Rapid sample preparation of dissolved inorganic carbon in natural waters using a headspace-extraction approach for radiocarbon analysis by accelerator mass spectrometry

Pan Gao, Xiaomei Xu, Liping Zhou, Mary A. Pack, Sheila Griffin, Guaciara M. Santos, John R. Southon, Kexin Liu

Limnol. Oceanogr. Methods 12:174-190 (2014) | DOI: 10.4319/lom.2014.12.174

ABSTRACT: We have established a high-throughput headspace-extraction method for the preparation of dissolved inorganic carbon (DIC) from water samples for radiocarbon (14C) analysis by accelerator mass spectrometry (AMS). Readily available septum-sealed screw cap vials were used for sample processing. Headspace-equilibrated gases with sample CO2 were transferred using a syringe and cryogenically purified on a vacuum line for graphitization and 14C-AMS measurements in the Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS/UCI). Systematic investigations have shown that the extraction process does not introduce contaminants that could bias the 14C measurements and that the 14C results for standards are consistent with their consensus values. Large numbers of duplicate measurements have established a precision of 1.7‰ for modern samples with an average background of ~43,400 radiocarbon year for graphite target samples > 0.3 mg carbon. Seawater samples collected from Newport Beach, California, and processed using the headspace-extraction method yielded 14C results in excellent agreement with published values obtained with conventional DIC stripping (≤ ± 2σ). The simplicity of our headspace-extraction approach allows its easy adaptation/implementation to any isotope lab, as we demonstrate here by a series of tests carried out at Peking University, China (PKU). With this innovative method, just 30 mL seawater per sample is needed. Coupled with the sealed tube zinc reduction method, 15 water samples can be prepared and graphitized in 1 day.