Cylinders containing natural gas standards

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As the cost of foreign oil rises, the discovery of new U.S. fuel sources becomes an economic necessity. Chemical and isotopic measurements of trace levels of gas emanating from sediment are a recognized but underdeveloped means to locate and appraise hidden petroleum and fuel gas reserves.

The ratio between the two stable isotopes of carbon, 12C and 13C, is systematically variable in natural systems. There is a preference for the lighter isotope in biological processes, so highly bioprocessed materials, such as hydrocarbon gases associated with a petroleum reserve, are measurably depleted in 13C. In fact, each individual compound will have a distinctive isotopic composition based upon its particular origin and formation history. Compound-specific isotopic measurements can thereby provide insights to the nature and capacity of the reserve.

While chemical measurements of natural gas are generally routine, the reliability of compound-specific isotopic measurements has been questionable. An attempt to address this issue was carried out in the early 1980s by several international organizations, including the Chevron Oil Company, Azienda Generale Italiana Petroli, and the International Atomic Energy Agency. This collaboration led to the collection of the so-called Natural Gas Standards (NGS), which were samples from three isotopically distinct sources of natural gas. While the availability of the NGS materials was a significant advance, the difficulty in chemical separations and isotopic analysis, as well as differences in data evaluation methods, led to wide dispersion in intercomparison results that prevented the assignment of precise isotopic values. This shortcoming limited the usefulness of these materials and created the need to characterize these materials with greater attention to method design and isotope metrology.

Eleven NIST isotope reference values (and four NIST informational values) for carbonaceous compounds in the three materials were based on measurements by outside laboratories using different methods in collaboration with NIST. These collaborators were Isotech Laboratories in Champaign, IL, and Exxon-Mobil Upstream Research Corporation in Houston, TX. The raw measurements were transmitted to NIST for evaluation and conversion to standardized results. Great attention was placed on assuring that all measurements were strongly traceable to the internationally-accepted carbon isotope ratio scale. For traceability, we utilized NIST RM 8563, the carbon dioxide isotopic standard material derived from petroleum that has a carbon-13 composition similar to the hydrocarbons in the NGS materials. We also utilized a standard computational method for converting the isotopic measurements of carbon dioxide into standardized carbon-13 results. This computational method is now accessible as a NIST web-based tool.

RMs 8559-8561 are supplied as compressed natural gas (about 2 grams) in 50 mL stainless steel cylinders, and are intended for use in instrument calibrations and for quality assurance in Gas Chromatography-Isotope Ratio Mass Spectrometry (GC-IRMS), a recently commercialized technique that combines the chemical specificity of gas chromatography with the isotopic specificity and sensitivity of isotope ratio mass spectrometry.

Besides helping petroleum exploration, these RMs may be used to improve the quality of GC-IRMS measurements of atmospheric methane, a greenhouse gas linked to climate change issues. Isotopic measurements, taken at strategic locations throughout a region, are used to help identify and apportion the individual sources of methane. The ability to compare consistent and reliable measurements across laboratories and time is critical to the strategies that will be formulated to address the issues.

For ordering information, please contact the NIST Standard Reference Materials Program.

For technical information, please contact Mike Verkouteren.