Indirect Tritium Determination by an Original 3He Ingrowth Method Using a Standard Helium Leak Detector Mass Spectrometer

A traditional helium leak detector mass spectrometer is applied in analytical chemistry. We report its straightforward use for the precise quantification of 3He and 4He concentrations at a sub-ppb level. Repetitive external calibrations with certified gases demonstrate long-term stability, reproducibility of 3% at the ppb range, and linearity versus helium concentration over 5 orders of magnitude. The mass spectrometric determination of 3He takes into account a H3+ contribution and has a 3He detection limit of 2 ppb. Since 4He induces an attenuation effect of the mass 3 signal, this apparatus is only suitable for the measurement of 3He/4He ratio greater than 10 Ra (atmospheric ratio Ra = 1.38 x 10(-6)). This mass spectrometric technique has found novel application in the determination of tritium concentrations with an original 3He ingrowth method. Here, samples are first purified cryogenically onto activated charcoal to eliminate tritium interfering during the 3He mass spectrometric determination step. Helium is also preconcentrated, and 3He is routinely determined around 0.05 ppb with a 20% relative uncertainty. This 3He measurement technique has been successfully applied in the field of tritiated waste drums as a nondestructive method. The total amount of tritium present in these 200-L drums can be determined as low as approximately 1 GBq.