Anal. Chem. 1903, 65, 331-337
331
Measurement of Seawater pC02 Using a Renewable-Reagent Fiber Optic Sensor with Colorimetric Detection Michael D. DeGrandpre Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
A new method, based on a renewablereagent fiber optic sensor, for measuring the partlal pressure of C02 (pC02)In seawater Is presented. The sensor operates by measuring the light intendty at the abwblng wavelengthsof a colorimetric acld-base indicator whkh b continuously delivered to the fiber tips through caplllary tubing. The light Intensity Is modulatedby pH changesthat occur when C02diffusesacross a gas-permeable membrane. The sensor operates both in a diffudon-dependent steady state and equlilbrlum regime depending upon the indicator flow rate. At low flow rates, an equlliklum model can be used to predlct the response of the sensor. The results Indicate that the sensor operates within the steadyetate reghe at fbw rateshigherthan approxbnately 0.2 pUmln. The optimal precision Is f0.8 patm from 300550 patm of C02, calculated from the response sendtlvlty and 9X the root mean square noke. Response tlmes (100 % ) range from 11 to 26 mln and depend upon the Indicator flow rate. Sen8ltlvlty to temperature and sample hydrodynamics is also discussed. The sensor performance was tested on a research c r u k , and these results are compared to the underway pC02measured dmultaneouslyby an infrared C02 analyzer.
INTRODUCTION The realization that modern society could significantly change global climate within the next century has led to a scramble to understand the natural mechanisms controlling globalclimate. Because of the complexbiogeochemical cycling of COz, the primary anthropogenically-producedgreenhouse gas, it is unknown what impact a global temperature increase would have on the COzcycle. The oceans are a huge reservoir of COzand can act as botha source and sink of COZdepending upon the region, season, and the occurrence of episodic events (such as El Niiio). Widespread mapping of the partial pressure of COZ (pCOz) disequilibrium between the atmosphere and ocean surface waters is vital for determining the present, and future, global COZair-sea flux. Long-term shipbased measurements are an intimidating endeavor because of their high costs, and it is, in general, preferable to moor instruments in strategic ocean locations. To accomplish this, the instrument must be simple, inexpensive, and low power yet be capable of measuring pCOz over a range of approximately 200-600 Matm with a precision of less than 176, unattended for 6 months to 1year. No instrument is currently available that fits these strict criteria. This work reports the initial stages of development of an instrument, based on a fiber optic sensor, for long-term autonomous pC0z measurements in surface ocean waters. Sensors for COZ have been extensively researched, a substantial part of the effort being devoted to the development of a COZ fiber optic sensor for biomedical applicati~ns.l-~ (1)Vurek, G. G.; Fuestel, P. J.; Severinghaus, J. W. Ann. Biomed. Eng. 1983,1 1 , 499-510. 0003-2700/93/0365-0331$04.00/0
These sensors typically use a fluorometric or colorimetric acid-base indicator immobilized or entrapped within a gaspermeable membrane at the fiber tips. Unfortunately, their sensitivities and precisions fall short of that required for seawater applications. The sensitivity (ApHIApCOz) is limited by the bicarbonate buffer, which is necessary for establishing the sensor equilibrium response.5 The use of fluorescent indicatorss-8 brought the response into the range needed for seawater measurements, yet the precision was either not sufficient(k7patm, &3% ,refs 6 and 7, respectively) or not reported.8 Stability, in all cases, was not extensively evaluated. While the fixed-fluorophore approach may still prove a viable solution, these sensors will probably suffer from the same problems characteristic of many of the fiiedreagent optrodes. The fixed-reagentsensorshave, in general, a significant drift induced by reagent loss and changes in the chemical