Determination of photochemically produced carbon dioxide in seawater

Emily M. White, David J. Kieber, Kenneth Mopper
Limnol. Oceanogr. Methods (XXXX) X:XXX-XXX

An analytical system was developed to determine low levels of photochemically produced carbon dioxide in marine waters. A closed system was used to prevent atmospheric contamination during sample preparation, irradiation, and analysis. To detect low levels of photoproduced carbon dioxide in seawater, background dissolved inorganic carbon (DIC) was removed before irradiation. To remove ambient DIC, samples were acidified (pH 3.0) and bubbled with low-CO₂ air. The pH was re-adjusted back to the original value and the resulting low-DIC seawater samples were transferred pneumatically to air-tight quartz tubes for irradiation. During analysis, samples were pneumatically transferred to a sample loop, injected, and acidified. Carbon dioxide was then stripped out, dried, and carried to a non-dispersive infrared CO₂ analyzer. Calibration was done with a series of aqueous carbonate standards (0.05 to 3 µmol L^-1). The detection limit, defined as three times the standard deviation of the experimental blank, was approximately 60 nmol L^-1. Method precision was largely dependent on the agreement between multiple injections from the same tube (± 2% RSD) and the reproducibility between different tubes ( 3 % RSD). This method was used to measure CO₂ photoproduction in a variety of waters (e.g., estuarine, lake) including the first direct measurements in marine waters.