CSU Thurgoona:Notebook/Dissolved Iorganic Carbon in headwaters/2009/12/20
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This is the DIC Page!
Pilot Study Results
This pilot work is included here as it demonstrates the viability of the approach required for development of a field portable instrument for this project. The core concept for DIC measurement is that hydrophobic ePTFE tubing is highly permeable to CO2 but not to liquid water. This means that water can be pumped into a length of this tubing that is enclosed in a larger tube, with the enclosing tube volume in turn connected to a licor-840 IRGA. A small diaphragm pump re-circulates the air between the outer tube and the IRGA and continuously measures CO2 abundance and water vapour concentration. As an aliquot of water is pumped into the ePTFE tube, it is acidified to a pH <2 driving all DIC to CO2. The CO2 rapidly diffuses through the ePTFE tube into the headspace and the build-up of CO2 to an equilibrium value is monitored by the IRGA over 12 minutes. At the end of analysis period the air in the headspace is re-circulated though a tube containing X13 molecular sieve that rapidly traps CO2 (and water vapour) out of the airstream, ready for the next sample introduction. DIC in the sample is directly and linearly proportional to the equilibrium CO2 concentration in the headspace (with the possibility of a secondary temperature effect). A manual ‘benchtop’ system, constructed to provide ‘proof of concept’ is shown in Figure 1B, using 20cm of ‘50% porous’ ePTFE tubing with a volume of 3ml (Figure 1A), and the results from calibration of the system is shown using waters of known DIC concentration from 5-115mgC/l are shown in Figures 1C and 1D. Triplicate analysis of each standard suggests a reproducibility of ±1-3% but most of this analytical uncertainty is the result of the manual acidification and injection of samples; reproducibility is likely to be better than 1% in an automated system. The range and precision of the technique is scalable simply by changing combinations of ePTFE tube length/diameter, volume of the re-circulating airstream and/or length of the IRGA optical bench (0-3000ppm or 0-20,000ppm ranges available).