Realistic measurement uncertainties for marine macronutrient measurements conducted using gas segmented flow and Lab-on-Chip techniques

Lab-on-Chip for In Situ Analysis of Nutrients in the Deep Sea

Microfluidic reagent-based nutrient sensors offer a promising technology to address the global undersampling of ocean chemistry but have so far not been shown to operate in the deep sea (>200 m). We report a new family of miniaturized lab-on-chip (LOC) colorimetric analyzers making in situ nitrate and phosphate measurements from the surface ocean to the deep sea (>4800 m). This new technology gives users a new low-cost, high-performance tool for measuring chemistry in hyperbaric environments. Using a combination of laboratory verification and field-based tests, we demonstrate that the analyzers are capable of in situ measurements during profiling that are comparable to laboratory-based analyses. The sensors feature a novel and efficient inertial-flow mixer that increases the mixing efficiency and reduces the back pressure and flushing time compared to a previously used serpentine mixing channel. Four separate replicate units of the nitrate and phosphate sensor were calibrated in the laboratory and showed an average limit of detection of 0.03 μM for nitrate and 0.016 μM for phosphate. Three on-chip optical absorption cell lengths provide a large linear range (to >750 μM (10.5 mg/L-N) for nitrate and >15 μM (0.47 mg/L-P) for phosphate), making the instruments suitable for typical concentrations in both ocean and freshwater aquatic environments. The LOC systems automatically collected a series of deep-sea nitrate and phosphate profiles in the northeast Atlantic while attached to a conductivity temperature depth (CTD) rosette, and the LOC nitrate sensor was attached to a PROVOR profiling float to conduct automated nitrate profiles in the Mediterranean Sea.

Lab-on-a-chip technology for in situ combined observations in oceanography

The oceans sustain the global environment and diverse ecosystems through a variety of biogeochemical processes and their complex interactions. In order to understand the dynamism of the local or global marine environments, multimodal combined observations must be carried out in situ. On the other hand, instrumentation of in situ measurement techniques enabling biological and/or biochemical combined observations is challenging in aquatic environments, including the ocean, because biochemical flow analyses require a more complex configuration than physicochemical electrode sensors. Despite this technical hurdle, in situ analyzers have been developed to measure the concentrations of seawater contents such as nutrients, trace metals, and biological components. These technologies have been used for cutting-edge ocean observations to elucidate the biogeochemical properties of water mass with a high spatiotemporal resolution. In this context, the contribution of lab-on-a-chip (LoC) technology toward the miniaturization and functional integration of in situ analyzers has been gaining momentum. Due to their mountability, in situ LoC technologies provide ideal instrumentation for underwater analyzers, especially for miniaturized underwater observation platforms. Consequently, the appropriate combination of reliable LoC and underwater technologies is essential to realize practical in situ LoC analyzers suitable for underwater environments, including the deep sea. Moreover, the development of fundamental LoC technologies for underwater analyzers, which operate stably in extreme environments, should also contribute to in situ measurements for public or industrial purposes in harsh environments as well as the exploration of the extraterrestrial frontier.