composition of the internal electrolyte solution and reagent support. Photodecomposition and leaching are the primary mechanisms for reagent loss while the internal solution and/or support matrix is sensitive to changes in sample osmolarity (liquids)and humidity (gases). It is evident their performance could be improved if the chemical microenvironment within the membrane could be controlled. One way of doing this, as demonstrated by Berman and Burgess,9J0is to continuously deliver the reagent solution to the fiber tips, essentially forming a membrane-based microoptical flow cell. Their renewable-reagent sensor significantly improved the sensitivity and precision achieved by the equivalent fixed-reagent fiber optic sensor. In addition, they showed that, by prudent choice of the flow rate and reagent composition,the sensor sensitivityand dynamic range could be controlled to suit the measurement conditions. The design retains the virtues of a sensor; that is it is a small single-ended device that can be remotely linked to the controlling instrument to perform measurements in situ. Because of the shortcomings of the fiied-reagent sensors and the promising characteristicsof the renewable-reagentsensors, efforts were concentrated on developing a renewable-reagent COZfiber optic sensor. As will be shown below, the ability to renew the reagent solution makes it possible to use a simple colorimetric indicator for detection of COz with the range and precision needed for surface seawater measurements. (2)Zhujun, Z.; Seitz, W. R. Anal. Chim. Acta 1984,160, 306-309. (3)Munkholm, C.; Walt, D. R.; Milanovich, F. P. Talanta 1988,35, 109-112. -.. _ _ ~ (4)Wolfbeis, 0.S.;Weis, L. J.; Leiner, M. J. P.; Ziegler, W. E. Anal. Chem. 1988,60,2028-2030. (5)Jensen, M. A.; Rechnitz, G. A. Anal. Chem. 1979,51,1972-1977. (6)Walt, D.R.; Gabor, G.; Goyet, C. Anal. Chim. Acta, in press. (7) Goyet, C.; Walt, D. R.; Brewer, P. G. Deep-sea Res. 1992,39,10151026. (8)Tokar, J. M.; Woodward, W. E.; Goswami, K. Oceans ‘891989,5, 1360-1365. (9)Berman, R. J.; Christian, G. D.; Burgess, L. W. Anal. Chem. 1990, 62, 2066-2071. (10)Berman,R. J.;Burgess, L.W. InProceedingsofSPIE-Chemical, Biochemical and Environmental Fiber Sensors; Lieberman, R. A., Wlodarczyk, M. T., Eds.; SPIE Belliigham, WA, 1990,Vol. 1172,pp 206-214. 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15, 1993
9.0
THEORY OF OPERATION The sensor operates by measuring the light intensity at the absorbing wavelengths of a pH-sensitive dye that flows into a gas-permeable membrane at the fiber tips. Light intensity is modulated by pH changes that occur when COZdiffuses acrossthe membrane. The response can be describedby either an equilibrium or steady-state model depending upon the dye flow rate. A large concentration gradient initially exists because no COS (or bicarbonate) is present in the indicator solution. The equilibrium model, analogous to the theoretical model for the potentiometric COzelectrode," can be used to describe the sensor response when the flow is sufficiently reduced to allow the solution to reach equilibrium with the ambient COz before leaving the sensor reservoir. At higher flow rates, aconcentration gradient is sustained over the entire membrane length, establishing a diffusion-limited steady state. The steady state is dependent upon diffusion coefficients, membrane permeability and thickness, kinetics of COZ dissolution and speciation, and the reagent and sample hydrodynamics. Only the equilibrium case will be derived here. The equilibriummodel is useful as a tool for determining the optimal reagent and solution properties and for determining if the sensor is actually operating in the equilibrium regime. Similar approaches have been presented to describe the equilibrium response of fiber optic pH,12NH3,13and COZ' sensors. Combination of the indicator and COZequilibria equations with the electroneutrality equation gives the following expression:
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I
400
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.
