Richard Socki and Tracey Jacksier 

Air Liquide          

The growth and evolving rolls of stable isotope techniques such as Continuous Flow-Isotope Ratio Mass Spectrometry (CF-IRMS) and Cavity Enhanced Spectroscopy (including cavity ring down spectroscopy, CRDS) techniques in areas including food authentication and environmental research have increased the need for isotopically reliable and stable working reference gas standards.  Both CRDS and IRMS reference gases are typically stored in high pressure compressed gas cylinders while connected for use as working gases for direct comparison to unknown samples during stable isotope measurements. Some of these compressed gases may contain more than one physical phase at common storage temperatures depending on the pressure and temperature (i.e., gas, supercritical and liquid), such as carbon dioxide (CO₂). During liquid-gas and gas-supercritical phase transitions, CO₂ can undergo isotopic fractionation in both C and O.  This phase transition can occur during routine cylinder handling, such as gas expansion or while subjecting the cylinder to cold temperatures without allowing the cylinders to come to thermal equilibrium prior to use. 

The objectives of this work were to determine if C- or O-isotope fractionation occurs when sampling CO₂ gas from single, supercritical, or dual phase compressed gas cylinders stored at different temperatures. The range of temperatures was selected to represent conditions that may be experienced as CO₂ cylinders stored outdoors were brought to indoor laboratories for use. These results provide a basis for recommending proper cylinder handling and storage procedures for CO₂ reference gas for proper stable isotope lab use. Results from these experiments clearly indicate that certain precautions should be taken when dealing with dual phase, supercritical and single-phase compressed CO₂ cylinders being used as IRMS working laboratory isotope reference gases.