Determination of I-129 in soil samples by DRC-ICP-MS

The use of ICP-MS for determination of I-129 in environmental samples is complicated by interference from Xe-129(+) originating from Xe present as an impurity in Ar plasma gas and by (IH2+)-I-127 derived from 127 I in samples. Collision cell and reaction cell techniques have been used to reduce such interference. Here, we used ICP-MS with the dynamic reaction cell (DRC) and axial field technology (AFT) to improve I-129 determination. The response characteristics of DRC-ICP-MS to test solutions with various I-129 and I-127 concentrations were investigated after instrument optimization. We eliminated Xe-129(+) interference by using O-2 as the reaction gas, and we effectively reduced (IH2+)-I-127 interference by applying a negative AFT voltage and removing traces of H-2 and H2O from the reaction gas. Consequently, the background count ratio (mass 129/mass 127) decreased to the 10(-8) level. However, the relationship between the signal at mass 129 and I-129 concentration was nonlinear, and we attributed the nonlinearity to sensitivity degradation due to abundant I-127 in the matrix. Using a Rh internal standard, we corrected the sensitivity degradation and obtained a linear relationship between mass 129/mass 103 and I-129 concentration. The detection limit for I-129 in a 1 mg mL(-1) I-127 matrix was 15.2 pg mL(-1), which is equivalent to an I-129/I-127 ratio of 1.5 x 10(-8). The developed method was used to determine I-129 in soil samples obtained from the land surrounding the nuclear fuel reprocessing plant in Tokai, Japan. The measured I-129/I-127 ratios (2.6 x 10(-8) to 4.7 x 10(-7)) agreed with the ratios determined by tandetron accelerator mass spectrometry.