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1.0,
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[H+13+ [H+12[Na+l- [H+I{K,[CO,(aq)l +
K , + K,([HAI,-
[ A 3 1 - 2K$l[COz(aq)l = 0 (1)
where [H+] is the hydrogen ion concentration, [Na+l is the concentration of base added to establish the initial pH (pHid, [COz(aq)l is the aqueous COz concentration, [HAIT is the total indicator concentration, [A-I is the concentration of the basic form of the indicator, K1 and K z are the HzCO3 acidity constants, and K, and Kaarethe water and acid-base indicator equilibrium constants, respectively. Equation 1, with [COZ(aq)] calculated from pC0z and Henry's law, is iteratively solved for [H+l using a bisection convergenceroutine written in QuickBASIC. The calculated [H+l is used with the sensor optical response, approximated by the Beer-Lambert relation 0
= 10-tbadHAI~
(2)
+ K,)
(3)
where a0 = K,/([H+I
and I is the detected signal, Io is the light intensity with no indicator present, e is the indicator molar absorptivity in the acid or base form, and b is the optical path length. An attenuation coefficient is defined as K
= tb[HAIT
(4)
Determination of the indicator solution blank (Io)requires that the dye be totally in the acid or base form or that a blank solution (i.e. pure water) be pumped to the fiber tips. The additional complexity and time required to introduce and flush a solution blank makes ita measurement impractical (11) Severinghaus, J. W.; Bradley, A. F. J. Appl. Physiol. 1958, 13,
51 - - 5-520. - - - -.
(12) Peterson, J. I.; Goldstein, S. R.; Fitzgerald, R. V.; Buckhold, D.
K.Anal. Chern. 1980,52, 864-869. (13) Arnold, M. A.; Ostler, T.J. Anal. Chern. 1986,58, 1137-1140.
0.2
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I
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I
1
I
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I
100
200
300
400
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600
PCO, ( c l a w Flgwo 1. Theoretical equlllbrlum curves calculated from eqs 1 and 2: (A) internal lndlcator solutlon pH dependence on pCOn;(B) sensor response for different lndlcator pKa's (pH,, = 9.0, K = 1.0).
for this application. Therefore, since Io is not measured, direct comparisonsbetween the theoretical and experimental results are not possible, but when eqs 1 and 2 are solved, the relationship between the detected light intensity and pC0z can be plotted for different solution and indicator properties. The results provide a useful basis for choosing the optimal operating conditions. The internal solution pH, calculated from eq 1using 0-600 patm of COB,is plotted in Figure 1A. Setting pHinit at 9.0, the pH drops from 9 to approximately 7 over the 600-patm change in COZ. Starting at a lower pHinitdrops the pH range of the response. Most of the pH change occurs from 0-200 patm of COZ,well below the COZrange generally expected in surface seawater. Assuming the internal solution quickly attains the equilibrium distribution of COz species, Figure 1A may represent a dynamic equilibrium where at high flow rates the internal COz remains in the lower COz portion of the response curves. Therefore, optimal sensitivity (ApH/ ApCOz) should be achieved with an acid-base indicator flowing at a rate which does not allow the inner solution to approach the equilibrium COz concentration, thereby operating in the most sensitive portion of the pH response curve. The equilibrium response curves for the base form ([A-I) of dyes with different pK,'s are shown in Figure 1B. These curves were calculated with PHinit equal to 9.0, [HAIT equal
ANALYTICAL CHEMISTRY, VOL. 85, NO. 4, FEBRUARY 15, 1903 999 0.9
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PCO, h.latm> Theoretical equillbrlum response curves for dlfferent attenuation coefficients (eq 3) (lndlcator pKa = 7.5, pHkn = 9.0). Figure 2.
