Determination of Trace Technetium-99 in Soil Samples by Isotope Dilution/Negative Thermal Ionization Mass Spectrometry

As an important long-lived and high-abundance fission product, ⁹⁹Tc dispersed into the environment because of the nuclear test in atmosphere, the nuclear fuel cycle and nuclear medicine.The isotope dilution mass spectrometry （IDMS）technology of measuring ⁹⁹Tc using ⁹⁷Tc as the spike was developed. The chemical separation procedure of Tc is established, and the recovery rate is more than 71% for the overall process. ⁹⁷Tc （⁹⁷Tc, 84.77%; ⁹⁸Tc, 15.03%; ⁹⁹Tc, 0.20%）was prepared by irradiation of natural ruthenium metal, then was calibrated by IDMS with ⁹⁹Tc standard material as the spike. The conventional positive thermal ionization mass spectrometry(TIMS) was inapplicable due to the high ionization potential of 7.3 eV for technetium, therefore based on MgCl₂+Ba(OH)₂ as the loading reagent, the negative thermal ionization mass spectrometry of technetium was established, and the ion current indensity of ⁹⁹TcO₄^-（M/z=163） reached 1.7×10^-12A. Then, ⁹⁹Tc in the soil samples was quantitatively determined by negative thermal ionization mass spectrometry and isotope dilution method. The combined standard uncertainty is better than 3%.