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

Abstract

The use of ICP-MS for determination of 129I in environmental samples is complicated by interference from 129Xe+ originating from Xe present as an impurity in Ar plasma gas and by 127IH2+ derived from 127I 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 129I determination. The response characteristics of DRC-ICP-MS to test solutions with various 129I and 127I concentrations were investigated after instrument optimization. We eliminated 129Xe+ interference by using O2 as the reaction gas, and we effectively reduced 127IH2+ interference by applying a negative AFT voltage and removing traces of H2 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 129I concentration was nonlinear, and we attributed the nonlinearity to sensitivity degradation due to abundant 127I in the matrix. Using a Rh internal standard, we corrected the sensitivity degradation and obtained a linear relationship between mass 129/mass 103 and 129I concentration. The detection limit for 129I in a 1 mg mL−1 127I matrix was 15.2 pg mL−1, which is equivalent to an 129I/127I ratio of 1.5 × 10−8. The developed method was used to determine 129I in soil samples obtained from the land surrounding the nuclear fuel reprocessing plant in Tokai, Japan. The measured 129I/127I ratios (2.6 × 10−8 to 4.7 × 10−7) agreed with the ratios determined by tandetron accelerator mass spectrometry.