Carbon isotopic analysis of dissolved inorganic carbon by laser spectroscopic technique

The analyses of the content of Dissolved Inorganic Carbon （DIC） in natural water and its stable carbon isotopic composition （δ13CDIC） provide an useful means for investigating the carbon cycle and its related biogeochemical processes. In recent years， the laser spectroscopic technology has provided us with a new method for isotopic analysis of DIC in water. In this study， we applied a TOC analyzer on-line with a laser Cavity Ring-Down Spectroscopy （CRDS） to determine DIC content and δ13CDIC value. The analytical precision and accuracy were discussed and both results determined by this system and the Isotopic Ratio Mass Spectrometer （IRMS） were compared. The results of the measurement of a large number of laboratory working standards and water samples with a wide range of DIC concentrations and δ13CDIC values show that such TOC-CRDS system has high analytical precisions. The relative standard deviation of DIC content test results are within 1%， and the precision of δ13CDIC is better than ±0.1‰ （1σ）. Measured δ13CDIC values by both TOC-CRDS and IRMS methods are close to each other， with a difference of mostly ≤0.3‰， an indicative of high accuracy for the TOC-CRDS system. Because the absorption spectrum is a function of gas concentration， however， the analytical precision and accuracy could be lowered for samples with lower CO2 yielding in the system. Thus， it is crucially important to make sure that the sample injected into the TOC analyzer has optimal carbon content. Moreover， it is better to use multiple working standards that have different δ13CDIC value to calibrate the measured values of water samples. Compared with the IRMS， the laser spectrometer is less costly and has advantages in fast， efficient determination of both DIC content and δ13CDIC value. In all， the machine is small and can be operated in the field， and it therefore has great potential to be one of useful methods in high-frequency monitoring of karst water， which benefits the study of karst dynamic system， karst critical zone processes and their responses to climatic and environmental changes.