Metered reagent injection into microfluidic continuous flow sampling for conductimetric ocean dissolved inorganic carbon sensing

Continuous autonomous measurement of total dissolved inorganic carbon (TCO₂) in the oceans is critical for climate change modelling and ocean acidification measurement. A microfluidic conductivity-based approach will permit integration of miniaturised chemical analysis systems into Argo ocean floats, for long-term, high-accuracy depth profiling of dissolved CO₂ with minimal reagent payload. Precise metering, suitable for sample acidification and CO₂ liberation, is addressed. Laser etched microfluidic snake channel restrictors and asymmetric Y-meters were fabricated, with channel dimensions down to ∼75 μm, to adjust metering ratios between seawater and acid simulants. Hydrodynamic resistances, from flow versus pressure measurements, were compared with finite element simulations for various cross-section profiles and areas. Microfluidic metering circuits were constructed from various resistance snake channels and Y-junction components. Sample to acid volume ratios (meter ratio) up to 100:1 have been achieved with 300 μm wide snake channels for lengths >m. At highest resolution, the footprint would be >600 mm². Circuits based solely on asymmetric Y-junctions gave maximum meter ratios of 16:1 with a footprint of <40 mm² and ∼0.2% precision. Further refinement is required to ensure the integrity of such small channels in integration of metering units into full TCO₂ analysis microfluidic circuits.