Isotope Fractionation of Gaseous-, Supercritical-, and Dual-Phase Carbon Dioxide in Pressurized Cylinders Exposed to Variable Temperatures.

This work examines the stable isotope fractionation of carbon and oxygen in gaseous, supercritical, and liquid carbon dioxide systems at temperatures from -27.1 to +43.5 °C. For pressurized single-, supercritical-, and dual-phase carbon dioxide, both carbon and oxygen isotope fractionations can be measured and are significant when subjected to variations within this temperature range. The δ 13C and δ 18O values ranged from -41.55 to -41.38 ‰ (VPDB) and -27.74 to -24.9 ‰ (VPDB), respectively, for gas-phase carbon dioxide from 9.3 to 39 °C. A pressure variation of 27.58 barg to 34.48 barg was measured throughout this temperature range. In order to evaluate the effect of supercritical formation and liquefaction on the stable isotope values, cylinders were filled to varying pressures. When stored at cold temperatures, the δ13C value as measured in the headspace of the liquid phase varied from -41.23 to -41.13 ‰ (VPDB) and -41.50 to -41.44 ‰ (VPDB) in the supercritical phase. The δ18O value was between -25.51 and -25.36 ‰ (VPDB) in the liquid phase and between -24.79 and -24.77 ‰ (VPDB) in the supercritical phase. Temperatures in these experiments were selected to mimic outdoor conditions (winter and summer) that stable isotope laboratory practitioners may encounter when storing compressed carbon dioxide cylinders containing stable isotope working reference gases. The carbon and oxygen isotope composition of carbon dioxide gas within these pressurized cylinders return to their precooled isotope values within ∼24 h when warmed to laboratory temperatures (∼24 °C). A headspace analysis performed immediately after the carbon dioxide cylinder was removed from the cold environment yielded δ13C values that were relatively enriched, while δ18O values were relatively depleted. This is likely an effect of 12C and 18O being preferentially partitioned in the liquid phase within the cylinder. As the cylinder warmed, both liquid and gas equilibrated, and carbon and oxygen homogenized isotopically. As the cylinder was heated into the supercritical phase, a slight opposite isotope effect at higher pressure and temperatures was noted. That is, a slight 13C depletion and 18O enrichment were observed in the gas phase. However, these isotope variations were just slightly outside of the analytical error. Additionally, a separate gas-phase carbon dioxide cylinder was kept at a constant laboratory temperature as a control. This carbon dioxide showed no measurable carbon or oxygen isotope variation throughout the duration of the experimental work. The measured isotope fractionation was significantly higher comparing the phase transition from the gaseous to liquid phase versus the gaseous phase to supercritical phase. The proper handling of pressurized carbon dioxide cylinders used as reference gases for an isotope ratio mass spectrometer includes using carbon dioxide at pressures of less than ∼34.88 barg to ensure that the gas is present as a single phase, storing the gas in a temperature-controlled environment, and allowing the gaseous carbon dioxide to equilibrate to ambient conditions for 24-48 h if storage in a controlled ambient environment is not feasible.

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 18O-water was added to natural-abundance, gaseous carbon dioxide. The δ18O-CO2 changed from -23.16‰ vs Vienna Pee Dee Belemnite (VPDB) to +109‰ 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 δ13C and δ18O remained consistent within ±0.04 and ±0.06‰, 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.

Compound-Specific Sulfur Isotope Analysis of Petroleum Gases.

We describe a simple, sensitive, and robust method for sulfur isotope ratio (34S/32S) analysis of ppm-level organic sulfur compounds (OSCs) in the presence of percent-level H2S. The method uses a gas chromatograph (GC) coupled with a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). The GC, equipped with a gas inlet and a valve that transfers the H2S to a thermal conductivity detector (TCD), enables a precise heart cut and prevents the saturation of the MC-ICPMS. The sensitivity and accuracy of the method are better than 0.3‰ for OSCs at a concentration of 25 pmol or 1.4 ppm, and better than 0.5‰ for concentrations ≥0.7 ppm of OSCs. An order of magnitude increase in sensitivity, with no effect on accuracy, can be achieved if the loop volume (0.5 mL) is changed to 5 mL. High concentrations of methane (95% v/v) and/or H2S (20% v/v) had no effect (within 0.5‰) on the precision and accuracy of the gas sample containing 2 ppm of OSCs after heart cut. The applicability and robustness of this method are demonstrated on a gas sample (10% v/v H2S) that was produced by pyrolysis of sulfur-rich kerogen. The results show good precision and reveal sulfur isotope variability between individual OSCs that may represent key processes during formation and degradation of OSCs.

The stability of 100 ppb hydrogen sulfide standards.

An experimental study has been performed to determine the factors affecting the stability of hydrogen sulfide calibration mixtures at concentrations below 1 ppm. Quantitative data was obtained that illustrates that material selection and surface treatment are major factors affecting the shelf life of low concentration reactive gas mixtures. With the proper material selection and surface treatment, hydrogen sulfide and carbonyl sulfide standard mixtures on the order of 50-100 ppb can be maintained for at least 18 months.