Development and application of mass spectrometric techniques for ultra-trace determination of ²³⁶U in environmental samples-A review

Measurements of the long-lived radionuclide ²³⁶U are an important endeavor, not only in nuclear safeguards work, but also in terms of using this emerging nuclide as a tracer in chemical oceanography, hydrology, and actinide sourcing. Depending on the properties of a sample and its neutron irradiation history, ²³⁶U/²³⁸U ratios from different sources vary significantly. Therefore, this ratio can be treated as an important fingerprint for radioactive source identification, and in particular, affords a definitive means of discriminating between naturally occurring U and specific types of anthropogenic U. The development of mass spectrometric techniques makes it possible to determine ultra-trace levels of ²³⁶U in environmental samples. In this paper, we review the current status of mass spectrometric approaches for determination of ²³⁶U in environmental samples. Various sample preparation methods are summarized and compared. The mass spectrometric techniques emphasized herein are thermal ionization mass spectrometry (TIMS), inductively coupled plasma mass spectrometry (ICP-MS) and accelerator mass spectrometry (AMS). The strategies or principles used by each technique for the analysis of ²³⁶U are described. The performances of these techniques in terms of abundance sensitivity and detection limit are discussed in detail. To date, AMS exhibits the best capability for ultra-trace determinations of ²³⁶U. The levels and behaviors of ²³⁶U in various environmental media are summarized and discussed as well. Results suggest that ²³⁶U has an important, emerging role as a tracer for geochemical studies.