Continuous-flow isotope ratio mass spectrometry method for carbon and hydrogen isotope measurements on atmospheric methane

Abstract

Abstract. We describe a continuous-flow isotope ratio mass spectrometry (CF-IRMS) technique for high-precision δD and δ 13 C measurements of atmospheric methane on 40 mL air samples. CH 4 is separated from other air components by utilizing purely physical processes based on temperature, time and mechanical valve switching. Chemical agents are avoided. Trace amounts of interfering compounds can be separated by gas chromatography after pre-concentration of the CH 4 sample. The purified sample is then either combusted to CO 2 or pyrolyzed to H 2 for stable isotope measurement. Apart from connecting samples and refilling liquid nitrogen as coolant the system is fully automated and allows an unobserved, continuous analysis of samples. The analytical system has been used for analysis of air samples with CH 4 mixing ratios between ∼100 and ∼10 000 ppb, for higher mixing ratios samples usually have to be diluted.

Simultaneous stable isotope analysis of methane and nitrous oxide on ice core samples

“…Nevertheless, the advent of continuous-flow isotope ratio mass spectrometry (IRMS) (Merritt et al, 1995), has led to the development of numerous analytical systems that only require small amounts of sample (e.g. Rice et al, 2001; al., 2002; Röckmann et al, 2003b;Brass and Röckmann, 2010) and subsequent analytical systems for ice core analyses (Bernard et al, 2006;Schaefer et al, 2006;Ferretti et al, 2005;Sowers, 2001Sowers, , 2006Sowers et al, 2003;Bock et al, 2010b;Behrens et al, 2008). These developments have resulted in a number of interesting findings in the past years Mischler et al, 2009;Sowers, 2001Sowers, , 2006Sowers, , 2010Sowers et al, 2003;Bernard et al, 2006;Ferretti et al, 2005;Bock et al, 2010a;Schäfer et al, 1998) which have, for example, ruled out a strong contribution of marine clathrate decomposition to paleoatmospheric methane changes as observed in ice core air.…”

“…The CH 4 IRMS system is a fully automated analytical set-up for δD(CH 4 ) or δ 13 (CH 4 ) analyses (Brass and Röckmann, 2010). This system has been used for various laboratory and atmospheric studies in the past (Vigano et al, , 2009(Vigano et al, , 2010Keppler et al, 2006Keppler et al, , 2008.…”

“…The CH 4 released is thus transferred to the CRYOFOCUS unit ( Fig. 1), while other condensable gases are still retained on the Hayesep D. The cooling/heating system is explained in detail by Brass and Röckmann (2010). The CRYOFOCUS unit is a 1/16 gas chromatography (GC) column (Poraplot Q) cooled to −158 • C where CH 4 is focused to obtain a narrow peak and remaining interferences are separated and removed (Brass and Röckmann, 2010).…”

“…), Length (L) = 360 mm) at a temperature of +1300 • C. During pyrolysis, CH 4 is converted to hydrogen (H 2 ) and carbon (C). C is deposited on the inner surface of the alumina tube, which promotes an efficient pyrolysis and H 2 production (Brass and Röckmann, 2010). For δ 13 C measurements, CH 4 is combusted to CO 2 in an alumina tube (1 mm i.d., L = 320 mm) at +900 • C. Three Nickel wires (0.25 mm outer diameter (o.d.…”

“…These values are then corrected for non-linearity when needed (see below). The reference air cylinders have been independently calibrated versus international standards as explained in Brass and Röckmann (2010) and Kaiser et al (2003). Linearity tests are performed at least twice a week on both IRMS systems by running twenty-five analyses of various volumes (5 to 40 ml for the CH 4 IRMS and 20 to 333 ml for the N 2 O IRMS) of reference air NAT335 (for N 2 O IRMS, 326 ppb) and NAT325 (for CH 4 IRMS, 1970 ppb) covering the investigated sample range.…”