Development of the He-3 mass spectrometric low-level tritium analytical facility at the IAEA

Here we present a mass spectrometric system specifically developed for the analysis of ultra-low-level tritium by the He-3 ingrowth method. The system was designed and developed in the Isotope Hydrology Laboratory of the International Atomic Energy Agency and consists of an off-line water degassing unit to remove pre-existing He-3 from sample water and a mass spectrometer system (Thermo Fisher Helix SFT) with a gas purification and separation system. The mass spectrometer system is equipped with a gas pipette system that inlets calibrated amounts of He-3 (99.995% He-3 spike) to accurately calibrate the mass spectrometer's sensitivity. The procedural blank level of He-3 in our system is extremely low and on the order of 10(-17) cm(3) STP, which enables us to quantify tritium in water samples (100 cm(3)) as low as 0.05 TU with an ingrowth time of 2 months. Quantification of even lower tritium levels are possible by loading more water and/or by increasing the ingrowth time (e.g., 0.01 TU with 400 cm(3) water stored for 4 months). We analysed a set of water samples from the Tritium Intercomparison Exercise by IAEA (TRIC 2018) and confirmed that our data were consistent with the expected values. We have also confirmed that our analysis of natural groundwater samples agree well with the data obtained by the conventional liquid scintillation counting (LSC) method.