Energy Resources Program
Dai, J., Xia, X., Li, Z., Coleman, D. D., Dias, R. F., Gao, L., Li, J., Deev, A., Li, J., Dessort, D., Duclerc, D., Li, L., Liu, J., Schloemer, S., Zhang, W., Ni, Y., Hu, G., Wang, X., and Tang, Y., 2012. Inter-laboratory calibration of natural gas round robins for δ2H and δ13C using off-line and on-line techniques. Chemical Geology 310-311, 49–55.
• The determination of the number and the molecular and isotopic composition of the gases used as reference materials.
• The choice of subsampling and sample distribution methods.
• The selection of the analytical methods used for the calibration of the gases.
• The evaluation of the analytical data.
The members of the technical advisory committee are listed below (Table 1). All aspects of the standard reference gas development were reviewed by the advisory committee. Due consideration was given to all recommendations, and every effort was made to comply with the consensus view of the committee. The USGS is indebted to all of the members of the advisory committee for their contribution to the successful development of these standards. Although the committee provided valuable oversight throughout the development of the standards; ultimately, the USGS is solely responsible for the scientific integrity of the standard gases and the process that led to their development.
Table 1: Natural Gas Standards Advisory Committee
*Left committee before project was completed.
Standard Gas Sample Selection
After extensive consultation with the technical advisory committee, the ideal molecular and stable isotopic compositions of the new standard reference materials were identified (Table 2). The targeted compositions and values represent a series of compromises among selection of specific compounds, identification of the most useful isotopic range, and the sheer volume of work required to provide a proper calibration. The inclusion of non-hydrocarbon gases (particularly N2 and CO2) was considered and determined not to be a priority given the availability of other standards for these compounds. Furthermore, heavier hydrocarbon homologues (specifically iso- and normal butanes and pentanes) were discussed as additional potential compounds of interest for inclusion in these standards. Although standard reference materials for the stable isotopic composition of butane and pentane gases do not currently exist, the consensus view of the committee was that the potential negatives associated with inclusion of these compounds outweighed the benefits. In particular, the heavier hydrocarbon components present a risk of phase separation under certain pressure and temperature conditions which complicates the storage, handling, and distribution of the standards. Moreover, it was difficult to justify the significant increase in the amount of analytical work required to calibrate these additional components when most analytical methods used to determine their stable isotopic composition are directly tied to methane, ethane, or propane.
It was determined that "synthetic" gas mixtures were more likely than produced natural gas samples to be able to meet the desired criteria (Table 2), and that, to the extent possible, the new gas standard mixtures should be fabricated from pure components with the desired isotopic compositions. The molecular compositions of the three mixtures were selected such that the gas components were equimolar on a carbon basis to facilitate the stable isotopic analysis of the standards by modern continuous-flow GCirMS methods.
Table 2: Target molecular and stable isotopic composition of new gas standards
Mix 1 – light: After an exhaustive attempt to identify a method for fabricating an isotopically light mixture that closely matched the target composition, it was determined that using a produced natural gas would be the most cost-effective method for obtaining this gas mixture. The closest match to the target composition for the isotopically light gas mixture that could be found was a produced natural gas sample from a biogenic gas field in Northern Colorado. It was concluded that this gas was the best material that could be obtained for the isotopically light end-member standard, given the fiscal and logistical constraints of the project. The mole % composition of the gas is typical of a natural bacterial gas and is labeled as USGS HCG-3 (U.S. Geological Survey Hydrocarbon Gas 3)
Mix 2 – intermediate: The intermediate gas mixture was created by locating sources of the pure component gases that had stable isotopic compositions that most closely matched the target compositions. These pure components were then blended together in the desired proportions to create the intermediate gas mix. Although the d2H of the ethane in this mix is significantly heavier (~50‰) than the target value, this was the lightest value that could be found. Given the difficulty associated with trying to create isotopically depleted compositions, it was determined that this was as close to the target values for the intermediate mix as was feasible. This intermediate gas mixture has been labeled USGS HCG-2.
Mix 3 – heavy: The heavy gas mixture was created by adding small aliquots of isotopically enrichCed (13C and 2H) methane, ethane, or propane into additional cylinders of the pure components that were used to make the intermediate mix. These heavy components were then blended together to make the heavy gas mix, USGS HCG-1.
The calibrated values for the gas standards were derived from analytical work performed at the USGS Energy Geochemistry Laboratory in Denver, Colorado and Isotech Laboratories, Inc. in Champagne, Illinois. The hydrocarbon gas analytes were directly measured against the inorganic carbonate standards NBS-19 and LSVEC for carbon and the SMOW-SLAP scale for hydrogen.
Both laboratories were provided duplicate pressurized stainless steel cylinders of the individual components (methane, ethane, and propane) that comprise the mixtures USGS HCG-1 and HCG-2, for a total of twelve (12) individual gas samples. These gases were prepared for dual-inlet isotopic analyses on a manual vacuum line. Briefly, this involved the combustion of the individual hydrocarbons to CO2 and water and the subsequent reduction of water to H2 by zinc reduction (400°C, 60 min).
