Preparation of natural methane samples for stable isotope and radiocarbon analysis

John D. Kessler, William S. Reeburgh

Limnol. Oceanogr. Methods 3:408-418 (2005) | DOI: 10.4319/lom.2005.3.408

ABSTRACT: We describe procedures for preparing methane samples from natural waters or sediments for radiocarbon (14C-CH4) analysis. These procedures also allow the determination of 2H/1H and 13C/12C ratios of this methane. Methane is extracted from natural samples in the field using Lamont radon stripping boards (Mathieu et al. 1988) and trapped using liquid nitrogen-cooled U-traps filled with HiSiv 3000. The field procedures eliminate the need to return large samples to the laboratory. The U-traps are returned to the laboratory where a new twostage purification-oxidation vacuum line is used to extract, purify, and oxidize the methane. The methane oxidation products (carbon dioxide and water) are analyzed for the natural content of 14C, 2H, and 13C with standard procedures. These procedures were evaluated under the conditions that methane was extracted from between 19 to 114 L water or 0.07 to 0.35 L sediment. No isotope fractionation was observed and the blanks are small (0.528 ± 0.39 µmol methane with radiocarbon content 14C/C = 96.1 ± 0.3 pMC [percent Modern Carbon] [Stuiver and Polach 1977]). Using 14C-accelerator mass spectrometry (AMS), these procedures can produce a successful radiocarbon measurement when at least 1.81 µmol methane are collected. Thus, 14C-CH4 measurements can be made when the dissolved methane concentration is between saturation (mM) and 15.9 nM for the water column or 5.2 µM for the sediment.