The SIF provides δ¹³C isotope analysis of CH₄ and CO₂ gases in headspace vials using a Thermo Scientific GasBench-PreCon trace gas system interfaced to a Delta V Plus IRMS.

Analysis of ¹³C in Carbon Dioxide

Stable isotope ratios of δ¹³C in carbon dioxide are measured using a ThermoScientific PreCon-GasBench system interfaced to a ThermoScientific Delta V Plus isotope ratio mass spectrometer (ThermoScientific, Bremen, DE).  For sample concentrations greater than 500ppm, CO₂ is sampled by a six-port rotary valve (Valco, Houston TX) with a 100µL loop programmed to switch at the maximum CO₂ concentration in the helium carrier gas; for lower concentrations, the entire CO₂ content is frozen in a trapping loop then released to the GC column.  The CO₂ is then separated from N₂O and other residual gases by a Poroplot Q GC column (25m x 0.32mm ID, 45°C, 2.5 mL/min). A pure reference gas (CO₂) is used to calculate provisional delta values of the sample peak.  Final δ¹³C values are obtained after adjusting the provisional values such that correct δ¹³C values for laboratory standards are obtained.  Two laboratory standards are analyzed with every 10 samples.  The laboratory standards are calibrated directly against NIST 8545.

Analysis of ¹³C in Methane
Stable isotope ratios of δ¹³C in methane are measured using a ThermoScientific PreCon concentration system interfaced to a ThermoScientific Delta V Plus isotope ratio mass spectrometer (ThermoScientific, Bremen, DE). 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. The CH₄ is separated from residual gases by a Poroplot Q GC column (25m x 0.32mm ID, 30°C, 2.5 mL/min).  After CH₄ elutes from the separation column, it is oxidized to CO₂by reaction with nickel oxide at 1000°C and then transferred to the IRMS.  A pure reference gas (CO₂) is used to calculate provisional delta values of the sample peak.  Final δ13C  values are obtained after adjusting the provisional values such that correct δ13C values for laboratory standards are obtained.  Two laboratory standards are analyzed with every 10 samples. The laboratory standards are commercially prepared CH₄ gas diluted in helium and are calibrated directly against NIST 8560.

Limit of Quantitation and Long-term standard deviation for CO₂ & CH₄ Analysis by Trace Gas-IRMS

CH4 : Limit of Quantitation: approx. 300 picomoles

           Long-term standard deviation: 0.2 ‰

CO2 : Limit of Quantitation: approx. 150 nanomoles (without cryofocusing)

           Long-term standard deviation: 0.02 ‰

Maximum measurable gas concentrations are dependent upon both concentration and isotopic enrichment.  Please contact us if you intend to submit enriched samples at concentrations more than ten times that of ambient concentration.