The design and testing of a robotic analyzer for autonomous TCO2 measurement from oceanographic moorings is described. The analyzer employs a conductimetric method of TCO2 measurement wherein CO2 from an acidified sample diffuses across a semi-permeable membrane into a NaOH solution decreasing the conductivity of the base. The instrument is capable of ~850 analyses over a period of at least six months. It is designed to operate to depths of at least 1000m. TCO2 calibration is based on in situ standardization throughout a deployment.
We report both laboratory and in situ tests of the analyzer. In the laboratory automated analyses over a period of 38 days at temperatures ranging from 8° to 25° C yielded a TCO2 accuracy and precision of ±2.7 ?mol/kg. In situ tests were conducted at the WHOI dock with a deployment of 8 weeks at in situ temperatures of 5°-13°C. The accuracy and precision of TCO2 analyses over the deployment period, based on in situ calibration, was ±3.6 ?mol/kg.
Laboratory tests of reagent and standard solution stability are also reported. Standards, based on Certified Reference Material were followed for periods of up to 2 years. In all cases TCO2 increased. Drift of the standards was the equivalent of ~1 to ?mol/kg per 6 months. The conductivity indicator solution was found to be stable for at least 2 months.