Underway determination of dissolved inorganic carbon in estuarine waters by gas-diffusion flow analysis with C4D detection

Abstract

The development and evaluation of a gas diffusion flow analysis system for the underway determination of dissolved inorganic carbon in marine and estuarine waters is described. Carbon dioxide produced when sample is injected into an acidic donor stream, diffuses through an efficient hollow fibre microporous membrane into an acceptor stream of ultrapure water, where the resultant changes in electrical conductivity are detected using a contactless capacitively coupled conductivity detector (C⁴D). The optimal parameters for the construction and operation of the C⁴D system are reported. Under field operational conditions, the flow analysis method had a linear calibration range for DIC of 0.2–10 mM, a limit of detection of 0.12 mM, repeatability of 0.46% RSD (n = 9 at 6 mM), a sample throughput of 90 h⁻¹ and excellent correlation with comparative analyses (R² = 0.9951, n = 16). The system was used to perform >250 determinations of DIC measurements underway during a short cruise on the Yarra River estuary, which demonstrated that DIC was conservative over much of the salinity gradient, with the exception of the low salinity region which exhibited the effects of respiratory CO₂ and other DIC inputs. The results illustrate the advantages of the use of rapid flow analysis techniques for chemical mapping in transient environments like estuaries.