Abstract
The oceans are a major sink for anthropogenic atmospheric carbon dioxide, and the uptake causes changes to the marine carbonate system and has wide ranging effects on flora and fauna. It is crucial to develop analytical systems that allow us to follow the increase in oceanic pCO2 and corresponding reduction in pH. Miniaturised sensor systems using immobilised fluorescence indicator spots are attractive for this purpose because of their simple design and low power requirements. The technology is increasingly used for oceanic dissolved oxygen measurements.We present a detailed method on the use of immobilised fluorescence indicator spots to determine pH in ocean waters across the pH range 7.6–8.2. We characterised temperature (−0.046 pH/°C from 5 to 25 °C) and salinity dependences (−0.01 pH/psu over 5–35), and performed a preliminary investigation into the influence of chlorophyll on the pH measurement. The apparent pKa of the sensor spots was 6.93 at 20 °C. A drift of 0.00014 R (ca. 0.0004 pH, at 25 °C, salinity 35) was observed over a 3 day period in a laboratory based drift experiment. We achieved a precision of 0.0074 pH units, and observed a drift of 0.06 pH units during a test deployment of 5 week duration in the Southern Ocean as an underway surface ocean sensor, which was corrected for using certified reference materials. The temperature and salinity dependences were accounted for with the algorithm, R=0.00034−0.17·pH+0.15·S2+0.0067·T−0.0084·S·1.075. This study provides a first step towards a pH optode system suitable for autonomous deployment. The use of a short duration low power illumination (LED current 0.2 mA, 5 μs illumination time) improved the lifetime and precision of the spot. Further improvements to the pH indicator spot operations include regular application of certified reference materials for drift correction and cross-calibration against a spectrophotometric pH system. Desirable future developments should involve novel fluorescence spots with improved response time and apparent pKa values closer to the pH of surface ocean waters.
Highlights
During the period 2002e2011, global average atmospheric carbon dioxide (CO2) concentrations increased by ~2.0 ppm per year; the highest rate of increase since monitoring began in the 1950s [1]
The research cruise was undertaken as part of the United Kingdom ocean acidification (UKOA) programme that investigated the effects of pCO2 gradients in surface waters on biogeochemical processes, calcification and ecosystem functioning
The temperature dependence (À0.046 pH CÀ1 from 5 to 25 C) and salinity dependence (À0.01 pH psuÀ1 over salinity 5e35) were accounted for using a calibration algorithm. This simplicity is an advantage compared to the individual calibrations required for ion-selective field-effect transistors (ISFET) and glass pH electrodes
Summary
During the period 2002e2011, global average atmospheric carbon dioxide (CO2) concentrations increased by ~2.0 ppm per year; the highest rate of increase since monitoring began in the 1950s [1]. We present an optode pH sensor based on fluorescent lifetime detection, for high resolution autonomous monitoring of surface ocean waters. The motivation for characterisation of an optode pH sensor for deployment in open ocean waters using fluorescent lifetime detection was based on the perceived advantages of this approach over other technologies, recently reviewed in detail by Rerolle [15]. The reagents may have limited lifetimes (~1 year) and specific storage requirements (e.g. exclusion of UV), while bubbles and particles introduced in the fluidic system can interfere with the quality of the pH measurement [39] Despite these potential issues, spectrophotometric SAMI-pH has been successfully deployed in situ for more than 2 years [40] and a microfluidic system with measurement frequencies of 0.5 Hz [35] has been demonstrated. Optodes are a newly emerging technology designed with both of these intended advantages
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