An Automated Microfluidic Analyzer for In Situ Monitoring of Total Alkalinity

Abstract

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 °C allowing for field use in most ocean environments. Each titration point requires approximately 170 μL of titrant, 830 μL 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 μmol kg–1. We further report a precision of 16 μmol 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.

Let’s Talk about Slime; or Why Biofouling Needs More Attention in Sensor Science

“…In a recent example, Sonnichsen et al developed an automated total alkalinity analyzer based on an optical microfluidic platform. 13 In order to protect the microfluidics from clogging and biofouling, the sample was passed through a 0.45 μm filter prior to entering the system. This enabled the deployment of the system for several weeks.…”

“…By the exclusion of bacteria from the sensing compartment, biofouling on the sensor unit itself can be avoided. In a recent example, Sonnichsen et al developed an automated total alkalinity analyzer based on an optical microfluidic platform . In order to protect the microfluidics from clogging and biofouling, the sample was passed through a 0.45 μm filter prior to entering the system.…”

High-Precision In Situ Total Alkalinity Analyzer Capable of Month-Long Observations in Seawaters

“…The duration of field deployments with the reported analyzers has been limited, with 23 days of TA data for the SAMI-alk; 29 6 days for the dual pH-A T sensor during a continuous deployment in Ka ̅ ne'ohe Bay; 27 and 15 days of field data for the microfluidic TA analyzer. 28 Despite the great advances made in recent years, improving the performance of in situ TA analyzers/sensors, particularly in terms of precision, accuracy, and duration of field deployment, remains a major challenge. An automated TA analyzer based on the automated single-point titration with spectrophotometric pH detection was developed in our earlier work.…”

“…To overcome these challenges, a number of automated TA analyzers/sensors have been developed. ,− Among these instruments, in situ TA analyzers/sensors are the best tools to obtain time series of TA data with high temporal resolution. To date, a few in situ TA analyzers/sensors have been reported, including the Submersible Autonomous Moored Instrument for alkalinity (SAMI-alk) developed by Spaulding et al, the dual pH-A T sensor developed by Briggs et al, and a microfluidic total alkalinity analyzer developed by Sonnichsen et al The SAMI-alk is based on the tracer monitored titration, and requires an accurate optical detection system .…”

“…To overcome these challenges, a number of automated TA analyzers/sensors have been developed. ,− Among these instruments, in situ TA analyzers/sensors are the best tools to obtain time series of TA data with high temporal resolution. To date, a few in situ TA analyzers/sensors have been reported, including the Submersible Autonomous Moored Instrument for alkalinity (SAMI-alk) developed by Spaulding et al, the dual pH-A T sensor developed by Briggs et al, and a microfluidic total alkalinity analyzer developed by Sonnichsen et al The SAMI-alk is based on the tracer monitored titration, and requires an accurate optical detection system . It has a precision of ±4.7 μmol kg –1 and an accuracy of −2.2 ± 13.1 μmol kg –1 , and it requires 12 min, 80 mL sample, and 4.5 mL titrant for a single measurement .…”