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

Abstract

Abstract. Firn and polar ice cores offer the only direct palaeoatmospheric archive. Analyses of past greenhouse gas concentrations and their isotopic compositions in air bubbles in the ice can help to constrain changes in global biogeochemical cycles in the past. For the analysis of the hydrogen isotopic composition of methane (δD(CH 4 ) or δ 2 H(CH 4 )) 0.5 to 1.5 kg of ice was hitherto used. Here we present a method to improve precision and reduce the sample amount for δD(CH 4 ) measurements in (ice core) air. Preconcentrated methane is focused in front of a high temperature oven (pre-pyrolysis trapping), and molecular hydrogen formed by pyrolysis is trapped afterwards (post-pyrolysis trapping), both on a carbon-PLOT capillary at −196 • C. Argon, oxygen, nitrogen, carbon monoxide, unpyrolysed methane and krypton are trapped together with H 2 and must be separated using a second short, cooled chromatographic column to ensure accurate results. Pre-and post-pyrolysis trapping largely removes the isotopic fractionation induced during chromatographic separation and results in a narrow peak in the mass spectrometer. Air standards can be measured with a precision better than 1 ‰. For polar ice samples from glacial periods, we estimate a precision of 2.3 ‰ for 350 g of ice (or roughly 30 mL -at standard temperature and pressure (STP) -of air) with 350 ppb of methane. This corresponds to recent tropospheric air samples (about 1900 ppb CH 4 ) of about 6 mL (STP) or about 500 pmol of pure CH 4 .

Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4ice core records

“…1 and Fig. S1, confirm previous results but largely extend the time coverage and are based on data with improved precision and accuracy (46)(47)(48). Our results are consistent with δ 13 CH 4 data from Greenland Ice Sheet Project 2 (GISP2) (49) and EDML (32) for the Holocene (MIS 1), the last termination, and the last glacial maximum (LGM) (Fig.…”

“…Note that there is a north-south IPD for δD(CH 4 ) of roughly −16‰ for the Holocene (46). Interestingly, glacial/ interglacial amplitudes in δD(CH 4 ) can be either of similar amplitude (LGM-Holocene: ∼14‰) as swings from stadial to interstadial conditions (e.g., during the glacial inception around 390 ka BP in Fig.…”

Interlaboratory comparison of <i>δ</i><sup>13</sup>C and <i>δ</i>D measurements of atmospheric CH<sub>4</sub> for combined use of data sets from different laboratories

“…This value is consistent with the intercomparisons between UB and IMAU reported by Bock et al (2010). Later UB modified the measurement set-up, but the measurements of same air samples before and after all modifications were in good agreement as presented by Bock et al (2014). The intercomparison in this study (Sect.…”

“…Later, a method based on a continuous-flow gas chromatography isotope ratio mass spectrometry (GC-IRMS) technique combined with combustion and pyrolysis furnaces became available (Merritt et al, 1995;Burgoyne and Hayes, 1998;Hilkert et al, 1999), which dramatically reduced time and effort in the laboratory and likewise the amount of sample air required (now typically 100 mL STP ). Such systems are now used in most laboratories worldwide to acquire δ 13 C-CH 4 and δD-CH 4 data in the current and past atmosphere (Rice et al, 2001;Miller et al, 2002;Sowers et al, 2005;Ferretti et al, 2005;Morimoto et al, 2006;Fisher et al, 2006;Umezawa et al, 2009;Brass and Röckmann, 2010;Sperlich et al, 2013;Schmitt et al, 2014;Bock et al, 2014;Brand et al, 2016;Röckmann et al, 2016). Although these systems use a similar measurement principle, they vary in the use of pre-concentration of CH 4 in sample air, GC separation and combustion/pyrolysis, data corrections and in the specific IRMS instrument among laboratories (see Schmitt et al, 2013, Sect.…”

Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample

“…To analyse isotopes of CH 4 and N 2 O on ice core samples, several methods using different extraction techniques have been developed (Sowers et al, 2005;Schaefer and Whiticar, 2007;Behrens et al, 2008;Melton et al, 2011;Sapart et al, 2011;Sperlich et al, 2013;Bock et al, 2014). In contrast, only a few techniques for ice core samples have been established to analyse trace gas species at the ppt level, such as ethane, propane, and methyl chloride (Aydin et al, 2002;Saito et al, 2006;Aydin et al, 2007).…”