Boreas: A Sample Preparation-Coupled Laser Spectrometer System for Simultaneous High-Precision In Situ Analysis of δ13C and δ2H from Ambient Air Methane.

We present a new instrument, ‘Boreas’, a cryogen-free methane (CH4) preconcentration system coupled to a dual laser spectrometer for making simultaneous measurements of δ13C(CH4) and δ2H(CH4) in ambient air. Excluding isotope ratio scale uncertainty, we estimate typical standard measurement uncertainty for an ambient air sample of 0.07 ‰ for δ13C(CH4) and 0.9 ‰ for δ2H(CH4), which is the lowest reported for a laser spectroscopy-based system and comparable to isotope ratio mass spectrometry. We trap CH4 (~1.9 μmol mol-1) from ~5 l of air onto the front end of a packed column, subsequently separating CH4 from interferences using a controlled temperature ramp with nitrogen (N2) as the carrier gas, before eluting CH4 at ~550 μmol mol-1. This processed sample is then delivered to an infrared laser spectrometer for measuring the amount fractions of 12CH4, 13CH4 and 12CH3D isotopologues. We calibrate the instrument using a set of gravimetrically prepared amount fraction primary reference materials directly into the laser spectrometer that span a range of 500 μmol mol-1 to 626 μmol mol-1 (CH4 in N2) made from a single pure CH4 source that has been isotopically characterised for δ13C(CH4) by IRMS. Under the principle of identical treatment a compressed ambient air sample is used as a working standard and measured between air samples, from which a final calibrated isotope ratio is calculated. Finally, we make automated measurements of both δ13C(CH4) and δ2H(CH4) in over 200 ambient air samples and demonstrate the application of Boreas for deployment to atmospheric monitoring sites.