EPSRC-SFI: An ocean microlab for autonomous dissolved inorganic carbon depth profile measurement

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CO2 concentration in the atmosphere has increased significantly since pre-industrial times leading to global warming. There is now a major concern that ocean absorption of CO2 may be saturating, leading to more rapid global warming and much more serious consequences than predicted. Understanding the ocean CO2 system is of fundamental importance for climate change models that inform our predictions but ocean measurement of CO2, particularly in the form of dissolved inorganic carbon (DIC), is severely lacking due to technical challenges. We need regular measurements, down to a depth of 2 km, from thousands of locations world-wide. Accurate field measurements of DIC up to now have involved large and expensive surface instruments, e.g. infra-red absorption or mass spectrometry, and their miniaturisation is not feasible at the required accuracy. The aim of this project is to develop a new method of measuring DIC that is accurate, but which can also be miniaturised so that worldwide float deployment becomes a possibility. At present, the Argo network consists of ~3000 untethered battery-operated floats located across the world's oceans. They operate autonomously, drifting at a park depth of 1.5 km and every 10 days they rise to the surface, measuring the temperature and salinity depth profiles on the way. This data is then transmitted to satellite and the cycle repeats. These two parameters can be measured instantaneously at each depth whereas DIC quantification requires time-consuming chemical analysis. In the laboratory, the standard calibration technique separates DIC from seawater as CO2 gas which then transfers across a membrane into a reagent (NaOH), resulting in a decrease in conductivity. With appropriate design and calibration, the measured change in conductivity can be converted to DIC concentration. The time required for gas exchange however prevents instantaneous measurement but with the Argo float cycle, there is a 10-day park window where this exchange could be allowed to occur, and with a large number of samples. Our objectives therefore are to miniaturise each of the functional units of the laboratory setup and integrate them into a single microfluidic lab on chip which can meet the severe size, power, cost and reliability limits imposed by the Argo float integration. This presents an immense challenge; microfluidics research up to now has focussed mainly on biomedical applications which have an entirely different set of criteria, essential ocean testing of ideas and refinements is very difficult and expensive, while technical challenges can appear insurmountable. Conductivity measurement is relatively simple in concept and is readily miniaturised. However, the accuracy is much lower compared to optical techniques and this is exacerbated by the need to use extremely small sample volumes, (~100 nL). The depth resolution depends on the number of samples collected, stored, and subsequently analysed within float rise and park times respectively. The ultimate preference is ~100 samples, giving a depth resolution of 20m. This requires 100 fluid circuits and at least 100 valves to be fabricated in a 10 x 10 x 2 cm device. Such high-resolution channel patterning creates major difficulties with regards to bonding and sample leakage between channels, exacerbated by the extremely harsh environment, high pressure and the long-term deployment. This situation is further challenged by the need to seal a membrane within a multilayer structure. The best membrane materials (gas permeable and ion blocking) are very hydrophobic and resist bonding to other materials. Finally, there is no nano/micolitre valve technology that could operate in an environment where pressures vary up to 200 atmospheres. Most of the limited research to date has focussed on pneumatic valves. In this project we need to discover and develop new stimuli responsive valve materials and find a way to incorporate these into multiple microfluidic channels.