to 1X 10-4 M, b equal to 0.2 cm and t equal to 5 X 104M-1 cm-1 ( K = 1.0). The theoretical curves for the acid form (not shown here) are inverted with respect to the base curves, with the light intensity decreasing for higher Con. The nonlinear response mirrors the nonlinear pH curves in Figure 1A for the high-pK, dyes because their pH response fallsin the 0-200 patm of C02 range where the pH versus C02 curvature is greatest. The lower pKa dyes' responses fall over the more linear pH range (C02 is greater than 200 /ratm) resulting in less sensitivity at low concentrations, but a greater linear dynamic range, than in the case of the high-pK, dyes. The attenuation coefficient K affects the sensitivity and the range of light transmission over which the sensor responds, as can be seen in Figure 2. These curves represent the base form of a dye with a pK, of 7.5 and pHinitequal to 9.0. The attenuation coefficients can be manipulated by the choice of dye or dye concentration, or by modification of the optical path length. The dye parameters offer more flexibility since it is difficult to change the optical path length with the present sensor configuration. The range of attenuation coefficients shown in Figure 2 could be obtained with t ranging from 2 X 104 to 5 X 104 M-' cm-l ([HAIT = 1 X M) or by dye concentrations ranging from 1 X 10-4 to 4 X 10-6 M (c = 5 x 104M-1 cm-1). It is also important to consider light throughput at the analytical wavelengths which, if a t very low levels, will limit the signal-to-noise and measurement precision. A transmittance from 0.25 to 0.50 gives the optimal photometric accuracy;14 therefore, the K = 1.0 curve would be optimal for measurement of the base form. If the acid and base forms of the dye are both being monitored, the dye concentration should be chosen by considering the optical throughput (Io) and dye molar absorptivivity a t both wavelengths. The dye should be selected so that its pK, fits approximately midway within the expected pH range12and strikes a balance between the sensitivity and dynamic range. When operating in the steady-state regime the pH will be higher than the equilibrium value; therefore, higher pK, dyes should be used under these conditions. Two dyes with intermediate pK;s, phenolred (PhR) (pKa = 7.5 a t 25 OC) and bromothymol blue (BTB) (pK, = 6.8 at 25 "C), and high base form extinction coefficients (PhR A = 560 nm, t = 5.64 X 104 M-l cm-1, BTB A = 590 nm, t = 2.75 X lo4 M-l cm-l) were evaluated over a range of flow rates to test these predictions. (14)Willard, H.H.;Merrit, L. L.; Dean, J. A. Instrumental Methods
of Analysis, 4th ed.; Van Nostrand Fteinhold Co.: New York, 1965;p 90.
Figure9. Renewabkeagent pCO2 sensor deslgn: (a)reagent delivery capillary: (b) reagent exit capillary;(c)fiber optic from source; (d) fiber optic to detection system; (e) white sillcone rubber membrane; (f) white silicone sealant; (g) epoxy; (h) O-ring; (I) sensor housing; (i)f l b r
optic cable. Note: individual components are not drawn to scale.
EXPERIMENTAL SECTION Sensor Design. The COz sensor described here is a modification of a design reported earlier.15 The sensor is constructed from a combination of fiber optics and capillary tubing potted together with a UV-curable epoxy (UV10, Master Bond, Hackensack, NJ) (Figure 3). A white silicone membrane (0.51-mm i.d., 0.94-mm o.d., SilasticRx-~OR,Dow Corning Corp., Midland, MI) is pulled over the assembly, forminga reservoir with minimal dead volume. The silicone membrane provides a gas-permeable barrier between the reagent and seawater and also acta as an efficient scatterer of light emitted from the source fiber. A 400pm core, 430-pm clad, 0.37 numerical aperture (NA) hard-clad silica fiber (HCN-M0400T-14,Ensign-Bickford Optica Co., Avon, CT) carries the light from a tungsten-halogen sourceto the sensor tip. The back-scattered incident radiation is collected with a 300-/rmcore, 330-pm clad, 0.22 NA fused-silica fiber (FHP 300/ 330/360,Polymicro Technologies, Phoenix, AZ). Reagent flows through a 1-m100-pm-i.d./170-pm-o.d.fused-silicacapillary tube to the fiber tips and exits through a 1-m 75-pm-i.d./150-pm-o.d. fused-silica capillary (TSP 100170and TSP 75150, respectively, Polymicro Technologies, Phoenix, AZ). What appears to be a random selection of fibersand tubes is actuallydriven by a desire to balance optical throughput and back-pressure with the total sensor size and reservoir volume. The fibers provide enough light throughput to give a reasonable signal-to-noise ratio while keeping the total sensor internal volume to a minimum (approximately 1.6 p L for the 