Autonomous and continuous analysis of ocean chemistry, in particular dissolved inorganic carbon(DIC)concentration profiles with depth, are of great significance with regard to ocean acidification and climate change. However, the development of suitable miniature in-situ analysis systems is hampered by the size, cost and power requirements of traditional optical analysis instrumentation. Here we report a low-cost alternative approach based on CO2 separation and conductance measurement in microfluidic cells that could pave the way to integrated lab on chip systems for long-term ocean float deployment. Conductimetric determination of dissolved inorganic carbon concentration, in the seawater range of 1000 –3000 mol kg-1, has been achieved using a microfluidic thin film electrode conductivity cell and a membrane-based gas exchange cell. After transfer across the membrane, the eluted CO2 reacted in a NaOH carrier, was drawn through a conductivity cell with a <1L interelectrode volume, where the change in impedance versus time was measured. Precision values, obtained from relative standard deviation,were ~ 0.2% for peak height measurements over extended time periods. This compares favourably with precision values of ~ 0.25% obtained using a large C4D electrophoresis headstage with similar measurement volume. The required sample and reagent volumes amounted to ~500L in total due to the incorporation of a planar membrane into a small volume exchange cell. This compares very favourably with previous attempts at conductivity based DIC analysis where total volumes between 5000L and 10000L were required. The use of long membrane tubes and macroscopic wire electrodes is avoided by incorporating a planar membrane (PDMS) between sample and exchange cell and by sputter deposition of Ti/Au multilayer electrode patterns directly onto a patterned thermoplastic (PMMA) manifold. Future performance improvement will require addressing membrane chemical and mechanical stability as well as further volume reduction and component integration into a single manifold.
Tweedie M., Sun D., Gajula D. R., Ward B. & Maguire P. D., in press. The analysis of dissolved inorganic carbon in liquid using a microfluidic conductivity sensor with membrane separation of CO2. ChemRxiv. Article.
