CH₄ & CO₂ Analysis by Trace Gas-IRMS

The SIF provids ¹³C isotope analysis of CH₄ and CO₂ gases in headspace vials using a Sercon Cryoprep system interfaced to an IRMS.

Analysis of ¹³C in Methane

Stable isotope ratios of ¹³C in methane are measured using a SerCon Cryoprep TGII trace gas concentration system interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK). Gas samples are purged from vials through a double-needle sampler into a helium carrier stream (20 mL/min), which is passed through a CO₂ scrubber (Ascarite) and a cold trap cooled by liquid nitrogen. Methane is oxidized to CO₂ by reaction with nickel oxide at 1000°C. The CO₂ is passed to the IRMS through a Poroplot Q GC column (15m x 0.53mm ID, 25°C, 3 mL/min). A reference CO₂ peak is used to calculate provisional delta values of the sample CO₂ peak. Final δ ¹³C values are obtained after adjusting the provisional values such that correct δ ¹³C values for laboratory standards are obtained. Two laboratory standards are analyzed with every 10 samples. The laboratory standards are CH₄ gas diluted in helium (e.g., 500 µL CH₄ in 12 mL He). The CH₄ is calibrated against NIST 8560.

Analysis of DIC ¹³C in Water

Water samples (4 mL) are injected into evacuated 12 mL septum capped vials (Exetainers, Labco, High Wycombe, UK) containing 1 mL 85% phosphoric acid, which forces the equilibrium between CO₂ and H₂CO₃ to gaseous CO₂. The CO₂ is purged from vials through a double-needle sampler into a helium carrier stream (20 mL/min). The gas is sampled by a six-port rotary valve (Valco, Houston TX) with a 250 µL loop programmed to switch at the maximum CO₂ concentration in the helium carrier. The CO₂ is passed to the IRMS through a Poroplot Q GC column (15m x 0.53mm ID, 25°C, 3 mL/min). A reference CO₂ peak is used to calculate provisional delta values of the sample CO₂ peak. Final δ ¹³C values are obtained after adjusting the provisional values such that correct δ ¹³C values for laboratory standards are obtained. Two laboratory standards are analyzed with every 10 samples. The laboratory standards are pure CO₂ gas (calibrated against an Oztech CO₂ standard) dissolved in de-ionized water.

Pricing

Sample preparation

Submit samples in septum capped vials preferably at atmospheric pressure (~100 kPa) or at pressures up to 200 kPa. Overpressurized vials may burst during shipping. Preferred vials are 12 mL Exetainer (Labco Ltd, High Wycombe, UK), but others are acceptable by arrangement. Vials should be evacuated prior to sample collection. Label each vial with a unique ID# as your data will reference your samples using this ID.

Shipping

Carefully package groups of exetainers in ziplock bags, small boxes, or their original boxes. Then package these sets in a larger sturdy box with styrofoam peanuts or bubble wrap. Make sure the samples are very secure, loose vials can shatter during shipping. These gas samples do not require refrigeration during shipping. Coolers will not be returned.

Analysis Order Form

Please include an Analysis Order Form with any samples you deliver to the Stable Isotope Facility. The SIF uses this form to track your samples and to contact you regarding receipt of samples, data, and invoicing. For mixed analysis requests, we recommend completing a separate form for each type of analysis.

Shipping and contact information:
David Harris
UC Davis Stable Isotope Facility
Department of Plant Sciences
One Shields Avenue, Mail Stop 1
Davis, CA 95616,
USA

Phone:(530) 754-7517, Fax: (530) 752-4361
E-mail: dharris@ucdavis.edu

Supplies

*Note: Labco has a new vial style that ends in ###W/W. We DO NOT recommend these vials when sending samples to the SIF because the vials are too wide for our autosampler rack. Supplier: Labco UK + International or Labco USA + Canada

References

Atekwana, E.A., Krishnamurthy, R.V. 1998. Seasonal variations of dissolved inorganic carbon and δ¹³C of surface waters: application of a modified gas evolution technique. Journal of Hydrology 205: 265-278

Tips

1) Avoid over-tightening vial caps. Vials will leak when caps are over-tightened, causing the septa to pucker and pull away from the vial. Turn caps until snug, but not so far that the septa start to pucker.

2) Do not apply silicone grease, adhesives, or sealant to the tops of the vials. These materials clog the autosampler needle. The process of removing these sealants increases the risk of losing your gas sample due to dislodged septum.