Spectrophotometric pH measurement in the ocean: Requirements, design, and testing of an autonomous charge-coupled device detector system

Abstract

A newly designed system for high quality discrete spectrophotometric measurements of pHT using a low-cost charge-coupled device (CCD) detector is described. Considerations and requirements for the choice of spectrophotometers with a CCD detector instead of scanning spectrophotometers with photomultiplier detector are elucidated. The presented system is evaluated in the laboratory for system accuracy and short-term precision and at sea for long-term precision and at-sea capability. Derived system characteristics are a (1s) short-term precision of ±0.0012 pH units and a (1s) long-term precision at sea of ±0.0032 pH units based on Certified Reference Materials (CRM). Such long-term precision is equivalent to a deviation of ±1.1 to 2.2 μmol kg-1 in total dissolved inorganic carbon (TCO2) and ±1.4 to 2.1 μmol kg-1 in total alkalinity (TA), depending on temperature and the TCO2/TA ratio. Overdetermination of the CO2 system (TCO2, TA, pHT) from surface-to-deep water profiles support the accuracy and precision assessment in comparison to earlier data. With careful design and testing, low-cost CCD spectrophotometers can be used for high accuracy pH measurements.