An autonomous in situ total Dissolved Inorganic Carbon sensor miniaturised utilising microfluidic lab-on-chip technology

The oceans have taken up ~25% of the carbon dioxide emitted into the atmosphere over the course of the industrial revolution. This has impacted the ocean carbonate system, including a measurable drop in oceanic pH. The ocean carbonate system can be determined from measurements of at least two of its four measurable parameters, pH, total alkalinity (TA), partial pressure of CO₂ (pCO₂), and total dissolved inorganic carbon (DIC). Of these, the DIC is the only one without a commercially available autonomous or underway sensor, a key requirement to enhance the spatial and temporal data coverage. DIC measurements are frequently combined with TA, and also often paired with pH for the lowest derived determination errors, to quantify the carbonate system. Here, we present results from the development, as well as laboratory and field tests, of a microfluidic DIC sensor based on the NOC lab-on-chip platform. This sensor autonomously analyses discrete samples via acidification of DIC to CO₂ and CO₂ transfer across gas permeable membrane into a NaOH (CO₂ acceptor) solution. The change in the NaOH conductivity from its reaction with CO₂ is recorded and is then related to the DIC concentration in the sample (conductimetric DIC determination). Sampling frequency was ~20 minutes and precision in laboratory tests was < 2 µmol/kg. The LOC DIC sensor was used as an underway system on board a ship in transit from the North Sea to Southampton, UK. It was also deployed with a network of other LOC sensors on seabed landers at 120 m water depth and on a Remotely Operated Vehicle as part of a large project aiming to detect an anthropogenic sub-seabed leak of CO₂. Comparison of sensor data with DIC analyses, of poisoned bottled samples on a VINDTA 3C, gave the sensor an estimated accuracy of ~10 µmol/kg. This demonstrates that the conductimetric LOC DIC sensor is capable of making frequent “weather” quality DIC measurements as defined by the Global Ocean Acidification Observing Network.