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

We present a new instrument, "Boreas", a cryogen-free methane (CH) preconcentration system coupled to a dual-laser spectrometer for making simultaneous measurements of δC(CH) and δH(CH) in ambient air. Excluding isotope ratio scale uncertainty, we estimate a typical standard measurement uncertainty for an ambient air sample of 0.07‰ for δC(CH) and 0.9‰ for δH(CH), which are the lowest reported for a laser spectroscopy-based system and comparable to isotope ratio mass spectrometry. We trap CH (∼1.9 μmol mol) from ∼5 L of air onto the front end of a packed column, subsequently separating CH from interferences using a controlled temperature ramp with nitrogen (N) as the carrier gas, before eluting CH at ∼550 μmol mol. This processed sample is then delivered to an infrared laser spectrometer for measuring the amount fractions of CH, CH, and CHD 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 (CH in N) made from a single pure CH source that has been isotopically characterized for δC(CH) 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 δC(CH) and δH(CH) in over 200 ambient air samples and demonstrate the application of Boreas for deployment to atmospheric monitoring sites.