Preliminary Study: Application of Off-Axis ICOS to Determine Stable Carbon Isotope in Dissolved Inorganic Carbon

Abstract:

CO₂ is one of the major causes for global climate change. Because stable carbon isotope ratio is used to trace carbon source, several analytical techniques likes IRMS (Isotope Ratio Mass Spectrometry) and LAS (Laser Absorption Spectrometry) were extensively used. Off-axis ICOS, a kind of LAS, has merits on long-term stability and field application, therefore it is widely being used in CCS (Carbon Capture and Storage) field. The aim of this study is to extend the application scope of OA-ICOS to determine dissolved inorganic carbon (DIC). Because OA-ICOS showed dependence of δ¹³C on CO₂ concentration, data processing is required. We tested CO₂ Carbon Isotope Analyzer (CCIA-36-EP, Los Gatos Research) with both reference gas (δ¹³C= –28.28‰) and aqueous solutions prepared by dissolving sodium bicarbonate standards (δ¹³C= –12.26‰ and +3.96‰). The differences of δ¹³C between reference and measurement values are plotted by CO₂ concentrations, then compared. At first, we checked the similarity between our curve pattern for reference gas and Guillon’s research (δ¹³C= –43.99‰) by other Analyzer. To analyze aqueous samples, more errors can be caused than gas analysis. The carbon isotope fractionation occurs during dissolving standard reagents and extracting DIC as CO₂ gas form. This effect is mixed with CO₂ concentration dependence effect, therefore the curve patterns are different with that for reference gas. Our experiments are done for various δ¹³C values. It could be an important point to use OA-ICOS to analyze DIC, too.