A new UV-Oxidation set up for AMS radiocarbon analysis for small dissolved organic carbon in marine and fresh water samples

Abstract

Radiocarbon measurements of dissolved organic carbon (DOC) can give us valuable information about origin and age of DOC, a major, yet little understood component in the global carbon cycle. One way to measure DOC in water is to remove dissolved inorganic carbon first, oxidize organic carbon with UV irradiation and ultimately analyses the formed inorganic CO2 for 14C. The main challenge of UV-Oxidation (UVox) methods is to extract the typically low concentrations of DOC with low blanks required for relatively high precision 14C measurements. A disadvantage of currently used UVox methods is that only one sample can be oxidized in a laborious process at the same time in large volume. Here we present a UV-Oxidation system where up to 12 water samples can be oxidized simultaneously in 12 separate quartz reactors arranged around a single UV lamp in a compact setup. The simple setup further uses helium instead of vacuum typically used by conventional extraction lines to speed up the extraction of the formed CO2 after oxidation. The key improvements of the new UVox setup are: 1) Reduced amount of water needed (30- 60 ml) as samples are measured for 14C with the Micadas gas ion source., 2) UV oxidation efficiency for standards is high (96%), 3) No KI trap is needed, 4) Required time for sample preparation of up to 12 samples is 4-6 h, 5) combined the CO2 from different reactors to one trap. We obtained 2.6 ± 0.6 µgC with F14C= 0.27±0.05 for processing blank. We will present the first measurments of DOC samples from Swiss lakes, Canadian Beaufort Sea and the reproducibility of the line.