Improved method for radium extraction from environmental samples and its analysis by thermal ionization mass spectrometry

Abstract

We have developed an efficient procedure for the chemical separation of ²²⁶Ra from environmental samples and its measurement by thermal ionization mass spectrometry. The original step in this procedure consists in a pre-concentration by co-precipitation of radium onto manganese dioxide. This pre-concentration step permits an efficient separation of radium from major elements including, most importantly, calcium. Thus, the present protocol provides a very useful technique for the precise and accurate analysis of radium-226 in carbonate samples. Furthermore, this procedure is also well-suited for seawater samples, since it does not require a preliminary evaporation stage, thereby eliminating problems due to the precipitation of large quantities of salts and permits the analysis of femtogram levels of ²²⁶Ra.

Reproducibility and accuracy of ²²⁶Ra measurements were estimated by replicate analysis of a reef coral skeleton and seawater samples. The external reproducibility obtained on the carbonate sample was ±0.55% at the 95% confidence level (n = 9) using between 300 to 600 fg of ²²⁶Ra. The chemical yield averages 80% for carbonate matrix. The accuracy of the measurements of ²²⁶Ra in the reef coral skeleton (dated by the ²³⁰Th–²³⁴U method) were assessed by comparing the measured ²²⁶Ra with the expected ²²⁶Ra activity assuming a closed system behavior for U-series. For seawater samples, the external reproducibility was 2.3% (2σ, n = 5) using less than 10 fg of ²²⁶Ra (i.e., ca. 200 ml of seawater).

The ability to extract with a high efficiency radium from carbonate and seawater samples allows wider investigations of the use of ²²⁶Ra either as a Holocene chronometer for carbonates or as a tracer of oceanic processes.