0.85-cm-long membrane used in this study). A large NA fiber is used to maximize collection of the diffuse white light source while a lower NA is used for the return fiber to match the detection system optics. The total sensor pressure drop with these tubing diameters and lengths is approximately 2 psi, and the smaller diameter exit capillary helps create more back-pressure to squeeze bubbles out of the gas membrane before they enter the view of the fibers. Instrumentation. The detection system for the laboratory evaluation of the sensor consisted of a fiber optic fluorometer (Model 11,Douglas Instrument Co., Palo Alto, CA) modified for absorbance measurements (Figure 4). Two 10-nm band-pass interference filters (OCA/Microcoatings, Westford, MA) and a dichroic beamsplitter (10QM20HL.4, Newport Corp., Fountain Valley, CA) were used to isolate the maximum absorbing wavelengths of the acid and base forms of the dye. Output from the photodiodes were sent to a data acquisition board (DAS(15)DeGrandpre, M. D. In Proceedings of SPZE-Chemical, Biochemical and Environmental Fiber Sensors IZ& Lieberman, R. A., Wlodarczyk, M. T., Eds.; SPIE Bellingham, WA, 1991;Vol. 1587,pp 60-66.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15, 1993
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I
8PGA, Keithley/Metrabyte, Taunton, MA) driven by a QuickBASIC (Ver. 4.0, Microsoft Corp., Redmond, WA) program and personal computer (PowerMate Portable, NEC Corp., Boxborough, MA). Data were acquired at lo00 (points/s)/channelover a 5-13 interval and then digitally averaged. Two methods were evaluated for pumping reagent to the sensor tip. For the majority of the data reported in this study the reagent was driven by gas pressure (high-purityNz or He) controlled by a low-pressureprecision regulator (Air Trol R-900-10-K,Controls for Automation, Waltham, MA). The acid-base indicator was enclosed in a vessel jacketed by a pH 10 borate buffer to help reduce COz contamination. The indicator was also pumped to the fiber tips using an infusion syringe pump (A-99, Raze1 Scientific, Stamford, CT) and a 1-mL gastight syringe (Model 1001, Hamilton, Reno, NV). The syringe was jacketed and continuously purged with COz-free Nz. All flow rates were measured gravimetrically. Reagents and Standards. Stock solutions of PhR and BTB (AldrichChemicalCo., Milwaukee, WI)were prepared in distilled water. Prior to use the dye solution was acidified and sparged of all COz using COz-freeNz and then brought to pH 9.0 with 0.1 M NaOH. Calibration gases were generated by mixing pure Nz and a 1600 ppm COz standard (Linde primary standard grade, Corp Brothers,Hyannis, MA) in different proportions using two mass flow controllers (FC-260, Tylan General, San Diego, CA). Gas concentrationswere confirmed to within 1ppm using an infrared (IR) COz analyzer (LI-6262,LI-COR, Inc., Lincoln, NE). Water samples were equilibratedwith the COz standards thermostated with a circulatingwater bath (RM3,Lauda, Germany). All initial evaluations were performed in distilled water at 23.1 "C.
1.1
,
n0) Detector A (acid form)
RESULTS AND DISCUSSION Sensor COz Response. The sensor calibration curves using different flow rates and monitoring the base form of PhR and BTB are shown in Figure 5. In general the best fit was obtained with a second-order polynomial. The offset shifta positive a t the slower flow rates as more dye is converted to the acid form before the indicator exits the membrane. The response curves for the acid form of PhR and BTB (A = 434nm) are not shown but have a similar nonlinear response for these flow rates. In Figure 5A the 0.66 pllmin response is linear over this concentration range while the high and low flow rates have slight upward and downward curvature, respectively. The upward curvature of the high flow rate response is attributed to stray light because a significant portion of the incident light is absorbed by the base form a t the low COz concentrations. With the low flow rate a decreasing slope is evident, as predicted by the theoretical curve ( K = 1.0) in Figure 2; therefore, the sensor solution may be a t or near equilibrium with the external p C 0 ~ . BTB response curves in Figure 5B follow a similar trend with
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PCO, (CLatn-4 Flguro 5. (A) Sensor response uslng a phenol red solution at different flow rates ([HAIT = 1 X lo4 M, pHwt = 9.0): (0)0.89 pL/min; (v) 0.66 pL/min; (V) 0.19 pL/min. (B) Sensor response using a bromothymol blue soiutbn at different flow rates ([HAIT = 1 X lo4 M, pi& = 9.0): (V)1.30pL/min; (0)0.49 pL/mln; (V)0.12pL/mln.