The isotopically light gas-standard (USGS HCG-3) is a produced natural gas from a biogenic gas field in northern Colorado. As such, it contains a mixture of methane, ethane, and propane and required splitting into the component hydrocarbon gases prior to offline combustion and analysis. The component isolation from this gas mixture was done at Isotech Laboratories by means of a preparatory GC system. This gas mixture was split into the individual hydrocarbon gases, combusted to carbon dioxide and water, and distributed to the USGS laboratory for analysis. A total of eighteen (18) glass ampoules (one carbon dioxide and one water sample for each of methane, ethane, and propane, each in triplicate) was distributed to the USGS laboratory for reduction of the water to hydrogen gas for d2H analysis of the hydrogen and d13C analysis of the carbon dioxide. Similar sample preparation (i.e., water reduction to H2) and analysis was performed at Isotech Laboratories, Inc.
Additionally, samples of the NGS gases (NIST RM 8559, RM 8560, and RM8561) were prepared and analyzed by the same methods as the USGS gas standards. This allows for the new reference gases to be directly tied to the NGS gases previously in circulation.
The carbon and hydrogen stable isotopic composition of the CO2 and H2 derived from the combustion of the hydrocarbon gases and subsequent reduction of water were measured by dual-inlet magnetic-sector isotope ratio mass spectrometry (Thermo MAT253). The CO2 was analyzed directly against CO2 produced from the H3PO4 digestion (30°C, 24h) of NBS 19. The H2 gases were analyzed against SMOW and SLAP prepared (reduced to H2) by the same method as the hydrocarbon-derived water. Each sample and primary standard was analyzed in replicate in order to establish the statistical significance of each result. The arithmetic mean of the replicate analyses of each individual component is taken as the calibrated value for the gas standards as reported in Table 3.
Table 3. Suggested reference values for the new USGS hydrocarbon gas mixtures. These values are solely based on offline preparation methodology and dual inlet analyses. The isotope values for carbon are relative to VPDB, and for hydrogen are relative to VSMOW. The mole percent compositions are not calibrated and are for informational use only.
Placing an Order
Sub-sampling of the standard gas tanks and distribution of the samples only takes place two times per year (assuming that there are orders to fill). This is typically in April and October of each year; however, exact dates vary. Orders will be accepted at any time. Given the limited quantity of gas available, purchasers are restricted to one purchase every three years. Gases are distributed in 50 mL stainless steel vessels equipped with one (1) standard ¼” NPT fitting and pressurized to approximately 50-75 psi. At this time, gases must be ordered as a set of all three standard reference materials. Individual reference gases may be offered for sale at some later date. Technical questions related to the gas standards and information regarding placing an order should be directed to:
Dr. Robert Dias
U.S. Geological Survey
P.O. Box 25046
Denver Federal Center, MS 977
Denver, CO 80225 USA
Tel: 1 303 236-9361
Fax: 1 303 236-3202
The next fill period will be April 2017; Cut-off for orders will be March 31, 2017
Individuals requesting to place an order for the gas standards will receive an order form to be completed in full and returned to Mirian Cuara via regular mail or fax. A user agreement form will then be sent to the purchaser for signature and must be returned to Ms. Cuara. Purchases by other agencies of the U.S. Government must be identified prior to completing the order form, as separate procedures apply.
In order to comply with the United States Economy Act, 31 USC 1535A, the USGS Technology Transfer Office requires that all purchasers of USGS natural gas standards complete and sign an agreement stating that the acquisition of the reference materials from the USGS falls within the authority of the agency to enter into cooperative research and development agreements. User agreement forms will be sent with blank order forms and should accompany each order.
The USGS will arrange and cover the cost of shipping for orders within the US and Canada. International orders are not currently being accepted. It is anticipated that international orders will be accepted in the near future; however, the purchaser will be responsible for making all of the arrangements and covering all of the expenses associated with international sample shipping. If special packaging and labelling are required, it will be the responsibility of the purchaser to notify the USGS and, if requested, supply any additional shipping materials. There are private companies that, for a fee, will make all of the arrangements for international shipping of hazardous materials such as these gas samples. The USGS may be able to refer purchasers to these third parties, but in no way endorses any particular company, nor is in any way responsible for their performance or actions. Currently, only orders from U.S. entities are being accepted for processing.
Payment for USGS natural gas standards can only be made by major credit card (Visa, Mastercard, or American Express). Credit cards will not be charged until the order has been filled.
Preliminary Certificate of Analysis
A full report on the development and certification of the natural gas standards is currently in preparation. A preliminary certificate of analysis containing the calibrated values of the gas standard is available here: USGS Natural Gas Standards Certificate
USGS Geochemical (Mineral) Reference Materials
USGS Reston Stable Isotope Laboratory
USGS Energy Geochemistry Laboratories
USGS Energy Resources Program
Other Links to Gas Standard Materials
Cambridge Isotope Laboratories
Indiana University Stable Isotope Reference Materials
International Atomic Energy Agency Reference Materials
NIST – National Institute of Standards and Technology Standard Reference Materials
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