Evaluation of Dissolved Inorganic Carbon in Seawater

Importance of the Topic

The rising concentration of atmospheric CO2 is a primary driver of global warming. Oceans act as major carbon sinks, absorbing more CO2 than terrestrial ecosystems. Accurate measurement of dissolved inorganic carbon (DIC) in seawater is essential for evaluating the ocean’s role in carbon sequestration and for advancing blue carbon research.

Objectives and Study Overview

This study evaluates the performance of the Shimadzu TOC-L total organic carbon analyzer, paired with the ASI-L autosampler, for rapid and precise quantification of inorganic carbon (IC) in seawater. Seawater samples from Tokyo Bay were spiked with known amounts of sodium carbonate to assess accuracy, linearity, and salt tolerance.

Analytical Methodology and Instrumentation

• Sample Preparation: Seawater spiked with 0, 1, 5, 10, and 20 mgC/L using sodium carbonate.
• Instrumentation: Shimadzu TOC-LCPH analyzer with ASI-L autosampler.
• Detection Principle: Phosphoric acid acidification converts IC to CO2, detected by non-dispersive infrared (NDIR) sensor.
• Calibration: Two-point calibration curve using 0–50 mg/L sodium carbonate/bicarbonate standards.
• Injection Volume: 50 µL per analysis.

Main Results and Discussion

• Linearity: Excellent correlation (R²=1.0000) between measured and added IC concentrations.
• Recovery: Rates between 101 % and 104 % across all spiked levels, demonstrating high accuracy in high-salinity matrices.
• DIC Conversion: Using a conversion factor of 3.67 (CO2/C), DIC values matched expected concentrations, confirming method reliability.

Benefits and Practical Applications

• Speed: Approximately 15 minutes per sample enables high throughput.
• Salt Tolerance: Unaffected by ~3.5 % salt content, ensuring accurate IC measurement in seawater.
• Automation: ASI-L autosampler supports unattended, multi-sample runs and improved reproducibility.
• Applications: Suitable for blue carbon research, oceanic CO2 uptake studies, environmental monitoring, and QA/QC in marine analytics.

Future Trends and Potential Applications

• Integration with in situ monitoring platforms for continuous DIC measurement in marine environments.
• Combination with isotope ratio mass spectrometry to investigate carbon cycling pathways.
• Development of portable or field-deployable analyzers for on-site assessments.
• Extension to other saline matrices such as estuarine water and industrial brines.

Conclusion

The Shimadzu TOC-L analyzer with ASI-L autosampler delivers a rapid, accurate, and salt-resistant technique for IC quantification in seawater. This approach enhances capabilities in carbon capture research, ocean acidification studies, and comprehensive carbon cycle analysis.