Using liquid chromatography-isotope ratio mass spectrometry to measure the δ13C of dissolved inorganic carbon photochemically produced from dissolved organic carbon

Abstract

Dissolved inorganic carbon (DIC) has long been recognized as the main identifiable product of dissolved organic carbon (DOC) photochemical remineralization. However, quantification of DIC photoproduction in natural waters has been hampered by low photoproduction rates (nM to µM h-1) relative to high DIC background concentrations (µM to mM). Here, we describe a novel method to determine the δ13C of photoproduced DIC in three southeastern US river water samples (Savannah, Altamaha, St. Marys) using liquid chromatography-isotope ratio mass spectrometry (LC-IRMS). Initial (< 4 h) DIC photoproduction rates were similar to those quantified using published methods that rely on extensive sample manipulation, but were 3.3 times higher than measured via the previous methods for prolonged exposures (> 14 h). Keeling plots indicated that the average dδ13C value for photoproduced DIC (-33.3 ± 1.4%ₒ) was depleted in 13C relative to bulk riverine DOC, consistent with the preferential photomineralization of lignin. This average was used with a simple isotope mass balance approach to calculate the concentration of photoproduced DIC. The LCIRMS measures δ13C-DIC shifts (typically±0.01 to 0.1%ₒ) more precisely than DIC concentration changes in the early stages of irradiation. Therefore, the combination of LC-IRMS analysis and our simple mass balance approach provides a new, higher precision method to quantify initial rates of DIC photoproduction and associated photochemical efficiency (i.e., apparent quantum yield) spectra in aquatic systems without any sample manipulation. A full description, sensitivity analysis and recommendations for the method are presented.