Enhanced Stability of Stable Isotopic Gases.

Pure gases and mixtures containing stable isotopes are used in a wide variety of applications including health care, food authentication, geochemistry, and environmental monitoring. It is therefore important to understand the role of moisture, which is one of the most critical impurities in compressed gas mixtures and pure gases, in their stability. Gaseous carbon dioxide (CO2) was used as a proxy for the evaluation of the effects of its isotopic composition, when in contact with moisture throughout the depletion of a cylinder's contents, as well as pressure regulation and long-term stability. To accentuate the detrimental effects of moisture on CO2 isotopic stability, enriched O-18-water was added to natural-abundance, gaseous carbon dioxide. The delta O-18-CO2 changed from -23.16 parts per thousand vs Vienna Pee Dee Belemnite (VPDB) to +109 parts per thousand vs VPDB. It was further demonstrated that with appropriate cylinder preparation to reduce residual moisture, source material purity with low moisture content, and pressure regulation (from 57.0 down to 0.5 bar), both delta O-13 and delta O-18 remained consistent within +/- 0.04 and +/- 0.06 parts per thousand, respectively, throughout the entire cylinder contents. Pressure reduction using a dual-stage regulator yielded statistically consistent results at the 99% confidence level from delivered pressures of 0.1-0.8 bar. Furthermore, the isotopic values remained consistent during a 1 year shelf-life study, illustrating the ability to utilize and regulate pressurized gases as working reference standard gases.