A new method for determining δ¹³C and δD simultaneously for CH₄ by gas chromatography/continuous‐flow isotope‐ratio mass spectrometry

Abstract

A continuous-flow technique has been developed to analyse the deltaD and delta(13)C values for CH(4) from gas samples, in a single run. This is achieved by splitting the sample gas stream and directing the streams simultaneously through a CuNiPt combustion reactor and an alumina pyrolysis reactor. The CO(2) from CH(4) combustion is trapped in a liquid nitrogen trap while the H(2) exiting the pyrolysis reactor is directed to the mass spectrometer for deltaD(CH4) determination. The CO(2) is then sublimed and directed to the mass spectrometer for delta(13)C(CH4) determination. Sample runs take approximately 10 minutes. This technique gives accurate delta(13)C(CH4) results to within +/-0.3-0.5 per thousand and deltaD(CH4) results to within +/-2-5 per thousand. Injection volumes between 0.5 and 2.5 microL of CH(4), equivalent to between 20 and 100 nmol CH(4), are required for accurate delta(13)C and deltaD analyses, respectively, using sample injection into a split flow with a split ratio of 10. This method provides rapid, accurate and reproducible results on multiple sample runs and is, therefore, an ideal method for analysing natural gas samples from a variety of sources.

Improving accuracy and precision of ice core δD(CH<sub>4</sub>) analyses using methane pre-pyrolysis and hydrogen post-pyrolysis trapping and subsequent chromatographic separation

“…If this cannot be achieved, one has to correct for any drift and signal dependency (e.g. Schmitt et al, 2003;Potter and Siemann, 2004;Bock et al, 2010a;Brass and Röckmann, 2010). In our case this is essential, because we observe a clear signal dependency of the δD(CH 4 ) values (Fig.…”

“…Air Controlé was cross-referenced to −93.6 ‰ with respect to Vienna standard mean ocean water (VSMOW) using bottled air from Alert station "Alert 2002/11" (Bock et al, 2010a;Poss, 2003;Marik, 1998), previously measured at the Institute of Environmental Physics in Heidelberg (IUP, University of Heidelberg, Germany). At the IUP two scales for δD(CH 4 ) co-exist; one is based on mass spectrometric measurements of water-derived H 2 (named MAT), and the other is based on methane-in-air gases measured using a tunable diode laser system (named TDLAS).…”

Gas chromatography flow rates for determining deuterium/hydrogen ratios of natural gas by gas chromatography/high‐temperature conversion/isotope ratio mass spectrometry

“…The deuterium/hydrogen (D/H) ratios of natural gas components (e.g., methane, ethane and propane) are valuable in geological and environmental studies1–4 due to their great usefulness for source evaluation. Since the technique of high‐temperature conversion (TC) was developed and proved to be able to convert H in organic compounds into hydrogen gas quantitatively,5 subsequently commercially available instruments6 using gas chromatography/high‐temperature conversion/isotope ratio mass spectrometry (GC/TC/IRMS) have been of more benefit to the determination of D/H ratios of natural gas3, 4, 7–10 than the previously available off‐line methods. Several studies have discussed the TC conditions that could have significant effects on the measured D/H ratios of various samples 5, 6, 8, 11.…”

“…Since the technique of high‐temperature conversion (TC) was developed and proved to be able to convert H in organic compounds into hydrogen gas quantitatively,5 subsequently commercially available instruments6 using gas chromatography/high‐temperature conversion/isotope ratio mass spectrometry (GC/TC/IRMS) have been of more benefit to the determination of D/H ratios of natural gas3, 4, 7–10 than the previously available off‐line methods. Several studies have discussed the TC conditions that could have significant effects on the measured D/H ratios of various samples 5, 6, 8, 11. Basically, before the ceramic tube (Al 2 O 3 ) in the TC reactor can be used for routine analysis, the tube must be graphitised (conditioned) by passing a relatively large volume of methane at the working temperature (>1400°C) through it 8, 9, 11.…”

“…Several studies have discussed the TC conditions that could have significant effects on the measured D/H ratios of various samples 5, 6, 8, 11. Basically, before the ceramic tube (Al 2 O 3 ) in the TC reactor can be used for routine analysis, the tube must be graphitised (conditioned) by passing a relatively large volume of methane at the working temperature (>1400°C) through it 8, 9, 11. This conditioning process is especially essential for hydrogen isotopic analyses of organic compounds with low carbon numbers 11.…”