ISO-CADICA: isotopic-continuous, automated dissolved inorganic carbon analyser

Quantifying the processes that control dissolved inorganic carbon (DIC) dynamics in aquatic systems is essential for progress in ecosystem carbon budgeting. The development of a methodology that allows high-resolution temporal data collection over prolonged periods is essential and is described in this study.

A novel sampling instrument that sequentially acidifies aliquots of water and utilises gas-permeable ePTFE tubing to measure the dissolved inorganic carbon (DIC) concentration and δ(13)C(DIC) values at sub-hourly intervals by Cavity Ring-down spectrometry (CRDS) is described.

The minimum sensitivity of the isotopic, continuous, automated dissolved inorganic carbon analyser (ISO-CADICA) system is 0.01 mM with an accuracy of 0.008 mM. The analytical uncertainty in δ(13)C(DIC) values is proportional to the concentration of DIC in the sample. Where the DIC concentration is greater than 0.3 mM the analytical uncertainty is ±0.1‰ and below 0.2 mM stability is < ± 0.3‰. The isotopic effects of air temperature, water temperature and CO(2) concentrations were found to either be negligible or correctable. Field trials measuring diel variation in δ(13)C(DIC) values of coral reef associated sea water revealed significant, short-term temporal changes and illustrated the necessity of this technique.

Currently, collecting and analysing large numbers of samples for δ(13)C(DIC) measurements is not trivial, but essential for accurate carbon models, particularly on small scales. The ISO-CADICA enables on-site, high-resolution determination of DIC concentration and δ(13)C(DIC) values with no need for sample storage and laboratory analysis. The initial tests indicate that this system can offer accuracy approaching that of traditional IRMS analysis.