A lab-on-chip sensor for in situ spectrophotometric measurement of seawater pH

Abstract

Accurate and high precision measurements of seawater pH are important for monitoring of ocean acidification and to fully understand the dynamics of the oceanic carbonate system. An optimisation and field testing of an in situ pH sensor, based on lab-on-chip technology, offers potential to address to this problem. The sensor employs a spectrophotometric method, with high precision (0.001 pH units) and accuracy (0.005 pH units) within the range of a certified TRIS buffer. In addition, it is a compact miniaturised field analyser that consumes minimal power (3 W) and uses low reagent consumption (2.2 μL per analysis). A newly developed Absorbance-Derivative method is used to eliminate indicator-induced pH perturbation; automatically correcting approximately ±0.08 pH units perturbation for each individual analysis. Another R-correction calibration method is also studied, resulting an accuracy of ±0.005 pH units, thereby providing an easy but also sufficiently accurate way to calibrate pH sensors. A recipe for preparing TRIS buffer with any pH value between 7.4 and 8.4, and with any salinity between 20 and 40 psu is resented, with initial verification suggesting an accuracy of ±0.004 pH units. The sensor has been field tested under various conditions, including vertical profiling, a brackish water moored deployment and a seawater moored deployment. A three-month pontoon deployment was performed, with an accuracy of ±0.003 pH units recorded over a 2 week period. However, biofouling in the inlet filter caused a diurnal offset up to 0.1 pH units, indicating that the next version of this pH sensor should focus on anti-biofouling technology.

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ORCID for Matthew Mowlem: orcid.org/0000-0001-7613-6121

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