An automated GC-C-GC-IRMS setup to measure palaeoatmospheric δ<sup>13</sup>C-CH<sub>4</sub>, δ<sup>15</sup>N-N<sub>2</sub>O and δ<sup>18</sup>O-N<sub>2</sub>O in one ice core sample

Abstract

Abstract. Air bubbles in ice core samples represent the only opportunity to study the mixing ratio and isotopic variability of palaeoatmospheric CH4 and N2O. The highest possible precision in isotope measurements is required to maximize the resolving power for CH4 and N2O sink and source reconstructions. We present a new setup to measure δ13C-CH4, δ15N-N2O and δ18O-N2O isotope ratios in one ice core sample and with one single IRMS instrument, with a precision of 0.09, 0.6 and 0.7‰, respectively, as determined on 0.6–1.6 nmol CH4 and 0.25–0.6 nmol N2O. The isotope ratios are referenced to the VPDB scale (δ13C-CH4), the N2-air scale (δ15N-N2O) and the VSMOW scale (δ18O-N2O). Ice core samples of 200–500 g are melted while the air is constantly extracted to minimize gas dissolution. A helium carrier gas flow transports the sample through the analytical system. We introduce a new gold catalyst to oxidize CO to CO2 in the air sample. CH4 and N2O are then separated from N2, O2, Ar and CO2 before they get pre-concentrated and separated by gas chromatography. A combustion unit is required for δ13C-CH4 analysis, which is equipped with a constant oxygen supply as well as a post-combustion trap and a post-combustion GC column (GC-C-GC-IRMS). The post-combustion trap and the second GC column in the GC-C-GC-IRMS combination prevent Kr and N2O interferences during the isotopic analysis of CH4-derived CO2. These steps increase the time for δ13C-CH4 measurements, which is used to measure δ15N-N2O and δ18O-N2O first and then δ13C-CH4. The analytical time is adjusted to ensure stable conditions in the ion source before each sample gas enters the IRMS, thereby improving the precision achieved for measurements of CH4 and N2O on the same IRMS. The precision of our measurements is comparable to or better than that of recently published systems. Our setup is calibrated by analysing multiple reference gases that were injected over bubble-free ice samples. We show that our measurements of δ13C-CH4 in ice core samples are generally in good agreement with previously published data after the latter have been corrected for krypton interferences.

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

“…The Centre for Ice and Climate (CIC) of the Niels Bohr Institute has reported δ 13 C-CH 4 measurements from ice cores (Sperlich et al, 2015) using a GC-IRMS system with measurement reproducibility of 0.09 ‰ (Sperlich et al, 2013). CIC also set up an offline combustion system for samples with a large amount of CH 4 , which is combined with DI-IRMS for δ 13 C-CH 4 and with either a high temperature conversion/elemental analyser (TC/EA) coupled to IRMS or laser spectroscopy for δD-CH 4 (Sperlich et al, 2012); the measurement reproducibility is 0.04 for δ 13 C-CH 4 and 0.7 ‰ for δD-CH 4 .…”

“…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

“…We set it to optimise pumping efficiency while limiting excessive water from entering the water trap. Low pressure is the main advantage of continuous vacuum extraction compared to melting under He overpressure and is necessary to extract gases with high solubilities in water at high extraction efficiency (Kawamura et al, 2003;Sperlich et al, 2013). During the melting of the ice sample the total pressure in the vessel (sum of partial pressures of H 2 O and air) ranges between 12 mbar at the start and 7 mbar at the end.…”

Automated simultaneous measurement of the δ13C and δ2H values of methane and the δ13C and δ18O values of carbon dioxide in flask air samples using a new multi cryo‐trap/gas chromatography/isotope ratio mass spectrometry system

“…On glacialinterglacial time scales of the last 800,000 years, the CH 4 mixing ratio ranged between 350 and 700 ppb, [24] which would require even larger sample volumes. Furthermore, the measurement of CH 4 isotope ratios in ice-core samples is technically challenging due to the requirement to quantitatively extract sample air from only a few 100 g of available sample ice, [25][26][27][28][29] often exhibiting large isotopic variations on centennial to glacial-interglacial time-scales. [28,30,31] Considerable progress in terms of reducing the amount of sample required for methane isotopic analysis was made in the mid-1990s with the advent of continuous flow and gas chromatographic (GC) techniques [32][33][34][35][36][37] using ultra-pure helium as carrier gas.…”

“…[33] Based on this principle, a number of semi-to fully automated devices have been built and described, all aimed at routinely analyzing the methane isotopes from air or ice-core samples with the required high precision. [16,25,26,[37][38][39][40][41][42][43][44][45][46] In this contribution we describe the development, system test and operation of the first isotope analysis system capable of a simultaneous determination of both carbon and hydrogen isotopes from CH 4 in air samples in a fully automated fashion. The usual flask size of the system is less than 5 L, in many cases only 1 L, so that the grab samples of the existing MPI-BGC observation program that are analyzed for many other parameters including greenhouse gas concentrations, their isotopes, and the ratios O 2 /N 2 and Ar/N 2 can now also be analyzed for both isotope ratios of CH 4 .…”