Direct radiocarbon measurements of dissolved inorganic carbon from environmental water using a gas ion source

An increasing demand for radiocarbon analysis of small samples has led to the development of various methods to further improve and simplify the CO₂ extraction needed for accelerator mass spectrometer (AMS) measurements. Here, the performance of a direct feeding system of CO₂ from dissolved inorganic carbon (DIC) from small water samples (<6 ml) and direct AMS measurement via a gas ion source coupled to a gas interface system (GIS) is presented and compared to the conventional preparation by graphitization that demands significantly larger samples (> 60 ml).

Seawater samples collected from Sargasso Sea, southeast of and lake water samples collected from Lake Constance have been prepared by both methods. The extraction of CO₂ gas from samples for GIS measurement is performed using a carbonate handling system (CHS-Ionplus AG) through purging the headspace, acidifying the water, and sparging out the CO₂. The preparation time is greatly reduced compared to conventional analysis that requires the labor-intensive graphitization step. The yielded ¹⁴C results from the direct CO₂ measurements are in good agreement with values obtained from graphite measurements. The observed deviation between the two methods is below the uncertainty of radiocarbon gas measurement (~7‰).

This new approach will facilitate understanding of carbon cycle dynamics in many different environments and applications where a high throughput (up to 80 sample/day) is required. The new method is suitable for groundwater, pore water, seawater, freshwaters from lakes, rivers and glacial settings. Furthermore, it enables the analysis of small milliliter-scale samples and those containing low DIC concentrations.