CleanEx: A Versatile Automated Methane Preconcentration Device for High-Precision Analysis of ¹³CH₄, ¹²CH₃D, and ¹³CH₃D

Abstract

The relative abundance of methane isotopologues offers key insights into the global methane (CH 4 ) cycle. Advances in laser spectroscopy enable routine high-precision measurements even for rare deuterated methane isotopologues, 12 CH 3 D and 13 CH 3 D, provided there are sufficiently high methane amount fractions and reproducible measurement conditions, which can be achieved by CH 4 adsorption−desorption techniques. We present a new cryogen-free automated preconcentration device�CleanEx�designed for quantitative extraction of CH 4 from large volumes of sample gas and for cleaning by stepwise temperature-controlled desorption to separate interferant gases. We show that CleanEx has the capability to preconcentrate methane by almost 2000-fold from ∼18 L of air. The performance is demonstrated in a range of methane amount fractions between 2 ppm (μmol mol −1 ), which corresponds to the present-day ambient air, up to 1000 ppm, representative for close to source or process conditions. Advantages over existing devices are a significantly larger primary adsorption trap and a secondary cryo-focusing step, which ensures separation of methane from major atmospheric compounds, i.e., O 2 , Ar, and CO 2 . We have demonstrated quantitative extraction of methane, with no significant isotopic fractionation and high repeatability of 0.2‰, 0.6‰, and 0.8‰ (n = 42) for the studied isotopologue ratios, 13 CH 4 / 12 CH 4 , 12 CH 3 D/ 12 CH 4 , and 13 CH 3 D/ 12 CH 4 , during cryogenic adsorption−desorption on HayeSep D material. The developed device in combination with a suitable laser spectrometer offers a robust and autonomous method for precise continuous monitoring of δ 13 C-CH 4 and δD-CH 4 in ambient air and optionally Δ 13 CH 3 D in process-derived methane.M ethane (CH 4 ) is both an important fossil and regenerative energy source and a potent greenhouse gas and air pollutant. This poses multiple research questions with respect to origin and conditions of formation in energy exploration 1,2 but also in atmospheric budgeting. 3 Isotope ratios 13 C/ 12 C and D/H measured through singly substituted 13

Fractionation of Methane Isotopologues during Preparation for Analysis from Ambient Air

“…For ambient air analysis, the use of laser based systems (with preconcentration sample preparation) to achieve high precision CH 4 isotope ratio analysis and high resolution IRMS measurements for multiply substituted CH 4 isotopologue ratios are growing analytical fields. ,,,, Both of these new areas of study rely on rigorous artifact-free sample preparation. This work highlights the tolerances for CH 4 recovery in cryogenic trapping systems when measuring isotopic ratios at high precision and suggests the mechanisms behind isotopic fraction which could help in the design of future sample preparation methods for minimizing isotopic fractionation before analysis.…”

“…HayeSep-D is a high purity macroporous divinylbenzene resin that is widely used for separation of light gases in gas chromatography. It has also been used for cryogenic preconcentration of halogenated trace gases, , nitrous oxide (N 2 O), CH 4 , ,, and more recently CH 4 from large sample volumes using a second, smaller HayeSep-D refocusing trap . Most of the trap tubing is helically wound onto a hollow copper standoff that is cooled by a Cryotel GT Stirling Cryocooler (CryoTel GT Sunpower Inc., USA); the trap configuration is shown schematically in Figure .…”

“…It has also been used for cryogenic preconcentration of halogenated trace gases, 26 , 27 nitrous oxide (N 2 O), 31 CH 4 , 23 , 24 , 28 and more recently CH 4 from large sample volumes using a second, smaller HayeSep-D refocusing trap. 32 Most of the trap tubing is helically wound onto a hollow copper standoff that is cooled by a Cryotel GT Stirling Cryocooler (CryoTel GT Sunpower Inc., USA); the trap configuration is shown schematically in Figure 1 . Temperature is measured by temperature controllers (Omega Engineering CNi32) recording the output of two type-E thermocouples attached inside the standoff and to the outside of the trap tubing by thermally conductive epoxy (Omegabond 101, Omega Engineering).…”

Sub-ppm Methane Detection with Mid-Infrared Slot Waveguides

“…Utilizing a LAS spectrometer based on a quantum cascade laser and a 76 m long multipass cell, methane has been traced down to impressive 12 ppt . Real-time monitoring of 13 CH 4 , CH 3 D, or even double-substituted “clamped” methane isotopes in ambient air has been realized in combination with a preconcentration unit. , NASA has deployed a LAS spectrometer with a sub-ppb limit of detection (LOD) on Mars to search for methane emissions from organic matter, and a shoebox-sized instrument with a LOD of 0.1 ppb has been recently demonstrated for drone-conducted measurements . However, when it comes to chip-scale devices, the state-of-the-art methane detection limits lie above 10 ppm, which translates into a 6 orders of magnitude difference in sensitivity compared to that of bulk instruments.…”