On the interference of Kr during carbon isotope analysis of methane using continuous-flow combustion–isotope ratio mass spectrometry

[摘要] Stable carbon isotope analysis of methane (δ¹³C of CH₄) onatmospheric samples is one key method to constrain the current and pastatmospheric CH₄ budget. A frequently applied measurement technique isgas chromatography (GC) isotope ratio mass spectrometry (IRMS) coupled to acombustion-preconcentration unit. This report shows that the atmospherictrace gas krypton (Kr) can severely interfere during the mass spectrometricmeasurement, leading to significant biases in δ¹³C of CH₄,if krypton is not sufficiently separated during the analysis. According toour experiments, the krypton interference is likely composed of twoindividual effects, with the lateral tailing of the doubly charged ⁸⁶Krpeak affecting the neighbouring m/z 44 and partially the m/z 45 Faraday cups.Additionally, a broad signal affecting m/z 45 and especially m/z 46 is assumed toresult from scattered ions of singly charged krypton. The introduced bias inthe measured isotope ratios is dependent on the chromatographic separation,the krypton-to-CH₄ mixing ratio in the sample, the focusing of the massspectrometer as well as the detector configuration and can amount to up toseveral per mil in δ¹³C. Apart from technical solutions to avoidthis interference, we present correction routines to a posteriori remove thebias.

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