An Automated Microfluidic Analyzer for In Situ Monitoring of Total Alkalinity

We have designed, built, tested, and deployed an autonomous in situ analyzer for seawater total alkalinity. Such analyzers are required to understand the ocean carbon cycle, including anthropogenic carbon dioxide (CO2) uptake and for mitigation efforts via monitoring, reporting, and verification of carbon dioxide removal through ocean alkalinity enhancement. The microfluidic nature of our instrument makes it relatively lightweight, reagent efficient, and amenable for use on platforms that would carry it on long-term deployments. Our analyzer performs a series of onboard closed -cell titrations with three independent stepper-motor driven syringe pumps, providing highly accurate mixing ratios that can be systematically swept through a range of pH values. Temperature effects are characterized over the range 5-25 degrees C allowing for field use in most ocean environments. Each titration point requires approximately 170 mu L of titrant, 830 mu L of sample, 460 J of energy, and a total of 105 s for pumping and optical measurement. The analyzer performance is demonstrated through field data acquired at two sites, representing a cumulative 25 days of operation, and is evaluated against laboratory measurements of discrete water samples. Once calibrated against onboard certified reference material, the analyzer showed an accuracy of -0.17 +/- 24 mu mol kg-1. We further report a precision of 16 mu mol kg-1, evaluated on repeated in situ measurements of the aforementioned certified reference material. The total alkalinity analyzer presented here will allow measurements to take place in remote areas over extended periods of time, facilitating affordable observations of a key parameter of the ocean carbon system with high spatial and temporal resolution.

Automated summary

We have developed an autonomous in situ analyzer for seawater total alkalinity, which is crucial for understanding the ocean carbon cycle and carbon dioxide removal efforts. The analyzer is lightweight, reagent efficient, and can be used on long-term platforms. It has been tested and proven accurate, with the ability to measure alkalinity in remote areas over extended periods of time.