downward curvature at low flow rates. No upward curvature is evident in the Figure 5B high flow rate curve because the molar absorptivity of BTB at 590 nm is approximately 2 X less than PhR at 560 nm, so the response is not a t the stray light limit. Assuming the low flow rates are near equilibrium, these data can be used with the theoretical model ([A-1 calculated from eq 1)to calculate the average path length of the sensor. The average path length is estimated to be 0.2 cm from this calculation. Table I shows the sensitivities and precisions calculated from the curves in Figure 5. Sensitivity was calculated as the ratio of the total signal change to the total concentration change. Precisions were calculated as 3X the root mean square (rms) (3a) baseline noise divided by the sensitivity. The rms noise was estimated by averaging approximately 200 measurements over a 1OOO-sinterval. Sensitivities and precisions for the acid form (A) and ratio data are also reported in Table I. The ratio precision was calculated by first ratioing A and B signals and then estimating 3X rms noise as before. As can be seen by these data, the dye flow rate significantly affects the sensitivity and range of light intensity over which the sensor responds. The highest sensitivity is achieved with intermediate flow rates for both PhR and BTB. It appears that with these intermediate flow rates the response falls in
ANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15, 1993
Table I. Sensor Sensitivities and Precisions (3 X rms Noise/Sensitivity) from Figure 6 (A 5 Indicator Acid Form, B = Indicator Base Form) flow rate (rcL/min)
sensitivity (mV/patm)
B
A
precision ( i p a t m of Cod A B ratio
Phenol Red
0.89 0.66 0.19
0.55 0.58 0.09
1.30 0.49 0.12
0.16 0.31 0.25
0.50 1.02 0.63
5.0 5.1 19.0
2.5 2.9 1.7
2.5 5.6 3.7
1.1 2.4 0.8
2.8 2.9 3.1
Bromothymol Blue
0.37 0.60 0.57
8.6 4.8 7.1
I
0.6
1
0.2
1
1.0
t
0.9 0.8
0.7
1
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Table 11. Time Required for the Sensor Full (100%) Response for Different Indicator Flow Rates flow rate (pL/min)
response time (min) Phenol Red
0.89 0.66 0.19
11 f 3 16 k 3 26 i 2 Bromothymol Blue
1.30 0.49 0.12
13f1 18 f 2 21 f 6
the large ApH/ApC02portion of Figure 1A. As the flow rate further increases,the solution residence time nolonger exceeds the diffusion rate through the membrane and the total C02 in the solution approacheszero, reducing the sensitivity. The difference in sensitivity between the acid and base form is due to the difference in molar absorptivities, the acid t being approximately half of the base e. The sensitivities of the PhR response at 0.19 pL/min for the acid form and at 0.89 pL/min for the base form are further diminished because they are at the stray light limit of the sensor signal. The measurement precision was as low as h0.8 gatm, obtained with BTB flowing at 0.12 pL/min (Table I). The higher flow rates generallyhad poor baseline stability, a result of working within the diffusion-dependent steady-state regime. Indicator flow oscillations, caused by variations in the sensor back-pressure or periodicity in the pump drive cycle, modulate the diffusion-dependent sensor signal and degrade the baseline stability. At low flow rates (near equilibrium), the response becomes insensitive to small flow oscillations and the baseline noise approaches the detector thermal noise limit. Precisions for the PhR and BTB acid response are poor due to the low light throughput at 434 nm and lower sensitivity (lower c). Therefore, signal ratios degraded the precision attainable with the base form signal. In addition, since the thermal characteristics of the two detectors and associated amplifiers were not matched, the dark signal drifts were poorly correlated and the ratio did not improve the long-term stability of the measurements, negating one of the basic benefits of performing ratiometric measurements. The times required for the sensor to come to full (100%) response after a step change in pC02 with the different dyes and flow rates are shown in Table 11. The response time variability did not correlate with positive or negative concentration changes, indicating that diffusion out of the sensor membrane was not a controlling factor. Therefore, the response time is probably dominated by the membrane flush out time. It is evident that to approach equilibriumwith the membraneused in this study, the solution requires a minimum residence time in the membrane of approximately 20 min. The slow response is not a critical factor in this application where a time resolution of 1-2 h w i l l be adequate. In cases
15
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20
25
30
Temperature
(OC)
Flguro 6. Sensor temperature sensitivity at dlfferent Row ram (same solutlon conditions as In Figure 5A, [CO,] = 370 wtm): (e)1.0 N mln; (V) 0.70 pl/mln; (V)0.36 FL/mln; ( 0 )0.25 pUmln.
