Isotopic analysis of atmospheric CH4 (δ13C and δ2H)

Isotopic analysis of atmospheric CH₄ (δ¹³C and δ²H)

Simultaneous isotopic analysis of atmospheric methane and carbon dioxide

Instrument combination for the analysis of air components. — **Figure 1:** Integrated System for the Analysis of Air Constituents” (iSAAC). At a flow rate of 5 ml/min ~ 100 ml of sample are consumed per isotope. The system is designed to consume very small amounts of sample out of 1 L glass sampling flasks.

**Figure 1:** Integrated System for the Analysis of Air Constituents” (iSAAC). At a flow rate of 5 ml/min ~ 100 ml of sample are consumed per isotope. The system is designed to consume very small amounts of sample out of 1 L glass sampling flasks.

Methane is one of the most potent greenhouse gases. Its sources and sinks are not yet fully understood and isotopic analyses are a good tool to further our understanding in this regard. The BGC-IsoLab has developed the “integrated System for the Analysis of Air Constituents” (iSAAC; Brand et al., 2016). ISAAC is designed to analyse the carbon and hydrogen isotopic composition of atmospheric CH₄, and the carbon and oxygen isotopic composition of atmospheric CO₂ from one 1 liter sample flask (Figure 1).

A total of 200 ml of sample gas are needed to analyse all isotopes of CO₂ and CH₄. Samples are introduced and transferred through an intricate array of acquisition and analysis lines. Four atmospheric CO₂ aliquots are taken during one sample aquisition and analysed against reference air on “Chiara”, the IRMS dedicated to isotopes of CO₂. Throughout the same sample run CH₄ is trapped cryogenically in two traps. The gas from one trap is oxidised to CO₂ and and analysed on “Chiara”, while the gas from the other trap is pyrolised to H₂ and analysed on “Diana” the IRMS dedicated to δ²H analyses. Details of the measurement principles can be found in Brand et al. (2016).

Figure 3: Q/C chart of iSAAC covering all measurements since 2013 of the Q/C syn. air tank “Arida”. (a) The δ13C and δ18O of atmospheric CO2 data is given on the JRAS-06 scale realisation of the δ13CVPDB scale. (b) The δ13C and δ2H data of atmospheric methane is given on the δ13CVPDB and δ2HVSMOW/SLAP scales, respectively. — **Figure 3:** Q/C chart of iSAAC covering all measurements since 2013 of the Q/C syn. air tank “Arida”. (a) The δ¹³C and δ¹⁸O of atmospheric CO₂ data is given on the JRAS-06 scale realisation of the δ¹³C_VPDB scale. (b) The δ¹³C and δ²H data of atmospheric methane is given on the δ¹³C_VPDB and δ²H_VSMOW/SLAP scales, respectively.

**Figure 3:** Q/C chart of iSAAC covering all measurements since 2013 of the Q/C syn. air tank “Arida”. (a) The δ¹³C and δ¹⁸O of atmospheric CO₂ data is given on the JRAS-06 scale realisation of the δ¹³C_VPDB scale. (b) The δ¹³C and δ²H data of atmospheric methane is given on the δ¹³C_VPDB and δ²H_VSMOW/SLAP scales, respectively.

Isotopic measurements of atmospheric CO₂ are standardised on the JRAS-06 scale (see JRAS). The measurement precision for δ¹³C and δ¹⁸O is usually better than 0.05 and 0.08 ‰, respectively. Hydrogen and carbon isotopic measurements of atmospheric CH₄ are standardised against methane standards that have been developed in the BGC-IsoLab (Sperlich et al., 2016). The δ¹³C and δ²H isotopic composition of methane gas was analysed against primary standards by Element-Analysis- (EA-) and High-Temperature Conversion- (HTC-) isotope ratio mass spectrometry. These standardised methane gases were then mixed into methane-free air and used to calibrate the working standard gas that is used for iSAAC. Thus the δ¹³C isotopic results of methane are given on the δ¹³C_VPDB scale, while the δ²H results are given on the δ²H_VSMOW/SLAP scale. Long-term measurement precision for isotopes of methane is 0.1 and 1.2 ‰ for δ¹³C and δ²H, respectively .

Isotopic analysis of atmospheric CH4 (δ13C and δ2H)

Further reading:

Isotopic analysis of atmospheric CH₄ (δ¹³C and δ²H)