  Continuous and autonomous measurement of total dissolved inorganic carbon
(TCO2) in the oceans is critical for modelling important climate change factors
such as ocean uptake of atmospheric CO2 and ocean acidification. Miniaturised
chemical analysis systems are therefore required which are small enough for
integration into the existing Argo ocean float network for long-term unattended
depth profiling of dissolved CO2 with the accuracy of laboratory bench
analysers. A microfluidic conductivity-based approach offers the potential for
such miniaturisation. Reagent payload for >3 yr operation is a critical
parameter. The precise injection of acid into sample, liberating CO2 from
seawater, is addressed here. Laser etched microfluidic snake channel
restrictors and asymmetric Y meters were fabricated to adjust the metering
ratio between seawater and acid simulants. Laser etching conditions were varied
to create a range of channel dimensions down to ~75 microns. Channel flow
versus pressure measurements were used to determine hydrodynamic resistances
which were compared with finite element simulations using a range of
cross-section profiles and areas. Microfluidic metering circuits were
constructed from variable resistance snake channels and dimensionally symmetric
or asymmetric Y-junctions. Sample to acid volume ratios (meter ratio) up to
100:1 have been achieved with 300 microns wide snake channel for lengths > 1m.
At the highest pattern resolution, this would require a footprint of > 600 mm2
(6 x10-4 m2). Circuits based solely on asymmetric Y-junctions gave meter ratios
up to 16:1 with a footprint cost of < 40 mm2 and precision values of ~0.2%.
Further design and fabrication refinements will be required to ensure the
structural and dimensional integrity of such small channels in future
integration of metering units into full TCO2 analysis microfluidic circuits.
