An Ultrahigh Precision, High-Frequency Dissolved Inorganic Carbon Analyzer Based on Dual Isotope Dilution and Cavity Ring-Down Spectroscopy
2015
Huang, Kuan | Cassar, Nicolas | Jonsson, Bror | Cai, Wei-jun | Bender, Michael L.
We present a novel method for continuous and automated shipboard measurements of dissolved inorganic carbon concentration ([DIC]) in surface water. The method is based on dual isotope dilution and cavity ring-down spectroscopy (DID-CRDS). In this method, seawater is continuously sampled and mixed with a flow of NaH¹³CO₃ solution that is also enriched in deuterated water (the spike). The isotopic composition of CO₂ (δ¹³Cₛₚᵢₖₑd_ₛₐₘₚₗₑ) derived from the DIC in the mixture, and the D/H ratio of the mixed water (δDₛₚᵢₖₑd_ₛₐₘₚₗₑ), are measured by CRDS analyzers. The D/H of the water in the mixture allows accurate estimates of the mixing ratio of the sample and the spike. [DIC] of the sample is then calculated from the mixing ratio, [DI¹³C] of the spike, and δ¹³Cₛₚᵢₖₑd_ₛₐₘₚₗₑ. In the laboratory, the precision of the method is <0.02% (±0.4 μmol kg–¹ when [DIC] = 2000 μmol kg–¹). A shipboard test was conducted in the Delaware Bay and Estuary. For 2 min average [DIC], a precision of <0.03% was achieved. Measurements from the DID-CRDS showed good agreement with independent measurements of discrete samples using the well-established coulometric method (mean difference = −1.14 ± 1.68 μmol kg–¹), and the nondispersive infrared(NDIR)-based methods (mean difference = −0.9 ± 4.73 μmol kg–¹).
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