensor reached 99% of full equilibrium with water samples (pCO2 200–1000 μatm) within 2 min, which is still the fastest response reported so

Acknowledgments

We thank the captain and his crews of the R/V Blue Fin for assistance and Dr. R.A. Jahnke for ship time. We also express our appreciation to Dr. S.Y. Liu of the World Precision Instrument for providing instrument support. Suggestion made by two anonymous reviewers helped to improve the quality of the paper.

Associate editor: Prof. C.T. Arthur Chen.

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Current address: College of Marine Science, University of South Florida, St. Petersburg Campus, 140 Seventh Avenue South, St. Petersburg, FL 33701, USA.

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