Microliter-scale seawater DIC measurement method using acidification-purge extraction coupled with hollow-core-waveguide-based TDLAS.

Dissolved inorganic carbon (DIC) is a key parameter for characterizing the seawater carbonate system, sediment pore-water biogeochemistry, and nearshore carbon cycling. However, conventional DIC analysis methods usually require milliliter-to tens-of-milliliter-scale samples and are unsuitable for high-resolution analysis of volume-limited samples. Here, we developed a microliter-scale DIC measurement method coupling acidification-purge CO2 extraction with hollow-core-waveguide-based laser absorption spectroscopy. The system integrates a sealed 1.0 mL micro-reaction chamber, a low-dead-volume gas-transfer pathway, an approximately 50 μL hollow-core waveguide (HWG) optical gas cell and employs a mid-infrared CO2 absorption line near 3728.41 cm-1, enabling reliable quantification of DIC in samples as small as 20 μL. A quantitative strategy was established based on Voigt spectral fitting, gas-phase CO2 calibration, BiHill fitting of transient release profiles, liquid-phase DIC calibration, and injection-volume correction. Under optimized acid dosage, carrier-gas flow, bubbling mode, and temperature, the gas-phase detection unit achieved a Gaussian-fit 1σ precision of 0.175 ppmv and an operational 3σ CO2 detection limit of 0.803 ppmv at an averaging time of 1 s, as derived from Allan deviation analysis. The method enabled reliable DIC quantification with the minimum injection volume of 20 μL and achieved an RSD of 0.446% at the recommended 50 μL injection volume. Validation against a TOC-L analyzer and applications to Bohai Bay surface seawater and Aoshan Bay intertidal pore-water samples demonstrated low sample consumption, field-compatible analytical capability in practice, and suitability for high-resolution DIC analysis of marine volume-limited samples.