A new method to determine carbon isotopic composition of dissolved inorganic carbon in seawater and pore waters by CO2-water equilibrium

Abstract

Rationale: Precise measurement of the carbon isotopic compositions of dissolved inorganic carbon (DIC) in water samples is very important for earth and environmental sciences, and the H3PO4 acidification method has long been applied for this purpose. Due to the large variation in DIC concentration in various types of water samples, however, it is difficult to sample the optimized amount of water for high-precision mass spectrometry analysis and in many cases this has led to poor data quality. In addition, when the amount of water sample is very limited or when the water has a complicated composition such as seawater and saline marine pore waters, it is often difficult to obtain high-quality data using the conventional method. Methods: Here we report a new method of obtaining CO2-water equilibrium for high-precision carbon isotope analysis of DIC in water samples. The instrument used was a Delta Plus XP stable isotope mass spectrometer coupled with an on-line a Gas Bench II sample preparation device. In general, 0.5 mL of water is sampled and equilibrated with CO2 in headspace. The CO2 is then transferred into the mass spectrometer and δ13C values are obtained. Results: The δ13C values for four marine pore water samples determined by this new method are only slightly different (<0.15%) from those determined using the conventional H3PO 4 method. The new method of obtaining CO2-water equilibrium also allows a constant amount of CO2 for every sample to finally enter the mass spectrometer and hence better precision can be achieved. Conclusions: This new method is suitable for the measurement of both oxygen isotopic composition in water and carbon isotopic composition of DIC for seawater and pore water samples. Another benefit of this new method is that the water sample can be re-used for other further analyses and this is critical when the amount of sample is limited. © 2012 John Wiley & Sons, Ltd.