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Characterisation and deployment of an immobilised pH sensor spot towards surface ocean pH measurements

Characterisation and deployment of an immobilised pH sensor spot towards surface ocean pH measurements
Characterisation and deployment of an immobilised pH sensor spot towards surface ocean pH measurements
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·1.075R=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 ?s 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.
Seawater, pH sensor, Fluorescence, Optode, Ocean acidification
0003-2670
69-80
Clarke, Jennifer S.
b52af005-821a-4b39-93b7-c0b66ce9d66c
Achterberg, Eric P.
685ce961-8c45-4503-9f03-50f6561202b9
Rérolle, Victoire M.C.
454aa1fc-b776-4df7-a763-721155a7be40
Abi Kaed Bey, Samer
a3568a3d-a024-4dd8-9ac3-87f324bf0821
Floquet, Cedric F.A.
9e0adc5e-10f1-4d67-81cc-63157949b704
Mowlem, Matthew C.
6f633ca2-298f-48ee-a025-ce52dd62124f
Clarke, Jennifer S.
b52af005-821a-4b39-93b7-c0b66ce9d66c
Achterberg, Eric P.
685ce961-8c45-4503-9f03-50f6561202b9
Rérolle, Victoire M.C.
454aa1fc-b776-4df7-a763-721155a7be40
Abi Kaed Bey, Samer
a3568a3d-a024-4dd8-9ac3-87f324bf0821
Floquet, Cedric F.A.
9e0adc5e-10f1-4d67-81cc-63157949b704
Mowlem, Matthew C.
6f633ca2-298f-48ee-a025-ce52dd62124f

Clarke, Jennifer S., Achterberg, Eric P., Rérolle, Victoire M.C., Abi Kaed Bey, Samer, Floquet, Cedric F.A. and Mowlem, Matthew C. (2015) Characterisation and deployment of an immobilised pH sensor spot towards surface ocean pH measurements. Analytica Chimica Acta, 897, 69-80. (doi:10.1016/j.aca.2015.09.026).

Record type: Article

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·1.075R=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 ?s 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.

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Published date: 15 October 2015
Keywords: Seawater, pH sensor, Fluorescence, Optode, Ocean acidification
Organisations: Ocean and Earth Science, Ocean Technology and Engineering

Identifiers

Local EPrints ID: 384032
URI: http://eprints.soton.ac.uk/id/eprint/384032
ISSN: 0003-2670
PURE UUID: 585f8f9f-d627-4fcb-a53e-99ea517b4460

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Date deposited: 16 Nov 2015 09:33
Last modified: 08 Jan 2022 09:30

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Contributors

Author: Jennifer S. Clarke
Author: Victoire M.C. Rérolle
Author: Samer Abi Kaed Bey
Author: Cedric F.A. Floquet
Author: Matthew C. Mowlem

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