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UC Davis Plant Sciences


Analysis of Methane (CH4) by GasBench-Precon-IRMS

The SIF provides δ13C and δ2H analysis of methane (CH4) in headspace vials using a Thermo Scientific GasBench-Precon interfaced to a Delta V Plus IRMS.

 

Analysis of 13C and 2H in Methane (CH4)
Stable isotope ratios of carbon (δ13C) and hydrogen (δ2H) in methane are measured using a ThermoScientific Precon concentration unit interfaced to a ThermoScientific Delta V Plus isotope ratio mass spectrometer (ThermoScientific, Bremen, Germany). Gas samples are purged from vials through a double-needle sampler into a helium carrier stream (20 mL/min), which is passed through a H2O / CO2 scrubber (Mg(ClO4)2 , Ascarite) and a cold trap (90 cm piece of coiled divinylbenzene 0.32 mm GS-Q column) cooled by liquid nitrogen. The CH4 is separated from residual gases by a GS-CarbonPLOT GC column (30 m x 0.32 mm x 3 μm, 30°C, 1.5 mL/min).  After CH4 elutes from the separation column, methane is either oxidized to CO2 by reaction with nickel oxide at 1000°C (δ13C), or pyrolyzed in an empty alumina tube heated to 1400°C (δ2H) and subsequently transferred to the IRMS.  A pure reference gas (CO2 or H2) is used to calculate provisional delta values of the sample peak.  Final δ-values are obtained after adjusting the provisional values for changes in linearity and instrumental drift such that correct δ-values for laboratory reference materials are obtained. Laboratory reference materials are commercially prepared CH4 gas diluted in helium or air and are calibrated against NIST 8559, 8560, and 8561. Final δ-values, delivered to the customer, are expressed relative to the international standards V-PDB (Vienna PeeDee Belemnite) for carbon and V-SMOW (Vienna-Standard Mean Ocean Water) for hydrogen.

 

Limit of Quantitation and Long-term standard deviation for CH4 Analysis by GasBench-Precon-IRMS

13C-CH4 : Limit of Quantitation: approx. 0.8 nanomoles

                   Long-term standard deviation: 0.2 ‰

 

2H-CH4 : Limit of Quantitation: approx. 2 nanomoles

                   Long-term standard deviation: 2 ‰

 


Maximum measurable methane content (without sample dilution or sub-sampling) is 100 nmoles for δ13C and δ2H, at natural abundance. Actual sample measurement limits are dependent upon both gas concentration and isotopic enrichment.  Please contact us if you intend to submit isotopically enriched samples at gas concentrations more than ten times that of ambient concentration.


References

C. Yarnes. 2013 δ13C and δ2H measurement of methane from ecological and geological sources by gas chromatography-combustion/pyrolysis-isotope-ratio mass spectrometry.  Rapid Commun. Mass Spectrom. 27, 1036–1044. DOI: 10.1002/rcm.6549




 
e-mail: sif@ucdavis.edu | phone: 530-752-8100 | fax: 530-752-4361
UC Davis Stable Isotope Facility | Department of Plant Sciences
One Shields Avenue | Davis, California, 95616 | USA