A Practical Cryogen-Free CO2 Purification and Freezing Technique for Stable Isotope Analysis

Since isotopic analysis by mass spectrometry began in the early 1900s, sample gas for light-element isotopic measurements has been purified by the use of cryogens and vacuum-line systems. However, this conventional purification technique can achieve only certain temperatures that depend on the cryogens and can be sustained only as long as there is a continuous cryogen supply. Here, we demonstrate a practical cryogen-free CO2 purification technique using an electrical operated cryocooler for stable isotope analysis. This approach is based on portable free-piston Stirling cooling technology and controls the temperature to an accuracy of 0.1 °C in a range from room temperature to −196 °C (liquid-nitrogen temperature). The lowest temperature can be achieved in as little as 10 min. We successfully purified CO2 gas generated by carbonates and phosphoric acid reaction and found its sublimation point to be −155.6 °C at 0.1 Torr in the vacuum line. This means that the temperature required for CO2 trapping is much ...