An optimized sampling system for highly reproducible isotope ratio measurements (δ13C and δ18O) of pure CO2 gas by infrared spectroscopy

Abstract

The characteristics of an optimized sampling system for measurements of isotope ratios in pure CO2 gas with Isotope Ratio Infrared Spectroscopy measurement systems that has achieved reproducible measurement of δ13 C and δ18O values with 0.02‰ reproducibility (1 σ) is described. The key elements of the sampling system revolve around almost identical treatment of sample and reference gases allowing two-point calibration of up to 14 samples, and appropriate flushing protocols to remove any biases from memory effects of previously sampled gases. Measurements are performed by the Isotope Ratio Infrared Spectroscopy system at a mole fraction of nominally 700 μmol mol−1 CO2 in air, by dilution of pure CO2 gas controlled by individual low-flow mass flow controllers (0.07 ml min−1), and with a feedback loop to control mole fractions to ensure that differences between references and sample gas mole fraction stay below 2 μmol mol−1. This level of control is necessary to prevent biases in measured isotope ratios, the magnitude of which has also been studied with a sensitivity study. The system has been validated using pure CO2 samples which range in δ13 C delta values of −1‰ and −45‰ vs VPDB-CO2, and in all cases measurement reproducibility over several days of testing of 0.02‰ or better (1 σ) was achieved for both δ13 C and δ18O, with negligible memory effects. The amount of sample gas used for each measurement was less than 5 ml of CO2 at (RTP), making the system easily deployable for isotope ratio value assignment of bulk CO2 gas, and adaptable to atmospheric mole fractions of CO2 in air, and for value assignments of standards. Using the sampling system described the measurement reproducibility of current Isotope Ratio Infrared Spectroscopy systems approaches measurement reproducibility that can be achieved with some IRMS systems.