Direct Quantification of Attogram Levels of Strontium-90 in Microscale Biosamples Using Isotope Dilution-Thermal Ionization Mass Spectrometry Assisted by Quadrupole Energy Filtering.

Although thermal ionization mass spectrometry (TIMS) has been employed for the high-precision analysis of isotope ratios, direct quantification of artificial mono-nuclide in the environment is difficult by even using isotope dilution (ID) due to the coexistence of the great magnitude of natural stable nuclides or isobars. In traditional TIMS and ID-TIMS, a sufficient amount of stable Sr doped on a filament is required to realize a stable and adequate ion-beam intensity (i.e., thermally ionized beams). However, the background noise (BGN) at / 90, detected by an electron multiplier, disturbs Sr analysis at low concentration levels due to peak tailing of a significant Sr ion beam dependent on the Sr-doping amount. Here, TIMS assisted by quadruple energy filtering was successfully employed for the direct quantification of attogram levels of an artificial monoisotopic radionuclide strontium-90 (Sr) in microscale biosamples. Direct quantification was achieved by integrating the ID quantification of natural Sr and simultaneous Sr/Sr isotope ratio analysis. Additionally, the measurement amount calculated by the combination of the ID and intercalibration was corrected for the net result amount of Sr by subtracting dark noise and the detected amount derived from the survived Sr, which are equivalent with the BGN intensity at / 90. Background correction revealed that the detection limits were in the range of 6.15 × 10-3.90 × 10 ag (0.31-1.95 μBq), depending on the concentration of natural Sr in a 1 μL sample, and the quantification of 0.98 ag (5.0 μBq) of Sr in 0-300 mg/L of natural Sr was successful. This method could analyze small sample quantities (1 μL), and the quantitative results were verified against authorized radiometric analysis techniques. Furthermore, the amount of Sr in actual teeth was successfully quantified. This method will be a powerful tool for measuring Sr in the measurement of micro-samples, which are required to assess and understand the degree of internal radiation exposure.