Performance evaluations and applications of a δ¹³C-DIC analyzer in seawater and estuarine waters

Dissolved inorganic carbon (DIC) and its stable isotope (δ¹³C-DIC) are important parameters for studying carbon cycling in aquatic environments. Traditional methods based on isotope-ratio mass spectrometers are labor-intensive and not easily deployable at field sites. Here we report the performance of a method that simultaneously measures DIC concentration and its stable isotope by using a CO₂ extraction device and a Cavity Ring-Down Spectroscopy (CRDS) detector. A multi-port valve is used to increase sample throughput and improve precision. The instrument achieves average precisions of better than ±1.95 μmol kg^-1 and ±0.06‰, respectively, for DIC and δ¹³C-DIC in seawater based on three injections for each sample. We also provide recommendations on how to precisely determine δ¹³C-DIC samples with a wide range of DIC content in different types of waters by examining injection volume and concentration effects. This technique was applied to study carbon cycling in the Delaware Estuary. It demonstrates that a simultaneous and precise determination of both DIC and δ¹³C-DIC is a powerful and effective approach for constraining the processes controlling aquatic carbon cycling and CO₂ fluxes. Both laboratory tests and field applications confirmed that this system can be used with high precision to study carbon cycling in various aquatic environments.