where a more rapid response is needed, the response time may be decreased by reducing the membrane thickness and internal reservoir volume. Temperature Sensitivity. Temperature is an environmental parameter that must be considered for almost any in situ measurement. The oxygen polarographic electrode provides a useful analogy for analyzing the temperature response of the COZ sensor16 when the sensor responee is limited by diffusion. It has been shown that the diffueiondependent 02sensors have significant temperature sensitivity ( 3 4 % increase in current/OC) because temperature affects membrane permeability, diffusioncoefficients,gas solubility, and solution equilibria. Because the COzsensor has the same temperature-dependent properties, similar temperature behavior should be expected when operated at steady state. Figure 6 shows the temperature response of the C02 sensor using different solution flow rates. At high flow rates, the response is diffusion-dependent and, as a result, very sensitive to temperature changes. Higher temperatures increase the permeability and diffusion rates, increasing the flux of C02 across the membrane, analogous to 02sensors. The increased C02 flux drops the pH, causing an increase in the signal at 560 nm. At low flow rates the slope of the temperature response actually changes sign. The negative slope is predicted by the equilibriummodel, shown in Figure 7. The curves in Figure 7 were calculated using the equilibrium constanta as a function of temperature which are known for PhR17 and the C02 equilibria.18 The calculations were made with 350 patm of C02and pHhit equal to 9.0. The change in pH, and hence, the sensor signal, is dominated by the C02 solubility which decreases with increasing temperature. The decreasing slope of the 0.36 pL/min flow rate curve probably indicates that the sensor is operating both at steady state and near equilibrium over the 15-30 "C temperature range-equilibrium being approached at the higher temperatures due to the increase in COZ flux. Subsequent mea(16) Hale, J. M. In Polarographic Oxygen Sensors: Aquatic and Physiological Applications; Gnaiger, E., Forstner, H., Eds.; Springer-Verlag: New York, 1983; pp 3-17. (17)Robert-Baldo, G. L.; Morris, M. J.; Byme, R. H.Anal. Chem. 1985,57, 2564-2667. (18) Goyet, C.; Poisson, A. Deep-sea Res. 1989, 36, 16361664.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15, 1993 0 h
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Figure 7.
PHinn
surements over a wider range of temperatures (10-30 "C) using BTB at 0.08 pL/min indicated a linear temperature coefficient of approximately 1patm/OC. Sample Flow Hydrodynamics. Sensitivity to sample flow or convection was tested by changing the rate at which the calibration gases were bubbled into the test water (while the sensor was operated with different flow rates). Bubbling rates ranged from 10 to 120 mL/min into the 250-mL sensor vessel. Signal changes were not significantly larger than the 3 X rms baseline noise over these extremes. The diffusiondependent (high indicator flow rate) measurements would be expected to be sensitive to flow hydrodynamics if the external viscous boundary layer were a significant resistance to mass transfer.16 The thick membrane (200 pm) likely dominates the mass-transfer resistance; therefore, the sensor is relatively insensitive to sample hydrodynamics even at the high indicator flow rates. Surface Seawater Measurements. The sensor performance was tested on a research cruise in the South Pacific Ocean during the spring of 1992. Two significant changes were made in the system before the sensor was used in the field study. First, the gas-driven pumping system was replaced with an infusion syringe pump so that very low (
