Boreas: A Sample Preparation-Coupled Laser Spectrometer System for Simultaneous High-Precision In Situ Analysis of...

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 a typical standard measurement uncertainty for an ambient air sample of 0.07‰ for δ¹³C(CH₄) and 0.9‰ for δ²H(CH₄), which are 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 500-626 μmol mol^-1 (CH₄ in N₂) made from a single pure CH₄ source that has been isotopically characterized 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.