Long‐Term Performance and Bulk‐Isotope Effects in Methane Clumped‐Isotope Measurements Using a 253 Ultra HR‐IRMS

Rationale Changes in instrument condition over time may affect methane clumped‐isotope measurements, requiring long‐term monitoring of calibration performance. Whether the 253 Ultra calibration framework remains valid over multiyear operation and applicable across natural bulk‐isotope compositions, particularly to low‐δD methane with weak 12 CH 2 D 2 signals, has not been systematically evaluated. Methods Long‐term performance of the Science Tokyo 253 Ultra was evaluated from 2019 to 2026 using reference‐gas zero tests, repeated working‐gas measurements, and independently prepared equilibrated methane gases. Equilibrated gases with different δ 13 C and δD compositions were used to test for possible bulk‐isotope dependence in Δ 13 CH₃D and Δ 12 CH 2 D 2 measurements. External consistency was assessed using nine shared methane gases measured across seven laboratories with 253 Ultra HR‐IRMS, Panorama HR‐IRMS, and mid‐infrared laser spectroscopy. Results Reference and working gases showed no temporal drift, and equilibrated gases measured from 2021 to 2026 remained consistent with the calibration framework established in 2020. No systematic dependence of Δ 13 CH₃D or Δ 12 CH 2 D 2 on either δ 13 C or δD was resolved, including for equilibrium gases with δD values as low as −335‰. Science Tokyo measurements were generally consistent with results from other laboratories across a wide range of bulk‐isotope compositions and nonequilibrium clumped‐isotope signatures. Conclusions The calibration framework established for methane clumped‐isotope measurements with the Science Tokyo 253 Ultra HR‐IRMS remained stable and applicable over multiple analytical campaigns. Validation measurements and interlaboratory comparisons demonstrated consistent results across a broad range of methane bulk‐isotope compositions, without the need for campaign‐specific or additional bulk‐isotope–dependent corrections, with δD values extending down to −335‰.