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.