Cryogen-Aided Chromatographic Purification of Millimole Quantities of Methane for Spectroscopic Clumped Isotope...

Quantum cascade laser absorption spectroscopy (QCLAS) is a fast and reliable method for analyzing the bulk (𝛿¹³C-CH₄, 𝛿D-CH₄) and clumped isotopic (∆¹³CH₃D and ∆¹²CH₂D₂) composition of methane. However, precise measurements of ∆¹²CH₂D₂ require 0.5 to 1.8 mmol of purified methane (equivalent to 15 to 40 mL at STP). We present a cryogen-aided preparative gas chromatography (GC) method with 1 h cycle time that quantitatively separates methane from complex gas mixtures containing N₂, O₂, Ar, CO₂, H₂O, and volatile higher alkanes.

The method employs two sequential Carboxen 1000 columns, precooled to -10°C and subsequently ramped to 150°C in two heating steps, coupled with cryofocusing on charcoal and cryocollection on silica gel. The method performance was evaluated using a well-characterized in-house methane reference gas, either diluted in air-He matrices or stored in Exetainer vials for up to 3 weeks. Purification tests on thermogenic and biogenic methane samples assessed the repeatability of the purification method across different natural gases.

Cooling of the GC columns to -10°C achieves complete chromatographic purification of millimole quantities of methane from contaminant gasses. The processing introduces no measurable bulk (𝛿¹³C-CH₄, 𝛿-CH₄) and clumped-isotope (∆¹³CH₃D and ∆¹²CH₂D₂) fractionation within the 1σ repeatability of the QCLAS system. The repeatability of methane isotope measurements from natural gas samples is comparable to that obtained in tests using the well-characterized in-house reference gas.

Chromatographic purification of millimole quantities of methane from synthetic and natural gas mixtures requires GC column cooling to at least -10°C to ensure sufficient separation from major air components. The developed method enables rapid and quantitative purification of up to 40 mL of methane without detectable isotope fractionation, demonstrating the applicability and robustness of the method for routine purification prior to QCLAS-based methane clumped-isotope analysis.