Efficient Collection of Methane from Extremely Large Volumes of Water for Natural Radiocarbon Analysis

Abstract:

Collecting sufficient amounts of natural methane sample for a high precision radiocarbon (¹⁴C-CH₄) analysis was previously unfeasible when sampling from low methane concentration waters like the open ocean. A new method incorporating dissolved gas extraction technology (Liqui-Cel^® membrane contactors) has been developed to circumvent the challenges that natural ¹⁴C-CH₄ sampling presents. With this method, adequate amounts of methane-carbon for a traditional ¹⁴C-accelerator mass spectrometry (AMS) analysis can be cleanly and efficiently extracted from 1000s L water in a few hours. This technique is currently being improved to enable sampling from > 11,000 L water in less than 1 hr. For transport from the field to the laboratory, each extracted gas sample is compressed into a small (1.68 L) high-pressure aluminum cylinder using an oil-free compressor pump. Due to the small size and portability of the sample cylinders, high resolution sampling plans composed of 30+ samples are possible even in remote locations. The laboratory preparation of these methane samples for ¹⁴C-AMS analyses is carried out on a new flow-through vacuum line. While the bulk water vapor and carbon dioxide (CO₂) are removed before the sample is compressed in the field, the residual trace amounts of these constituents are cryogenically removed from the sample in the initial phase of the vacuum line. Carbon monoxide in the sample is quantitatively oxidized at 290°C to CO₂ and cryogenically removed. Finally, the sample methane is quantitatively oxidized at 950°C to products CO₂ and water and then cryogenically isolated. The new vacuum line technique achieves low blanks and purifies and oxidizes the methane contained in the extracted gas sample with high efficiency. At an AMS facility, an aliquot of the methane-produced CO₂ is graphitized and analyzed for radiocarbon content using traditional ¹⁴C-AMS. Supporting dual-inlet isotope ratio mass spectrometry measurements are conducted to determine both the δ¹³C of the methane-produced CO₂ and the δ²H of the methane-produced water.